TonyA

Recently my chainrings started wobbling (running out of true), with about a 1/8" maximum play. The whole assembly, i.e. both of the chainrings and the spider, wobble as a unit.

I took my 3-pc bottom bracket apart today, and the axle looks perfectly straight. I guess it wouldn't have to be off by much to produce that 1/8" deviation out at the edge of the larger chainring. But the axle is an incredibly stout, short piece of steel, which looks like it would tear the frame apart before it would bend even a hair's width. Still, I can't think of any other explanation for the wobble. Any advice? It's a square tapered axle.

The crank and chainrings look straight, too, and it's hard to see how they could be distorted so as to cause this. Or could they? They're Sugino, all alloy.

neurocop

Does the wobble loosely (indicating that the axle/bearings are sloppy), or does it wobble without looseness (indicating that the chainwheel or axle ends...or both are bent)?

TonyA

No, it's tight. I can't make it deviate by pushing the edge of the chainring, only by rotating the crank.

There's no play in the bearings. They were adjusted tight last Fall, as I learned to do in this thread, and the bearings and cups were shiny and new-looking when I took it apart today, and I made sure it was tight again when I put it back together.

reptilezs

since it is the whole spider. remove the crank and reinstall it in one of the 4 possible positions. looking for the best run out

TonyA

Ah! Great idea. I didn't think of that. I'll try it tomorrow. Thanks.

Kimmo

Nice tip.

Leebo

Check chainring bolts?

TonyA

Using reptilezs' trick, I got the run-out down from about .080" to about .050" -- still not very straight, but enough to get rid of the chain noise that was bothering me.

There must be something wrong with the geometry of the tapered faces of the axle or the crank. I noticed today that the drive crank goes too far onto the axle; so far that the cap makes contact with the end of the axle and I can't get a quarter into the slot to turn it. I've got an old axle that I could put that crank on to see if it mates up any better. If it does, I might just need to buy an axle. If not, I guess I'll have to get a new crankset and a cartridge BB.

Thanks, everyone.

Hendo252 

Perhaps a shim under the right bolt(s) will help?

Brass shim-stock is widely available (hobby or industrial supply place), or a chunk of beer can will often work...

Kimmo

Pretty sure the crank will have deformed rather than the BB spindle. But it really doesn't amount to much; I'd just go the beer-can shim and keep my $. Maybe just get a BB that takes bolts instead of nuts for aesthetics' sake.

Bike Gremlin

My friend's mountain bike had his front chainrings wobbling. A good old hammer fixed it - works fine now. Don't know how he bent them, but hammer put them back in line. Really small wobble, doesn't affect gearing and ride anymore. Mind you, there was no play in the bb, no pedals problem, just chainring causing wobble.

Kimmo

I wouldn't use a hammer. But if the spider's not that beefy, you could prolly just tweak an arm or two with an adjustable wrench... you'd need to be careful you're not just bending the very end, though - you want to tweak the arm from near its base, but only by a poofteenth.

TonyA

More good ideas! Thanks, everyone.

I'm a little afraid to try bending the spider; I tend to be a klutz at things like that. But I'll definitely try shimming the bolts if I can get them off.

Or, if I can't get them off, maybe try using epoxy putty to build up the tapered faces of the crank mounting hole? (and then file them down flat and where they should be) That's something I could pull off without breaking anything, and it could potentially take care of not only the run-out but also the fact that the crank goes too far onto the spindle.