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Bent crank or bottom bracket, am I being too precise?

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Bent crank or bottom bracket, am I being too precise?

Old 08-24-09, 08:55 PM
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entplex
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Bent crank or bottom bracket, am I being too precise?

Just built up a new bike and I noticed the chainring wobbles a bit. I measured the wobble with a dial indicator and from one side to the other of the chainring there is a difference or 40/1000th of an inch. Basically I used a magnetic base to attach the dial indicator to my frame, set it to zero on the point where the chainring swings out most, then rotated the crank 180 degrees (this happened to the point at which the chain ring swings the most inward) and measured it again. With the chain ring off, there is a difference of 25/1000th of an inch on the crank spiders at 180 degrees difference. This appears to be on the bottom bracket, because I can change the orientation of the crank on the spindle and the outs/ins are at the same spots relative to the spindle. Clearly my bottom bracket spindle is some how bent or the square tapper is off or something. The cranks are Sugino RD2 paired with a Sugino BB3 BB. Yes this is a fixed build, so please put the whole bicycle classism thing aside for a second and give me technical feedback. My new chainring already shows wear in the place where it was mounted at the low spot. I know the above measurements are very precise, but it is visually noticeable and I'm worried that the chainring might warp over time with the high amount of torque it is subjected to riding fixed. The big question is am I being overly critical or is this something to actually worry about?
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Old 08-25-09, 12:51 PM
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No, I don't think you're being overly critical. You are the judge of what is important on your bike.

I think the spindle is bent. Replace.
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Old 08-25-09, 01:45 PM
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.040 sounds like a awful lot to me. I would view anything more than .010-.015 TIR to be excessive on a new build with undamaged parts. Are the parts new so that you can make a warranty claim?
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Old 08-25-09, 01:46 PM
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fuzz2050
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Nothing to worry about, but I would like to see a picture of the setup you used to measure a wobble of 40/1000th of an inch
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Old 08-25-09, 02:03 PM
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Originally Posted by fuzz2050 View Post
Nothing to worry about, but I would like to see a picture of the setup you used to measure a wobble of 40/1000th of an inch
You've never seen a magnetic mount dial indicator?

http://www.ptreeusa.com/layout_products.htm Look at the 5th item down, measures relaibly to the nearest 1/1,000th of an inch.
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