Bent Chainring causing chain to attempt changing gears

Question

I was riding my road bike last night when something strange happened. I stopped at a stop sign. (I didn't have time to slow down so I was in my highest (fastest) gear. When I started I had to press a lot of power into my pedals to get the bike moving again. I was pulling up on the handle bars to press more power into the pedals.

A minute or two later I noticed that my bike was trying to change gears on me. I have non indexed shifters so I expected that I was just between gears. I adjusted the leavers but could not find a spot (in my current gear) where the chain was not trying to shift.

Upon further inspection I noticed that my biggest front chainring was slightly bent. I am not sure if posting a picture would be useful because it is hard to see when it is not spinning. It seems like it is bent maybe 2-6mm towards the frame of the bike.

Questions:

1. Do you think that I bent my chain ring while pedaling? It is possible that it was bent before but something else is now causing my problem (although I don't know what). Is it possible that I bent my chain right by pedaling hard?
2. What is the solution to this problem? Can I take a hammer to my chain ring and pound it back into shape? I think that if I do this AND it was bent by hard pedaling then it is likely that it will just be rebent by future hard pedaling.
3. If I need to replace the chain ring how complex is this task? Is this something relatively easy that I could do at home or do I need to take it to my LBS? If I take it to my LBS how much should I expect to pay?

Answer 1

1. no, it's almost impossible that you bent it just by pedaling. Exacerbate an existing bend, yes, but bend a straight ring, no.

2. Bending it back into shape is possible, but be careful - bending metal back and forth weakens it. Do not pound it with a hammer - if you dent it you are also thinning it and making it longer. Which means it will be permanently bent. Use an adjustable wrench set to grip the chainring and try to bend around the attachment bolts, not just at the end of the wrench jaws.

3. A new chainring should be $20-$50, depending on what exactly you get. If your chainrings are not removable that means a new crankset but it also means your crankset is cheap, so it should be in that price range as well.

To replace it yourself is easy and should require a 5mm allen key and a thin coin. There will be four or five bolts on the outside of the chainring, and a matching slotted nut on the inside. You can buy a special tool to grip that nut, but I find a small coin is usually enough. Taking the chainring off before trying to straighten it is not a good idea as there's too much risk of dishing the chainring.

Answer 2

It's unlikely the chainring was bent by pedaling alone... The forces are linear. However, if you were badly cross-chained and put a lot of pressure on the pedals it might aggravate an existing condition to the point you'd start auto shifting.

As for straightening the chainring... If it's steel, no problem. I've done a bunch of them with nothing more than a crescent wrench. Get directly above the ring so you're looking down at it absolutely vertically, and tweak it gently till it goes straight. There should be no side-to-side play. If the chainring is removable, you can simply unbolt it and tap it straight with a hammer on a flat surface. However... If it's aluminum you might do better to buy a new one. Even a fairly small bend may cause cracking and eventual failure.

Answer 3

As other users have suggested, using an adjustable crescent wrench is the way to go. I recently fixed a bent chainring, and was able to get it nearly perfect with a crescent wrench.

I would also recommend taking a piece of cardboard and cutting a slit in it for the chainring to go through. Tape the cardboard to the bike frame, and as you move the pedals you can see whether the chainring goes through the slit in the cardboard. This will help you get the chainring better than you could with visual inspection alone.