Bottom bracket sizing problem

"Rob" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message 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?

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?

The 73mm and 68mm shimano BBs are almost the same - the difference is in the
shim which goes on the end and an extra 5mm on the LHS spindle.

I've used this to my advantage on our tandem - different LH and RH cranks
need different spindle lengths, and fortunately it's the LH one which needs
to be longer.

Anyway it sounds like you've got a similar thing here. If you put a 113mm
long 68mm BB in with the same shim as you've got now your LH will move in
4mm and your RH out 1mm, and all will be about right.

cheers,
clive

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.

Quoted message 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.

Quoted message said:

What can I do to make my cranks my symmetrical?

Quoted message said:

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

John Barnett's exhaustive bike repair manual has a table of "relative
chainline", giving the 107mm "MM" axle as a reference. As luck would have
it, the relevant chapter (a 400k .pdf) is available for download from his
website:

http://www.bbinstitute.com/BM5%20chap%2010.pdf

page 10:7

According to this table, the 107mm and 110.5mm axles have the same relative
chainline, which must mean that the difference is in the amount of axle
overhang at the left of the unit. In other words, a 107mm unit should bring
the left crank in by 3.5mm without affecting your chainline. I think that's
as close as you can reasonably expect to get to symmetry.

Quoted message said:

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

Shimano used to supply one with the fitting instructions for their bottom
brackets, though I haven't seen one in a long while. Perhaps a local bike
shop might still have one kicking around.

James Thomson

Clive George said:

If you put a 113mm long 68mm BB in with the same shim as you've
got now your LH will move in 4mm and your RH out 1mm, and all
will be about right.

I think you've got that muddled. The critical difference between a 68mm and
a 73mm unit is that the axle in the latter case is shifted leftward 2.5mm
relative to the RH flange, so in the case of identical axle lengths,
installing a 68mm unit in a 73mm shell will shift everything rightward by
2.5mm. Going to the 113mm axle will then add a further 3mm to the
chainline, so the right crank ends up 5.5mm further right.

James Thomson