Most modern mountain bikes have derailleur gears. These bike gears operate by shifting the bike chain up and down a cassette of cogs on the rear wheel, each cog offering a different gear ratio.
The name gives away the method; the derailleur arm simply “de-rails” the chain from one cog to the next.
Shifting Gears on a Mountain Bike
The derailleur arm is moved inwards and outwards by the release or tensioning of a cable which is operated by a gear shifter, normally located on the bicycle’s handlebar.
The amount of cable released or taken in is controlled by the “indexing” within the shifter and allows just enough movement of the derailleur arm to shift one gear at a time with each click of the shifter.
Problems normally arise when the cable tension is too loose or too tight. This commonly happens on new bikes after the first few rides which is why bike shops usually offer a free mini service shortly after purchase.
Most rear derailleurs operate with the release of the cable allowing the gears to move down the cassette to a smaller cog and increasing cable tension to move the chain up to a bigger cog.
The smaller the cog the more forward momentum is achieved with just one revolution of the crank but greater effort is required to turn the crank. The reverse is true and as more cable tension is applied the chain climbs up the cassette to a larger cog.
Larger cogs are designed for climbing, the crank requiring less effort to turn but forward momentum is reduced.
Why Bicycle Gears Start to Slip
Over time cables will stretch and the normal shifting problem is that the derailleur tries to drop the gear down to a smaller cog and will often drift between two gears. To rectify this problem cable tension needs to be increased:
- On the back of a derailleur there is a barrel adjuster, a screw device located around the cable housing where it enters the derailleur.
- Turn the adjuster anti-clockwise to increase cable tension and solve problems caused by cable stretch.
- Whilst the process is simple, fine tuning is often required to get the cable tension just right. Small quarter-turn adjustments are recommended until perfect gear changes are achieved.
- Occasionally there will be too much tension in the cable causing the gears to climb up to the next biggest cog and normally slipping back down again.
- Correct a tight cable by releasing cable tension with a clockwise turn of the barrel adjuster.
Additional Problems Affecting Bicycle Gear Adjustment
Other shifting problems can be caused by dirty rear derailleur jockey wheels. These are the two small cogged wheels within the derailleur arm. If these wheels are covered in congealed oil and road muck, the buildup of debris can be enough to move the chain off the cog. As with all good bike maintenance, the solution is to keep the derailleur clean.
Rusty cables or blocked cable housings can result in slow release of chain tension preventing a gear from dropping to the next smallest cog.
If releasing cable tension by the above method does not solve the shifting problem, check the condition of the cables and housings.
Occasionally the shifting problem will be the result of a bent derailleur arm. If all other adjustments fail to rectify a shifting problem the cyclist should ask a local bicycle shop to check derailleur alignment.
Worn components can also result in poor gear shifting. Chains should be replaced frequently and chain rings and cogs replaced when worn.
Tips on Adjusting Bicycle Gears
- Use a bike stand if available to adjust and check gear changing or ask a friend to hold the rear wheel off the ground whilst shifting gears up and down the cog.
- On the trail, the gear cable can be tightened using the barrel adjuster on the handlebar. See the picture below.
- Many people find it difficult to remember which way to turn the barrel adjuster. Imagine a mean old aunt as a reminder that “anti”-clockwise means “tight!”
Stop Those Bike Gears Slipping
Simple tightening of the gear cable with an anti-clockwise turn of the barrel adjuster usually resolves most gear changing problems. If not, check cable housings, chain and cogs for wear and see if the rear derailleur arm is bent. If all else fails, consider converting the bike to a single speed to eliminate any need for gear changing in the first place.