Bike 'jumping' when pedaling hard or high gears

[takiguy]

Hello,

I just bought a used Trek Mountain Track 800 for $120. I didn't get a good chance to test it beforehand, and I have very little experience doing anything with bikes other than replacing a punctured tire.

The problem is: whenever I am putting a lot of pressure on the pedals and pedaling hard -- ie on hard uphill climbs -- the pedals will 'jump.' It happens a lot when rear gears are in the 4 highest (4,5,6,7), and front is in the highest (3).

I looked at the chainring, and the teeth are a little bent. The "cogs" on the rear cassette look ok. I don't know very much about the chain itself other than it doesn't look bent :\.

Is it jumping because the teeth are bent on the chainring? Should I get a wrench/vice and try to straighten the teeth? Is there something wrong with the chain? Derailleur? I'm not sure how to fix much of anything, and I am just now learning all the parts of the bicycle, but I bought this bike partially for riding and partially to learn how to fix things (which I am not very good at). What can I do? Thanks.

 

[kdelong]

There are three things to suspect when you experience the jumping that you are describing. The most likely problem is a worn chain. You can check this by leaning your bike against the wall, putting a slight tension on the chain by pressing down on the pedal, and measuring the top of the chain. 12 links should measure 12 inches exactly, when measured from the center of the first pin to the center of the last pin. If it measures 12-1/8 inches, then it is worn and should be replaced.

The next most likely cause is a worn cassette. Normally the most heavily used cassette cogs wear first and this is the jumping tends to occur.

The last item that would cause this is worn chain rings. Once again, the most heavily used chainring will be where the jumping tends to occur.

If a worn chain has been used for a long time, it may have caused undue wear on the cassette cogs and the chain rings. So if you change the chain, it may not cure the problem and you might have to look into changing the cassette and/or chainrings to eliminate the jumping.

I don't know which chainring you have but some chainring manufacturers make their chanrings with offset teeth or twisted teeth to improve their shift characteristics. You might really muck things up if you start straightening teeth that are meant to be bent/twisted.

There are two other possible causes but they rearely occur and this is excessive flexing of the frame where it actually causes the rear shift cable to stretch and move the rear derailleur. The other possibility is a poorly adjusted derailleur, but that would show up all the time rather than when you are pedalling hard.

The bad thing about buying a used bike is that you usually don't know how it was ridden and the amount of maintenance that the previous owner did to it. If he was not diligent, it is now up to you to rehabilitate the bike.

 

[threaded]

The teeth are not 'bent' they are 'hooked' from wear. It sounds like the whole lot is worn out so suggest a new set of front and rear cogs and new chain.

 

[takiguy]

Where should I go to get the best chainring/chain/cassette for my buck? Which ones should I look at getting for my bike (Trek Mountain Track 800)? Should I bother trying to install myself or will it be too difficult?