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A crooked bottom bracket?
I have a LeMond Zurich (all steel frame; I think they're now part steel,
part carbon fiber). This is the third year I've been riding it. It has Shimano Ultegra components and a triple front chainring crankset. I've pretty much always have a hard time dialing in the front shifting. Recently, my front crank on the drive side has been coming loose. I even had to use blue loctite to try to stop the front crank from coming loose. Because it was loose, I would look down sometimes and thought I saw the front chainrings wobbling relative to the frame. I chalked this up to perhaps a worn bottom bracket/crank connection. I purchased a new bottom bracket and new (Truvative) cranks to try to fix this problem. After I installed everything, I was having a bear of a time getting the front end to shift correctly. Specifically, when in the middle chainring and the large cog (or second to largest cog) on the rear cassette (12x27, 9 speed), the chain would jump to the bottom chainring. The chain and the rear cassette are new. (I had previously ordered a new wheel.) The chainrings have shift pins, which my old -- but not original -- chainrings on my Ultegra crankset did not. What looks like what was happening was the cranks wobble just enough to cause the chain to jump from the middle to the small chainring when in the large cogs of the rear cassette. Now, I've confirmed this -- the front chainrings really are wobbling. I put cardboard up against the downtube and put a line on it. The large chainring wobbles inside the line and outside the line with each complete turn of the cranks. The largest travel (farthest from the downtube) appears to coincide with the drive side crank; the smallest at the other end (when the non-drive side crank would be aligned with the paper). The amount of wobble is small; about 1/16 inch, but it's enough to affect shifting. I've confirmed this by comparing the distance from the non-drive side crank to the downtube and the distance from the drive-side crank to the downtube (former smaller than latter). What could cause chainring wobble? The crankset and bottom bracket are both new. Could the bottom bracket be crooked relative to the downtube? I do not see any metal breaks or anything other type of broken frame parts -- the frame looks darn new. Thanks. -- Bob M remove ".x" to reply |
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#2
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A crooked bottom bracket?
"Bob M" wrote: Now, I've confirmed this -- the front chainrings really are wobbling. I put cardboard up against the downtube and put a line on it. The large chainring wobbles inside the line and outside the line with each complete turn of the cranks. The largest travel (farthest from the downtube) appears to coincide with the drive side crank; the smallest at the other end (when the non-drive side crank would be aligned with the paper). The amount of wobble is small; about 1/16 inch, but it's enough to affect shifting. I've confirmed this by comparing the distance from the non-drive side crank to the downtube and the distance from the drive-side crank to the downtube (former smaller than latter). What could cause chainring wobble? The crankset and bottom bracket are both new. Could the bottom bracket be crooked relative to the downtube? I do not see any metal breaks or anything other type of broken frame parts -- the frame looks darn new. Thanks. -- Bob M remove ".x" to reply Do the crankarms actually wobble, or just the rings? I've corrected misallignment caused by bent chaingrings by carefully tweaking the rings with a large adjustable wrench until they are laterally true, with good results. Desired result: Rings are flat and perpendicular to the bottom bracket spindle and parallel to each other. If plane described by the sweep of the crankarms is not parallel to frame then...? -- Mykal C |
#3
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A crooked bottom bracket?
Sheldon Brown has some tips at
http://sheldonbrown.com/straighten-chw.html to help troubleshoot this problem. Tom Ace |
#4
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A crooked bottom bracket?
On Mon, 05 Sep 2005 16:46:38 -0400, "Bob M"
wrote: What could cause chainring wobble? The crankset and bottom bracket are both new. Could the bottom bracket be crooked relative to the downtube? Howdy, Let's assume for the moment that the bottom bracket is not properly aligned with the frame but that the BB axle is straight... Unless I'm missing something, the chain rings would still not wobble. If the BB axle were bent, well then you would have the rings wobble. If you are convinced that the crank is not bent, I would suggest removing the BB axle and rolling it on a smooth surface. If it wobbles, that is your problem. All the best, -- Kenneth If you email... Please remove the "SPAMLESS." |
#5
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A crooked bottom bracket?
On Tue, 6 Sep 2005 08:32:45 -0700, Mykal
wrote: [cut] Do the crankarms actually wobble, or just the rings? I believe both the crankarms and the rings wobble, but I need to take another look at it. I've corrected misallignment caused by bent chaingrings by carefully tweaking the rings with a large adjustable wrench until they are laterally true, with good results. Desired result: Rings are flat and perpendicular to the bottom bracket spindle and parallel to each other. If plane described by the sweep of the crankarms is not parallel to frame then...? I don't know -- It's very bizarre. -- Bob M remove ".x" to reply |
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A crooked bottom bracket?
On 6 Sep 2005 08:53:29 -0700, Tom Ace wrote:
Sheldon Brown has some tips at http://sheldonbrown.com/straighten-chw.html to help troubleshoot this problem. Tom Ace Thanks, Tom. -- Bob M remove ".x" to reply |
#7
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A crooked bottom bracket?
On Tue, 06 Sep 2005 12:50:15 -0400, Kenneth
wrote: On Mon, 05 Sep 2005 16:46:38 -0400, "Bob M" wrote: What could cause chainring wobble? The crankset and bottom bracket are both new. Could the bottom bracket be crooked relative to the downtube? Howdy, Let's assume for the moment that the bottom bracket is not properly aligned with the frame but that the BB axle is straight... Unless I'm missing something, the chain rings would still not wobble. If the BB axle were bent, well then you would have the rings wobble. If you are convinced that the crank is not bent, I would suggest removing the BB axle and rolling it on a smooth surface. If it wobbles, that is your problem. All the best, Well, both cranks and BB are brand new. I have another new BB I can put on to check. -- Bob M remove ".x" to reply |
#8
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A crooked bottom bracket?
On Wed, 07 Sep 2005 07:44:24 -0400, "Bob M"
wrote: On Tue, 06 Sep 2005 12:50:15 -0400, Kenneth wrote: On Mon, 05 Sep 2005 16:46:38 -0400, "Bob M" wrote: What could cause chainring wobble? The crankset and bottom bracket are both new. Could the bottom bracket be crooked relative to the downtube? Howdy, Let's assume for the moment that the bottom bracket is not properly aligned with the frame but that the BB axle is straight... Unless I'm missing something, the chain rings would still not wobble. If the BB axle were bent, well then you would have the rings wobble. If you are convinced that the crank is not bent, I would suggest removing the BB axle and rolling it on a smooth surface. If it wobbles, that is your problem. All the best, Well, both cranks and BB are brand new. I have another new BB I can put on to check. Howdy, I actually think that a bent BB shaft is more likely to be caused by some defect of manufacturing than caused by something from riding. Given that you have another, I'd put it in to check. Good luck with it, -- Kenneth If you email... Please remove the "SPAMLESS." |
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