Shimano Road Disc Pad Compatibility Question

[pbarry]

New to road hydro discs and recently made a trade on the forum: My Dura Ace BR-R9170 flat mount calipers for post mount BR-RS785 calipers. We agreed to keep our respective pads as we both thought they were interchangeable..

However, when the BR-RS785s arrived, I installed the DA pads in the 785 calipers to check before sending mine along, and the pads were way too small side-to-side.

What I've found, but would enjoy some real world wisdom:

JO2A and JO4C + others for the BR-RS785, [Same pattern as XTR.]
LO2A and KO2Ti + others for the BR-R9170. [Also for BR-RS805 and BR-RS505]

Pads are incompatible between the flat mount and post mount calipers, right?

Thanks for any help figuring this out.

[dem]

You are correct.

The post mount calipers are based on the XT/XTR design with the larger pads.

The flat mount calipers use the smaller road pads.

Additionally, you cannot use the new Ultegra/DA rotors with your XT/XTR pads - there is not enough swept area (would anyone like 4 NIB Ultegra rotors I can't use!??!)

I eat rotors and pads like candy, so I'm using cheap discobrakes pads and SLX rotors these days.

[pbarry]

Thanks so much! Your insight is a huge help. This stuff has a little learning curve.

I'll pm about the rotors.

[radsmd]

Originally Posted by dem

Additionally, you cannot use the new Ultegra/DA rotors with your XT/XTR pads - there is not enough swept area (would anyone like 4 NIB Ultegra rotors I can't use!??!)

.

I did not know this. Currently using the newer ultegra rotors with 785 calipers without known issues, but never thought to check that the swept area is the same.

[DRZRM]

I just wanted to confirm, I'm actually going to use a post mount fork on a flat mount frame, and I have a 785 front caliper and a Dura Ace flat mount rear.

I thought I could use standard Shimano MTB rotors front and rear. Is the case that a 160 XTR rotor won't work with a Dura Ace caliper? Or is the problem only with road rotors and MTB calipers?

[rheosibal]

Originally Posted by dem

Additionally, you cannot use the new Ultegra/DA rotors with your XT/XTR pads - there is not enough swept area

Originally Posted by radsmd

I did not know this. Currently using the newer ultegra rotors with 785 calipers without known issues, but never thought to check that the swept area is the same.

Same here, using the RT900 rotors (DA) with RS785 calipers without issues. Didn't know they were incompatible?

[Jaybee]

Originally Posted by rheosibal

Same here, using the RT900 rotors (DA) with RS785 calipers without issues. Didn't know they were incompatible?

I think there is still enough swept area that the bike will stop, but you would get more swept area and thus better performance out of the XT series rotors. Of course, this is complicated by the fact that Shimano doesn’t make a 140mm 6-bolt rotor that is officially compatible with their post mount calipers.

[dem]

Correct - the issue is the shape of the rotor surface material.

The XT/XTR pad is 13.25 mm wide.

The widest point of the Ultegra rotor is 15.5ish. But the narrowest point is 10.2mm

The pad would be sweeping over the aluminum cooling fins at that point, not rotor material.

Red line in this pic