Using shorter BB on Deore DX triple crank

Russell Seaton wrote:

I am in the process of transferring parts from my Trek 520 touring
frame to a Redline touring frame. I decided to replace the bottom
bracket. Crankset is a Shimano Deore DX triple with 45-42-20
chainrings. Rear cassette is 7 speed 14-34 or 12-28, depending.
Chainstays are about 17 inches or more long. Whatever is right for a
touring frame. Old bottom bracket was about 124mm by my measuring.
Sheldon Brown's Harris website shows 122.5mm as the appropriate length
bottom bracket for my Deore DX crankset. Trek 520 frame had 135mm
spacing. New Redline frame has 130mm spacing. Rear axle will have a
couple washers removed and redished to fit the 130mm Redline spacing
better.

I put a 122mm Shimano U53 bottom bracket in last night and it looks
OK. Based on just looking at how the chainrings and cassette line up.

Usual chainline for a "road" triple is 45 mm.

See: http://sheldonbrown.com/chainline

But I am wondering if I can use a shorter bottom bracket. I have on
hand 118mm, 115mm, 113mm, 110mm bottom brackets.

I know the bottom bracket length is supposed to get the chainrings
centered on the cassette cogs. Probably have the middle chainring
line up with the middle cassette cog. But the chainstays are long so
the chain angle will not be too awful. And the inner chainring will
rarely be used. And when the inner ring is used, it will most likely
be with just the larger 3 or 4 cogs. So I am thinking it might be
best to move the chainline in.

This approach can often improve average chainline for a road triple. On
the frame I built myself, I set up the 52-42-28 Biopace crank as close
inboard as it can be without rubbing on the chainstay. The chainline of
the 52-42 is similar to that of a typical double.

This works out well for me, as I only very rarely use the 28 chainring,
and when I do it's only with the 2 or 3 largest rear sprockets (12-22
6-speed freewheel.)

The problem you may run into is finding a front derailer that will move
far enough to the left to reliably shift onto the granny gear. The
Brown uses a 30-year old Sun Tour Cyclone, that just barely does it. It
is my impression that currently available front derailers, though they
shift much better, won't generally move as far to the left.

http://sheldonbrown.org/brown

Sheldon "10 Speed Ergo/6 Speed Freewheel" Brown
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Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

russell- I put a 122mm Shimano U53 bottom bracket in last night and it looks
OK. Based on just looking at how the chainrings and cassette line up.
But I am wondering if I can use a shorter bottom bracket. I have on
hand 118mm, 115mm, 113mm, 110mm bottom brackets. BRBR

Two thinmgs to consider-
-is the line drawn from the middle ring to the middle of the cogset parallel to
the top tube?

-Will the front der have enough inward travel to get the chain to the small
ring? w/o hitting the frameset?

Use the BB that is the best combo of above.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"

russell- I put a 122mm Shimano U53 bottom bracket in last night and it looks
OK. Based on just looking at how the chainrings and cassette line up.
But I am wondering if I can use a shorter bottom bracket. I have on
hand 118mm, 115mm, 113mm, 110mm bottom brackets. BRBR

Two thinmgs to consider-
-is the line drawn from the middle ring to the middle of the cogset parallel to
the top tube?

-Will the front der have enough inward travel to get the chain to the small
ring? w/o hitting the frameset?

Use the BB that is the best combo of above.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"