Bottom bracket axle length

<[email hidden]> a écrit:

Quoted message said:

have just discovered that the bottom bracket on my MTB has lots of
play and is in need of replacement. On measuring it I also discover
that it has a rather strange size: 68mm shell and 121mm axle (in case
it's important the bike is a Scott Elite Racing MTB from ~4 years
ago). It has the shimano Octalink 8 spline interface and I think it's
probably around the Deore or LX level (cartridge BB) given the level
of components on the rest of the bike (Note: I would check the label/
stamp on the existing BB but I don't currently have the correct tool
in my toolbox to remove it!). It seems that the majority of BBs have
shorter axles of 118 or 113 mm, and the fact that mine is 121mm gives
me essentially no choice for the replacement unit. I had hoped to
replace it with something of an XT or XTR level but it seems this is
not available with a 121mm axle. A few questions:

1) Is there a reason why longer axles aren't offered in the high end
BBs? Is it down to the increased weight or something?

From the technical documents on the Shimano site, it looks like the 121mm
axle corresponds to one specific Deore crankset, the FC-M531. I don't know
why that crank was designed to use a longer axle than the costlier groups,
or the previous Deore model.

XTR Octalink bottom brackets use a different spline pattern than the other
MTB groups, so that wouldn't be an option anyway.

Quoted message said:

2) Am I measuring the axle length correctly? I assume, logically, that
it is the distance between the ends of the axle, but I guess it could
the distance between the base of the Octalink splines or something?

End to end is correct.

Quoted message said:

3) I assume that replacing with a unit with a shorter axle would wreck
the chainline and is not adviseable?

You could almost certainly get away with an XT 118mm unit. The chainline
specified for the FC-M531 is 50mm. Most other groups have two options for
chainline - 47.5mm and 50mm. Assuming the 121mm axle isn't wildly
asymmetric, the 118mm axle should still put your chainline in the right
ballpark.

Quoted message said:

4) I am considering using this as an opportunity to replace the
chainset as well which is also quite worn. I notice that the high
end chainsets come with the BB included, but don't specify
dimensions for the BB. They appear to be universal and work
by moving spacers around to get the right shell width.

Right.

Quoted message said:

How does this system cope with different axle lengths? Is it
adjustable or is it one standard size? If so what is the standard
size.

There's just one size. The axle is permanently attached to the right crank,
and the left crank attaches to the other end with pinch bolts. The purpose
of the spacers is to adjust for different bottom bracket shell widths. I
don't know how long the axle is, but it's immaterial: there's no choice, and
it's not comparable to other systems.

Quoted message said:

5) Does the group have experience on home fitting of these
high end chainsets.

I don't, but I'd be inclined to have the shell faced.

James Thomson

Quoted message said:

Hi all,

have just discovered that the bottom bracket on my MTB has lots of
play and is in need of replacement. On measuring it I also discover
that it has a rather strange size: 68mm shell and 121mm axle (in case
it's important the bike is a Scott Elite Racing MTB from ~4 years
ago). It has the shimano Octalink 8 spline interface and I think it's
probably around the Deore or LX level (cartridge BB) given the level
of components on the rest of the bike (Note: I would check the label/
stamp on the existing BB but I don't currently have the correct tool
in my toolbox to remove it!). It seems that the majority of BBs have
shorter axles of 118 or 113 mm, and the fact that mine is 121mm gives
me essentially no choice for the replacement unit. I had hoped to
replace it with something of an XT or XTR level but it seems this is
not available with a 121mm axle. A few questions:

1) Is there a reason why longer axles aren't offered in the high end
BBs? Is it down to the increased weight or something?

2) Am I measuring the axle length correctly? I assume, logically, that
it is the distance between the ends of the axle, but I guess it could
the distance between the base of the Octalink splines or something?

It's end to end for square taper axles. I would expect the same to be true
for Octalink, though I'm not sure,

Quoted message said:

3) I assume that replacing with a unit with a shorter axle would wreck
the chainline and is not adviseable?

Assuming symmetrical BBs, a 118mm BB would only bring the chainline in by
1.5mm. That's practically nothing for a multi-gear system. I would
consider trying one if there was enough chainring clearance.

Remember that, though worse for some, the new chainline would actually be
/better/ for some gear combinations: middle chainring to larger rear
sprockets, for example.

Quoted message said:

4) I am considering using this as an opportunity to replace the
chainset as well which is also quite worn. I notice that the high end
chainsets come with the BB included, but don't specify dimensions for
the BB. They appear to be universal and work by moving spacers around
to get the right shell width. How does this system cope with different
axle lengths? Is it adjustable or is it one standard size? If so what
is the standard size.

5) Does the group have experience on home fitting of these high end
chainsets. I understand the advice is that you should get the LBS to
"face" the BB shell on the frame to ensure it fits. How essential is
this? I ask because my LBS is some distance away and as a result I
prefer to do things myself where possible.

With BBs in general (internal type rather than external or integrated), the
bearings may bind from a side effect if the cups are correctly torqued and
the shell's threads or faces are in less than ideal condition. If lucky,
using blue Loctite Threadlocker will let you get away with less torque. Use
plentifully and leave it to set one or two days before riding. Degrease
first.

With a 'British' BB, the right cup will be fine but the left one will
tighten itself if installed too loose.

~PB