Disc brake mounting systems

Disc brake mounting systems

Warning! The dimensions that I present here still need final testing. There may be some configurations that are challenged. This is true in the current market and the problems that exist now do carry through to what I’m proposing. 

I’ve been doing a ton of work with disc brake standards lately. this, for my own new bike and because recent proposed “standards” needed correction.

This post is intended to pass on what I’ve come up with and communicate to the industry a path that can get us through a difficult time.

Below, find new specifications for Post Mount, Flat Mount, and International Standard mounting of calipers for the front and rear of a bicycle.

Attention was paid to the original intent of these systems, where they broke down, and rebuilding them to work for the future and custom configurations while not destroying the past. Some of the dimensions here are very close to(but not equal) those specified by Shimano and SRAM. By systematizing them, they can be more easily interchanged among different caliper and adapter types, offering the end user the most options and the builder far more options than previously specified.

The basis of this systematization is the 57 degree path, offset, and radii of the flat mount positions. This is formed by the 140, 180, and 203mm positions (the 160 slightly off and needing of correction). From there, each system interconnects to the flat mount by way of adapter angles. It’s remarkably simple and has been applied by Shimano and TRP consistently.

International Standard rear connects to the Post Mount system by a 7 degree translation while the Flat Mount system connects to Post Mount by a 17 degree translation. I am capitalizing on this. If manufactures stay true to this, we have some real value. Flat mount front via 16 degrees. IS front via 4 degrees.

I needed to do this now, and I’m actually rushing, to get good information out to the market and industry before a flood of new and bad specifications emerge for FM160 and FM180. Hopefully, I can get some help spreading this information (or correcting it) from higher level industry folks. At the very least, I’m hoping to ensure that the small tooling, frame parts, and framebuilders make parts that work with what has been rolled out.

We are all in this together and we will all benefit from bikes designed as well as possible.

Yes. There is a larger deviation from the official PM160 specification to what I’ve shown here. Further investigation will reveal that that specification has been the big problem for any brake standards moving forward well since Manitou came up with it years ago. This should have been fixed 20 years ago. Still, the IS160/FM180 adapters that exist will be slightly off and should be updated. IS really has the most problems but other than special cases gone. They are usable. This effects very little of the current performance market.

Note: I do use IS160R/PM180 adapters with a 160mm rotor for the rear of my custom hardtails. PVD StepDown. It’s pretty slick. Looks great, works great, lightweight. We still have uses for all of these parts. It’s about choices!

One important detail here is that the 203mm rotor needs to be abandoned in favor of the 200mm rotor and IS160/PM200 adapters be generated. This small change will allow so much more to be done with these systems. I can’t push this change ahead in any meaningful way but maybe other will see here why this move can be so valuable.

Here is the existing Post Mount standards. Comparing the locations, you can see that the maximum deviation my proposal is from the defined position is about 1/3 of a millimeter. It’s not really much and shouldn’t cause much issue as the industry has been using this as a system even though it isn’t defined that way, causing increased misalignment to already compromised pad positioning. I fix that shift.

Flat mount is the flavor of the week and getting all the attention. Because the pad angles of post mount are steeper than it’s sizing scheme represents and the pad angle of flat mount is shallower than the translation angle produces, not much correction is possible to make up for pad twist at extremes. This is something for caliper manufacturers to address at a later date.

Both Shimano and TRP follow the 17 degree system.

For front Flat Mount, things are a little less pretty. There is a bit of interference with bolt locations as things change with the adapters. In addition to that, the ascetics of the larger adapter solutions kinda suck. Thus, just one adapter will be preferred for crossover, the FM140F to PM140, 16 degree adapter. To step up, the FM140F to PM160, 2 degree adapter is used. This doesn’t look good though. Both Shimano and TRP follow these angles.

With International Standard, only IS160 is specified but a sensible construction can be made to allow use of the IS160R/PM160R adapter in any rotor configuration. The placement will match the positioning that the originally defined PM160 caliper takes, rather than that in my refined system. Further, stepping up or down with other adapters will cause some shift, the problem with the PM160 placement. In a perfect world, new adapters would be made to the new streamlined spec.