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Is my BB bent or my crank set?

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Is my BB bent or my crank set?

Old 10-18-18, 09:28 AM
  #1  
blaubner
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Is my BB bent or my crank set?

Saw another user suggest this to determine if the BB is bent or the chainrings/spider.

Rotate the crank until the largest chain ring is furthest from the frame. Note the position of the non-drivetrain crank arm. Take the non-drivetrain crank arm off, flip its orientation 180 degrees to match the other crank arm, then reinstall it. Rotate the crank again until the largest chain ring is furthest from the frame. If the original position of the non-drivetrain crank arm was 7 o'clock, and the new position of the non-drivetrain crank arm is still 7 o'clock - bent bottom bracket. If the positions are 7 o'clock and 1 o'clock respectively - bent chain ring or spider.

Is my understanding correct?

Best,
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Old 10-18-18, 10:16 AM
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Asked someone who can see it in person, yet?

I'd have to see it, myself.
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Old 10-18-18, 10:37 AM
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The tapers on axles and in arms are not always perfectly cotaperial (coaxial) to the axel or perpendicular to the rings/spider arms. So remounting an arm in a different rotational position on the axle can cause some slight offness, sometimes. I don't consider this to be any big deal though, just what I've both read and seen. Either way it's about the rings being flat to the perpendicular of the axle and that is often correctable with some carefully placed leverage. Andy (who just yesterday better aligned a ring for less chain rub within the ft der cage).
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Old 10-18-18, 11:01 AM
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chainring can be bent.. too

pull the BB axle out and test it, like picking a pool cue,
roll it on a 'known to be flat' surface..

you can do that with chain rings too. lay them on a flat surface.





..
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Old 10-18-18, 11:58 AM
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Great, thanks for the advice everyone.
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Old 10-18-18, 01:51 PM
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Just had an idea.

First, if you can fasten/bold/duct tape something to the bottom of the downtube (like a 2x4) so that you can check the chain rings (use the 2x4 to hold a credit card steady close to the chainring) you can spin the crank and tell if the chainring is running true.
Second, you should be able to put a yardstick or something against the chainring and see where the chainline is relative to the rear cluster.
If your chainrings run true and the chainline is correct, you can check the crank arms and pedal for whether they're correct with this approach:
With the bike firmly held in a bike rack (you could even have the wheels on the ground), use the goniometer (angle meter) in your cell phone to check that the bike is plumb. Then measure the angle of the pedal at bottom dead center. Put the pedal at top dead center and compare the angle. They should be the same. If not, you have a bend or misalinged crank and/or pedal spindle.
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Old 10-18-18, 02:06 PM
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Nothing on TV?
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Old 10-19-18, 07:54 AM
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I'd tighten all the chain ring bolts too. A cartridge or an, old time, (separate parts) ball bearing bottom bracket axle are pretty tough. Unless they're worn out. A worn out of either, would give you a wobbly pedaling experience. I'd check your front deraller alignment too. Look over it with a straight edge along the right side of the cage. cheers
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