BB axle length - how crucial is it really?

Hi,
looking at chainset specs, they all specify bottom bracket axle widths
to the last millimeter & beyond. As far as I understand, that ensures
that the chain will run absolutely straight from the centre chainwheel
(if an odd number of sprockets) to the centre sprocket (if an odd
number of sprockets), or a slight deflection (~2mm) if either are even
numbers, and therefore minimise maximum chain deflection at the more
extreme gears.
Obviously that's the ideal situation, but as the chain will be
deflected for most gear selections, how important is the exact match
between chainset and bottom bracket?

There's obviously a reason for my question: I've finally got round to
rebuilding a slightly crash-damaged Dalesman I bought a while ago, and
after some effort getting the torn gear hanger repaired, I've now
discovered that the 122mm sealed-unit bottom bracket has been Loctited
in.

I'm quite literally stuck with it.
But am I stuck with my choice of triple chainset? I haven't found a
46/36/26 that fits 122mm, but could I get away with more common
118mm/115mm/113mm?

Thanks,
Al

Al, Cambridge, UK wrote:

I'm quite literally stuck with it.
But am I stuck with my choice of triple chainset? I haven't found a
46/36/26 that fits 122mm, but could I get away with more common
118mm/115mm/113mm?

You are more likely to get away with a chainset requiring a shorter BB
than one which requires a longer one. One aspect is to keep the inner
ring from fouling the chainstay. A secondary concern is that you will
be increasing the q-factor, ie the distance between the pedals. This
may be bad for you, it may be just fine.

9mm difference is essentially two gears. It depends also where the
extra is located. If it is on both sides of the BB then the difference
will be less.

You may also have issues with the front derailleur being misaligned,
though this should be correctable by moving it up or down.

and one last thing (silly question) - are you sure it is locktited in,
and not that you are turning it the wrong way?

...d

In article . com
Al, Cambridge, UK wrote:
snip
There's obviously a reason for my question: I've finally got round to
rebuilding a slightly crash-damaged Dalesman I bought a while ago, and
after some effort getting the torn gear hanger repaired, I've now
discovered that the 122mm sealed-unit bottom bracket has been Loctited
in.

That just means you need decent quality tools and a bit more effort to
remove it. In the worst case you may need to apply some heat, so will
need to repair the paint. I'd rather have scruffy paint and a good
chainline than good paint and a poor chainline. Having said that you
may find that the gears work fine with the longer axle. Or you could
convert the chainset to a quad, then the chainline would be right :-)

Al, Cambridge, UK wrote:
Hi,
looking at chainset specs, they all specify bottom bracket axle widths
to the last millimeter & beyond. As far as I understand, that ensures
that the chain will run absolutely straight from the centre chainwheel
(if an odd number of sprockets) to the centre sprocket (if an odd
number of sprockets), or a slight deflection (~2mm) if either are even
numbers, and therefore minimise maximum chain deflection at the more
extreme gears.
Obviously that's the ideal situation, but as the chain will be
deflected for most gear selections, how important is the exact match
between chainset and bottom bracket?

Depends how much deflection you have already in the combinations you like
to use much, particularly the ones you put strong load on (can you feel
friction through the pedals?), and whether this is already at the limit of
what you find acceptable, and whether you would be prepared to use
different combinations instead, possibly fitting different chainrings and
sprockets to enable this.

For instance, a "too-long" BB would make using the middle ring + largest
two rear sprockets poorer to use, but on the other hand would improve
performance from the granny ring to the smaller rear sprockets.

There's obviously a reason for my question: I've finally got round to
rebuilding a slightly crash-damaged Dalesman I bought a while ago, and
after some effort getting the torn gear hanger repaired, I've now
discovered that the 122mm sealed-unit bottom bracket has been Loctited
in.

I'm quite literally stuck with it.
But am I stuck with my choice of triple chainset? I haven't found a
46/36/26 that fits 122mm, but could I get away with more common
118mm/115mm/113mm?

A 118 type may be OK, but I would first try a longer spanner/wrench or
some heat to get the existing BB out. I wouldn't fancy trying a 115 or
113 type.

Might even be a problem for the front mech if you go way to far, and it
wouldn't be nice to have feet sticking out very much further.

~PB

In article
Pete Biggs wrote:
snip
For instance, a "too-long" BB would make using the middle ring + largest
two rear sprockets poorer to use, but on the other hand would improve
performance from the granny ring to the smaller rear sprockets.

That's a great way to increase the stress on your drive train.

"Rob Morley" wrote in message
t...
In article
Pete Biggs wrote:
snip
For instance, a "too-long" BB would make using the middle ring + largest
two rear sprockets poorer to use, but on the other hand would improve
performance from the granny ring to the smaller rear sprockets.

That's a great way to increase the stress on your drive train.

Eh? It's a great way to decrease the stress on your drive train if you
normally use the gears whose performance is improved.

clive

In article
Clive George wrote:
"Rob Morley" wrote in message
t...
In article
Pete Biggs wrote:
snip
For instance, a "too-long" BB would make using the middle ring + largest
two rear sprockets poorer to use, but on the other hand would improve
performance from the granny ring to the smaller rear sprockets.

That's a great way to increase the stress on your drive train.

Eh? It's a great way to decrease the stress on your drive train if you
normally use the gears whose performance is improved.

Small front to small back puts higher loads on the chain and sprockets
no matter how good the chainline is.

Rob Morley wrote:

For instance, a "too-long" BB would make using the middle ring +
largest two rear sprockets poorer to use, but on the other hand
would improve performance from the granny ring to the smaller rear
sprockets.

That's a great way to increase the stress on your drive train.

The granny ring would be better aligned with the smaller sprockets than
usual, so the small-small combinations wouldn't be as bad as usual. It is
more efficient to use larger sprockets in general*, but only when the
chain angle is OK.

* To do with the amount the chain liks pivot; separate issue to lateral
deflection.

~PB

In article
Pete Biggs wrote:
Rob Morley wrote:

For instance, a "too-long" BB would make using the middle ring +
largest two rear sprockets poorer to use, but on the other hand
would improve performance from the granny ring to the smaller rear
sprockets.

That's a great way to increase the stress on your drive train.

The granny ring would be better aligned with the smaller sprockets than
usual, so the small-small combinations wouldn't be as bad as usual. It is
more efficient to use larger sprockets in general*, but only when the
chain angle is OK.

* To do with the amount the chain liks pivot; separate issue to lateral
deflection.

Is it proven to be more efficient? Although the links rotate less there
are more links rotating. I was thinking more of increased wear because
of higher loads and fewer teeth.

Rob Morley wrote:
In article
Clive George wrote:

That's a great way to increase the stress on your drive train.

Eh? It's a great way to decrease the stress on your drive train if
you normally use the gears whose performance is improved.

Small front to small back puts higher loads on the chain and sprockets
no matter how good the chainline is.

I don't believe the loads would be lower when using middle front to large
rear when the chainline is very poor. The friction would be so bad that
you'd feel it through your feet.

Large sprockets are more efficient to use in general, but I think only
when the chainline is reasonable.

Note I'm not suggesting using the very smallest sprocket with the granny
ring (unless the chainring is incredibly far out), but the next few along
would be more useable than usual, that's all.

~PB