Bicycle Mechanics - BB axle length...

Hallo forum community,

I am (still) in the progress of building my dream cyclocross/allround bike. As Holland is flat, flat, then flat again, I go for a singlespeed setup in front.

The components:

- Shimano FC-M752 (170mm) crankset with 44t. chainring
- Rohloff CC 8290 chainhelp for singlespeed
- Shimano CS-6500 cassette, 12-25
- Shimano FH-M752 rear hub, 135mm
- Shimano RD-6500-SS (short cage) rear derailleur
- Shimano CN-HG73 chain

The problem:

The Shimano BB-ES71 bottom bracket is available in 113 or 118mm, but which one do I need for a smooth running drivetrain? 113 or 118? The BB-tube in my frame is 68mm wide.

Thanks in advance for your input.

Regards,
Timo

113mm is more likely to work. Double setups range from 108 to 118, and triple start at 113 (few work at this length) and go up. You can check your chain line and figure out which side of the spider to put the ring on. Steve

Great, that what I'll try! Thanks Steve :)

Refer also to Barnett's Manual. There is a section that discusses BB spindle lengths, a table of BBs and their numbers, and the change in chainline that occurs with each spec.

I have just been through a quite difficult process in correcting the Q-factor on my randon bike, and confusion reigned supreme on this subject. The offets on each side of the spindle influence both chainline and Q-factor.

You might also consider referencing Shimano's websites and seeing what BB spec they match the chainring/crank to.

I agree that 113mm will probably give you a straighter chainline than 118mm. You will need to check this for your specific bicycle, spindle, and crankset, but you will probably get a better chainline if you mount the single chainring in the inner, rather than the outer, position on the spider.

Despite not needing a wide gear range, consider using a double chainring in front, in a half-step (3-tooth drop) configuration. If nothing else, this will let you obtain a straighter chainline by using the inner ring with the inner and middle cogs and the outer ring with the outermost cogs. The front derailleur cage may also provide some protection against throwing the chain during a full-range shift across the cogs.

Originally posted by John E Despite not needing a wide gear range, consider using a double chainring in front, in a half-step (3-tooth drop) configuration. If nothing else, this will let you obtain a straighter chainline by using the inner ring with the inner and middle cogs and the outer ring with the outermost cogs. The front derailleur cage may also provide some protection against throwing the chain during a full-range shift across the cogs. [/B]
...Thanks John,
However, I won't need the derailleur cage for protection against throwing the chain, because that's what the Rohloff devise is for. Also, compared with a triple setup, the single setup in front (including a non-STI left hand breaklever) weights 235 grams less. I know, I am on a diet, but all weigth saved on the bike won't come back on the belly and I really don't need more than 5 out of 9 cogs available in the rear. Holland is incredibly flat :)