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Help: Bent Chain Ring

Old 05-29-10, 03:05 PM
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Help: Bent Chain Ring

Hi there,
I just got a new (vintage) Dahon folding bike bike, but didn't know it had a bent chain ring before I bought it on ebay.
Can you advise if this is repairable or replaceable and with that type of chain ring (are there standard ones)?
(And how much this might cost - trying to get some refund for the cost of fixing this).

Pictures attached.

thanks,
Amy
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Old 05-29-10, 06:32 PM
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In order to begin looking for a new chainring, you'll need to determine the chainring BCD (bolt circle diameter). Go to www.sheldonbrown.com and read up. Your crank might be a little more difficult as it does not appear to be a standard crank in appearance, though that does not mean that it doesn't use a standard BCD. With that said, I'm a little concerned that your crank spider is bent along with the chainring. That's a big bend on a thick steel chainring. If the crank spider is bent, you will want to replace it as well.

Assuming that all you need is a new chainring, stell chainrings can be had quite cheap (~$10) assuming you can find one that fits. The one I've seen available are meant for cruisers and BMX bikes so they might not work. Aluminum chainrings will run from ~$25-40 in the size you need, but again might not use the BCD you need. If you can measure your crank and post the specs, we can likely find a part that will work. You could also look for a folding bike-specific website which may carry special parts for your Dahon.
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Old 05-29-10, 07:14 PM
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You should be able to get a chainwheel replacement without a big problem and the pic shows a pretty straightforward replacement - assuming you can get by the rust! Just measure from the center of the crank spindle to the center of a chainwheel bolt and multiply by 2 for the bolt circle diameter, then start searching. You might want to disassemble the chainwheel first to make sure you can get it apart and to see clearly what the requirements would be for a replacement (thickness, recess for the bolt, etc).

Forgot to mention. Is a chainring bent that badly straightenable? Yes. Are you likely to be able to get it back in to good condition and have it remain that way? Maybe.

Last edited by cny-bikeman; 05-29-10 at 07:31 PM.
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Old 05-29-10, 07:16 PM
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The chainring looks to be removable... If so you should have no trouble straightening the thing. Clamp it in a vise and work gently with a large crescent wrench... Or put it on a hard, flat surface and tap it straight with a hammer.
I have straightened slightly-bent rings on the bike with a crescent wrench or a big pair of vise-grips. Steel is easy...Aluminum might crack.
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Old 05-31-10, 11:29 PM
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I was able to take off the chain wheel and with the crescent wrench, a c-clamp and a hammer, bang it back into pretty good shape (so thank you all).
I found that when I did that, the chain wheel was a little too close to the frame, and so then was trying to take the spider off to inspect and the bottom bracket - however, I had problems with taking the spider off...it could have been the rust, but I took the nut off and tried to slide the spider off and it didn't work? Could it have been treaded?

Question: Do you guys have experience with taking the bottom bracket off a vintage Dahon (or the spider component)? If not, I will repost? (I have posted in the dahon.com forum already)

Amy
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Old 05-31-10, 11:29 PM
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Thanks, but another Q

I was able to take off the chain wheel and with the crescent wrench, a c-clamp and a hammer, bang it back into pretty good shape (so thank you all).
I found that when I did that, the chain wheel was a little too close to the frame, and so then was trying to take the spider off to inspect and the bottom bracket - however, I had problems with taking the spider off...it could have been the rust, but I took the nut off and tried to slide the spider off and it didn't work? Could it have been treaded?

Question: Do you guys have experience with taking the bottom bracket off a vintage Dahon (or the spider component)? If not, I will repost? (I have posted in the dahon.com forum already)

Amy
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