determining correct BB spindle length

zencycle said:

How does one determine what bottom bracket spindle length to get? I'm
asking what I need to measure on the bike to make sure I get a length
that will give me a good chainline.This is for a newer road racing
frame that had no bottom bracket when I got it so I have nothing to
compare it to. I'm using a square taper crankset (FSA carbon) and have
found FSA ultimax BBs online at reasonable prices. I would prefer
having the big chainring line up with middle of the cogset. Is there a
measurement or combination of measurements I can make to come to a
reasonable number?

In your case, the crank is recent enough that you should be able to
dig up a recommended spindle length. FSA can probably tell you pretty
quickly if you call them. Recommended spindle lengths are a length
chosen to give decent chainline when used alongside the rear
drivetrain parts the crank is intended for, ie a 130-spaced road hub
in your case. The trick with recommended spindle lengths is that
sometimes they'd be a little longer than would give optimal chainline,
in order to pre-emptively work around the many frames with clearance
issues.

zencycle said:

How does one determine what bottom bracket spindle length to get? I'm
asking what I need to measure on the bike to make sure I get a length
that will give me a good chainline.This is for a newer road racing
frame that had no bottom bracket when I got it so I have nothing to
compare it to. I'm using a square taper crankset (FSA carbon) and have
found FSA ultimax BBs online at reasonable prices. I would prefer
having the big chainring line up with middle of the cogset. Is there a
measurement or combination of measurements I can make to come to a
reasonable number?

The usual litany-
Thread matches frame, spindle matches crank.
FSA singles are 108, doubles 113, triples 118 all symmetric.

Yes, that is counterintuitive (you'd expect that 5mm overall on a
symmetric spindle would add only 2.5mm per side). Crank designers have a
wide latitude of choices about the relationship of spindle to rings to
pedal. You can't look at an unknown crank and 'see' the spindle length
readily.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Nate Knutson said:

zencycle said:

How does one determine what bottom bracket spindle length to get? I'm
asking what I need to measure on the bike to make sure I get a length
that will give me a good chainline.This is for a newer road racing
frame that had no bottom bracket when I got it so I have nothing to
compare it to. I'm using a square taper crankset (FSA carbon) and have
found FSA ultimax BBs online at reasonable prices. I would prefer
having the big chainring line up with middle of the cogset. Is there a
measurement or combination of measurements I can make to come to a
reasonable number?

In your case, the crank is recent enough that you should be able to
dig up a recommended spindle length. FSA can probably tell you pretty
quickly if you call them. Recommended spindle lengths are a length
chosen to give decent chainline when used alongside the rear
drivetrain parts the crank is intended for, ie a 130-spaced road hub
in your case. The trick with recommended spindle lengths is that
sometimes they'd be a little longer than would give optimal chainline,
in order to pre-emptively work around the many frames with clearance
issues.

Thanks, I actually posted the question to their tech support before I
posted here, but whoi knows when/if I'll hear back.

A Muzi said:

zencycle said:

How does one determine what bottom bracket spindle length to get? I'm
asking what I need to measure on the bike to make sure I get a length
that will give me a good chainline.This is for a newer road racing
frame that had no bottom bracket when I got it so I have nothing to
compare it to. I'm using a square taper crankset (FSA carbon) and have
found FSA ultimax BBs online at reasonable prices. I would prefer
having the big chainring line up with middle of the cogset. Is there a
measurement or combination of measurements I can make to come to a
reasonable number?

The usual litany-
Thread matches frame, spindle matches crank.
FSA singles are 108, doubles 113, triples 118 all symmetric.

Yes, that is counterintuitive (you'd expect that 5mm overall on a
symmetric spindle would add only 2.5mm per side). Crank designers have a
wide latitude of choices about the relationship of spindle to rings to
pedal. You can't look at an unknown crank and 'see' the spindle length
readily.
--
Andrew Muziwww.yellowjersey.org
Open every day since 1 April, 1971

Except that they offer the ultimax in six different sizes. I tried a
Specialized 113 I had lying around but the chainline ended up with the
big ring lined up on the 14 (12x23 9sp) and I couldn't push the
'fixed' cup side any further into the BB shell, but i'm thinking the
103 they offer would be a bit short. I don't want to spend $100 on yet
_another_ bike part I'll never use.This is why I was hoping for some
sort of silly measurement like 'hub spacing times .75 +\- Xmm times
the number of cogs you want to offset....' or some [censored] like
that.Since this is a JIS standard, is there a spec regarding how far
into the crank arm the spindle is supposed to seat?