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Fixing A Bent Crank Spider

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Fixing A Bent Crank Spider

Old 10-20-08, 08:43 PM
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Fixing A Bent Crank Spider

Campagnolo Nuovo Record crank and rings - right now, there is a bit of a wobble as you spin the crank (i'm guessing it's about 1-2mm and seems limited to one arm). It's not a lot but it means the chain rubs after shifting and the front shift lever has to be adjusted just a smidge.

Is it possible or advisable to bend the arms of the spider to make the chainring run true? Are they too brittle to reform? If this is do-able, how does one do it, with a flat-jawed wrench?

Could it be a bent chainring? After removal from crank, the ring should lay flat on a flat surface, correct?

Could it be improperly installed and torqued chainring bolts which caused a warp? I can remove the rings and check the runout on the spider arms.
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Old 10-21-08, 05:53 AM
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Chainrings bend way easier than the spider. I'd be checking the chainring first. I'd be surprised if you even could bend the spider.
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Old 10-21-08, 07:07 AM
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I've seen tools made to straighten bent crank spider arms but they are pretty rough and really not a good idea for a high quality crank.

I'd remove the chainring and lay it on a known flat surface like a piece of glass to see if that's where the problem is. If the problem is really in the crank spider, perhaps the ring could be shimed to make it run true rather than trying to bend the spider arm.

Last edited by HillRider; 10-21-08 at 07:11 AM.
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Old 10-21-08, 12:41 PM
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I'm sure you guys are correct. I'm also worried the crank spider would crack rather than bend - the mounting flange is machined with a very sharp corner thaty's likely to crack during any attempt to reform it.

In thinking further on this, I realized I don't even know if it's the spider or misalignment of the spindle. It wouldn't take very much at the spindle-crank interface to cause a 1 mm wobble at the OD of the ring.

I'll check the two rings for flatness and shim as necessary. Now, which hardware store was it that carried a good selection of shim stock ...?
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Old 10-21-08, 03:20 PM
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Are you sure it's the spider that's bent? Have you measure total +/- runout between the 5 arms? Most likely it's the chainring that needs bending and is also A LOT easier to bend back as well.

Using the FD cage as a guide, use a cresent-wrench on the chainring and bend it inwards or out as necessary.
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Old 10-28-21, 04:05 PM
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Check the inner chainring too

Originally Posted by DannoXYZ
Are you sure it's the spider that's bent? Have you measure total +/- runout between the 5 arms? Most likely it's the chainring that needs bending and is also A LOT easier to bend back as well.

Using the FD cage as a guide, use a cresent-wrench on the chainring and bend it inwards or out as necessary.
Very old post I know.. knowledge is timeless.. i have a 1976 Bob Jackson with the same issue. I'm pretty sure the spider is off ( maybe from new..?) Though it's been in a wreck or two.. checking BOTH chainrings to see if they deflect in the same place should tell you if the spider is out of spec..I won't say bent.. Mine is. I did rotate the crank 180° and got the same result. I banged it with a rubber mallet but lacked the will to really smack it.. It's my winter roller bike so I won't pursue it.
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