Bicycle Mechanics - Best way to figure out best bb length?

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d2create
05-04-08, 09:55 AM
I did a search and couldn't figure out the best way to calculate what bb length is needed for good chain line.
I'm starting a new build and i have the frame measurements. Will have the rear hub (sram i-motion 9) soon. Have not yet decided on a crank but it will be a single chain ring.

bikeman715
05-04-08, 10:19 AM
the chain ring need to clear the chain stay by 5 mm at it closes point.
HillRider
05-04-08, 10:29 AM
Actually, the crank defines the required bottom bracket length. Once you have chosen a crank, the bb should be based on what the crank manufacturer says.

d2create
05-04-08, 01:04 PM
Actually, the crank defines the required bottom bracket length. Once you have chosen a crank, the bb should be based on what the crank manufacturer says.

But wouldn't rear hub spacing also factor in?
roadfix
05-04-08, 01:57 PM
But wouldn't rear hub spacing also factor in?In most cases, no. I think you're over-thinking this too much. Just go with the recommended BB for your crankset and you should be fine.
operator
05-04-08, 02:48 PM
But wouldn't rear hub spacing also factor in?

Not unless the hub was deliberately dished over and respaced. The chainline is listed as 48,9 mm in the technical docs. All you need is a crankset and a bb that will give you that chaInline and you're ready to go.

Manufacturers usually will state what length bb with $CRANK_MODEL will give $CHAINLINE_VALUE.