Bicycle Mechanics - BB Axle Length?

I'm building a new bike, and I'm about to install the cranksets.

I'm going to crank on some Sugino RD2 cranks, and it specifies a 103MM BB for a 45MM chainline.

I have an old 108MM JIS BB and I was wondering if there's actually any problems with installing a BB with a longer axle than what's recommended?

Would the crank even fit properly or would it be "loose" on the taper? Would the axle protrude out?

The length of the BB spindle has nothing to do with whether the cranks will fit it. All it affects is the distance between the edge of the BB shell and the crank, which changes the chainline, so you won't be able to use that BB if you need a 45mm chainline.

Also, it has one crankset, unless it's a tandem. Crankset generally refers to both cranks.

The length of the BB spindle has nothing to do with whether the cranks will fit it. All it affects is the distance between the edge of the BB shell and the crank, which changes the chainline, so you won't be able to use that BB if you need a 45mm chainline.

Also, it has one crankset, unless it's a tandem. Crankset generally refers to both cranks.

I had a feeling that it might only affect the chainline. So then, would it be possible to use a shorter BB and get an even smaller chainline?

There is no need to guess with chain line. If the new crankset specs 45mm with a particular BB then you are already half way there to getting the chain line correct. Sheldon Brown shows you how to calculate the rear chain line for your bike. Figure that out. Is it 45mm? If it is then use the stock setup. If it isn't then you need to add or subtract BB spindle length to get the same chain line at the front. Of course you have to take clearances into account when going shorter and FD chain line specs if you are working on a multi-speed bike. Al

I had a feeling that it might only affect the chainline. So then, would it be possible to use a shorter BB and get an even smaller chainline?

It would, but do you know what chainline you need?

There is no need to guess with chain line. If the new crankset specs 45mm with a particular BB then you are already half way there to getting the chain line correct. Sheldon Brown shows you how to calculate the rear chain line for your bike. Figure that out. Is it 45mm? If it is then use the stock setup. If it isn't then you need to add or subtract BB spindle length to get the same chain line at the front. Of course you have to take clearances into account when going shorter and FD chain line specs if you are working on a multi-speed bike. Al

Ok thanks for the pointer. I'll check Sheldon's site out.


It would, but do you know what chainline you need?

Well I don't want it too short because then they're won't be enough clearance. I was kind of thinking more of getting a perfectly centered chainline, and running a shorter BB to really get the least amount of angle on the chain.

Are you using a multi speed free wheel/cassette or a single cog?