Bent front sprocket

Question

My chain just started to slip off my front gears and it happens within a minute of riding. I discovered that the middle sprocket is bent. This leads me to my questions:

1. How much should a LBS charge to fix / replace this? I don't want to get ripped off.
2. What would cause the sprocket to bend? The only event that I think could have caused this is I was recently riding down a street very fast. The light turned yellow, so I quickly stopped, attempting to shift down at the same time. I had to stop before I could shift down in time, and this left the chain in a shifting state. When I started up again, the chain fell off for the first time.

Thanks,

Answer 1

Before answering your question I will add the caveat that without seeing your problem and without knowing the exact spec of the bike, this 'quote' could be wrong. For one, you might not have replaceable chainrings. But, assuming you do have replaceable chainrings the cost would be somewhere around 30-35$ (10$ labor, 20-25$ for chainring) depending on the quality of the chainring (shift ramps/machining/material). It could be much more if you were to be using something like Shimano XTR or SRAM XX.

A way to see if they are replaceable is to inspect the crankset and find either chainring bolts holding the chainrings on or rivets holding it together.

If it is riveted the shop might use a slotted screwdriver or a set of channel locks, if possible, to bend the chainring back into place. This depends on the amount of clearance provided by the other chainrings. If that is not possible then they might have to remove the crank arm and hammer it straight using a mallet. With those possibilities in mind it might run you 10-20$ for the fix.

When going to your LBS be sure to identify the issue you want solved and express that you simply want that one problem solved. Often the reason a problem fixed seems more expensive than it should is that the mechanic has to spend the time IDing the issue. Time is money in any business. By identifying the problem right away you have saved time and therefore money. Be sure to get a quote before leaving the bike.

I believe the cause for your bent chainring has been addressed already. Starting from a half shift or shifting under load can cause this. The less expensive the drive train the more likely it is for this to happen. A riveted crankset w/ stamped chainrings will bend quite easily while a more expensive crankset with replaceable chainrings that are not of the stamped variety will be more resistant to bending.

As a note, I am in Canada ... therefore this prices might not be entirely reflective of the prices in your specific area.

Answer 2

Admittedlty I've never done it before but changing a front chain ring should be pretty easy to do. You can pick up a new or second hand one on your favourite internet listing/auction site, then it should just be a case of disengaging the chain and undoing the bolts that attach the rings. Or am I missing something?

Answer 3

I have bent the teeth on a front chainring before by accidentally dropping the bike on the ground and it hit a rock just on the end of a tooth. I was able to bend it back slowly using a pair of pliers, but I would recommend getting the whole ring replaced. I am unsure as to the price these days, but the thing you need to know when ordering is the BCD - Bolt Centre Diameter.
It is very easy to replace the rings. All you need is an Allen key (usually 5mm) and a large slot screwdriver. You use the Allen key to remove the bolts. You can then slip the large chainring off the crank arm and then the middle one. You might have to take the pedals off first to get the middle ring off if it is a small diameter. When putting the new ring on, you may need to use the large slot screwdriver to stop the bolt from spinning when nearly tight. There are slots on the back of the "nut" for the slot to go into. I also like to put some WD-40 on the bolts and nuts before putting them back together, making later removal easier.

Answer 4

Yeah, sounds like it might have bent when you were starting while half shifted, basically shifted under extreme load. I've no idea how much it would cost to fix, but to avoid something like that in the future, you should get used to shifting while stopped.

Basically, you shift (only one gear at a time!) then stand up, lift your read wheel, and pedal (usually just half a rotation would be enough). It's easier to do with toe clips/clipless pedals, but it's certainly doable even on platforms.

I often end up having to do this when I try to catch a light, and don't make it - a hard stop before the intersection, in a really high gear. After a while, it becomes really easy to shift, and make sure you start off at a reasonable gear.