Bike System Troubleshooting Notes

Common bike issues, with causes, things to check for, explanations, and fixes.

Synopsis

Wheel

My tire went flat, or keeps going flat after patching or replacing! (For wheels with inner tubes.)

Issue Check Fix
Something sharp punctured the tire Carefully look and feel for any remaining splinters along the outside and inside of the tire Remove the splinter, and replace or patch the tube
Low pressure caused a pinch flat Two mirrored punctures appear on the tube Replace or patch the tube. Try to keep the tire inflated
Tube creased during installation caused a pinch flat Two mirrored punctures appear on the tube Improve installation technique: examine tube before seating tire; add soapy water; give the tube more body during installation
Tube not contained within tire before inflation explodes A long tear appears on the tube Check around the tire bead before inflation
Rim tape leaves valve hole exposed The tube has a hole at the base of the valve; the rim valve hole has a sharp edge Replace the rim tape, or add extra protection
Mis-positioned or wrong size rim tape/strip Tube has a puncture facing inward; the rim tape doesn’t fit the channel, or isn’t straight Re-position or replace the rim tape
Valve is leaking or damaged Hold the valve underwater, or check a removable core for tightneses Snug down a removable core, replace a damaged removable core (if the tip is damaged), or replace the tube
Tire sidewall has hole Look for frays or holes in the tire sidewalls; rubbing brake pads are a common cause Patch the tire sidewall with a tire boot or other patch, or replace the tire
Presta valve used with schrader rim Look for extra clearance around the presta valve, or if a schrader will fit Add a presta → schrader adapter sleeve
Debris inside tire Look and feel around the tire, rim, and tube for any debris Clean out the tire, tube, and rim
Sharp edge inside rim Look around the rim for sharp edges, exposed spoke ends or nipples, or torn rim tape Sand down any edges, and replace any torn rim tape

Brakes

My brakes are rubbing against the rim while I’m biking

Issue Check Fix
Brake pads have too little clearance for the rim’s variation No brake adjustment avoids pad rubbing, while allowing braking within 1/2 the lever’s travel True the wheel
Wheel is mounted off-center Check the rim’s centering in the fork or dropouts, and the wheel for a dish if adjustment doesn’t work Reinstall the wheel; look for anything preventing the axles from fully seating
Brake is mounted off-center, or is loose During braking, one pad touches the rim before the other Remount the brake and center it
Cable/housing moves the brake out of centered position Move the housing by hand to see how the brake responds to this Recenter the brakes, or trim or shift the housing
Brakes can’t re-open all the way Pull the brakes open, seeing if they budge Lubricate or fully service the brakes

My brakes are squeaking or noisy when I brake (rim)

Issue Check Fix
Pad needs toe-in Braking is super noisy and squeaking! At the rim, pads are parallel or tilted to the back of the bike Adjust the pads; sometimes a shim at the back can angle the pad
Pad dirty or glazed over Braking is slow, and makes a whooshing sound. Pad surface shiny, hard, or cracked Resurface the pad or replace them
Rim is dirty Braking is slow, and makes a whooshing sound. Scrub the rim with degreaser or isopropyl
Pad has embedded metal/debris Braking makes a rubbing sound. Pad has splinters in it. Pick debris out of the pad, or replace them

My rim brakes are not working well

Issue Check Fix
Brake pads mis-mounted Check that the pad lands on the rim, without a significant angle or cant Adjust the pad to the rim
Cable and housing run seized up Check levers for difficulty, and compare to squeezing brakes by hand Replace cable and housing, or lubricate
Housing worn out Squeeze lever hard after pads close on rim, looking and feeling for housing flex/gumminess Replace the housing, and run no more housing than is needed
Pad dirty or glazed over Pad surface shiny, hard, or cracked Resurface the pad or replace them
Rim is dirty   Scrub the rim with degreaser or isopropyl
Brake is mounted off-center See if one pad touches before the other while the brake closes, or the arms appear tilted Center and re-mount the brake
Wheel is mounted off-center Check the rim’s centering in the fork or dropouts, and the wheel for a dish if adjustment doesn’t work Re-install the wheel, or correct any dish
Brake pivots seized or too tight Test the brakes by squeezing them by hand, and pulling them back open Lubricate all pivot points with a light lubricant; adjust any tension nuts and service the brakes as needed
(less common) Lever pivot seized up or too tight Pull the lever with the brake or unhooked Loosen any hinge nut, and lubricate the hinge pin against any bushing
(less common) Levers and brakes are mismatched Compare the brake and lever specs; examine the lever for short vs. long pull (25mm vs 35mm from hinge to cable) Pair the brake and lever correctly

My disc brakes are not working well

Issue Check Fix
Housing worn out, or
cable and housing run seized up		 Squeeze lever hard after pads close on rim, looking and feeling for housing flex/gumminess

Check levers for difficulty, and compare to moving brake arm by hand		 Replace housing

Replace cable and housing, or lubricate
Pads worn See if lever achieves braking after 1/2 of the travel; check if pads appear distant from the rotor; measure pads for under 1mm in thickness Advance pads with knobs and adjust the caliper mount to take up pad wear.

Replace the pads if necessary
Wheel mis-aligned Rotor rubs against pads abruptly after wheel is installed Re-install wheel into dropouts, and see if rotor is somewhere better
Caliper seized Move caliper arm by hand, and feel for difficulty or an incomplete/slow return Service the caliper
Caliper mis-aligned or mis-mounted During braking, look for pads hitting rotor at an angle, or offset from braking strip; see if rotor flexes side-to-side Re-mount the caliper; correct any misplaced washers
Hydraulic lines need bleed Squeeze lever and feel for squishyness before braking occurs Bleed air out of hydraulic lines
Hydraulic pistons dirty Squeeze lever and feel for edge of resistance before braking, or slow return Clean pistons

My disc brakes are scraping as I ride

Issue Check Fix
Rotor out of true Observe caliper interior: watch for rotor swerving to rub against a pad or caliper body True the rotor by bending it where it swerves
Pads, caliper, or wheel mis-adjusted Observe caliper interior: see if pads or caliper don’t leave enough room for rotor Remount wheel; back off pads; remount caliper
Pad spring bent into too-closed position Pads are always slightly touching rotor, and can be pushed away into the caliper Bend spring back open, or replace

My disc brakes are squealing or squeaking when I brake

Issue Check Fix
Dirty or ineffective pad/rotor surface Look for a shiny or oily brake pad surface Resurface pads and rotor with sandpaper and clean with isopropyl; replace pads if needed
Worn pads expose pad base, springs, or caliper body Check for worn-through pads (<1mm in thickness); look for rub marks on the spring; look for a stationary pad flush with the caliper Adjust the pads and caliper body, or replace the pads
Hub or rotor has play Pinch the rim and rotor, and wiggle back and forth — feel for movement Service a loose hub, and tighten down a loose rotor

Chain

Chain is skipping on rear gears

Issue Check Fix
Chain is stretched Use a chain tool to check for stretch, or compare to a new chain Replace the chain
Cassette/freewheel worn Look for asymmetric and widened teeth; compare teeth between less and more-used gears; use a cassette-checker Replace the cassette or freewheel
Chain/cogs dirty or under-lubricated Look for grime and grease on the chain and cassette Clean or replace the chain and cassette
Chain is stiff or has misformed links Watch for skips as the chain passes through the derailleur cage wheels; feel for any stiff links Replace a damaged chain. Pry open any stiff links formed to splice with a flathead
Rear derailleur is misadjusted See if the chain is rubbing against a neighboring gear Adjust the rear derailleur and shifter

Front Drive

My front shifting isn’t consistent

Issue Check Fix
Derailleur is not aligned with gears Test all front shifts up and down, with rear derailleur in low, middle, and high gears Re-calibrate shifter and derailleur
Derailleur mounted at wrong height or angle Position outer edge of derailleur cage over the high (big) chainring: check for 2mm in height above tooth tops, and parallel to chainring Adjust derailleur angle or height, by loosening clamp and tightening in its new position
Limit screws interfere with shifting Low limit (small chainring): check for loose shift cable with shifter in low gear

High limit (big chainring): feel for difficulty/wall at shifter		 Low limit: adjust so the front low gear is reachable, but without derailing the chain.

High limit: adjust at or just shy of the high position
Misindexed shifter Check for cable slack after the low gear; check for extra shifting positions after the high gear Reset the shifter, and adjust the cable in the pinch bolt
Housing or cable worn out or dirty, or
Derailleur’s hinge pivots gummed up		 Derailleur: pull the cage out from the frame, let go and let it settle, the push it back in. Movement indicates gummed-up pivots

Housing/cable: Unhook the shift cable and pull slack out manually; feel for difficulty. Or shift down (smaller gear), and push cage in; movement indicates friction in the system.		 Derailleur: put a thin lube on each pivot point. Four pins and two ends makes for eight rubbing points

Housing/cable: lubricate or clean housing inside. Look for snapped housing, grime under bottom bracket, eyelets touching cable, missing end caps
Shifter seized up Feel for any difficulty taking up or releasing the cable. Brifters and trigger shifters frequently have issues releasing gears Disassemble the shifter and clean out the mechanism with WD-40 and brake cleaner, and add lubrication
Bottom bracket loose, or
Chainring bent, or
Crank bent or eccentrically mounted		 Bottom bracket: pull crank into frame, feeling for click

Crank or chainring bent or mismounted: watch for run-out/eccentricity as the crank rotates		 Bottom bracket: tighten or service bearing assembly

Crank or chainring: tighten loose chainring bolts; remount crank; check equal shimming around chainring; replace any damaged parts
Derailleur pivots worn out Watch derailleur cage for flex during shifting; feel for any side-to-side play Adjust shifting to compensate, or replace the derailleur
Derailleur cage is bent or damaged Look for bends or kinks in the derailleur cage Bend back into shape, or replace the derailleur
Chain worn out Derailleur cage is centered over the next gear, but no shifting occurs. Check the chain with a wear tool Replace the chain
(less common) Cable/housing run has too much flex Push cage into frame, looking for any movement. Look for issues in the housing and bottom bracket cable guide  
(less common) Pinch bolt loose or stripped Adjustment always needs to be re-done; pinch bolt can tighten indefinitely; bolt or nut threads are flattened and shiny Replace pinch bolt and nut
(less common) Incompatible equipment Cage shift steps don’t match chainring spacing; chain hits cage top or bottom; cage can’t reach all gears  

Rear Drive

My rear shifting isn’t consistent

Issue Check Fix
Derailleur is not aligned to the gears Compare vertical alignment of derailleur guide wheel and cassette gear Change barrel adjusters, or move shift cable through derailleur pinch bolt
Derailleur hanger is bent Compare alignment of derailleur cage and cassette/frame; use derailleur hanger alignment gauge Bend derailleur hanger back into alignment, or replace a separate hanger
Housing or cable worn out or dirty
Derailleur hinge pivots gummed up		 Derailleur: push the arm into the frame, let go and let it settle, then pull it back. Movement indicates gummed-up pivots

Housing/cable: Unhook the shift cable and pull slack out manually; feel for difficulty. Or shift up (smaller gear), and pull arm out; movement indicates friction in the system.		  
Wheel’s hub is loose Pinch rim, and rock in-and-out of frame; feel for click or movement Tighten or repack wheel hub
Cassette freehub body or freewheel is loose Pinch biggest cog and rock in-and-out of frame; feel for click or movement Service or replace cassette hub body or freewheel
Cassette is loose or not tightly stacked Pinch two cogs, and pull them apart; gears will flex apart separate Add spacer or tighten cassette lockring
Shifter seized up Feel for any difficulty taking up or releasing the cable. Brifters and trigger shifters frequently have issues releasing gears Disassemble the shifter and clean out the mechanism with WD-40 and brake cleaner, and add lubrication
Misindexed shifter Check for cable slack after the high gear; check for extra shifting positions after the low gear Reset the shifter, and adjust the cable in the pinch bolt
B-screw is set too far or too near Too near: gears rumble in lowest gears, or guide pulley in contact with chain and cog

Too far: in high (small) gears shifting is sluggish, or guide pulley aligned under next gear but chain snakes to previous gear		 Adjust b-screw to have 5-6mm between guide pulley and low gear. Check shifting in high gears.
Wheel mis-mounted Check rim centering in dropouts Remount wheel
Limit screws interfere with shifting High limit (small gear): check for slack cable, with shifter is in high gear

Low limit (large gear): feel for difficulty/wall at shifter		 Back limit off; with shifter in high or low position, adjust high or low limit to be at or just shy of derailleur

Wheels

My tire went flat, or keeps going flat after patching or replacing! (For wheels with inner tubes.)

When removing your tire and tube, it can help to mark:

Once the tube is out, look for the puncture. It may be visually obvious; if not, pump in a little air and listen. If the culprit cannot be seen or heard, fill up a sink with some water, and submerge the tube (the whole thing at once, or rotate through sections): bubbles will give the hole away. Mark the source with a sharpie (silver works best), so you can find it again later.

Once the leak is found, the location and orientation (outside, inside, or side) on the tube can sometimes help you find what caused the flat. Sometimes the cause is obvious, like a shard of glass poking through the tire. Other times it’s not, like rim tape that barely covers over a spoke hole, and gives way under pressure.

Here are some frequent causes:

Something sharp punctured the tire

Look around the inside and outside of the tire for anything sharp embedded in it. Sometimes it’s easier to see or feel something from the inside, where it’s cleaner and there’s no tread; other times, the debris is only visible from the outside, and barely pokes into the inside. Carefully remove anything you do find — tweezers or small screwdrivers work well. If you find a hole in the tire, but there is no debris, the puncture may have been transient and the debris remains on the road.

Low pressure caused a pinch flat

A tire with low pressure can “bottom out” while riding: a curb, pothole, or something else presses the tire all the way to the rim. This pinches the inner tube, causing a puncture. While there isn’t a physical cause that sticks around (like a shard of glass), pinch flats usually have two, mirrored punctures — these are the two spots that were brought together during the pinch.

Tube creased during installation caused a pinch flat

Inside the tire, a twisted, creased, or bunched inner tube can pinch when inflated: any fold in the tube will come under pressure as it is inflated, and the pressure against the pinch causes the tube to self-destruct. As with pinch flats from low pressure, flats from internal creases usually have two, mirrored punctured. The pinch may be obvious as the tube is removed.

For next time, before installing the tube be sure to add a little air in: this will give the tube some body, so it can resist twists and creases. When installing the tube, it is easier (but slower) to seat one tire bead, then stuff the inner tube into the tire before seating the remaining bead; with this method, the tube can be checked and fixed before inflating.

Sometimes, adding soapy water into the inside of the tire can help the tube inflate despite any creases: the friction between the tire and tube is reduced, and the tube can slip into a uniform shape as it inflates. (This soapy water trick also helps reduce tire bulges and wobbles.)

Tube not contained within tire before inflation explodes

Sometimes a part of the tube slips underneath one of the tire beads during installation, and remains outside of the tire. During inflation, this part has no support from the tire and rim, and explodes leaving a long tear. (This tends to be more common with skinnier tires). Before inflation, work around the tire and pull the beads away from the rim to check for any exposed tube.

Rim tape leaves valve hole exposed

Rim tape comes with a hole for the tube’s valve; this is supposed to line up with the rim’s drilled valve hole, and protect the tube from its edge. Misalignment or stretching of the rim tape may leave the tube exposed to the rim edge, which at pressure is sharp enough to bite into the tube. This stretching is especially common with rubber rim tape.

This can cause a flat right at the base of the valve, where it’s not possible to patch a tube. Replace the tube, and replace or realign the rim tape.

(A fast and simple alternative to replacing the rim tape is to add extra protection for a new tube around the valve’s base. Find an extra inner tube (rim tape and tube liner also work), and cut a section to the length of a band-aid and the width of the rim tape. Then, folding this patch in half, cut a small hole in the middle and slide it over the tube’s valve before installing.)

Mis-positioned or wrong size rim tape/strip

Sometimes the rim tape isn’t positioned well inside the rim, or the rim tape doesn’t fit the rim. This can expose the tube to a well inside the wheel, or to the edges of spoke holes.

Inspect the rim tape for its fit in the rim, and for any deviations from straight. Mis-positioned tape may be difficult to evaluate, as the edge of the tape may cover a spoke hole, but not have enough body to prevent it from caving in.

Replace or reposition the tape or strip; something with adhesive that can hold itself in place during installation is preferable.

Valve is leaking or damaged

Some valves have removable cores that unscrew from the stem; these valves have flats on ether side, and a seam between the stem and the core. There are three parts in a valve with removable cores: the stem, which attaches to the tube and passes through the rim; the valve core, that screws into the stem; and a valve needle and screw tip, that compress to open the valve.

A loose valve core lets air out of the tube, causing it to deflate. This can happen slowly, and present as a tube with a slow leak. Try snugging down the valve with a core removal tool or pliers, and check the valve underwater.

The valve’s tip can also be bent or ripped off, causing a leak. This damage often occurs during inflation, as the pump is removed: be sure to pull the pump off the valve in a straight path away. Replace the valve core, if possible, or the tube.

Tire sidewall has hole

The brake pad (or something else) can wear against the side of the tire, and slowly wear a hole. When pressurized, the inner tube is left unsupported and blows out. Look for any frayed areas around the edge of the tire; check the brake pads for pad alignment, and look for any other contact (fenders, chainstays, etc).

The tire can be replaced, or temporarily patched with a variety of things (tube patch, tire boot, dollar bill, cardboard, or duct tape).

Presta valve used with schrader rim

There are two kinds of valves: schrader valves (squat, flat) and presta valves (thin, screw cap). Schrader valves are bigger, and can’t fit into presta rims as-is; a presta rim can be drilled out to fit a schrader valve.

Presta valves can fit into schrader rims, but the gap around the valve leaves the tube unsupported, and can cause the tube to blow-out. There are special valve nuts and adapter sleeves that fill this gap and support the tube.

Debris inside tire

Sometimes debris can make its way inside a tire, and puncture the tube from the inside. Take care to keep your tube and tire clean during installation.

Sharp edge inside rim

Punctures can come from issues on the rim itself. The rim tape can wear and expose sharp edges underneath; the rim can be dented (sometimes by a tool while being worked on), or carry defects created during manufacturing. Check the rim’s channel for any edges. Examine the rim tape for any tears. Look under the rim tape for edges around spoke holes, spoke ends that stick out, and spoke nipples with any sharp bumps or raised edges.

Sand or file any edges down. Replace any torn rim tape.

Brake

Whenever troubleshooting cable-actuated brakes, remember that brakes can be actuated and tested by hand: first, at the brake itself; then, at any point along the cable system where the cable is open; at the point where the cable goes into the brake lever; and finally, using the brake levers themselves. The cable can also be unpinched from the brake, and the lever to cable-end can be isolated, by pumping the lever and pulling on the end of the cable.

My brakes are rubbing against the rim while I’m biking

Brake pads have too little clearance for the rim’s variation

As a wheel goes out of true, the rim’s left-to-right movement grows. When the pads keep the same clearance from the rim, the widening travel of the rim causes the brakes to rub. An ideal rim has under 1 millimeter in left-to-right travel, but more can work just fine.

A brake lever should reach effective braking at about 1/2 of the lever’s travel. If the levers have some clearance, try letting out the brakes (tighten a barrel adjuster, or let cable out through the brake) to see if adding clearance resolves the situation. If the brake levers need to travel too far (close to the handlebars) to brake with this new clearance, it may be time to true the wheel.

Wheel is mounted off-center

When a wheel is re-installed at an angle, a rim slanted to one side will be closer to one side of the brake. This can cause the pads to rub.

Examine the rim placement in the center of the drop-out or fork, to see if the wheel is centered horizontally and vertically. Try to re-mount the wheel and make adjustments. If it’s hard to find the right spot, something may be causing this. Sometimes the rim is out-of-true or dished; sometimes the wheel shifts as it is being clamped; sometimes debris or an errant quick release spring has found its way between the axle and the drop-out; finally, sometimes the frame is built out-of-true or has become slightly bent over time.

Brake is mounted off-center, or is loose

Effective braking is not achieved until both pads touch the rim; if one pad touches the rim before the other, the levers have to travel further than necessary. This can also create a sponge-y feel from the added resistance until the other pad closes. Sometimes the brake can shift over time (after a bump), or be mounted too loose to hold its position. Check the pads on both sides of the wheel as it revolves: if they appear tilted to one side (one side only rubs), try re-centering the brake.

For caliper brakes: loosen the brake’s mounting bolt, adjust the brake position, then snug it down. This snugging sometimes rotates the brake a little. Some brakes have a square nut at the brake’s base, where a wrench (usually 90-degree cone wrench) can hold the brake during tightening. In other situations (there may be no square nut, or it’s too hard to reach), hold the brake while tightening against this rotation, or over-adjust to compensate for it. With dual-pivot brakes, there is often a lateral adjustment screw for smaller adjustments.

For most cantilever and v-brakes, there is an adjustment screw on the base of each brake arm. These adjust spring tension on each side: tightening pushes the arm further away from the rim (this is almost always the right direction). Adjusting each side by a quarter or half turn usually works well. Remember to pump the brakes with each adjustment; the arms don’t usually move with small adjustments to spring tension, but will settle where they want to go.

After this re-centering, often pads need to be adjusted as well.

Cable/housing moves the brake out of centered position

Sometimes, the brake can’t hold its position because the housing is pushing or pulling the brake out of position.

Sometimes tugging the housing upwards or downward can help reveal the housing’s influence on the brake. For caliper brakes, try to recenter the brake or trim the housing. For v-brakes, the housing may push into the brake more than the adjustment springs can compensate for; trim or re-routed the housing. On some bikes, the housing can be shifted through eyelets to change the tension on the brakes.

Brakes can’t re-open all the way

Sometimes brake pads rub because the brake is unable to open fully. Try pulling the two brake arms apart, to see if they open any more and create more room around the rim. See the causes around seized or gummed-up brakes, housing, and brake levers in the issue section below about “My brakes are too hard to pull, or won’t re-open all the way (rim).”

My brakes are squeaking or noisy when I brake (rim)

Pad has insufficient toe-in

Brake pads sometimes make a high-pitched squeal during braking, when the heel of the pad contacts the rim first: because the pad is flexible, the back edge of the pad will compress and decompress at a rapid rate, creating noise. To test this, press the brake in, and observe where and how the pad on each side comes into contact with the rim. If the pad appears tilted in towards the back, it needs to be adjusted.

If the pad has a swivel post, angle the pad so the front edge of the pad contacts the rim first. Sometimes placing a shim (thin cardboard) under the pad’s heel makes positioning the pad easier.

Some pads don’t have posts that can swivel (this may be a worthwhile upgrade). In this case, you can resurface the pads (with a file or sandpaper) to create this angle; purchase or make up an angled shim or washer; or bend the brake arms themselves.

Pad dirty or glazed over

With age or dirt from the road, brake pads can glaze over, making for a louder brake. A pad can look a metallic shiny, hard, or cracked.

Sometimes a pad needs to be replaced. Other times, sanding or filing the pad exposes a usable braking surface.

Rim is dirty

The rim can build up a layer of dirt, making braking more slippery—and less effective. Scrub the with degreaser or isopropyl alcohol, and wipe them clean. (Take care to not get degreaser on the drivetrain!) Try to do this whenever the pads are replaced or resurfaced.

Pad has embedded metal or debris

Sometimes the pad can accumulate metal splinters and other debris from the street. These become embedded in the pads, and rub against the rim. This debris can make noise, and wear out the rim prematurely.

The pads can be replaced, or the debris can be picked out by hand. Sometimes it’s hard to see what’s in the pad: sand or file the brake to make the debris more visible.

My rim brakes are not working well

Many causes can make rim brakes less effective. This section includes issues like:

Brake pads mis-mounted

A pad that does not seat well on the rim will be less effective. When the pad doesn’t seat fully on the rim (part of it may sit below the rim), it has less surface area in contact and will brake slower. When the pad comes in at an angle, only part of the pad may flatten against the rim; this will also cause some sponginess in the levers while the brake pad flattens against the rim.

Adjust the pad, until it seats better.

Cable and housing run seized up

Worn-out, dirty, or corroded housing and cable can add friction to the cable and housing run, making it difficult to actuate or release the brakes. Some areas tend to cause more difficulty than others: ends of housing; a missing end cap; any places where the housing may be snapped or bent; any eyelets or metal parts where the cable passes through, like braze-ons or brake ends; for v-brake systems, the joint between the housing and the brake’s noodle is suspect, and the inside of the noodle itself.

To test for worn cable or excessive friction, compare the difficulty of actuating the brakes by hand at the rim, to the difficulty at the lever. If the brakes feel fine on their own, the cable and housing run is likely the issue. (Often housing/cables are worn out because they are doing too much work; the brake may be stiff as well.)

Replace the cable and housing run. Sometimes in a pinch, lubricating the cable and housing liner can fix things. (A favorite trick is to rotate the bike so the cable is near-vertical, and use the cable to guide lubricant into the housing.)

Housing worn out

As housing ages, it provides less and less support for the cable as it pulls; any loops or bridges become areas where the cable can flex against the housing.

To check for worn out housing, squeeze the brake levers until the pads make contact with the rim. Then, squeeze hard: worn-out housing feels gummy at the levers at this point, and will (visually) flex as you squeeze. (Note that the pads also may not be fully seated, causing similar feelings at the lever.) Another way to check is to develop a feel for worn-out housing, by bending it between your fingers.

Replace the housing, taking care to run no more housing than you need.

Pad dirty or glazed over (same as last section)

With age or dirt from the road, brake pads can glaze over, making for a louder brake. A pad might look a metallic shiny, hard, or cracked.

Sometimes a pad needs to be replaced. Other times, sanding or filing the pad exposes a usable braking surface.

Rim is dirty (same as last section)

Whenever the pads are cleaned, it’s a good idea to clean the rims too. Scrub them with degreaser or isopropyl alcohol, and wipe them clean. (Take care not to get degreaser on the drivetrain.)

Brake is mounted off-center

Effective braking is not achieved until both pads touch the rim; if one pad touches the rim before the other, the levers have to travel further than necessary. This can also create a sponge-y feel from the added resistance until the other pad closes. Sometimes the brake can shift over time (after a bump), or be mounted too loose to hold its position. Check the pads on both sides of the wheel as it revolves: if they appear tilted to one side (one side only rubs), try re-centering the brake.

For caliper brakes: loosen the brake’s mounting bolt, adjust the brake position, then snug it down. This snugging sometimes rotates the brake a little. Some brakes have a square nut at the brake’s base, where a wrench (usually 90-degree cone wrench) can hold the brake during tightening. In other situations (there may be no square nut, or it’s too hard to reach), hold the brake while tightening against this rotation, or over-adjust to compensate for it. With dual-pivot brakes, there is often a lateral adjustment screw for smaller adjustments.

For most cantilever and v-brakes, there is an adjustment screw on the base of each brake arm. These adjust spring tension on each side: tightening pushes the arm further away from the rim (this is almost always the right direction). Adjusting each side by a quarter or half turn usually works well. Remember to pump the brakes with each adjustment; the arms don’t usually move with small adjustments to spring tension, but will settle where they want to go.

After this re-centering, often the pads need to be adjusted as well.

Wheel mis-mounted

A related issue to out-of-center brakes is an out-of-center wheel. When a wheel is re-installed at an angle, a rim slanted to one side will be closer to one side of the brake. This causes the same issue as brakes being out of center, where one side has to travel further than the other.

Examine the rim placement in the center of the drop-out or fork, to see if the wheel is centered horizontally and vertically. Try to re-mount the wheel and make adjustments. If it’s hard to find the right spot, something may be causing this. Sometimes the rim is out-of-true or dished; sometimes the wheel shifts as it is being clamped; sometimes debris or an errant quick release spring has found its way between the axle and the drop-out; finally, sometimes the frame is built out-of-true or has become slightly bent over time.

Brake pivots seized or too tight

The pivot points in a brake can seize up with gunk and corrosion, preventing smooth action of the brakes; the brake can also be assembled too tightly, creating friction. This can make it difficult to pull the brakes to closed, or for them to reopen again; often, the cable can pull the brakes closed through resistance, but the brake’s internal spring can’t return them to fully open.

To test whether the brakes are seized, try squeezing the brakes into the rim by hand. Feel for difficulty in closing them, and look for difficulty in returning to fully open; try pulling them open manually, and see if they open up more.

To fix this, try cleaning and lubricating any points where the brake arms pivot, or loosening the assembly holding the brake arms together (if this is possible and applies to the brake model). Here are some tips specific to each brake type:

(less common) Lever pivot seized up or too tight

Sometimes the lever itself is causing an issue; the hinge pin may be over-tightened, or has seized. To test this, try actuating the lever without any load on it: unhook the cable (from the brake or lever), or squeeze the brake by hand to create cable slack.

Fix with lubrication, a service, or loosening an over-tightened hinge nut.

(less common) Levers and brakes are mismatched

Sometimes a well-meaning biker will pair a short-pull brake lever with a long-pull brake; the cable won’t travel as far as needed when the lever is pulled, and braking can’t be achieved at half the lever’s travel.

Check the brake’s specification for its pull rating. Levers can be checked by measuring them: short-pull levers have 25mm between the hinge point and the cable pull, while long-pull lever have 35mm.

My disc brakes are not working well

A few issues can make disc brakes less effective. This section includes issues like:

Housing worn out, or cable and housing run seized up (mechanical)

See discussions of worn housing and cable in rim brakes section.

Pads worn (mechanical)

As pads wear out, the brake lever must travel farther to engage the brakes. Watch the pads move as they brake, ideally with a backlight (paper over a phone flashlight provides a diffuse light). If there is extra space between the pads and the rotor, take this up with any pad adjustment knobs. Note that many brakes (single-arm moves one pad, with one stationary pad), adjusting for worn pads also involves adjusting the caliper position to shift the worn moving pad closer to the rotor.

After adjusting the pads and mounting, check that:

Wheel mis-aligned

Sometimes the pads appear mis-adjusted, but the wheel is out of alignment. The wheel may be mis-centered in the drop-outs or fork. Try re-centering the wheel, to see if it can be re-seated in a place where the brakes work fine. Otherwise, mount the wheel using the weight of the bike to help it into a position that seems consistently achievable, and adjust the brakes to match.

Caliper seized (mechanical)

Inside a mechanical caliper, the rotary mechanism (translates rotary motion into the pad squeezing) can become seized or gummed up. Dirt can accumulate in the mechanism (a common mechanism uses ball bearings trapped in teardrop-shaped canal), or on the arm’s bushing. This can make the caliper hard to actuate, or prevent the internal spring from returning the brake.

To test this, actuate the brake arm by hand; unpinch the brake cable and see how difficult it is to pull; note how well the arm returns to its starting position. Difficult or grimey action, or a weak return mean the caliper needs a service.

Caliper mis-aligned or mis-mounted

Sometimes the caliper isn’t aligned well, from prior or current work. Using a backlight, watch how the pads come into contact with the rotor as the brakes are pulled.

Some things to check:

Hydraulic lines need bleed

Air can build up in the hydraulic lines, becoming a compressible pocket. Feel for any squish at the start of the lever pull. Bleed the brakes to remove this air, following the procedure for your specific caliper and lever model.

Hydraulic pistons dirty

As hydraulic brakes work, a seal slips back and forth over the brake pistons; this allows movement while containing internal fluid. The sides of the hydraulic can accumulate dirt, adding friction to the seals. The sides of the pistons can be cleaned with a q-tip or rag, using the same kind of hydraulic fluid.

My disc brakes are scraping as I ride

Rotor out of true

A sufficiently out-of-true rotor will scrape against the pad or caliper body at some part of its rotation. If the rotor wobbles excessively, the rotor needs to be trued or replaced; mark any areas that need work with a sharpie, and bend them back into alignment. In a pinch, sometimes the pad being scraped can be backed off a bit.

Pads, caliper, or wheel mis-adjusted

Sometimes scraping can be caused by a wheel that hasn’t been fully or evenly seated in the bike. Other times, past work on the pad or caliper has placed it with too little offset for typical variation in the rotor or wheel/hub.

Pad spring bent into too-closed position

Many disc brake pads have a small spring, that pushes the pads back open when the brake is released; these are wedge-shaped, with delicate fingers that fit around the pad. This spring can be pressed into a too far closed position, leaving the pads able to fall into the middle of the caliper and rest on the rotor.

A sign that the spring may be bent too closed, is that the pads always seem to be close to or touching the rotor. Check with a small screwdriver whether the pads can be pressed away from the rotor. Try bending the spring into a more open position.

My disc brakes are squealing or squeaking when I brake

Dirty or ineffective pad/rotor surface

Disc brake pads can accumulate dirt and glaze over, making a surface that squeaks when pressed against the rotor. Remove the pads, and look for a shiny and glazed-over surface, or an oily pad.

Clean and resurface the pads and rotor, then bed them in together.

Worn pads expose pad base, springs, or caliper body

When the pads wear out, the pad’s metal substrate or the springs that keep the pads open and apart can rub against the rotor, causing squeaking. Check the pads for 1mm or more of thickness, and check for signs of wear on the base or springs. Replace any worn pads.

In calipers where only one pad moves, the pad on the non-moving side can wear down until the rotor rubs against the caliper body, causing a scraping noise during braking (or during wheel rotation when the rotor is out of true). Advance the pad on this side, or replace if too worn. If the rotor is also out of true, it will now scrape against the pad instead.

Hub or rotor has play

A rotor or hub with play/wiggle causes the rotor to be unstable during braking; this can cause squeaking during braking, or scraping. Check the rim for side-to-side play in the wheel’s hub, and the rotor for play.

Chain

Chain is skipping on rear gears

Chain is stretched

As a chain wears, it stretches out: the pins and inner plate bushings slowly wear against each other during use, creating extra space between inner plate pins. This stretching out prevents even contact with gear teeth. In a common situation, as the chain is pulled forward at the top of the cog, fewer links are in firm contact around the gear; without as much grip, the chain can skip over the teeth. (Chain and sprocket wear is a subtle subject, and different outcomes can occur depending on how these wear together or separately.)

Check the chain with a chain-checker tool: any chain that exceeds 0.50% stretch should be replaced. The cassette or freewheel may need to be replaced together.

Cassette/freewheel worn

A stretched-out chain will wear out the rear gears to match. When a chain is replaced, this wear pattern doesn’t match any more. (Gears that are used frequently and smaller gears wear out faster than others.)

Examine the cassette for uneven or widened wells, or sharp teeth. A sprocket-wear check tool may also be helpful. (Wrap the teeth around a cog; press the arm into a well, and put pressure on the chain; the far end of the chain should lift easily off the cog, if it’s in good shape, but bind otherwise.)

Chain/cogs dirty or under-lubricated

Dirt can build up on the cogs and chain, and it can bind the chain and cog together. Clean and lubricate the chain and cogs; consider replacing an exceptionally rusty chain.

A chain with a faulty link can’t bend correctly around the derailleur and cassette, and can skip. Faulty links can be caused by a mis-installed quick-link, by a link that was re-pressed by hand but remains too tight, or by a link that was damaged (jammed or twisted, etc).

Inspect each link to check that it is straight and un-mangled, and that can flex easily; watch the chain as it travels through the derailleur cage for any skips. A damaged or rusty chain should be replaced; a quick-link can be reinstalled or replaced.

A too-tight link formed while splicing a chain back together can be fixed: open the adjacent inner link slightly, using a flathead screwdriver to slightly twist until it moves freely.

Rear derailleur is misadjusted

When the rear derailleur positions the chain too close to the next gear, the side of the chain can rub against it and be pulled upward. This can cause rapid shifting, and sometimes skipping.

Check that the derailleur guide wheel is below each cog, and that the side of the chain has clearance from the next gear.

Front Drive

My front shifting isn’t consistent

Be sure to check the derailleur’s mounting height and angle first, before investing time in sensitive cable, housing, and limit adjustments.

Perfect shifting can be difficult with the front derailleur. Shifting issues covered in this section can include:

Derailleur is not aligned with the gears

There are a few conditions that are good ways to check shifting adjustment, and good places to start adjusting from. First, when the front derailleur is in its lowest (smallest) gear, and the rear derailleur is in its lowest (biggest) gear, the inside of the derailleur cage should be just shy of rubbing against the chain. (When resetting the shifting, start by positioning the derailleur over the low gear using the limit screw; then pull the shift cable through to taught, and snug it down.) Second, when the front and rear derailleur are both in their middle gears (this applies only to a 3x), the front cage should be about centered over the chain. Finally, when the front derailleur is in its highest (biggest) gear, and the rear derailleur is in its highest (smallest) gear, the chain should not rub against the cage; the cage should be close enough into the chainring to prevent the chain from escaping.

Test shifting by positioning the rear derailleur in a middle gear, and shift through the front range; make any adjustments using barrel adjusters, limit screws, or the cable’s position at the pinch bolt. Once this is working, test the front’s range again with the rear derailleur in the biggest and smallest gears. Shifting should be smooth, and happen quickly; the chain should seat fully on the destination chainring immediately (the plates don’t ride on the teeth, and the chain doesn’t fall off); the chain should have clearance against the derailleur cage in most positions (a small amount of chain rub is acceptable when fully cross-chained, in a big-big combination on the front and back, or small-small).

Limit screws are a common point of confusion in adjusting shifting characteristics. With friction shifters, the limits define the low and high range, and their setting is important. With indexed shifters however, the limit screws have a limited impact on indexed positions and mostly provide guardrails. In most situations, the limit screws need to be set at or just away from the indexed positions.

The low limit can be used to hold the derailleur away from the bottom bracket, when the lowest derailleur position leaves excess space between the cage and chain; this prevents the cage from shifting off the low chainring if the cable fails or comes loose. Adjusting the low limit is a useful trick for positioning the derailleur when beginning calibration.

The high limit should not intrude on an indexed position, as it will prevent the shifter from reaching it (or make this difficult). Instead, it should be set at or just shy of the index position. This does two things. First, it holds the cage in and prevents the chain from pushing the cage open and derailing during a shift into the high gear. Second, it prevents the rider from shifting past the high gear. With trigger and integrated shifters, the shift lever on the handlebars has additional shifting range after the indexed position has been reached; the lever can click into position, and continue to drive the shift cable and derailleur with an extra nudge. This is easy for a rider to do accidentally, and can allow the chain to continue derailling off the chainring.

Derailleur mounted at wrong height or angle

The derailleur’s mounting height and angle are crucial for safe and consistent shifting. In positioning for height, there should be about a 2mm gap between the bottom edge of the derailleur cage and the chainring teeth (rotate the entire chainring around to ensure this clearance). In positioning for angle, the cage should be parallel to the chainring; positioning the derailleur cage over the biggest chainring is a good place to check this.

A derailleur mounted too high can let the chain slip under the cage, in the high gear; it can also snag the chain in lowest gear. When set too low, it will rub or strike the high gear. A derailleur mounted at an angle can shift too much, pushing the chain off the chainring; at an angle, the cage has reduced width, causing chain rub in some positions.

Before adjusting, mark the previous derailleur height and orientation on the downtube, with a sharpie or tape. Butt the tape up against the mount, and draw a line through the mount and tape with a marker (this records the angle). Check the derailleur again after tightening it down; make fine adjustments when it is semi-tight.

Limit screws interfere with shifting

On the front derailleur, mis-adjusted limit screws can interfere with shifting. As a limit screw is tightened down, it hems the derailleur’s possible range inwards: a tighter low limit (small-gear end) keeps the derailleur away from the frame, and a tighter high limit (big-gear end) keeps it closer to the frame.

The low limit should position the cage as close to the chain as possible, while still allowing shifting into the low gear when the rear derailleur is in its highest gear. When the low limit is set too tight, the shifter can click into its low position, but the derailleur cage can’t move to the indexed position; instead, it is held away from the frame while the cable goes slack. To check for this, when the shifter is in the low gear, feel the cable for any slack. Alternatively, try loosening the limit screw, and watch for any cage movement. Adjust the limit to find the balance between two things: first, with the rear derailleur in the high (small) gear, make sure the front low gear is reachable. Second, with the rear derailleur in the low (big) gear, make sure the chain doesn’t derail shifting into the front low gear.

The high limit should be set just at the high shifting position. When the high limit screw is set too tight, it prevents an indexed shifter from shifting into a gear, or can make it very difficult. The derailleur may flex to reach this gear, and the cage may not reach its ideal position. When the high limit is set too loose, the derailleur cage can move further out than it needs to, in some cases allowing the chain to derail off the outer chainring.

Misindexed shifter

The shifter may be offset from the chainrings, with an extra position at the low or high end, and a missing position on the opposite end. Look for extra cable slack in the low gear, or extra positions after the high gear. Start over, with the shifter set to a selection that matches the chainring.

Housing or cable worn out or dirty

Derailleur’s hinge pivots gummed up

(Similar description as in rear derailleur shifting issues) Dirt, corrosion, or under-lubrication can hinder the derailleur’s hinge and spring mechanism, causing shifting issues. This excess friction can come from the derailleur pivots or the shift cable run.

When shifting into higher (bigger) gears, the cable pulls the derailleur cage away from the frame; the shift lever and cable are (usually) strong enough to pull through resistance in the pivots or cable/housing run. When shifting into lower (smaller) gears, the shifter lets the cable out, and an internal derailleur spring takes up the slack until the cable is taught.

The spring is not as strong as the cable, and this excess friction in the system can prevent the derailleur from fully returning. This creates different shifting positions for each direction; this can make shifting adjustments difficult.

Check the derailleur on its own: press the derailleur arm in by hand and let go gently, then press the cage into the frame. Any extra movement is a sign of a gummed-up derailleur; any difficulty in moving the arm is a sign that the derailleur pivots are binding. Fix this by applying a light lubricant to all pivot points, and work it in; a deeper clean may be needed.

Check the derailleur, cable run, and lever together by shifting into a lower gear, then press the derailleur cage into the frame: if the cage can move and settles in a new place, this is a sign of excess friction somewhere in the system. Feel for the difficulty at the shifter and at any point along the cable system where the cable is exposed. If necessary, check the cable run on its own, by undoing the cable from the derailleur pinch bolt and pulling the cable end as the shifter lets it out.

If the issue is in the housing, inspect the run for any issues. Suspect places are:

Replace, clean, or lubricate any worn or dirty sections of housing and cable.

Shifter seized up

(Same description as in rear derailleur shifting issues) Inside the shifter, dirt can build up and prevent the shifter from working easily. This is similar to issues with derailleur pivots and excess friction in the housing: often shifting down and pulling cable into the shifter is doable, but shifting up and releasing cable from the shifter causes problems when the derailleur spring can’t pull through the shifter’s gunk. This tends to happen more with integrated/brifters and trigger shifters.

Try testing the shifter in isolation. A good test is pulling on the end of a loose or disconnected cable to see how easily the shifter takes it in and out.

Lubricate or service the shifter to resolve the issue. (Integrated drop-bar shifters are difficult to disassemble. These can be flushed out with a cleaner, like brake cleaner or WD-40, then followed by a lubricant.)

Bottom bracket loose

Chainring bent

Crank bent or eccentrically mounted

All of these can cause the chainring to wobble as it rotates through the derailleur cage; an inconsistent shifting destination makes for inconsistent shifting!

A loose bottom bracket leaves the spindle with play, and the crank arm and chainring can wobble too. Check for this by pulling/pushing the crank end into the frame: any movement means the bottom bracket should be tightened, serviced, or replaced.

Similarly, a bent or mismounted crank or chainring will have runout relative to the derailleur cage. With the crank, try re-mounting it after cleaning out the spindle mount. With chainrings: look for any loose chainring bolts, misplaced or inconsistent spacers, gunk sandwiched in the crank’s mount tabs, and any raised edges of the mount tabs. These all skew the chainring.

Derailleur pivots worn out

Derailleur arms swing around pivots to move the cage. With enough grit and too little lubrication, these can wear to the point where the cage has enough side-to-side play to make shifting inconsistent; the cage has trouble shifting in both directions, and it may be difficult to adjust the derailleur so both directions work well. Some newer front derailleurs have pivots that screw in: check that these are tight.

Try pressing the derailleur arm side-to-side, and see how it wiggles; or try shifting up, and watch the cage as it pushes against the chain. Significant flex means the derailleur may replacing.

Note that a worn derailleur may shift fine in the stand, but not on the road. The top section of the chain has more tension when driving the bike and rider, and the cage may flex more than it did in the stand. Always take a bike for a test ride when adjusting a derailleur like this.

Derailleur cage is bent or damaged

A bent derailleur cage can change shifting characteristics, by angling the cage or narrowing it; the damage can make the two sides of the cage inconsistent, and no single angle or height works for upward and downward shifting. Some cages can be bent back into straight; others need to be replaced.

Chain worn out

A worn-out chain is more flexible, and can bend around the derailleur cage during the shifting. If the cage shifts well over another gear and the chain is able to snake around it, check the chain for wear and replace if necessary.

(less common) Cable/housing run has too much flex

Sometimes there is too much flex in the cable/housing system for effective shifting. When shifting, the cage tries to push a chain under tension; when there is flex in the housing, the chain pushes back and takes up this flex, instead of the derailleur firmly moving the chain. As with the gummed-up pivots/housing issue, the derailleur doesn’t have a stable position and makes shifting adjustment difficult.

This flex can come from anywhere in the cable and housing system: brake housing that’s been used for shifting, and worn out; shift housing that’s gummy; a housing end that’s snapped or frayed and can bend in; a bottom bracket cable guide misroutes the cable; etc.

To check for this, shift up to a higher gear, then press the derailleur cage into the frame. If the derailleur pivots and cable run have easy movement on their own but the cage still flexes, there may be an issue in the system. Sometimes this happens only while riding, when the chain is under more tension from driving the bike and the rider; in the stand, this issue may not occur.

(less common) Pinch bolt loose or stripped

The pinch bolt holds the shift cable in a carefully chosen spot; this spot is important, because the only other way to adjust shifting is with barrel adjusters. If the pinch bolt is loose or stripped, it can’t pinch the cable tightly enough. Shifting may slip out of adjustment, sometimes immediately and other times after some riding. This is especially common during adjustment, when the pinch bolt hasn’t been tightened quite enough before a test shift.

Some signs of an issue with the pinch bolt include: shifting characteristics changing suddenly during adjustment or while riding soon after; making the same adjustment repeatedly, with no change in shifting; a pinch bolt that can be endlessly tightened down, and seem to hold.

Check the pinch bolt and nut threads for wear. A stripped bolt or nut with have flatter, shinier threads. Check the cable channel and washer for excess dirt; clean this off. Watch the cable as it is tightened down, to see if it walks somewhere else; mark the cable (with a sharpie) and observe how the mark shifts during adjustment and tightening. Develop a feel for how tight the pinch bolt should be, or use a torque wrench to measure it more precisely—4-6nm is typical.

(less common) Incompatible equipment

Check that the derailleur is compatible with the bike. Some important aspects:

Inconsistent rear shifting can be caused be several different things. Sometimes, there is just one issue; other times, there are several overlapping issues. Teasing out which ones are causing issues can take some work.

Symptoms covered in this section include:

Derailleur is not aligned to the gears

When the derailleur is well adjusted left-to-right, the upper cog in the derailleur cage (the guide pulley) is vertically aligned below each gear on the cassette; the chain side does not rub against the neighboring gear; shifting is consistent and responsive in both directions.

When the derailleur is set too far towards the wheel, shifting towards lower gears (bigger) positions the chain too far past the selected gear; this direction is responsive and fast. But shifting towards higher gears (smaller) positions the chain shy of the selected gear. The chain may shift slowly; stay put until the shifter moves again; shift, then return to the previous gear.

When the derailleur is set too far away from the wheel, shifting issues occur in the opposite direction: shifting towards lower (bigger) is fine, but shifting towards higher (smaller) has issues.

Visually inspect the vertical alignment of the derailleur’s guide wheel with the current gear, or compare the shifting in both directions. If the derailleur appears misaligned, or shifting is fast in one direction but hesitant in the other, adjust the derailleur cage:

Derailleur hanger is bent

The derailleur hanger can bend in an impact, placing the derailleur and its cage at an angle relative to the gears and frame. This angled cage imparts a tilt or twist to the chain before it is pulled onto the cogs. An angled cage also has shorter left-to-right widths with each shifting step than its cogs; these steps don’t align with the gear spacing. With smaller hanger bends or lower cassette speeds (5 or 6 speeds), this misalignment may not matter; with significant bends or higher speeds (11 or 12 speeds), it usually will.

A bent hanger can cause shifting issues like slow shifting, skipping over some gears, or skipping between gears. These issues can show up in all gears, or sometimes in just a few gears where no amount of adjustment makes shifting work everywhere. (Issues tend to show up in larger gears, when the guide wheel is closer to the cassette and leaves less room for the chain to adapt.)

A bent hanger can be checked quickly by eye, or with the derailleur hanger alignment tool. Visually, the derailleur’s guide and tension wheels should form a straight line with the gears; for significant bends, this will be obvious—but it may not be for slight bends, and for twists. A derailleur hanger alignment tool is excellent at measuring bends of all sizes; take care to measure it against the same spot on the rim, rotating the wheel around as you measure. A small gap of 2 or 3mm is not worth correcting, and suggests another cause to shifting issues.

While reinstalling the derailleur, take care to rotate the derailleur clockwise or pull back the b-tension assembly; it is common for this to get pinched or bent while reinstalling the derailleur.

Housing or cable worn out or dirty

Derailleur hinge pivots gummed up

(Similar description as in front derailleur shifting issues) Dirt, corrosion, or under-lubrication can create friction that hinders the derailleur’s hinge and spring mechanism, causing shifting issues. This excess friction can come from the derailleur pivots or the shift cable run.

When shifting into lower (bigger) gears, the cable pulls the derailleur cage towards the wheel; the shifter and cable are (usually) strong enough to pull through resistance in the pivots or cable/housing run. When shifting into higher (smaller) gears, the shifter lets the cable out, and an internal derailleur spring takes up the slack until the cable is taught.

The spring is not as strong as the cable, and this excess friction in the system can prevent the derailleur from fully returning. This creates different shifting positions for each direction; this can make shifting adjustments difficult.

Check the derailleur on its own: press the derailleur arm in by hand and let go gently, then pull the arm away from the frame. Any extra movement is a sign of a gummed-up derailleur; any difficulty in moving the arm is a sign that the derailleur pivots are binding. Fix this by applying a light lubricant to all pivot points, and work it in; a deeper clean may be needed.

Check the derailleur, cable run, and lever together by shifting into a higher gear, then pull the derailleur arm away the frame: if the arm can move and settles in a new place, this is a sign of excess friction somewhere in the system. Feel for the difficulty at the shifter and at any point along the cable system where the cable is exposed. If necessary, check the cable run on its own, by undoing the cable from the derailleur pinch bolt and pulling the cable end as the shifter lets it out.

Check the derailleur, cable run, and lever together by shifting into a lower gear, then press the derailleur cage into the frame: if the cage can move and settles in a new place, this is a sign of excess friction somewhere in the system. Feel for the difficulty at the shifter and at any point along the cable system where the cable is exposed. If necessary, check the cable run on its own, by undoing the cable from the derailleur pinch bolt and pulling the cable end as the shifter lets it out.

If the issue is in the housing, inspect the run for any issues. Suspect places are:

Replace, clean, or lubricate any worn or dirty sections of housing and cable. Note that worn cable and housing is often caused by a stiff shifter.

Wheel’s hub is loose

(Note that a loose hub, freehub body or freewheel, and cassette stack all cause the gears to wobble; these have different signs and solutions, so take care in distinguishing them.) A loose hub has extra space in its bearing assembly, leaving the wheel and rim with side-to-side wiggle room. The gears, mounted on the wheel, wobble along with the wheel as it rotates. The derailleur can’t shift consistently with wobbling gears because the ideal shifting position is always moving. As with many shifting issues, higher speeds are more sensitive. Note that shifting issues may be intermittent and random, because it depends on how the wheel is wobbling.

To check for a loose wheel hub, pinch the rim (from underneath, bracing against the seatstays) and press from side-to-side: a loose hub has a click or a wiggle. When pedaling, the gears may swim back and forth.

Adjust or repack the hub to fix this.

Cassette freehub body or freewheel is loose

A worn freehub body or freewheel can develop play, and wobble as it rotates. As with a loose hub, the derailleur can’t shift consistently when the destination is always moving. (A freehub body is attached to the wheel; the cassette slides onto this.) Inside, a worn or misadjusted bearing set causes this issue.

To check for this, pinch the back of a gear, and move it in-and-out of the wheel: if it clicks or wiggles, the freehub body or freewheel is loose. These can be replaced, or disassembled and serviced.

Cassette is loose or not tightly stacked

A loose cassette has side-to-side play; as with a loose hub, freehub body, or freewheel, a loose cassette is a moving destination for the derailleur, and causes inconsistent shifting. The cassette is pressed onto the freehub body by the lockring. There are two common issues. First, the lockring may be loose, allowing the cassette to move around. Second, the cassette’s total height may be too short for the freehub body, leaving extra room to move around—even if the lockring is tight. (This can happen when the cassette is missing a spacer. For example, a ~2mm spacer is often used to place a 10 speed cassette on an 11 speed hub.)

To check for a loose cassette, pinch two neighboring gears, and try to pull them apart and back together. If they can be separated, the cassette is loose. Note this is very similar to having a loose freehub body, but in this case the gears don’t move all together.

Check the lockring to see if it’s tight, or look into adding a spacer; the cassette and freehub may be incompatible.

Shifter seized up

(Same description as in front derailleur shifting issues) Inside the shifter, dirt can build up and prevent the shifter from working easily. This is similar to issues with derailleur pivots and excess friction in the housing: often shifting down and pulling cable into the shifter is doable, but shifting up and releasing cable from the shifter causes problems when the derailleur spring can’t pull through the shifter’s gunk. This tends to happen more with integrated/brifters and trigger shifters.

Try testing the shifter in isolation. A good test is pulling on the end of a loose or disconnected cable to see how easily the shifter takes it in and out.

Lubricate or service the shifter to resolve the issue. (Integrated drop-bar shifters are difficult to disassemble. These can be flushed out with a cleaner, like brake cleaner or WD-40, then followed by a lubricant.)

Misindexed shifter

The shifter may be offset from the derailleur, with extra positions at the low or high end, and missing positions on the opposite end. Look for extra cable slack in the high gear, or extra positions after the low gear. Start over, with the shifter set to a selection that matches the derailleur.

B-screw is set too far or too near

The b-screw is important for adjusting rear shifting characteristics. A b-screw set too close (screw is backed off) can position the guide pulley too close to the lower (bigger) gears, causing rumbling as the chain contacts two gears at once.

A b-screw set too far (screw is tightened down) can position the guide pulley far away from the smaller gears, making shifting is less effective and inconsistent; the chain has enough room to flex around the guide pulley and remain on a previous gear after shifting.

Most derailleurs need a gap of at least 5-6mm between the biggest cog and the guide pulley. Adjust the b-screw to find the right spot; if this magical place does not exist, the derailleur may be incompatible with the cassette’s size. While adjusting the b-screw, it is easier and saves the screw head to rotate the derailleur clockwise around the hanger until the spring assembly is not engaged.

Wheel mis-mounted

A wheel mounted at an angle (left-to-right or vertically) will be misaligned with the derailleur. This creates a shifting issue: the chain passes between two different lines, defined by the pulleys in the derailleur and the gears in the cassette.

Re-center the wheel, and make any shifting adjustments. Note that the wheel may have mounting issues of its own: a loose skewer, an out-of-true rim, a quick-release spring that has snuck around the axle, drop-outs that are too tight for the axle width, or an excessively greasy dropout or skewer face.

Limit screws interfere with shifting

Limit screws set too close can prevent shifting into gears on either end of the range. When the high limit screw is set too far in: the shifter can reach the high position (smaller gears) and let out cable, but the limit will hold the derailleur up and keep it from shifting into higher (smaller) gears. The cable will remain slack. When the low limit screw is set too far in: the shifter can’t reach the low position (bigger gears), and will feel like hitting a wall. The cable will be taught, and the derailleur is positioned by the shifter.

Try loosening the limit, and see if these gears become accessible. Both limits should be set just at or after the shifting positions. With the derailleur shifted to the high or low gear, tighten the limit screw until it touches the derailleur and back it off a touch. (In the high gear, the derailleur will start to move; in the low gear, the limit will feel like hitting a wall.)

The high limit is useful as a starting point for adjusting shifting. Otherwise, it has no purpose in regular operation, though it can provide a safety guardrail if the shifter or cable fails and lets the derailleur out. (If the derailleur swings too far out, the chain can derail onto the axle and pinch. This stops the drivetrain abruptly.)

The low limit does have an important safety purpose, in keeping the derailleur away from the spokes and from shifting the chain into the space behind the biggest gear. With trigger and integrated shifters, the shift lever can continue to move the derailleur after clicking into the low gear. This is easy for a rider to do accidentally on many shifters.

(less common) Chain is worn out

A chain that is worn out is more flexible, and can snake around the derailleur after it shifts to remain on the previous gear.

Look at the derailleur from behind, when the chain remains on a previous gear; if the derailleur’s guide wheel is aligned below with the next gear and no other adjustments fix this, the chain may be too flexible. Check the chain with a checker tool, and replace if necessary.

(less common) Shift housing is incorrectly sized

The shift cable and housing run needs to be unconstrained in all orientations of the handlebars as they turn, and the derailleur as it shifts and rotates around the cassette. If the shift housing at the handlebars is too short, turning can pull on the cable and move the derailleur.

The last stretch of housing running to the derailleur needs to have a graceful, consistent loop. It shouldn’t become taut as the derailleur shifts and rotates; it should avoid getting caught around the quick-release knob; it should not have a bend or kink at the turn into the derailleur.

Try to find the happy-medium between too long and too short. If the loop is too long, it will be a little less efficient/accurate, and is easier to snag on something.

(less common) Pinch bolt loose or stripped

(Same description as front derailleur issue.) The pinch bolt holds the shift cable in a carefully adjusted spot; this spot is important, because the only other way to adjust shifting is with barrel adjusters. If the pinch bolt is loose or stripped, it can’t hold the cable tightly enough. Shifting may slip out of adjustment, sometimes immediately and other times after some riding. This is especially common during adjustment, when the pinch bolt hasn’t been tightened quite enough before a test shift.

If the shifting characteristics change suddenly during adjustment or while riding soon after adjustment, a loose or stripped pinch bolt may be the cause. Some signs include making the same adjustment repeatedly, with no change; marking the cable (with a sharpie) and observing how the mark’s distance from the pinch bolt during adjustment and testing can help confirm this.

Check the pinch bolt and derailleur threads for wear; a pinch bolt that is stripped can be endlessly tightened down, and seem to hold before giving way. Check the cable channel and washer for excess dirt; clean this off, and consider sanding it to roughen it. Develop a feel for how tight the pinch bolt should be, or use a torque wrench to measure it more precisely—4-6nm is typical.

(less common) Rear derailleur, chain, gears, or shifter is mismatched and incompatible

The derailleur, chain, cassette, and shifter must be compatible in a few important aspects. Signs that these parts aren’t compatible include:

The shifter and derailleur have a shift or pull ratio, that describe how much cable movement corresponds to derailleur movement. This needs to match! The shifter needs to move the derailleur at increments that match the cassette spacing. Look for a derailleur that moves at different steps than the cassette spacing. Do any research to find out the pull ratios of these two parts.

Chains are described by speed-count: the chain needs to be wide enough to seat on the gear teeth, but not so wide that it rubs against neighboring gears or part of the derailleur cage. (Note that some newer drivetrains have brand-specific chain geometries.)

Derailleurs have a max-tooth count; this is the biggest cog that the derailleur can swing around. If a derailleur is used with a bigger cog, the guide pulley may rumble against the low gear (even when the b-screw is set all the way) or the cage may snag against the cog’s teeth. Compare the cassette and the derailleur specs to understand if they’re compatible. Sometimes an extension arm can increase a derailleur’s capacity.

(less common) Derailleur worn out, damaged, or missing a part

The pivots on most derailleurs are pinned in place, and cannot be replaced. Over time, these can wear until the derailleur can flex and make shifting inconsistent; for any shifted position, the derailleur cage can be in range of places instead of one spot. Try flexing the arm in various directions, to see if it can move in ways other than its pivoting axis. Replace a derailleur with loose or damaged pivots.

The guide and tension wheels inside the derailleur cage need to spin freely, run straight, and have little side-to-side play. Otherwise, the wheels can twist or tilt the chain, scrape against the derailleur cage, or slide inside the cage as the derailleur tries to shift. Lift the chain away from the guide and tension wheels, and check that the wheels spin easily and have minimal side-to-side play/twist. Lubricate any stiff wheels. Wheels and internal bushings with wear should be replaced.

The derailleur cage can bend and deform, from an impact or from getting tangled up in the wheel, causing significant shifting and derailment issues. Check that the cage plates on either side are straight, that the tension and guide wheels are in a straight line, and that the cage is perpendicular to where it rotates around the derailleur.