Learning to adjust the gears yourself can really add to your enjoyment of cycling. However, if you feel like you're getting in over your head, bring your bike to a qualified mechanic.
Adjust the Front Derailleur
1. Hang the bicycle from a hook or bungee cord, suspended so the pedals and wheels can rotate freely, or use a repair stand if you have one.
2. Find the front derailleur: Starting with the left shift lever, follow the cable until it enters a mechanism near the front gears. This is the front derailleur.
3. Locate the derailleur adjusting screws, two small adjacent screws on top of the mechanism. One should be labeled "L" for low gear; the other should be marked "H" for high gear.
4. Turn the pedals forward and shift into the small chain ring. If the chain does not move into the small chain ring crisply, loosen the adjusting screw labeled "L" by one turn.
5. Loosen the "L" screw an additional turn if you still can't shift. If the chain comes off, tighten the "L" screw.
6. Shift into the big chain ring once you have set the inner limit. If the chain does not go into the big chain ring, loosen the "H" screw. If the chain goes too far and comes off, tighten the "H" screw.
7. Shift the rear derailleur. The chain should not rub the front derailleur when you're in the big ring in front and in the small cog in back. Likewise, it shouldn't rub when you're in the small ring in front and the big cog in back.
8. Fine-tune your adjustment as necessary.
You can follow the same steps for a triple crank set. However, if you have Rapidfire or STI shifting, you may need to take the additional step of fine-tuning your cable tension. Generally, when the chain is on the small chain ring, the front derailleur cable should have a small amount of slack - it shouldn't be completely taught.
Adjust the Rear Derailleur
1. Find the rear derailleur: Starting with the right shift lever, follow the cable to the pulley mechanism near the back gears.
2. Look for two screws. As with the front derailleur, there is one screw for Low and one for High.
3. Turn the pedals forward and shift into your smallest cog. If the chain doesn't shift into the smallest cog, loosen the "H" screw. If it overshoots the small cog, tighten the screw.
4. Shift into the largest rear cog. Again, if it doesn't shift, loosen the "L" screw. If it goes too far, tighten the screw.
Once the limits are set, you may need to fine-tune the index shifting. See "How to Fine-Tune a Bike's Rear Index Shifting."
Pay particular attention when adjusting the "L" limit on the rear derailleur. If you don't set this limit properly and shift past the largest cog at speed, you can cut off the drive-side spokes and your wheel could collapse.
Be careful of fast-spinning wheels as you work. Getting yourself caught in the spokes hurts; be mindful of fingers and tools.
Tips from Users:
carefull by The guy who recently did this
Don't pedal when going down steep trails that are rougher than your bike can handle. You will not be able to maneuver your bike around the big, bike-mangling, body-crushing pothole you are heading toward.
When heading directly toward a large tree or similar stationary object on your bicycle be sure to either: a) veer away to safety, or b) decrease forward momentum by desisting pedaling and applying brakes. Failing a) and b) follow c) which entails ensuring clean underpants are worn to avoid embarrassment at the hospital.
How to Adjust Bike Gears
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