Bottom bracket spindle length for a Triple

Joe Roach said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle length
is required depending on the diameter of the seat tube - but I can't see
why. In what way does the seat tube diameter affect clearance or chainline -
bottom bracket shell size I can understand (it's 68mm) but why do I need the
longer spindle because the seat tube is 34.9mm?

Because the FD is further out.

Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)

Yes, sort of.
It depends on which BB you purchase. The Chorus and Record BBs are
asymmetrical, so you can get by with a shorter BB, 111mm for a triple.

The centaur and below BBs are symmetrical, so typically you need a 115, but
on bikes with fairly small seat-tubes (I think 1.125" is the limit) you can
get by with a 111.
No question, with your 34.9mm seat-tube you'll need the 115.5 if you use
Centaur or below.

Confusing? Sure is to me;-]

-pete

"Joe Roach" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle
length is required depending on the diameter of the seat tube - but I
can't see why. In what way does the seat tube diameter affect clearance or
chainline - bottom bracket shell size I can understand (it's 68mm) but why
do I need the longer spindle because the seat tube is 34.9mm?

Joe Roach said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle length
is required depending on the diameter of the seat tube - but I can't see
why. In what way does the seat tube diameter affect clearance or chainline -
bottom bracket shell size I can understand (it's 68mm) but why do I need the
longer spindle because the seat tube is 34.9mm?

If the seat tube is large and the spindle is short, the FD hits the tube
before it is "in" far enough to shift to the small ring. In close
cases, a little judicious filing on the part of the FD that hits the
tube will make the difference.

I'm managing with a campy triple, 111 spindle, and ~29mm seat tube, but
it took about 0.5 mm of filing on the FD.

Mark J.

Joe Roach said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle length
is required depending on the diameter of the seat tube - but I can't see
why. In what way does the seat tube diameter affect clearance or chainline -
bottom bracket shell size I can understand (it's 68mm) but why do I need the
longer spindle because the seat tube is 34.9mm?

Because a front changer cannot manage a shift to the inside ring when
the changer itself hits your fat seat tube. Moving the chainline out is
the obvious solution.
--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

"Joe Roach" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle
length is required depending on the diameter of the seat tube - but I
can't see why. In what way does the seat tube diameter affect clearance or
chainline - bottom bracket shell size I can understand (it's 68mm) but why
do I need the longer spindle because the seat tube is 34.9mm?

Thanks all for the replies. The inner ring is already close to the chain
stay so I didn't thing the FD would be an issue but I forgot that the new,
smaller ring will be closer because it's smaller and that will affect the
FD. I'll go ahead and order the parts.

Joe Roach said:

"Joe Roach" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle
length is required depending on the diameter of the seat tube - but I
can't see why. In what way does the seat tube diameter affect clearance or
chainline - bottom bracket shell size I can understand (it's 68mm) but why
do I need the longer spindle because the seat tube is 34.9mm?

Thanks all for the replies. The inner ring is already close to the chain
stay so I didn't thing the FD would be an issue but I forgot that the new,
smaller ring will be closer because it's smaller and that will affect the
FD. I'll go ahead and order the parts.

Number of teeth of inner ring doesn't matter.

Lou
--
Posted by news://news.nb.nu (http://www.nb.nu)

Joe Roach said:

I'm thinking of moving from a compact double to a triple chainset on my
Campag set-up. I see that a 111mm or a 115.5mm bottom bracket spindle length
is required depending on the diameter of the seat tube - but I can't see
why. In what way does the seat tube diameter affect clearance or chainline -
bottom bracket shell size I can understand (it's 68mm) but why do I need the
longer spindle because the seat tube is 34.9mm?

Inward travel of the FD to ensure the chain can get to the smallest
ring, is why, 115.5mm BB for OS seatubes.