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Old 05-29-07, 11:01 AM   #1
TallRider
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chainring wobble when BB spins - what are most likely causes?

I've noticed on a number of different bikes that I've worked on, that when you spin the cranks (without the chain connected), the chainrings wobble side-to-side. It means that the chainrings are not perfectly in a plane when the bottom bracket axle rotates. This side-to-side play can contribute to front derailer rub.

Anyway, there's a number of causal variables, and I'm curious as to which are the most likely (before I go through the long process of switching lots of things to see is it that? no, maybe the other thing.
  1. bottom bracket taper is not machined perfectly in-line with the axle
  2. crank is press-fit onto the bottom bracket taper at slightly off angle (not sure if this is possible with square-taper interface)
  3. the taper interface in the crank was forged/machined/etc. at slightly wrong angle
  4. crankarm spider arms are not perfectly in the same plane with the crankarm (seems unlikely)
  5. chainrings are bent (quite possible, but easy to test and rule out)

How often is the taper machined or forged into the crank at a slightly wrong angle? How often is the taper on a square-taper BB at a slightly wrong angle with respect to the BB axle?

Are there any other variables I've missed here? Thanks all.
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Old 05-29-07, 11:05 AM   #2
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Is it possible that without the chain that you are looking at the play in the bearings. It will appear magnified at the chainring distance from center. The bottom bracket could simply be wobbling around the largest blob of grease.

Once the chain is on, the crank may be pulled tight up against the rear bearings.

Just a thought
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Old 05-29-07, 11:46 AM   #3
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Quote:
Originally Posted by timcupery
I've noticed on a number of different bikes that I've worked on, that when you spin the cranks (without the chain connected), the chainrings wobble side-to-side. It means that the chainrings are not perfectly in a plane when the bottom bracket axle rotates. This side-to-side play can contribute to front derailer rub.
Actually it even does this when the chain IS connected! ;-)

I have an article on this topic, believe it or not!

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

Sheldon "Planarity" Brown
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Old 05-29-07, 12:42 PM   #4
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yes, without the chain it's simply easier to see

No play in the bearings in any of the cases I'm looking at, and if there were play in the bearings you wouldn't notice it when just spinning the cranks under no load, unless it were major, major play.

In all of the cases where I've seen this kind of non-planarity of the chainrings, the BB is installed correctly and in good shape.

Here are Sheldon's possibilities from his article:
  • The bottom-bracket axle may be bent
  • The spider may be bent
  • The chainrings may be bent

It seems likely to me that in some of these instances, the taper may have been manufactured wrong on either the BB spindle or the crank. But I'm not sure if that ever happens.
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Old 05-29-07, 02:30 PM   #5
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All right, so I think I've isolated that the problem is caused by the combination of the crank taper and the BB axle taper being forged/machined slightly off-kilter. Here's why I think I've isolated that:

I marked the area of the crank spider and chainrings that was closest to the frame in the rotation-wobble.
I removed the crankarms and rotated them 180 degrees on the BB spindle and re-tightened them. Just to check. The wobble was gone.
To make sure that this wasn't simply a function of the crankarm being pressed onto the BB axle at a slightly wrong angle, I re-rotated the crankarms 180 degrees (back to their original position on the BB spindle) and the wobble was back again, same as before. Then I re-rotated them 180 degrees again, and again, the wobble disappeared.

The crank in question here is a used Shimano 600 130mm-bcd double crank, from around 1994, but the tapers appear to be in good shape. The BB was a brand-new Nashbar-brand (made by RPM) square-taper. I'd be very, very surprised if the BB spindle was bent.

I'm going to try and re-mount the crank on a couple of different bottom brackets (Shimano UN-73 113mm and 107mm) and see if there's wobble.

But at this point it appears that there's slight variation in the taper angle with both the crank and the BB spindle, probably as a result of manufacturing.
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