Bicycle Mechanics - Correct BB length?

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sethco
10-22-10, 07:12 PM
I just replaced the cranks (and bb) on a hybrid bike and the bb spindle is too short. How can I figure out the correct length so I only have to order the part once (well, once more)?

The bike is a Scott SUB 10, it has a Deore rear hub w/ disc brake and it had a mountain triple (Shimano FC-M521) with a 190mm Q-factor which is just too wide for me. I replaced it with a FSA Vero compact double and Sugino 103mm square taper bb (the crank specs 103mm).

Everything fits, shifting between rings is fine and there is clearance on the chainstays but the chainline is too far in and when I am in the small chainring and smallest three cogs the chain rubs on the large chainring.

I want to move the line out just enough to for the chain to run freely but no further as I am trying to minimize the Q.

Thanks!

badamsjr
10-22-10, 07:21 PM
If chain rub is the only problem with the 103mm BB, you could try a 107mm (2mm wider, each side). It should help, but may not completely do away with the rub. I think the next wider BB would be about 111mm or 113mm, which would be 4mm and 5mm wider (each side) respectively. If you get a 107mm (for example), and it only rubs a little, you could put a 1mm spacer between the BB and the drive side shell to move it slightly to the right. This would also work with the wider BB's.
sethco
10-23-10, 01:31 PM
Thanks badamsjr.

I thought about this and did some measurements and I think I will need a much longer bb, maybe 118mm. That will increase the Q-factor but still be better than where I started.

The current setup has a chain line of 41/46mm which is exactly the Shimano road standard, but the rear hub is a 135mm mountain hub and the old triple was probably around 50mm out to the middle ring (I need to measure this). I bet that I'll need to get the inner ring out to almost 50mm for the small ring to work across the while cassette.

sethco
10-23-10, 01:36 PM
Does anyone have a recommendation on how large a bb spacer I can use without endangering myself or the threads in the bike's bb shell? Universal Cycles has them available 1mm - 2.5mm, is 2.5mm bigger than I should use with a Shimano UN54?

Thanks!
AEO
10-23-10, 01:50 PM
the general rule is 7 threads engaged.