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Discussion Starter · #1 ·
So does anyone know the difference between the "A" and "B" chainrings for Shimano cranks (specifically Dura Ace, but I think it's the same through the line) - I haven't held the two side by side to compare. Is there some reason an "A" won't work with a "B"? (Shimano sort of suggests they won't...) - I thought it had to do with the size of the inner ring you were shifting from, but maybe not since you can get the 42 as both an A or a B...
 

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Shimano has said not to mix A and B rings. I was informed that the pin placements on the 53T is indeed different from the A and B therefore cannot work in conjunction (slopping upshifts) Maybe someone on this board has successfully tried an A with a B?

There are two types of 53's
"A" that will work with 42T
"B" that will work with 39T

50 and 52's are all "B" type
 

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Discussion Starter · #3 ·
So would that apply only to Shimano inner rings? (I am guessing they have a tooth profile which caters to the matching big ring) Other non-shimano inners are effectively uniform around the circumference, so would it not matter (A vs. B) with those? I haven't looked too closely at my current inner (it's a 39b) - I wonder if the difference is the position of the teeth relative to the chainring bolts (so that things line up just so with the pins on the big ring...)
 

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No problem mixing.

For many years, I've mixed Shimano A and B rings and never had any shifting problems. It's possible that a matched set of rings will make a ham-fisted shift under full power just a bit faster and smoother than an A-B mix. I'm generally not doing these sort of shifts, so I wouldn't know.

As you suspect, the A-B match is about tooth profiles and pick-up pins. A few teeth on each ring have a lower profile than the rest of them. These are the "gates" for the chain to come onto, or leave a ring. The inside of the big ring also has pins which are supposed to pick the chain up during a shift. For all this to work as advertised, Shimano says the gates and pins should be in a certain relation to one another - which only an A-A or B-B match will give you. Matched rings might shift marginally better than mixed ones, but as I said above, I was never able to detect the "improvement."
 

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As stated above it is about the pin locations, but you can mix and they will work. Last summer I had to replace my 42 and 53 rings. I replaced them both with A's, but when I would get into the 3 big rings in back the chain would fall off the big ring up front. Very strange since it would even do this in a bike stand. My LBS couldn't believe it until they saw for themselves. I had to put on a 53 B because the shop didn't have any more A's. The B works fine, maybe not as smooth shifting as a matched set, but still well within reason. I think the A ring was probably bent from the factory. Very strange. I nor the guys at the shop had ever seen anything like this. Props to the LBS for swapping rings for me. One of the advantages of buiyng local.
 

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You can only tell the difference when to shift between big and small while standing (or on a 10% hill).
Going from small to big ring may take an extra second to finish. The only difference is the location of the pick up pins.
 

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Mr.Loops said:
Shimano has said not to mix A and B rings. I was informed that the pin placements on the 53T is indeed different from the A and B therefore cannot work in conjunction (slopping upshifts) Maybe someone on this board has successfully tried an A with a B?

There are two types of 53's
"A" that will work with 42T
"B" that will work with 39T

50 and 52's are all "B" type
This is a pretty sloppy way to do things, with no complex calculations or theories being involved - but I slapped my 52T off my old triple onto my new double early in last year and it worked just fine with the 39T standard inner that came with the cranks. Rest of the drivetrain was Ultegra, the shifters having around 3500 miles in them when I made the swap.

I rode it with the 39/52 for around 4,000, give or take a mile or three. Couldn't tell any difference and shifting was as good as I expect. Have since fitted a 38 (Salsa) with 50T (TA), and the same shifters / der work just as good there too.

Hope that helps

Dereck
 

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Discussion Starter · #8 ·
After looking at a bunch of pics, and comparing an A ring to a B ring, it looks like the A ring has two sets of pins located 180 out of phase with one another, while the B ring has three sets of pins located 120 degrees away from one another.

I got a good deal on an older 53 "A" ring, and I plan on using it with a non-shimano inner ring, so based upon the replies I have seen here, I should be just fine. (Couldn't pass up $9 for a new DA 53t ring!)
 
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