import600

How do you figure out this number? Its a sekai 2400 frame, with a 68mm shell, and the BB installed is 127mm. This is obviously too wide for my cranks. I'm just curious if there is a method as to figuring what size BB diameter I need. Thanks.

666pack

you need to figure that out yourself based on what chainline your cranks give you.

raster

I'm also curious about the correct way to do this. When I built mine, I determined the chainline by measuring the rear hub, as detailed by Sheldon. I then bought the BB that would give me that chainline.

import600

how do you buy a BB for a crank that isn't installed? I don't know how to figure how far the crank slips on. Is it just guess work??

operator

Put your current crankset on, put the cog on. Slap a giant metal ruler on the front chainring and eyeball it. Then measure the offset.

You can also use a caliper and measure the chainline from teh center of the bb to the middle of the chainirng and then measure the chainline of your hub.

That should give you an exact answer.

operator

The manufacture should tell you what chainlie you get with what length bb. Supposedly.

import600

thanks, I would have to say to myself duh. That does help for my sugino's. but not for my older cranks. Good info.

dizzy101

If you have the crank, the BB and the frame, just try to put them together and measure the distance from the middle of the frame to the center of the chainwheel.

Also keep in mind that there has to be clearance between the chainring and the frame--this often limits how much shorter the BB can get.

You can also always fiddle with the chain line by adjusting stuff at the rear wheel, with spacers and stuff.

(It's good to finally see a bike topic in this forum. in between the flame wars and fashion advice.)

na975

go to business cycles website, click on references and there you will see a diagram showing you where to measure.

import600

Is there a recomended absolute minimum for this? What is the clearance on most track bikes?

I think I found a BB that should work theoretically, cross my fingers here, I ordered it off of harriscycles. Just gotta wait for the UPS guy to see if this works. For this frame, I'm using sugino cranks, They suggest a 103mm BB. Now it seems a 103 will work but the chainring will clear the frames with only a couple mm to spare. It's close, probably around 5 or 6mm. It seems to me it should be fine, a bent chainring would bind it up on the frame absolutly. But when would you ride around with a bent one? ya know? I suppose if something were to happen in the middle of the ride, I'd be ****ed. lol.

Thanks for the info. Just trying to expand my knowlege here. Ha.

operator

At least 1mm to account for frame flex. I have about 2mm on my converted road bike with a dura ace double and a 103 dura ace bb.

It's not the chainring that's actually a problem with the conversion, it's the chainring bolts hitting the chainstay. Every bike will be different.

import600

Good info. Thanks guys. Once the mail comes in and I install this thing, I'll try to take pictures for anyone else who has the same question.