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Triple crankset (minus granny ring) on a double setup?(6 posts)

Triple crankset (minus granny ring) on a double setup?winstonc
Jan 15, 2004 1:43 PM
Say I have a triple crankset in my possession and want to use it on a bike that's set up for a double -- the bottom bracket is the normal length for a double. If I get rid of the granny ring, will the other two rings be positioned the same, laterally, as if I had a double crankset?
No (nm)Chen2
Jan 15, 2004 2:08 PM
depends. . .jimPz
Jan 15, 2004 3:36 PM
without knowing what crank & what BB length you're using, it could (by accident ) line up that way.

depends. . .winstonc
Jan 15, 2004 5:49 PM
Say it's an ultegra triple crankset with a double (109.5mm) bottom bracket.

I would think that the chainline differences between a double and a triple setup were managed by the bottom bracket spindle, not differences in the crank itself (except for the presence of holes for the third ring, of course). It seems easier to design things this way.

If the cranks ARE different, does anyone have any information on what exactly the specifications are? For example, for a given crank, are the numbers for the distance between the end of the spindle and the center of the outer chainring? (This precision only applies to octalink and ISIS, of course)
Can you still get BB spindles separately?Cory
Jan 16, 2004 8:59 AM
I haven't modified a bike in awhile, but back when I used to do that, you could go to almost any bike shop and order a BB spindle in at least a few lengths. I've swapped from double to triple and vice versa just by going up or down a few millimeters on the spindle length. Dunno if you can still get the parts, though, now that everything comes in sets and stuff isn't as serviceable as it used to be.
re: Triple crankset (minus granny ring) on a double setup?StmbtDave
Jan 16, 2004 11:01 AM
I suspect that with a triple crank and the proper BB, the middle chainring will be lined up with the middle of the rear cluster. With a double crank and the proper BB, the middle of the rear cluster will be lined up half way between the two front chainrings.

Since the crank arm is mounted between the outer two chainrings in both configurations, I would expect that the triple crank on the double BB would result in a correct chain line.

The thing to watch out for would be whether there is enough clearance between the inner chainring mounting studs and the frame, even if the inner chainring isn't mounted.