How to shop for replacement bottom bracket?

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.

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

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


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.

On 12 Aug 2003 12:32:22 GMT, (AndresMuro) wrote:

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 wrote in message . ..
On 12 Aug 2003 12:32:22 GMT, (AndresMuro) wrote:

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.