Bizarre Bottom Bracket Baffles Bad Bicyclist

Borrall Wonnell said:

Blliteration aside,

Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle)
that seems to be off-center on my '02 MTB (with 68mm BB shell). I
think I have installed it correctly...the fixed cup (DS) is screwed in
as far as it can go, such that the flange is pressing against the side
of the BB shell. The free cup is inserted as far as it can go (with a
couple of threads showing). The BB is square taper.

The gap between crankarm and chainstay is larger on the NDS by about
2-5mm. I swapped the crankarms and placed them in various positions on
the spindle to rule them out as a source of the problem...in all cases,
the gap was larger on the non-drive side.

Assuming the frame is symmetrical (it appears to be, and is in good
condition), is this kind of discrepancy acceptable/expected? Have I
installed the BB wrong? Am I doing something else wrong?

Thoughts are appreciated,
Dave

Sounds like you need to reverse the spindle to me. Most spindles are
asymmetric, with one side (normally the drive side) longer.

If this is one of those cartridge BBs for which the fixed cup cannot be
removed or moved to the other side of the cartridge, then it sounds like
your spindle is inappropriate for your crankset.

Mark

Borrall Wonnell said:

Blliteration aside,

Are bottom brackets self-centering?

Yes and no. Most do not provide for lateral position adjustment,
which is what I think you're looking for.

Quoted message said:

I've got a 68mm BB (110mm spindle)
that seems to be off-center on my '02 MTB (with 68mm BB shell). I
think I have installed it correctly...the fixed cup (DS) is screwed in
as far as it can go, such that the flange is pressing against the side
of the BB shell. The free cup is inserted as far as it can go (with a
couple of threads showing). The BB is square taper.

By any chance, was it a Shimano unit that had an unthreaded shoulder
about 2mm wide next to the right flange? There's one design of
UN-series BB that I've encountered which is intended specifically for
use with BB-mounted front ders; using it with a tube-mounted fder will
cause the cranks to be 2mm farther to the left than normal for the
spindle length.

Quoted message said:

The gap between crankarm and chainstay is larger on the NDS by about
2-5mm. I swapped the crankarms and placed them in various positions on
the spindle to rule them out as a source of the problem...in all cases,
the gap was larger on the non-drive side.

Assuming the frame is symmetrical (it appears to be, and is in good
condition), is this kind of discrepancy acceptable/expected? Have I
installed the BB wrong? Am I doing something else wrong?

I hink the BB that caused this problem for me was a UN52 or UN53; I've
swapped over to the cheaper UN25 (and competitive units) as a result.

I will note, however, that as long as the sprockets don't hit the
chainstay, there's no real problem with having the cranks off center
by an amount this small.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

Quoted message said:

I hink the BB that caused this problem for me was a UN52 or UN53; I've
swapped over to the cheaper UN25 (and competitive units) as a result.

The BB I am using is a generic Chinhaur (or something like that).
After my post last night, I swapped it out for a new Shimano UN52 of
the same specs (68/110)...and found the same problem. Hmmm...maybe
I'll ask my LBS for a sample of the UN25.

Quoted message said:

I will note, however, that as long as the sprockets don't hit the
chainstay, there's no real problem with having the cranks off center
by an amount this small.

Part of the reason I'm concerned about the difference is that I plan on
'upgrading' my crankset to a 48-36-26 (still a Shimano crank w/square
taper)....and I am a bit concerned about clearance between the frame
and large ring. I'll probaby change the BB to a 113mm (still within
chainline recommendations).

Cheers,
Dave

Borrall Wonnell said:

Blliteration aside,

Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle)
that seems to be off-center on my '02 MTB (with 68mm BB shell). I
think I have installed it correctly...the fixed cup (DS) is screwed in
as far as it can go, such that the flange is pressing against the side
of the BB shell. The free cup is inserted as far as it can go (with a
couple of threads showing). The BB is square taper.

The gap between crankarm and chainstay is larger on the NDS by about
2-5mm. I swapped the crankarms and placed them in various positions on
the spindle to rule them out as a source of the problem...in all cases,
the gap was larger on the non-drive side.

Assuming the frame is symmetrical (it appears to be, and is in good
condition), is this kind of discrepancy acceptable/expected? Have I
installed the BB wrong? Am I doing something else wrong?

Thoughts are appreciated,
Dave

I had the exact same thing after I changed the chainset on my bike.
Different chainsets may need different spindle lengths. My spindle
length was originally 110mm and the NDS (left) appeared to be
approximately 4mm further out from the frame than the other (right) side.

I then got the next length down, 107mm, and everything is just about OK.
If you want some proof, then take your BB down to your LBS and compare
it to a number of different sizes. You will probably find that the
amount that the drive side spindle protrudes from the BB shell is the
same for different lengths.

That was my experience with Shimano BB's anyway. Hope that helps.

Borrall Wonnell says...

Quoted message said:

Blliteration aside,

Are bottom brackets self-centering? I've got a 68mm BB (110mm spindle)
that seems to be off-center on my '02 MTB (with 68mm BB shell). I
think I have installed it correctly...the fixed cup (DS) is screwed in
as far as it can go, such that the flange is pressing against the side
of the BB shell. The free cup is inserted as far as it can go (with a
couple of threads showing). The BB is square taper.

The gap between crankarm and chainstay is larger on the NDS by about
2-5mm. I swapped the crankarms and placed them in various positions on
the spindle to rule them out as a source of the problem...in all cases,
the gap was larger on the non-drive side.

Assuming the frame is symmetrical (it appears to be, and is in good
condition), is this kind of discrepancy acceptable/expected? Have I
installed the BB wrong? Am I doing something else wrong?

Thoughts are appreciated,
Dave

An Isis BB I installed recently had a spacer and an o-ring that were
supposed to be used or left off depending on your BB shell width and
desired chainline. The o-ring is only there to cover up the gap on the
non-drive side cup because it can't be fully inserted on a 68mm shell.
The spacer was for the drive side 68mm shell. With a 73mm shell, both
were left off. The new outboard bearing cranksets all have spacers on
the drive side for MTB models with a 68mm shell and I believe MTB's have
the drive side crankarm further from the BB than the non-drive side by
about 5mm to accommodate triple chainrings and wide, thick chainstays to
clear wide tires. For Isis and Octalink, MTB BB's have spindle widths
of about 113mm and narrower 108mm spindles for road doubles. There are
other typical spindle widths for road triples and downhill MTB's. The
only way to know for sure the correct spindle width is to know the
manufacturer specs, because some cranksets don't follow the typical
patterns. Are you sure you have the correct spindle width? Were there
spacers included that maybe you should have used?

Quoted message said:

The
only way to know for sure the correct spindle width is to know the
manufacturer specs, because some cranksets don't follow the typical
patterns. Are you sure you have the correct spindle width? Were there
spacers included that maybe you should have used?

I wasn't aware of any of this before I took the crankset apart.
According to Sheldon, the recommended BB spindle length for my crankset
is 110-113mm. I measured the BB and was able to physically confirm the
110mm length. The spindle is the same length on either side of the BB
shell (about 21mm). No spacers were included...this is the original
crankset, unfortunately I never examined the spacing before stripping
the frame.

I just measured the gap between the crankarm and chainstay on both
sides of the frame, from the center of the pedal spindle. The
crankarms are roughly parallel with the chainstay at this point.
Here's what I came up with:
DS: 6 mm gap
NDS: 13 mm gap!!

Wow, a 7mm difference....I never thought it was that large.

Even worse...I swapped the crankarms around so that the chainrings were
on the NDS and re-measured. This time, the gaps were 8mm and 11mm (DS,
NDS respectively). Arggh! Ok. Obviously my crankarms aren't
perfectly identical, otherwise I would have had the same gap
measurement after switching them around. Something else must be fishy.

My next guess...the square taper is slightly larger on the NDS, and as
a result the crankarm cannot be installed as close to the frame as on
the DS.

I don't think I can bend the crankarms (and even if I could, it
wouldn't be accurate). I *could* try filing down the inside of the NDS
crankarm to make it slide down the tapered spindle a little further.
Alternately, I could try filing down the spindle itself. I'm not a fan
of either approach. The 7mm gap isn't a dealbreaker for me...I'm just
wondering why the gap exists, and if it is a common 'problem' for
square taper designs.

Cheers,
Dave