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Old 10-04-06, 09:48 AM   #1
BikeToWork
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Straightening Bent Middle Chainring

I bought a 34T 104mm Rocket middle chainring for my bike which has a 4 arm LX crankset. The chainring is about 2 weeks old. I suspect it was bent when I bought it since I have not crashed the bike or hit the chain ring on anything. The chain kept skipping when I was on the middle chainring and would autoshift to the small, despite the proper adjustment/allignment of the front deraiileur. When I looked down I saw the chain wobble badly on the middle chainring when I spun the crank. Now I'm wondering how to straighten it with the big chainring in the way. I've used a crescent wrench on other bent chainrings in the past with marginal success but that was always the big ring. Is there a better way? How to get around the big chainring blocking the middle one? Any advice?

Thank you in advance.
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Old 10-04-06, 10:07 AM   #2
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I'd remove the big ring, use washers if needed to tighten up the bolts in the absence of the big ring. Crecent wrench is what I've used in the past as well.
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Old 10-04-06, 12:02 PM   #3
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My crank is fine and the other rings are true. It's just the middle chainring that is wobbling badly.

Speaking of going back to square one, the chainline rule of thumb is that on the middle chainring in the front and cog 5 of a 9 speed cassette in the rear, the chain should be a straight line, correct? The bike I own is supposed to use a 113 mm length spindle (stock) but that was with the 7 speed cassette and if I actually used one that long the chain line would be WAY off. I have to use a shorter spindle to get closer to a straight chain line.
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Old 10-04-06, 12:09 PM   #4
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Not sure if it may help, but there are dedicated chainring straightener tools like http://tinyurl.com/mnowq. I *think* Park has one, but can't find it.
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Old 10-04-06, 12:21 PM   #5
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I've always just used an adjustable crescent wrench. But you'll probably want to remove the large chainring and just bolt the middle chainring on by itself to keep it stabilized while wrenching.
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Old 10-04-06, 12:25 PM   #6
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An adjustable (crescent) wrench will definitely work, provided you are patient and go slow.

However, I've always managed to do it without removing the 44t ring - in fact it provides you a visual guide for straightening the bent ring.

BTW the crescent wrench technique works on brake discs too.
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Old 10-04-06, 12:26 PM   #7
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Originally Posted by timcupery
I've always just used an adjustable crescent wrench. But you'll probably want to remove the large chainring and just bolt the middle chainring on by itself to keep it stabilized while wrenching.
You have checked that the BB bearings aren't knackered? You've also made sure all the crankbolts are done up tightly? And you didn't leave out any chainring bolt spacers or washers when doing the replacement job? It would be very unusual for a chainring to be bent out of the shop -- they just aren't that soft.
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Old 10-04-06, 01:02 PM   #8
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Yes, I checked everything you mentioned in your reply. It is the chainring itself that is bent. If I had crashed the bike or hit the chainring on something I would know it but this ring has only been on 4 rides and it has misbehaved the same all along.

My bike is an old Trek 990 which came with a 7 speed cassette. Since I made it into a 9 speed I've always gone through drive trains fast and the shifting, especially shifting up to larger chainrings from small and middle chainrings has been rough. I'm not sure whether the geometry for the 9 speed cassette is the same since the gears are narrower, but the chain line never looks right. Do others have experience with this. Thanks for all the helpful advice.
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Old 10-04-06, 01:11 PM   #9
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If your gearshift are rough, that may be a clue as to how the chainring (and most likely just the teeth) are bent out of shape. It sounds like you need to go back to square one, and look particularly at how the chainline is when the chain is on the middle chainring, and on the fifth cog on the rear. If there is a kink in the chainline, you have a problem. Did you change the BB cartridge for a new one of different spindle length when you undertook the project?

You may also need to look at your riding style... do you ease the pressure on the pedals (but not pedalling of course) when you make you shifts?
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Old 10-04-06, 01:26 PM   #10
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Quote:
Originally Posted by BikeToWork
Yes, I checked everything you mentioned in your reply. It is the chainring itself that is bent. If I had crashed the bike or hit the chainring on something I would know it but this ring has only been on 4 rides and it has misbehaved the same all along.

My bike is an old Trek 990 which came with a 7 speed cassette. Since I made it into a 9 speed I've always gone through drive trains fast and the shifting, especially shifting up to larger chainrings from small and middle chainrings has been rough. I'm not sure whether the geometry for the 9 speed cassette is the same since the gears are narrower, but the chain line never looks right. Do others have experience with this. Thanks for all the helpful advice.
The best way to check if a ring is bent is to lay it on a piece of glass and look for high spots. I would remove the crank and check to see if maybe it is bent and not the ring. Assuming you're replacing the ring because the other was bent too???
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