Bottom Bracket Info

Dave said:

Can anyone help - I have check the Sheldon Brown website but
unfortunately on this occasion it has not helped me.

How about the Glossary? It contains lots of info along with the
definitions. Also check out the Park Tools site.

Quoted message said:

I have just managed to get the old bottom bracket assembly from my
wife's bike - it is the cup, ball and spindle type and took some
shifting.

After inspection I have decided that I need a new one, and to replace
it with a sealed unit.

So I have started looking around and can see that basically there are
2 measurements to be taken into account when buying one is the
spindle length and the other is a measurement is called the "shell
width". What is the shell width? I am guessing that it is the width
of the body as part of the frame into which the sealed unit is
screwed but could someone confirm this. It is usually 68mm but can
also be 70mm or 73mm.

Yes, the bottom bracket shell is the barrel-shaped part of the frame that
holds the bottom bracket.

Quoted message said:

The next question is how do I decide what spindle length to buy. The
one I have taken off is about 127mm (so I guess that is 127.5mm
looking at available lengths), but I am guessing that would have been
matched to the front chainring, and as I am putting on a new
chainring should I buy the length that is recommended with that which
is 116mm or 122.5mm or stay with a 125.5mm. What difference will it
make besides putting the chainring further from the frame and how
might that affect things.

Spindle length is matched to the crankset. You won't need a different
length spindle if just changing the chainring itself rather than the whole
crankset, aka chainset. But you might do if getting a different model of
crankset. See the manufacturer's recommendations.

Placing the chainring further out may spoil the chainline, the angle of the
chain. The straighter the chain is between the chainring and rear
sprockets, the more efficient the drive is. It also places the rider's feet
further apart - which will be uncomfortable once past a certain point.

~PB

"Dave" <[email hidden]> a écrit:

Quoted message said:

What is the shell width? I am guessing that it is the width of the body
as part of the frame into which the sealed unit is screwed but could
someone confirm this.

That's right. Just lay a ruler across the bottom bracket shell of the frame.

Quoted message said:

It is usually 68mm but can also be 70mm or 73mm.

Right. 70mm corresponds to Italian threading. There are one or two obsolete
widths you might also come across - some older Raleighs, for example, used
very wide shells and 26tpi threading.

Quoted message said:

The next question is how do I decide what spindle length to buy. The one I
have taken off is about 127mm (so I guess that is 127.5mm looking at
available lengths), but I am guessing that would have been matched to the
front chainring, and as I am putting on a new chainring should I buy the
length that is recommended with that which is 116mm or 122.5mm or stay
with
a 125.5mm. What difference will it make besides putting the chainring
further from the frame and how might that affect things.

The axle length is matched to the cranks, so yes, go with the recommendation
of the crank manufacturer unless you have good reason to depart from their
advice. Bottom bracket length affects the distance between the pedals (which
can be an ergonomic issue), and the distance of the chainring(s) from the
centre line of the frame. The latter affects mechanical efficiency, gear
shifting quality, and in extreme cases, the tendency of the chain to stay on
or fall off the chainring(s).

http://www.sheldonbrown.com/chainline.html

James Thomson

Quoted message said:

Spindle length is matched to the crankset. You won't need a different
length spindle if just changing the chainring itself rather than the
whole crankset, aka chainset. But you might do if getting a
different model of crankset. See the manufacturer's recommendations.

Just to explain a bit more:

Cranks (and spiders) vary in shape, and so the position of the chainrings
can be different from one crankset to another in relation to interface
between crank and bottom bracket. Therefore different BBs are required to
put the rings in the same place.

The rear hub width is a factor as well, but it can be ignored if the
crankset is designed for the size of hub you have. This is often the case
thanks to the amount of standardisation there is. It's not always possible
or necessary to do anything about it anyway (within reason).

~PB

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

Quoted message said:

Quoted message said:

Spindle length is matched to the crankset. You won't need a different
length spindle if just changing the chainring itself rather than the
whole crankset, aka chainset. But you might do if getting a
different model of crankset. See the manufacturer's recommendations.

Just to explain a bit more:

Cranks (and spiders) vary in shape, and so the position of the chainrings
can be different from one crankset to another in relation to interface
between crank and bottom bracket. Therefore different BBs are required to
put the rings in the same place.

The rear hub width is a factor as well, but it can be ignored if the
crankset is designed for the size of hub you have. This is often the case
thanks to the amount of standardisation there is. It's not always

possible

Quoted message said:

or necessary to do anything about it anyway (within reason).

~PB

Thanks for your response. I think I understand now.

Basically, I have bought components that work as a set on my bike to put on
my wife's as some things needed changing. So bought a new rear set and
derraileur, front chainset and derraileur. These are matched according to
the papers that accompany them. The only thing I hadn't bought was the
bottom bracket as I needed to get the old one off first to see what it's
condition was ([censored]), and having read the info with that came with the other
parts it just raised this question that I think has now been answered.

Dave