unbending a bent spider?

Hi,
I got a used Shimano 105 group for a song, and everything's working
great except that the SPIDER of the right crankarm is bent, so the
chainrings wobble. I'm definitely sure that the chainrings themselves
aren't bent, have laid them out on a flat table and there's no obvious
problem, but when I put them on the spider and tighten 'em down, it
doesn't lay flat...

Any tips on fixing this? Would bending it back into shape weaken it
too much (it appears to be bent about 2 mm)? I imagine I could bend it
back into shape with a vise, but what part would I want to try bending?
I dunno how to align it properly other than by trial and error, which
could be very tedious...

Thanks,
Dan Lenski

Dan Lenski wrote:

I got a used Shimano 105 group for a song, and everything's working
great except that the SPIDER of the right crankarm is bent, so the
chainrings wobble. I'm definitely sure that the chainrings themselves
aren't bent, have laid them out on a flat table and there's no obvious
problem, but when I put them on the spider and tighten 'em down, it
doesn't lay flat...

Any tips on fixing this?

Yes, I have an online article on exactly this topic.

See: http://sheldonbrown.com/straighten-chw.html

Sheldon "Clothespin" Brown
+------------------------------------------------------+
| A billion here, a couple of billion there -- |
| first thing you know it adds up to be real money. |
| --Sen. Everett McKinley Dirksen |
+------------------------------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Awesome! I hadn't seen that article before. Thanks a lot, Sheldon.

On Mon, 01 Aug 2005 13:22:18 -0400, Sheldon Brown
wrote:

Dan Lenski wrote:

I got a used Shimano 105 group for a song, and everything's working
great except that the SPIDER of the right crankarm is bent, so the
chainrings wobble. I'm definitely sure that the chainrings themselves
aren't bent, have laid them out on a flat table and there's no obvious
problem, but when I put them on the spider and tighten 'em down, it
doesn't lay flat...

Any tips on fixing this?

Yes, I have an online article on exactly this topic.

See: http://sheldonbrown.com/straighten-chw.html

Sheldon "Clothespin" Brown

Sheldon! What in tarnation are you doing? Sitting there typing all day, article
after article, ready to preempt any stupid bike question anyone might ask.

What freeking orifice are you pulling this stuff out of.

What PLANET are you from.

I am awed.

Ron

On 1 Aug 2005 08:43:00 -0700, "Dan Lenski" wrote:

Any tips on fixing this? Would bending it back into shape weaken it
too much (it appears to be bent about 2 mm)? I imagine I could bend it
back into shape with a vise, but what part would I want to try bending?
I dunno how to align it properly other than by trial and error, which
could be very tedious...

Bending aluminium: bad idea. Even to keep using it when it's been bent
once is probably a bad idea.

Jasper

Jasper Janssen wrote:
Bending aluminium: bad idea. Even to keep using it when it's been bent
once is probably a bad idea.

Jasper

A good point. I figure that since the spider arms aren't actually
bearing my weight, the worse thing that could possibly happen is that
the weakened spider arm bends or breaks again under the load of
pedaling... since I'm a poor grad student, the risk is worth it :-)

I'll of course inspect the crank for cracks carefully before replacing
it.

Jasper Janssen wrote:
On 1 Aug 2005 08:43:00 -0700, "Dan Lenski" wrote:

Any tips on fixing this? Would bending it back into shape weaken it
too much (it appears to be bent about 2 mm)? I imagine I could bend it
back into shape with a vise, but what part would I want to try bending?
I dunno how to align it properly other than by trial and error, which
could be very tedious...

Bending aluminium: bad idea. Even to keep using it when it's been bent
once is probably a bad idea.

Jasper

Bent an FSA crank spider. Sent it back under warranty. They just bent
it back and shipped it back to me. Said it happens all the time.

-n

Jasper Janssen wrote:
On 1 Aug 2005 08:43:00 -0700, "Dan Lenski" wrote:

Any tips on fixing this? Would bending it back into shape weaken it
too much (it appears to be bent about 2 mm)? I imagine I could bend it
back into shape with a vise, but what part would I want to try bending?
I dunno how to align it properly other than by trial and error, which
could be very tedious...

Bending aluminium: bad idea. Even to keep using it when it's been bent
once is probably a bad idea.

Not so. I've straightened a bent aluminum crank arm. And an aluminum
(non-replaceable) derailleur hanger. And aluminum brake arms. And an
aluminum handlebar bag support. (That was one I made and heat treated
myself. It's been reshaped many times.) And aluminum rims. And a
couple aluminum rear pannier racks... etc.

None of these have caused problems in thousands of miles of riding.

- Frank Krygowski