Bottom bracket sizing problem

Rob said:

I have an old steel specialized rock hopper that I use as my about town
bike. The bottom bracket (not the original) is now about to fall apart.
It is a UN72 for 73mmm shell and the spindle is stamped 110E. Measuring
with the vernier claipers I get 111mm, so I guess its about right.

About a year ago I purchased what was probably one of the last square
taper LX chainsets as the specialized chainset had finally died. Since
then I have noticed that the lefthand (non drive) crank is approx 5mm
further away from the chain stay than the righthand crank. That is 15mm
gap for the left and 10mm for the right when measuring from the inside
of the crank along the middle of the peddle spindle to the chainstays.

What can I do to make my cranks my symmetrical?

Is it causing some sort of physiological problem, pain or something? If
not, why muck with it. If the BB is cooked, a 115mm will work and
they are symmetrical.

Quoted message said:

Do I buy a 107mm spindle or a 113mm spindle?

How do I know that the length variation will only affect the left hand
side?

Are there any decent engineering drawings that show the dimensions of
shimano BB's?

Thanks.

Rob said:

I have an old steel specialized rock hopper that I use as my about town
bike. The bottom bracket (not the original) is now about to fall apart.
It is a UN72 for 73mmm shell

That's odd. I would have expected a 68mm BB on an mtb frame. Perhaps
someone with more familiarity with Specialized can elucidate.

Quoted message said:

and the spindle is stamped 110E. Measuring
with the vernier claipers I get 111mm, so I guess its about right.

About a year ago I purchased what was probably one of the last square
taper LX chainsets as the specialized chainset had finally died. Since
then I have noticed that the lefthand (non drive) crank is approx 5mm
further away from the chain stay than the righthand crank.

Well, if there's a 73mm BB in a frame that was supposed to wear a 68mm
unit, that might be part of the problem.

Quoted message said:

That is 15mm
gap for the left and 10mm for the right when measuring from the inside
of the crank along the middle of the peddle spindle to the chainstays.

Was it symmetrical with the Specialized crank set?

Quoted message said:

What can I do to make my cranks my symmetrical?

First, before you do anything, look at the most important issue in BB
length selection. Check the chainline. It it's right, and if the
73mm installed BB really is the right width for the shell, then your
best bet is to simply ignore the slight assymmetry and keep riding.
If it's wrong, fix it and recheck the spacing...but bear in mind that
as long as the left crank clears the chainstay, the exact spacing on
that side is usually not important.

Quoted message said:

Do I buy a 107mm spindle or a 113mm spindle?

Both will move *both* cranks. If the BB shell width is correct now,
and you install another of the same width, the assymetry will remain,
it will just be by a different amount.

Quoted message said:

How do I know that the length variation will only affect the left hand
side?

It won't.

Quoted message said:

Are there any decent engineering drawings that show the dimensions of
shimano BB's?

Not to my knowledge. Others may have that information.
--
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I measured the shell, and it's 73mm.

I perceive the cranks not being symetrical as being a bad thing. I will
check the chain line, however, I seem to have it in my mind that it's
the left that is too far away rather than the right that is too close.

This is important to me since I have a muscle imbalance in my left leg
and a bit of foot pronation that can cause bad tracking of the knee. I
am keen to eliminate any other potential problems.

thanks.

Rob said:

I measured the shell, and it's 73mm.

I perceive the cranks not being symetrical as being a bad thing. I will
check the chain line, however, I seem to have it in my mind that it's
the left that is too far away rather than the right that is too close.

This is important to me since I have a muscle imbalance in my left leg
and a bit of foot pronation that can cause bad tracking of the knee. I
am keen to eliminate any other potential problems.

Consider reinstalling the original Specialized left crank, if it was
doing the job. The mismatch is purely a cosmetic issue.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

Rob said:

About a year ago I purchased what was probably one of the last square
taper LX chainsets as the specialized chainset had finally died. Since
then I have noticed that the lefthand (non drive) crank is approx 5mm
further away from the chain stay than the righthand crank. That is 15mm
gap for the left and 10mm for the right when measuring from the inside
of the crank along the middle of the peddle spindle to the chainstays.

What can I do to make my cranks my symmetrical?

Do I buy a 107mm spindle or a 113mm spindle?

How do I know that the length variation will only affect the left hand
side?

The important issue is whether the chainline is correct. If so, don't
worry about the purported lack of symmetry.

--

John ([email hidden])

Rob said:

I measured the shell, and it's 73mm.

Good. You've got that part okay in your 110E bb then.

Quoted message said:

I perceive the cranks not being symetrical as being a bad thing. I will
check the chain line, however, I seem to have it in my mind that it's
the left that is too far away rather than the right that is too close.

This is important to me since I have a muscle imbalance in my left leg
and a bit of foot pronation that can cause bad tracking of the knee. I
am keen to eliminate any other potential problems.

thanks.

The "E" in the bb marking is a clue: it's the version intended to
accomodate the "E" type front derailleur, which includes a bracket that
mounts partly under the right side of the BB. If your front derailleur
doesn't have that, I wonder if you can just add, say, 2.5mm of spacers
there in its place? That should eliminate the 5mm difference.