Bottom Bracket Sizing

NLee1875 said:

I am trying to replace my old Dura-Ace double crank (model 7402 ) on my road bike with a mountain
bike Shimano XT triple crankset.

The current bottom bracket is stamped 68x113.

The Shimano XT crank allegedly also uses the same 68x113 -- but the chain line doesn't line up
correctly. The positioning of middle chain ring on the XT is where the outer chain ring of the
Dura-Ace use to be.

Thats more or less as it should be. Stock Shimano spec for road chainline is 43.5 mm. This is
measured halfway betwixt the two rings, so the outer ring would wind up at about 46 mm.

Standard chainline for a "mountain" triple is 47.5 mm, only 1.5 mm different.

Quoted message said:

Should I be looking to replace the bottom bracket with a 68x107 to make my new set up work on a
road frame?

Certainly not. This would likely cause the small chainring to hit the chainstay, and you might not
be able to get the front derailer to move far enough inward for good shifting.

See: sheldonbrown.comchainlineOpen ↗

and: sheldonbrown.combbsizeOpen ↗

Sheldon "Where's The Problem?" Brown +-----------------------------------------------------+
| Life is the art of drawing sufficient conclusions | from insufficient premises. --Samuel Butler |
+-----------------------------------------------------+ Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041 harriscyclery.comharriscyclery.comOpen ↗ Hard-to-find parts shipped Worldwide
captainbike.comcaptainbike.comOpen ↗ sheldonbrown.comsheldonbrown.comOpen ↗

nlee- << The current bottom bracket is stamped 68x113.

The Shimano XT crank allegedly also uses the same 68x113 -- but the chain line doesn't line up
correctly.

<< The positioning of middle chain ring on the XT is where the outer chain ring of the Dura-Ace
use to be.

On the triple or double, the middle of the cogset should line up on the middle of the crank
rings...to preserve spacing, chain stay spacing, etc, the big ring of the triple gotta go
somewhere...

Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
(303)440-3535 vecchios.comvecchios.comOpen ↗ "Ruote convenzionali costruite eccezionalmente bene"

NLee1875 said:

Should I be looking to replace the bottom bracket with a 68x107 to make my new set up work on a
road frame?

That's what I do, and I always use mountain-bike cranks in order to get better gearing options.

Quoted message said:

BTW.. The XT crank works with 68x113 bottom bracket on my mountain bike frame just fine. Is this
because the rear triangle spacing on the mountain bike is wider than a road bike?

yes, of course. The cassette width is the same, and the axle is 5mm longer, which means that the
cassette on a mountain bike is splayed to the right 2.5mm compared with a road bike. OK, a 107 cuts
a bit more off than the needed 113, but it's close enough, especially if you use the mountain bike
cranks as a double.

--

David L. Johnson

__o | Do not worry about your difficulties in mathematics, I can _`\(,_ | assure you that mine
are all greater. -- A. Einstein (_)/ (_) |