Bicycle Mechanics - Fixing A Bent Crank Spider

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Mike Mills
10-20-08, 08:43 PM
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.

Steev
10-21-08, 05:53 AM
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.
HillRider
10-21-08, 07:07 AM
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.

Mike Mills
10-21-08, 12:41 PM
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 ...?
DannoXYZ
10-21-08, 03:20 PM
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.