Shimano Road Chainring Question

David White said:

I know that Shimano makes "A" and "B" style 53 tooth road rings. I have read
that the "A" should go only with a 42 tooth inner ring and the "B" with a 39.
My question is what, really, is the difference between "A" and "B" and can I
use an "A" with a 39 tooth inner. Thanks.

After looking at a pair of them, it appears that there are differences
in the precise layout, angles and shapes of the ramps and pins, and
possibly some tweaks in the tooth shaping, to optimise the shifting.
Given that all of these features were added lonmg after the der system
had become common, having them less than optimally implemented will
cause nothing more than a slight delay in upshifts to the big ring, if
even that. Yes, it's nice to use them correctly; if you need to ride
*today*, use what's at hand and don't worry about it.
--
Typoes are a feature, not a bug.
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David White said:

I know that Shimano makes "A" and "B" style 53 tooth road rings. I have read
that the "A" should go only with a 42 tooth inner ring and the "B" with a 39.
My question is what, really, is the difference between "A" and "B" and can I
use an "A" with a 39 tooth inner. Thanks.

Dear David,

See this short thread, where Sheldon Brown, Peter Chisholm, and Mike
Jacoubowsky broke Shimano's heart back in 1999:

http://tinyurl.com/e84ye
or
http://groups.google.com/group/rec.bicycles.tech/browse_frm/thread/64b8f42973085a33/1ff986509cb09252?q=%22shimano+a%22&rnum=1#1ff986509cb09252

Cheers,

Carl Fogel