The problem with flat mount brakes

27/10/24 15:09 Filed in: Gear

My first issue with flat mount brakes was why did we require another standard?
When disc brakes came out they were mostly IS - International Standard (some back then were proprietary and meant you were stuck with whatever brake you were supplied with). IS were bad - left/right movement of the caliper required shims or filing material off - and the brakes didn't actually mount on there, rather the appropriate adaptor did, and then the caliper mounted on that adapter. Back in the IS days, I purchased a brake mount machining kit because it was essential on almost every bike's swingarm I ever worked with (forks were cast and so better - but not perfect). I've updated the kit to work with PM and flat mount brakes (thanks Park for offering the update kit) but haven't had to use it. The range of adjustments and the quality of alignment on carbon frames means using one is extremely rare. I've also machined off part of the adaptor using the tool when the frame alignment was off but there didn't appear to be much of the tab to remove to get where I needed.

Next up came PM - post mount. And PM is still how it's done on the mountain bike. PM uses a bolt on the axis of the rotor (IS was perpendicular to this axis) and oval holes in the caliper to permit some side-to-side movement in the caliper body. To move to a larger rotor is easy with an adaptor that spaces the caliper further from the axle and some longer bolts (some adaptors use 4 bolts and move the caliper backwards as well as upwards to be more compact). I like PM. Early disc road bikes were PM because flat mount hadn't come along yet.

And then flat mount came for road bikes. For the front brake using flat mount, a literally flat adaptor bolts to the threaded holes on the caliper which in turns bolts to the fork via blind threaded holes - very similar to PM, but different in that it sits right on the fork (also worth noting that flat mount works with 140 or 160 mm rotors only - PM works with 140, 160, 180, 200 and 200 and some variations like Shimano's 203 mm rotors). At the rear, however, the mounting bolts pass up through the chainstay and into the caliper. The bolts have to move with the caliper and so tiny adjustments become markedly harder than for PM adjustments. If the bolt head at all sticks to the chainstay because of paint, dirt, a non-flat surface or any reason then the caliper can be pulled into a slight angle that will cause brake rub. Even with everything set well it is easy to get a little brake rub on a flat mount system but easy to avoid this with a PM system.

One tiny issue is that the chainstays of the frame can be thin or thick. The bolts have a short threaded section, so they need to be sized to work with the frame. When I purchased my Campagnolo gruppos, I had to figure out what length bolts to order. I also ordered 2 of each thinking they were sold individually, but I got 4 of each as they are sold in pairs. Campagnolo lists sizes in 5 mm increments and one of the bikes required 49.5 mm bolts according to the brand guidance, which is 0.5 mm from the limits of the 45-50 mm bolts. I ordered them, they worked. I was slightly worried. At only a couple of Euro per bolt, it wouldn't have been the end of the world.

Now some stylish, high-end and race-oriented mountain bikes are using flat mount disc brakes. Great if you don't actually like stopping. I say that because my hardtail now has 180 mm rotors on it, my trail bike 200 mm rotors and my DH bike has 220 mm rotors. Bigger rotors are much better for heat management. Lots of hardtails had 140 mm rotors on them but I'm with Enduro magazine in thinking that the big rotor goes on the back wheel so you can speed control on steep slopes safely.

Michael Hanslip Coaching

If you want to go faster, you have to pedal harder

The problem with flat mount brakes