Bicycle gears consist of a chain that engages with chainrings at the front and a number of cogs at the rear. This is the most complex system on a bike, but each part can be serviced with a little know-how.
The chain keeps falling off
Adjust the derailleurs or tighten the chain
A chain that keeps falling off is either too loose (in the case of a hub-gear or fixed-gear bike), or the derailleurs are badly adjusted and move the chain right off the cogs when you change gears.
- To tighten a loose chain on a hub- or fixed-gear bike, use a wrench to loosen the axle nuts on the rear wheel. Pull back firmly on the wheel, keeping it centred in the bicycle frame, then retighten the bolts. If your wheel has a tensioning adjustment, turn the screw to increase or decrease chain tension.
- For a bike with derailleurs, you need to adjust the limiting screws that change the extremes of how far the derailleurs can move from left to right. These are sometimes marked “H” (which controls how far away from the frame the derailleur will travel) and “L” (how far towards the frame it will move).
- If your chain jumps off the rear cogs to the inside of the cog, give the “L” screw a couple of turns clockwise and try again. If it jumps off to the outside, turn the “H” screw clockwise. Tweak the screws until the chain runs smoothly and you cannot force it to jump off the cog when you change gears.
My chain won’t change gears properly
Check the chain and rear derailleur
If your chain doesn’t make clean, definite gear changes, the chain may be dirty or your rear derailleur might be poorly adjusted.
- A clean, well-lubricated chain (see below) is far less likely to jump gears. Brush it with degreaser to remove any dirt, rinse it off with water and apply lubricant sparingly.
- If your chain still skips after it has been cleaned and lubricated, you need to adjust the derailleur to keep your chain in place on the cogs. If your chain has trouble shifting towards the spokes (that is, from a smaller cog to a larger cog), turn the cable barrel-adjuster counterclockwise. If shifting away from the spokes is the problem, turn it clockwise. Turn the adjuster half a turn at a time while turning the pedals with your hand, until the shifting is smooth.
Why does the chain keep skipping while I’m riding?
Check for worn parts
A chain will skip if the gear cogs are worn.
- If your derailleurs are adjusted correctly, check the cogs for teeth that are rounded, shark-fin shaped, broken or bent. If you see this type of wear or damage, the cogs should be replaced (a job best left to a professional).
- A worn chain is easy to diagnose. Using a ruler, take a measurement from one rivet to the same rivet 10 links away — the measurement for a new chain should be 25 centimetres (10 inches). If it is within two millimetres (1/12 of an inch) of this, your chain is still serviceable, but any more and you will need to replace the chain. This job is best carried out by a mechanic, but it is possible to do yourself. Use a chain tool to remove the chain. Take the chain to the bike store; buy and install an identical chain.