proper bottom bracket size
 

I have a cup & cone style bottom bracket that I would like to replace for a cartridge unit. I would like to use a Shimano square taper cartridge (UN54?). The crankset is an Sakae triple and the rear spacing is 135.

The bottom bracket shell width is 68mm. English/ISO threading.

The spindle on the old bottom bracket is marked as 3S (YST-3S, specifically). On the Sheldon Brown site, 3S has an over-all length of 121.5, but when I measure the spindle with my calipers, I get 125 on the dot.

http://www.sheldonbrown.com/bbsize.html

What width bottom bracket should I go with?

Sheldon's chart refers to Sugino/SR bottom bracket spindles, so your YST spindle may not match. Go with your measurement.

OTOH, if you have room to allow the right crank to move in 3 or 4mm, you could go with the shorter spindle. If I were working on it, I'd try it out and hope I could exchange the BB if it didn't fit. You make the call.

thanks.

when measuring spindle length, one would measure the overall length ("D"), correct?

http://i52.tinypic.com/2cooa6q.jpg

That is what I am measuring and getting 125mm.

yes, that is correct. But it can be off by a mm or 2 based on how it was manufactured.

If you can go back in time and see how close the chainrings were to the frame before you took it off, it would tell you if you can safely go one size down.

For safety, I usually take the closest size that is exactly the same or slightly bigger than the measurement of the old spindle. This promises you that your chainrings will not grind the chainstay.

The one thing about switching to cartridge that I have never been able to reconcile exactly is that many loose ball spindles are not actually symmetrical. So A and C are not always identical. So, while you should pick a spindle based on chainline, you may end up with your left crank arm being in a slightly different place than before. Not a big deal unless you are really sensitive to your Q factor I guess though.

thanks.

the spindle is indeed asymmetrical.

I'll reinstall everything tomorrow and see what clearances I am working with here.

It depends upon the diameter of the tapered ends (look up difference between ISO & JIS tapers).

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