Although widely claimed that Shimano A and B type chainrings are interchangeable with an unnoticeable penalty in performance, in reality things are not as perfect. They say a picture is worth a thousand words so with actual photos I aim to show what is the difference between the two types.
Shimano’s front derailleur/chainring shifting is an industry standard. No grind, no hesitation, as soon as you move the lever the chain obediently and imperceptibly moves between the chainrings. Electronic Di2 shifting makes it even better. In order to do that Shimano, had actually machined “gates” and pins on the big ring to assist and “pick up” the chain.
In their search for perfection Shimano has taken front shifting a step further in their top of the line components – Dura Ace. Outer chainrings are available in two types – A and B.
- Type B: by far the most popular one and also the default on Ultegra cranksets; it comes in 52 and 53 teeth and must be paired with 39 tooth inner chainring.
- Type A: comes in 53 as well as 54 and 55 teeth; as per Shimano’s advice must be installed together with a 42 small ring.
- Type E: a VERY rare combination: 56/44
There is NOTHING unusual about the 39, 42, and 44 tooth chainrings. It is the outer big ring that has special gates and pins machined into it for smooth and fast shifting. Since the 53/39 cranksets are dime and a dozen, flatlanders and time-trialists alike may opt for a 42 inner ring for tighter gearing. The more inquisitive like myself are aware of the A and B type Shimano story so we go on Google and search if it is possible to mix and match. And what do you know, the internet says it is, so it has to be true!
60% of the time it works every time.
Yes, but no! In short: 60% of the time it works every time. In my experience 3 out of 4 shifts were even better and quicker than with the 39 tooth ring – no surprises there, the chain travels a shorter distance. However, the horror is that 1/4 of the shifts that do not work as intended. The chain just sits in between the two chainrings and with a loud grind, it wrestles against the ramps and pins, until it makes the shift almost a full revolution later. It does this both during an upshift and downshift, although it is quite more pronounced when the chain is desperately going for the big ring. If you are doing the shift under even moderate power, or when trying to make a move and accelerate out of the saddle you can hit your knee(s) at the back of the stem, slam the top tube with your nether regions or even worse – tumble face first forward (although the last one takes a little more than a misbehaving chain). When seated during a time-trial this would cost you valuable seconds and can throw off your pacing.
At first I thought it was due to a slight misadjustment of the front derailleur, however, I experienced the “limbo” shifts on another bike with an almost new and clean Dura Ace 7900 directional chain installed correctly. As soon as I changed the 42 ring for the original 39, everything was perfect again.
Even the great Sheldon Brown doesn’t provide a reasonable explanation, so I got to searching and I finally came across a close up view of both A and B type 53 Dura Ace 7900 outer chainrings (thanks to eBay seller Mr_dura-ace for the pictures).
The Shimano A and B Type Chainrings
First, both chainrings have two surfaces: a darker colored inner one and a polished one where the teeth are. There is a circular “edge” going around the darker metal part.
- The B type ring has 2 pairs of gates and pins, together with a single gate and pin between the two.
- The A type ring has the “edge” much closer to the teeth and has only 2 pairs of deeper gates with a pin at the end of each one.
So what happens when you swap a 42 on your 53/39 B type crankset? The bigger inner ring (such as 42,44) simple covers the “edge” so the chain would have to rely on the very shallow gates to direct it to the pickup pins. As I mentioned at the “right” moment and/or under power it will not work.
The A type ring addresses all of the above. That being said a 53 A and a 39 inner ring is not a combination I have tested.
Apparently the A and B type chainrings by Shimano are not just a marketing gimmick. If you are to believe other’s opinions you can mix and match components without a penalty in functionality. Although installing a 42 inner ring on your standard 53/39 B type crankset would not be the end of the world, be aware than occasionally shifting might feel like a dropped chain. I am almost certain that 99% of sold units out there are your humble 53/39 B type so the cheapest/easiest solution for closer gearing is to switch only the inner ring and (learn to) live with it. Therefore most of the information regarding compatibility is assumed and misleading, be aware of that (when you are trying to diagnose where did the perfect Shimano shifting go after you changed chainrings).
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