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Bicycle Mechanics - Bottom bracket length for compact (50-34) crank on Hardtail?

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rollo

01-18-06, 09:45 AM

I was thinking of moving a Ritchey pro compact road crank over to my hardtail.
I’m concerned that the larger chain ring will contact the chain stay.
Will a triple bottom bracket work better or not at all?

Thanx

sch

01-18-06, 10:46 AM

34t cw will have a 2.7" radius, so you can measure back from the center of the bb axle about 6mm out from the edge of the bb shell and see how much clearance you have from the chainstay at that distance. Remember that riding torque will cause the cw to deflect a little side to side. It is likely to be close as ATB frames assume small cws. The 50t cw has a 4" radius and is going to be about 12mm from the edge of the bb shell. OTOH if you just use your triple length axle it will clear ok. You will have to raise the FD up a bit.
Edit: assumption you were going to shorten your axle to double length. Steve

TallRider

01-18-06, 10:52 AM

I think that ISIS double-cranks for road bikes are supposed to go with 108mm bottom brackets. For correct chainline on a mountain bike (with 5mm wider rear hub spacing), you should go with a 113mm bottom bracket spindle. This will also ensure that the chainrings are a little bit further out from the stays.