How to shop for replacement bottom bracket?

My suggestion is that you get the right size bottom bracket for your crank. Measure the length of
your spindle. Then, subtract a few millimeters. Ideally, you want to get a spindle that will allow
the crankset to sit far enough away from the frame so that inner chainring will spin freely w/o
touching the frame. Look at your crank right now and see approximately how many mm you could shorten
the spindle and still have plenty of clearance. Then buy a bb with a spinlde of that length.

Andres

Quoted message said:

Subject: How to shop for replacement bottom bracket? From: eric34 [email hidden] Date: 8/11/2003
9:51 PM Mountain Standard Time Message-id: <[email hidden]>

Heres what I have:

A shimano bike with a relatively modern BB (well, within 15 years old..) It probably used to have
LX or 1 group lower groupo.

Heres the problem. When I had the bike shop rebuilt the bike 3-4 years ago, I tried to fit an old
crank-set that I had and found the spindle too short. The shop was nice enough to find me one that
had long spindles. However, discovering the crank-set was bio-pace, and pretty worn and slightly
warmed rings, I put on new cranks. Of course, it stuck out too much. The same bike shop did not
want to put another spindle on it without putting a whole new BB on it. ($50 canadian, ~$35us).
Another bike shop was nice enough to "work my front derailler" to work. It worked fine until now.
The derailer is at its outer limit, and is getting "old". (I guess it goes out of line more often
than I want to adjust).

My question is:

I want to find a BB online, or a front derailer online. (maybe both!). However, I can't figure out
what size to buy. Can someone tell me how to determine the right size? I'm sure its a generic
Shimano mountain BB.

Also, the BB works great, and I really enjoy the extra Q-factor (my feet points naturally outward
quite a bit..) Is there an "extra reach" front derailer I can get? (Clamp on) Right now, its a
generic LX triple.

(AndresMuro) said:

Ideally, you want to get a spindle that will allow the crankset to sit far enough away from the
frame so that inner chainring will spin freely w/o touching the frame. Look at your crank right now
and see approximately how many mm you could shorten the spindle and still have plenty of clearance.
Then buy a bb with a spinlde of that length.

I don't think you are correct. You need a bb for a triple crankset that permits the center ring to
be 45 mm from the centerline of the frame. That will give a proper chainline for optimum rear
shifting. BB length is specific to the crankset.

I agree with what you are saying. However, the bb disigned for a particular crankset will place it
so that the inner chainring will not touch the chainstay, yet it will be close. This will allow the
front deraulier to reach and shift through all three rings. If you get a bb that is too long it will
place the crankset far away from the chainstay. A front deraulier may have difficulty reaching the
outer chainring, which is the situation that the original poster was facing.

Older bbs were designed in such way that they needed longer bbs. Newer ones require shorter bbs. The
poster has a bb that puts the crank far away from the frame, he can get a shorter one that will
bring the crank closer to the frame. The shimano un 72 bb, is made in all sizes, from 103 to 128 (I
think). A 103 would work with a new dura ace. the 128 will probably work with an old triple
crankset. Most cranks will need something in between.

Andres

Paul Kopit <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

(AndresMuro) said:

Ideally, you want to get a spindle that will allow the crankset to sit far enough away from the
frame so that inner chainring will spin freely w/o touching the frame. Look at your crank right
now and see approximately how many mm you could shorten the spindle and still have plenty of
clearance. Then buy a bb with a spinlde of that length.

I don't think you are correct. You need a bb for a triple crankset that permits the center ring to
be 45 mm from the centerline of the frame. That will give a proper chainline for optimum rear
shifting. BB length is specific to the crankset.