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Discussion Starter · #1 ·
Anyone with this bike, and still having the Shimano XT calipers - what is the caliper adapter arrangement you were setup with?

I've just gotten interested in going from 160 to 180, but when I look at my setup I got very confused. I want to know if this is a proper setup or if the lbs did something wonky.

I am also making the assumption that this change is technically sound/available for my bike frame. I have never changed rotor size before.

1. My front has no adapter.
2. My rear has an adapter SM-MA-F180P/P2.
3. Both rotors are 160 (SM-RT81-S)
4. Both brakes appear to line up fine with the (original) pads.

My understanding based on this very useful CC page is that I have the wrong rear adapter.

Shimano Disc Brake Adapters | Competitive Cyclist

The Shimano adapters all have a model number that starts with SM. The Mount Adapter acronym is MA. Front brake is F. Rear brake is R. Rotor Size is 140, 160, 180, or 203. Caliper Mount Type will be S for ISO/International A, and P will represent Post. And the last letter will be for the frame or fork mount type: S for ISO/A, P for Post, B for Boxxer, Z for Marzocchi 20mm Axle as well as QR20Plus and DH ISO. Thus an adapter that reads SM-MA-F-203S/P is Shimano Small Parts Mount Adapter Front 203mm Rotor with ISO caliper on a Post type fork.
So this means I would have a Front caliper adapter on my rear. Now it does appear like I need a rear adapter because both the front and rear calipers line up fine. (as best I can tell). With a rear adapter, it would be unable to fit the rotor.

1. I'm concerned about the risks of using the incorrect adapter.

Ironically, that page shows the incorrect adapter for SM-MA-R160 - so I dont know what it looks like. I can't seem to find a SM-MA-R160P/P, which is what I would need today. (maybe the SM-MA-F180P/P2 serves this dual role?)

2. As for the 180 sizing, I think I would need:
SM-MA-F180P/P2 - For the front
SM-MA-R180P/P - for the rear (but this doesn't exist)

This review DOES appear to have the same adapter on the rear as mine but I can't help but wonder what adapter I need to get to 180.

Any ideas?

· Registered
1,762 Posts
You have the correct adapter on your bike. Generally speaking (there are lots of exceptions) a front 180 is the same as a rear 160 adapter. They both space the caliper 10 mm off the post mounts to work with a bigger rotor.
I would try upgrading the front rotor to a 180 and leave the rear at 160 if you are looking for more brake power. A 180 on the rear is likely overkill for that bike. You may also need to double check chainstay clearance or a 180 rotor on your bike (it may hit, since it is an XC bike, not intended for big brakes).
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