Clearing up FM140

Flat mount has been the topic of the week in PVD land. I can’t wait until this is over.

The flat mount specification was released a few years ago and had everyone pulling out their hair. It is a fine system but it was specified so poorly and in such a confusing way that it caused a lot of problems. Many problems have been caused by the confusing and strange lateral locating of the bolt slots for FM140.

Adding to this, SRAM went rouge and added their own change to the specification with an additional millimeter of slot to the inside. Ugh! That was a good thing but…UGH!

Below, I’m posting several prints with the important measures in a clear way. It is shown for 148mm rear and 110mm front spacing but it is simple enough to adjust to another spacing as the references should be clear and make sense. I would advise most folks to follow the SRAM specification but I’ve posted Shimano also as it is their baby.

Notice, the true reference of the specification is the inner face of the rotor, which has different locations front and rear with regard to the axle end. Thus, we see systematization of 12.20mm from the inner rotor face to the reference position of the slots. If you know where the rotor is, you know where the caliper mount is.

Remembering that the rotor of a 135mm rear hub is 3.50mm closer to the hub end (15.25mm) compared to it’s socket-type counterparts of 142/148mm. As this is a dead hub width, it shouldn’t come up often and wasn’t shown here.

Also, be careful how your tool is positioned relative to the axle end. The end of the hub is different than the inner dropout socket face. Many rear sockets are 3.5 mm deep and many front sockets are 2.0 mm deep. This can mess with a fool. Measure your parts and your tool to ensure alignment of the stars.

Simple enough. So simple, it would have been nice for Shimano and SRAM to specify this way.

Tolerances were left out of this print for clarity.

Mark at Paragon Machine Works has been making framebuilding parts to the 2mm slot Shimano specifications. Make sure to understand which parts you are getting and how to set them up.

A special note to users of the SRAM FM caliper and all these specifications. It looks like SRAM calipers are shifted 0.3mm from center as designed. This will use up that amount of the 1.0mm on the outboard side of the slot. Fair warned is fair armed.