Post mount vs flat mount

Why do gravel bikes have flat mount brakes? Surely post mounts are a more flexible design since you can put bigger rotors on them?

Fashion

Because most Gravel and Road frames are limited to 160mm rotors anyway.

Also, when the postmounts are inside the rear triangle, they’re a pain in the ass to align properly.

Also, post mount is neater and transfer the vast majority of it’s force directly into the structure (compression) rather than partial shear.

It’s just an all round better design when the max rotor size is limited to 160mm.

post mount relies on threads in the frame, think cross threading/stripped
flat mount, the threads are in a replaceable bracket

Surely PM is compression directly in to the fork/stay, so the bolt is acting in shear mainly to hold it aligned.

FM must be much the same. Bolt angles of both are pretty close to being parallel to the tangent of the disc.

IS2000 has bolts acting in shear by the braking force.  (edit: actually IS2000 to PM adapter will have all the bolts acting partially in shear – at least looking at both my bikes, that’d be the case, whereas PM rear would be close to no shear braking force)

Anyone ever broken brake mount bolts by braking?  (especially an IS2000 brake bolt?)

Shear is a strong orientation. Tension might be a not so good angle.

I imagine that FM exists mainly so roadies etc. can run 140/160 rather than enduro/dh spec rotors on frames far more slender than a bike that would be labelled “a rig”. Could bolt PM from the other direction I spose, but that’d still force seatstays to be oddly shaped like one some MTBs, cos PM is so huge. And roadies seatstays are so slender they look like they’ll bend just by looking at them wrongly.

One think I found when getting some new brakes for the old ‘cross bike is that (almost) no one does an adaptor to let you run a flat mount brake on a Post Mount bracket. If you have a flat mount bike, you can get post mount adaptors. As things like GRX now only come in flat mount, it’s going to force some people (me included) to look at changing frames…

@chipps or just swap the caliper to a shimano post mount one (or even a hope Shimano compatible caliper). Both options cheaper than a frame unless you want a new one 😀.

There are adapters, A.S. Solutions do one, there are limitations with what frames they work on though as it needs more clearance than a post mount caliper would.

I got the AS Solutions one as shown above. Required some work on the milling machine to get it right (IS to flat mount) but pleased it worked in the end as despite everyone claiming that post mount isn’t going to be obsolete any time soon, the range of calipers that work with sti type shifters in post mount is already limited.

That Arkose in my garage is looking a little tired… Might be time for something new with GRX 🙂

when i bought my latest bike the fact that of my final two choices one had flat mount and the other had post mount meant that i had to abandon the thought of buying the outdated post mount equipped bike as i would always have had that niggling feeling that my brakes could have been that little bit better.
sadly that meant spending an extra £1500 but let me tell you brake calipers look much better directly mounted rather than hanging off a lump of twisted metal.
to be honest the brakes wernt the deal breaker but as a long time sucker for ‘go faster’ marketing i did stop and think about it for a moment.