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Double vs triple chainrings...(3 posts)

Double vs triple chainrings...TFerguson
Dec 13, 2003 11:26 AM
Shimano chairings are A type and B type depending on whether they are intended to be used with a triple set up or a double. As far as I can see, the only really specific one is the center ring on a triple because it "needs" to help the chain lift off the little ring. The rest would be inter-changable. Maybe you wouldn't want a triple middle as an inner doulble because the ramps would increase the chance of chain suck. Anything else to look out for?
TF
more than that....C-40
Dec 13, 2003 1:31 PM
Apparently there is a difference in the location of the ramps and pins on the big chainring, depending on whether it's intended to mate with a 39 or 42 inner ring.

Actually, using a triple middle ring as the inner ring of a double should work fine because the pins on the ring improve the shift up to that ring from a smaller one, but have no effect on the shift to a larger ring.

It's really the big ring that should have the correct pin location relative to the inner ring.
Incorrect...satanas
Dec 14, 2003 9:05 AM
A type and B type *does not* refer to double versus triple, but rather to what chainrings will give the best shifting when used together.

I can't recall which is A and B right now, but for instance 39T outers (let's call them A for the sake of discussion) work with 50, 52, and 53T A type outers because the ramps and cutouts line up exactly. The 42T inner (B type) lines up exactly with the 53T *B type* outer, but not with the A type. Why Shimano didn't just make the inner different is beyond me...

It doesn't really make all that much difference whether the ramps match exactly, or whether there are ramps at all - unless you shift a lot under high load. As this is just asking to break something - or crash if the shift fails badly - I can't see why one would want to do this routinely.

I think that the ramps mainly serve to make the shift smoother, not to make it happen. I've done plenty of front shifting under (moderate) load with unramped rings with no major issues...

Middle rings with reduced height teeth are more likely to lose the chain to the inside and will wear out faster too.

None of this is likely to be crucial.