Bicycle Mechanics - Crank Arm Q Difference Between Sides

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Werkin
05-23-11, 09:35 AM
What is an acceptable difference in distance between the drive and non-drive crank arm end to the seat post tube for a SRAM crank/GXP bottom bracket? As an example: the drive side arm 1mm greater than the non-drive side.

I've been able to reduce the distance difference on tapered square spindles with temperature treatments, and reduced the difference by .5mm on a GXP with a few hammer taps, just wondering what's an acceptable tolerance. This is with proper frame BB shell preparation and road double crankset.

fietsbob
05-23-11, 09:38 AM
If the inside of the crankarm clears the frame , by say a CM, it's good.


You get the chainline right,
and crank arm falls where it may, that's secondary.
Kimmo
05-23-11, 09:58 AM
I'm with the OP, but then I'm a tad anal about symmetry. Chainline doesn't count for a whole lot on multispeed drivetrains, particularly if the study I saw cited in another thread saying there's no loss in efficiency across a whole 9spd cassette is to be believed.

rloftus
05-23-11, 10:18 AM
On the GXP, if you have a 68mm BB shell, you can move both 2.5mm spacers to one side or the other if a 2.5 mm shift makes Q factor better and doesn't adversely affect chainline or chainring clearance.