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crank wobble

Old 10-07-15, 11:15 PM
  #1  
ednotmilkman
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crank wobble

Hi,

This question is about my road bike that I use both for long tours, and riding fast with the local club. I've been using the original Sora triple crank designed for 3 x 8 gearing for 13 years now with the original 52 tooth big ring and 4 or 5 year old middle ring. Might be time to replace the big ring! Also the square taper bb is about 6 years old. I forget what the spindle length is, but shell is for sure 68mm. Crank has had some noticeable wobble in the big ring for years. Didn't really concern me until it was finally time to replace the original front derailleur. The old one had wider cage due either to being completely worn out or because it was for 8 speed chain. Could not find a new triple derailleur designed for 8 or 9 speed chain so had to buy one listed for 10 speed chain. Now that I have the new narrower cage derailleur on, chain shifts great, but the crank wobble causes chain to rub in half of the 9 gears when in big ring if I adjust rub out of all the gears in middle ring. Not a problem when leisurely touring but riding fast with the club makes me constantly have to shift front rings instead of staying in big ring up all 9 gears.

I don't feel any loose play at all in the crank due to loose or worn bottom bracket.

It wobbles when spinning by hand on the stand about as much as when pedaling hard. Chainrings are not the problem either since it has wobble on same part of crank when I rotate big ring to another of the 5 bolt holes. I am semi-retired and have time to enjoy working on my old bike instead of just buying a new bike and the latest components.

So my questions are:

1) where might this wobble be originating from, bent crank, bent spindle on bottom bracket or BB shell frame out of exact line? I guess shell being out of line would require new frame, oh my, but thinking it through ............. shell out of line would make the whole thing out of line, but would not wobble!

2) Should I, or even can I, check each of these possibilities separately without buying or going to LBS to borrow, then install new crank and BB as a test?

3) I am watching ebay for used cranks to upgrade to, but am not sure if keeping a square taper cartridge BB is wise vs. going to the newer external bearing systems like Hollowtech or GXP even though they are stiffer and should help with chain ring wobble. Looks like durability of these external bearings is much poorer than the $15 Shimano square taper inner cartridge bearings?

4) if I purchase a newer crank designed for a 10-speed setup, are the 3 rings closer together than what my current 9 speed front shifter is set for, which would cause even more alignment problems unless I put little spacers in between the crank bolts?

Edwin
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Old 10-08-15, 02:22 AM
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Originally Posted by ednotmilkman View Post
So my questions are:
Originally Posted by ednotmilkman View Post
1) where might this wobble be originating from, bent crank, bent spindle on bottom bracket or BB shell frame out of exact line? I guess shell being out of line would require new frame, oh my, but thinking it through ............. shell out of line would make the whole thing out of line, but would not wobble!
ID where/what is causing the wobble, would take it apart, re-build and see if you can ID the exact cause of the issue (I'm thinking worn BB spindle)

Originally Posted by ednotmilkman View Post
2) Should I, or even can I, check each of these possibilities separately without buying or going to LBS to borrow, then install new crank and BB as a test?
Why not, you seem more than capable, if you have the tools, all it will cost you is time.

Originally Posted by ednotmilkman View Post
3) I am watching ebay for used cranks to upgrade to, but am not sure if keeping a square taper cartridge BB is wise vs. going to the newer external bearing systems like Hollowtech or GXP even though they are stiffer and should help with chain ring wobble. Looks like durability of these external bearings is much poorer than the $15 Shimano square taper inner cartridge bearings?
The frame, then crank determine the BB, you can't choose to swap BB type, this choice is made when you choose the crank.

For durability of external BB's, that's old news, they are reliable now, much more user friendly, and easier to work with than non-external types

Originally Posted by ednotmilkman View Post
4) if I purchase a newer crank designed for a 10-speed setup, are the 3 rings closer together than what my current 9 speed front shifter is set for, which would cause even more alignment problems unless I put little spacers in between the crank bolts?
8/9/10 speed cranks are close enough in spec that they can be used for the same application. The rings distances haven't changed, and if you look at Shimano 10 speed rings, they have 9/10 speed marked on them.

If your looking at getting a new crank Shimano currently offers Claris (8 speed) 2403, Sora (9 Speed) 3503 or Tiagra (10 speed) 4703 triple cranks, Other makes are available.

For the issue you had with the 10 speed FD, Shimano offer a triple version of these in Claris (8 speed), Sora (9 Speed) and Tiagra (10 speed) options.
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Old 10-08-15, 02:24 AM
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Shim it or shave it.

Chainrings are quite easy to true/straighten, but the spindle tend to be sturdier. So either stick some shims under the low spot chainring bolts, or remove chainring and carefully file the chainring seat on the high arms a little thinner.

I would NOT replace a functioning square taper with an external BB system. There is simply too much evidence of short life on external units.
Since the root cause of your issue isn't tied to your BB, there is not need for that replacement. The probable cause is that the spindle is a bit deformed. Either the arms, or the axle hole might be off alignment.
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Old 10-08-15, 06:10 AM
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Sounds like the crank spider is a bit warped.
Aluminum ones can be bent a bit, with some grunting.
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Old 10-08-15, 06:15 AM
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Try removing the both cranks and turning the spindle from the non-drive side with your fingertips while watching the drive side end. If it's bent, you should see an eccentric wobble. Since the spindle is hardened steel, it's unlikely that this part is bent. (You wouldn't survive the collision needed to bend it.)

More likely, your chain rings and/or crank spider are bent. Remove the rings and lay them on a flay surface to check for warping. While they are off the crank, rotate the crank, and check the spider for wobble. Tape a pencil to the down tube with the point near the inner side of the spider and determine if each arm passes the pencil point at the same distance.

When you find the offending part, you should be able to true it carefully with hand tools. Been there, done that.
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Old 10-08-15, 07:36 AM
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Originally Posted by habilis View Post
Tape a pencil to the down tube with the point near the inner side of the spider and determine if each arm passes the pencil point at the same distance.

When you find the offending part, you should be able to true it carefully with hand tools. Been there, done that.
That is such a great idea! Simplicity and immediate results! I applaud you.
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Old 10-08-15, 08:11 AM
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If the wobble is only present on one chainring, it's more likely to be the ring than the crank or bottom bracket. In that case, replacing the ring should fix it.
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Old 10-08-15, 08:30 AM
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Originally Posted by 60_tooth_ring View Post
That is such a great idea! Simplicity and immediate results! I applaud you.
This method also can turn your fork or seat stay into a pretty accurate truing stand. No need to dismount the wheel.
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Old 10-08-15, 10:31 AM
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Originally Posted by jimc101 View Post
........
For the issue you had with the 10 speed FD, Shimano offer a triple version of these in Claris (8 speed), Sora (9 Speed) and Tiagra (10 speed) options.
And 7 speed: A073
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Old 10-08-15, 12:15 PM
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Originally Posted by 60_tooth_ring View Post
That is such a great idea! Simplicity and immediate results! I applaud you.
Simple, maybe - but not complete. The fact that one arm is off could be either the spider, the spindle, or a machining defect that results in the arm not sitting at perfect right angle to the spindle. It is sometimes helpful to rotate the crank arm on the spindle 90 degrees at a time to see if that resolves the problem. AS for chainring vs. spider, if both chainrings move at the same point then one can conclude that the spider is bent. Of course it's perfectly possible that more than one problem may be present.
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Old 10-20-15, 10:41 AM
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OK, Thanks everybody, Got almost more answers that I needed. I did take the crank off and rotate it 90 degrees on spindle to see that it is the crank, not spindle that is off. Have not tried the pencil as a "truing stand"to see if just one or all crank arms are off center. Big chain ring looked flat when I set it on table. Chain rings need replacing anyway.

I'll try straightening this up later if I need it on another bike project; just ordered a used Hollowtech Ultegra triple crank and threaded external BB off Ebay.
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