Bottom bracket length

Tim Hall said:

I've just got a new chainset from Byercycles for my fixer. However,
as the fixer is the fruit of the contents of my bits box, it has a
long bottom bracket on it, such that the chain line is not good.
Trying to measure over the ends of the spindle shows it about 133mm.

The chainline of the sprocket is 40mm. The chainline of the chainring
is 50mm. (measurements subject to getting a steel rule I can see the
divisions on properly).

So it seems I need a new bottom bracket. Is a 113mm bottom bracket
10mm shorter on the drive side only, or is it shared between either
side?

I *think* modern Shimano cartridge BBs have symmetrical axles (same amount
sticking out each side), but I'm having trouble finding info to verify that.

Your old BB could be asymmetrical. Measure each side.

~PB

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

Quoted message said:

I've just got a new chainset from Byercycles for my fixer. However,
as the fixer is the fruit of the contents of my bits box, it has a
long bottom bracket on it, such that the chain line is not good.
Trying to measure over the ends of the spindle shows it about 133mm.

The chainline of the sprocket is 40mm. The chainline of the chainring
is 50mm. (measurements subject to getting a steel rule I can see the
divisions on properly).

So it seems I need a new bottom bracket. Is a 113mm bottom bracket
10mm shorter on the drive side only, or is it shared between either
side?

I have just recently done the whole drive on my wife's bike and read in my
books that when you get a new chainset you needed to ensure that the bottom
bracket was the correct length for it.

So basically you need to know the make and model of your new chainset (which
I assume you do) then hopefully there will be a website that will tell you
what you need.

Mine did not - so as it is Shimano I contacted the UK distributer and asked
them. I then bought the correct length.

Dave

Tim Hall said:

I've just got a new chainset from Byercycles for my fixer. However,
as the fixer is the fruit of the contents of my bits box, it has a
long bottom bracket on it, such that the chain line is not good.
Trying to measure over the ends of the spindle shows it about 133mm.

The chainline of the sprocket is 40mm. The chainline of the chainring
is 50mm. (measurements subject to getting a steel rule I can see the
divisions on properly).

So it seems I need a new bottom bracket. Is a 113mm bottom bracket
10mm shorter on the drive side only, or is it shared between either
side?

Tim

To answer your question, it depends.

You probably already know most/all of this, but just to lay out some
options:

In addition to thread standards for the BB "cups" into the bottom
bracket, there are two standards for the taper at each end of the BB
axle, ISO (used by most, but not all European manufacturers), and JIS
(used by Shimano and many other).
You need to establish which one your new chainset uses, and buy to
match that.

You should then establish if the BB axle you know have is symmetrical
or asymmetrical. If it's a cartridge type, you can probably do this
with the BB in situ - just measure how much axle protrudes out of the
face of the cartridge on each side. If it's the older seperate cup/
bearings/axle type, the only way to be sure is to pull the axle and
measure it off the bike.

Having established those things, and already knowing how much you need
to shorten the chainline at the sprockets, you can figure out all of
the required specs for the new BB/axle.

You might also consider using chainring bolt spacers to shift the
chainring in towards the seat tube (depends on the design of your
chainset), and or shifting the spacing of the hub on it's axle to
increase the chainline at the back (may require some re-dishing of the
rear wheel).

Any of these later suggestions may be more work than just getting the
right BB length... ;-)

hth,

bookieb

In article <[email hidden]>, Pete Biggs

Quoted message said:

I *think* modern Shimano cartridge BBs have symmetrical axles (same
amount sticking out each side), but I'm having trouble finding info
to verify that.

I don't have any Shimano BBs but all the ones I use in the shop are
longer on the chainwheel side.

--
T h e - e x t e n d e r ! ! ! !

Dave said:

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

Quoted message said:

I've just got a new chainset from Byercycles for my fixer. However,
as the fixer is the fruit of the contents of my bits box, it has a
long bottom bracket on it, such that the chain line is not good.
Trying to measure over the ends of the spindle shows it about 133mm.

The chainline of the sprocket is 40mm. The chainline of the chainring
is 50mm. (measurements subject to getting a steel rule I can see the
divisions on properly).

So it seems I need a new bottom bracket. Is a 113mm bottom bracket
10mm shorter on the drive side only, or is it shared between either
side?

I have just recently done the whole drive on my wife's bike and read
in my books that when you get a new chainset you needed to ensure
that the bottom bracket was the correct length for it.

It also needs to line up with the rear sprocket(s). When you buy a modern
mountain bike chainset, for instance, the manufacturer assumes you'll be
using a modern mountain bike hub and cassette too, and recommends a BB to
suit that with the chainset.

Quoted message said:

So basically you need to know the make and model of your new chainset
(which I assume you do) then hopefully there will be a website that
will tell you what you need.

Mine did not - so as it is Shimano I contacted the UK distributer and
asked them. I then bought the correct length.

Things aren't so straightforward when using a chainset with a different type
of hub from the one the chainset manufacturer has in mind, especially with a
fixed gear. You either have to find out a lot of specs or take
measurements.

~PB