Postmount brake calipers are the standard on mountain bikes. They are by no means the first disc brake standard and they are unlikely to be the last, but they are having a good run. Why?
- Easy to install.
- Easy to adjust.
- In most frame designs the caliper bridges between chainstays and seatstays, spreading the load over a big area, so you can design a light frame around it. For the fork, they require posts that usually stick out a bit, but nothing too dramatic.
- They work well, so why mess with it?
So when the first gravel bikes were built, they also used postmount brakes. Why? Well, when trying something new, you need to work with what’s available. Postmount brakes already existed, they worked, so that was an easy decision. See below the postmount brakes on the first generation Exploro:
But then the usual bike industry itch started: Why not have a new standard? The reasons?
- “New is better”, even if it isn’t.
- Having a separate standard for road means parts are not interchangeable. So you could price both to market, and for example charge more for a road brake than a mountain bike brake.
- Road bikes get more aesthetic consideration, and the postmount brake is a bit bulky.
In this case, it was Shimano who came up with flatmount to solve a non-existent problem. The basic caliper stayed exactly the same, the inner workings did too, all that changed was that the two tabs sticking out of the caliper to mount it were now moved “under” the caliper and accessed from the other side.
Of course, as a customer, you shouldn’t care about #1, won’t care for #2, and as for #3, you decide. Is the difference earth-shattering?
To me, the visual difference for the rear caliper is negligible, while structurally the brake forces are now transferred only into the chainstays.
On the front you do get a somewhat cleaner look. But this photo is of the 3T Fango fork from the second generation Exploro, which integrates the mounting bracket for the caliper inside the fork leg. On other forks, there is a mounting bracket hanging off the back of the fork leg, making it less aesthetically pleasing.
In fact, this mounting bracket is one of my big frustrations, almost an afterthought to the flatmount design (“Oh, the bolts now come from the other side. Hm, the fork blades are already there. Well let’s just make a plate, mount the caliper on there so it looks exactly like a post mount brake, and then bolt that onto threads you put into the fork”). See below a postmount brake (left) and a flatmount brake with front fork bracket (middle). In fact, Campagnolo realized how silly this was an made specific flatmount front calipers with the bracket integrated (right):
Now, to be fair, in the case of carbon frames, the flatmount did create a side benefit for the frame. While the fork still requires threaded inserts into the carbon (or a complete workaround of the flatmount design), the frame no longer requires threaded posts. Instead, the “posts” are simply thru-holes that the mountain bolts go through as they thread into the caliper.
As any composites engineer will tell you, we can embed alloy parts into a carbon structure, but we’re happier if we can find a way not to. Prevention is better than cure and galvanic corrosion is just one of the “diseases” you want to avoid as a composites engineer.
The take-aways:
Did we need flatmount? No, but it’s here and it’s a done deal as Shimano only offers its road/gravel groups with flatmount brakes. Since no frame maker has the luxury of telling customers that Shimano road/gravel groups won’t work, virtually all performance road/gravel frames are now flatmount. That it is a marginal improvement in some ways and a marginal deterioration in others has become irrelevant.
Can I still find postmount gravel frames? Probably. As with other changes, this change first went slowly and then quickly. When enough frame makers moved Shimano’s way, consumers saw that and legitimately considered flatmount the “future-proof” option. Postmount gravel frame sales tanked overnight, leaving lots of inventory in stores and manufacturers’ warehouses. And with sales close to zero, it takes a long time to clear those out.
Is my old postmount gravel frame not “future-proof”? There is actually some good news, if you look for it. With SRAM’s new AXS road groups, you will find only flatmount brakes on their website. But they do offer RED AXS postmount brakes. Why this is such a secret and not on SRAM’s website, I don’t know. But it looks like this:
So you can still upgrade your postmount frame to the most advanced groupset available today, go with electronic 12-speed shifting, be it a regular 2×12 or even the Mullet with an Eagle rear derailleur. The part number to search for online is 00.7018.392.000 for the left and 00.7018.392.001 for the right lever/caliper set. It’s also available as part as a complete groupset. But again, you won’t find it on SRAM.com as of today.
It is also possible to use those new SRAM Force AXS and Red AXS levers in combination with the previous generation Force1 calipers that many people will have on their early-generation postmount gravel bikes.