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Postmount vs. Flatmount brakes – Gravel Bike Tech

This may seem like a finished debate already, and for many riders it will be. But it provides use good lesson on how to judge "innovations" in the future. Plus there's a nice bonus at the end for people who own a postmount gravel frame today.

Posted by Gerard Vroomen - 28 October 2019

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?

  1. “New is better”, even if it isn’t.
  2. 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.
  3. 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.

More Gravel Bike Tech HERE.

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23 responses to “Postmount vs. Flatmount brakes – Gravel Bike Tech”

  1. P

    Peter Kumble says:

    12 February 2022

    Good summary and yes, finding a solution for a problem that did not exist is silly. Ok, here is my question: I own a Ritchey Outback 2019 with post mounts. I have Sram Rival on it and it works pretty well. I am a Campy guy on my road bikes. Campy Ekar came out after 2019 so I did not have the option for this when building the bike. Ekar is only flat mount. What is the adapter that I will need to install Campy flat mount Ekar onto my Ritchey post mount frame? I wrote to Campy to ask if they offer post mounts; no was their answer. I wrote to the shop who built my Ritchey to ask what is the correct adapter and their reply was that they were not exactly sure. Help please.

    Reply
    1. Gerard Vroomen says:

      12 February 2022

      Yep, post-purchase service is important but often hard to find. So here a little pre-purchase service from 3T for you 🙂 The adaptors you need are for example these: https://www.assolutions.ca/flat-mount-caliper-to-post-mount-adapter-guide/

      Reply
  2. a

    anthony wood says:

    11 February 2022

    I’ve taken a Santa Cruz Stigmata ‘22 warranty replacement frame for a cracked 2016 Stigmata. The post mounts are now flat. So, my sram red 22 hrd seems to be incompatible. My sram red 22 calliper on the rear doesn’t even align with the mount holes-atleast 15m too wide! Does anyone know what I can do? Is their a flat mount adaptor that can fit my old calliper or do I need a completely new rear calliper? Many thanks

    Reply
    1. Gerard Vroomen says:

      11 February 2022

      There are adaptors like these: https://trpcycling.com/product/flat-mount-to-post-mount-adapter/

      Reply
  3. P

    Paul Bowen says:

    20 December 2020

    Hi, this is my experience of the flat mount postmount brake scenario.
    I the last year I have purchased 2 genesis bike a cda 20 and a cda 10 both were originally sold 2016 as cyclo-cross bikes both running lower end shimano Claris and sora grousets with mechanical disc brakes. The front composite fork on both models is postmount, the frame at the rear is flatmount but comes with the post mount adapter cda 20 160mm cda 10 140mm.
    For the cda 20 I purchased a tiagra 4720 hydraulic groupset but had to discard the calipers and fit some basic shimano postmount calipers taken from some mountain bike brake setup M365 running B10s resin pads(€3.99 pair so cheap to run) , these work perfectly with no issues whatsoever(12000km covered 3rd set of pads went in October) , I could have fitted the flatmount caliper to the back but would have either had to run a 140mm disc or purchase the flatmount 160mm adapter, I could have also changed the front fork to flatmount but decided to stick to the cheapest option of postmount brakes. I have since learned there is a Canadian company producing postmount to flatmount adapters for 40 Canadian bucks plus postage but there are still issues. Currently I am not aware of shimano or sram producing postmount to flatmount adapters! Plenty of flatmount to postmount but not the other way.
    For the cda 10 I have gone the same route but this time its shimano grx St 400 shifters that are connecting to the M365 calipers all postmount, rear 140mm disc this time as the adapter was already fitted. The big difference between these builds is almost exactly 1 year apart finding parts in 2020 is far more difficult out of stock is a familiar sight!
    In conclusion connecting the modern shimano hydraulic Road shifters to postmount mountain bike calipers is straightforward and works without issue and can make big savings in the wallet area 😂

    Reply
  4. T

    Tom Stuart says:

    16 December 2020

    I just had XT mountain bike brakes fitted when I upgraded to GRX instead of the flat mount ones

    Reply
  5. H

    Honza says:

    27 October 2020

    Just out of curiosity … Where did the 51mm International Standard (IS) mount go in this article?

    Reply
    1. Gerard Vroomen says:

      27 October 2020

      Not really a standard that was ever in serious use for gravel bikes. As I mention at the start, post mount is not the first and the last disc brake standard. But it was the first to be applied for gravel bikes. So I filed the IS standard together with the spoon brake and rim brake.

      Reply
  6. T

    Tim says:

    01 September 2020

    I’m looking for some technical support regarding my 3T Rear Axle Free Hub, I need to know what is the set screw size for the ring clamp that holds the freehub in place on the axle. The rear ring clamp has three very small set screws, unlike the front has a single and large set screw – none of my hex keys seem to fit.

    Can some one help me please.

    Reply
    1. A

      Andrea Bortolin says:

      01 September 2020

      Dear Tim,
      from your description, I assume you need support for a Discus C35 (Pro or Team or LTD) or C25 Pro or C30 Team wheelset, could you please confirm that?
      For these wheelsets, Freehub+axle can be removed from the hub by loosening with a 1.5mm allen key the three small screws on the rear ring clamp.
      The ring clamp that holds the freehub in place on the axle can be removed with a 20mm hexagonal socket. 
      In case you need support for our last generation wheelset https://www.3t.bike/en/178-wheels, freehub removal is toolless: you just need to remove by hand the cap in front of the freehub and pull the freehub out.
      Hope this helps and please don’t hesitate to come back to me if you need any further support

      Reply
  7. B

    Bryan says:

    10 June 2020

    I recently purchased one of your frames in the used market, a 2017 Exploro Team which is post mount that I purposefully bought because my current component selection is from my previous frame and post mount. when mocking up the rear caliper to the rear rotor I noticed the shimano post mount caliper would not reach my 140 rotor. what adapter do I need to make this happen or I’m destined to run 160 rotors?

    thank you in advance for your time

    Reply
    1. Gerard Vroomen says:

      10 June 2020

      Hi Bryan, A postmount in principle is designed for a specific rotor size, and in this case it’s 160mm. you can go bigger with adaptors if absolutely necessary, but not smaller. However, 160mm is the minimum I would recommend for gravel anyway (which is of course also why I decided to make it a Postmount 160mm setup).

      Reply
  8. R

    Rasmus says:

    30 October 2019

    As a customer I don’t agree that I couldn’t care less about different standards for MTB and road. For example, I am pretty certain that there are perfectly valid reasons for wanting to put MTB chainrings on road cranks (for example, heavy duty touring bikes), and, of course, matching road shifters and MTB derailleurs. Or even MTB front, road rear or vice versa.

    The thing that really makes me feel a bit left out because new standards are newer, is actually the 15mm front axle on my Exploro. Road wheels for 15 mm axle have always been kind of rare, and now MTB wheels for 15mm non-boost seem to become rarer for each day. Luckily I got my hands on a pair of 3T C60 and a pair of DT Swiss XMC 1200 Spline 30 while they were both available with 15×100 hubs.

    Thinking of it, adapting a 12mm axle to a 15mm fork doesn’t really seem to be a very hard thing to do, but I’m not enough of an engineer to figure out if such an adapter would be a disaster structurally or something like that. Maybe it already exists?

    Reply
    1. Gerard Vroomen says:

      30 October 2019

      It’s maybe not clearly phrased, but what I meant to say is that while charging different prices to different customers may be great for gear makers, it’s not something that is at all in your interest. It’s a bad trade-off, getting something that is a potential negative for you in exchange for something that could be a positive.

      Reply
    2. C

      Chris from Swiss says:

      15 November 2019

      There are adapters for this available on the market. Google for “15mm to 12mm Fork Thru Axle Conversion Kit”

      Reply
  9. P

    Pascal says:

    29 October 2019

    Hey Gerard, nice article! Thanks for the technical information on flatmounts brakes. I always asked myself why it came up and went so popular. For me it has always been an “improvement” that hadn’t been necessary.
    Personally I even prefer the more bulky style on the gravel bikes. It suits better to offroad character thos bikes should have! I wouldn’t like to waive on my 3T Luteus II – Nicolai Argon CX – SRAM Force 1 combination.
    Technically I didn’t notice any markable differences between my gravel disc bike (PM) and my road disc bike (FM).

    Cheers and ride on!

    Pascal

    Reply
  10. J

    Jacob Hansen says:

    28 October 2019

    Hi Gerard,

    Slightly off-topic, but do you know if I can re-use my existing Force1 HRD brake calipers with the Force AXS brake levers or am I forced to buy Red AXS brake calipers + levers since they are the only ones available in post-mount?

    Thanks
    Jacob

    Reply
    1. Gerard Vroomen says:

      29 October 2019

      I checked with SRAM and yes you can do this. The old Force1 HRD brake calipers are compatible with the new Force AXS and Red AXS shift/brake levers. Of course connecting it all and setting it up may be a job best left to the capable bike store.

      Reply
      1. J

        Jay says:

        02 April 2021

        Hey Gerard, love the article. Do you know if the reverse is true? I.e. Old force 1 HRD levers compatible with the new red axs post mount brakes?

        Thanks!

        Reply
        1. Gerard Vroomen says:

          06 April 2021

          Hi Jay, I don’t know, that you should really check with SRAM.

          Reply
  11. Stefan Kasteren van says:

    28 October 2019

    Dear Gerard, I agree with your statement, however I have one issue with this new flatmounted front Force1 breaks on the 3T Exploro. ( don’t get me wrong, I love the 3T from day one) The advantage of this bike is to swap wheels in a second to get an different bike feeling. I have a set 700c and the original 650b (c25) now every time when you swap the wheels you have to adjust a litle bit the breaks. As it is flatmounted i noticed that the threaded is damaged as you direct mount on the carbon fork. The bike is just 3 months old and i always make use of a moment allen key and still it is damaged. It just holds. What to do?

    Reply
    1. Gerard Vroomen says:

      28 October 2019

      Hi Stefan, but this is nothing specific to the Fango fork though. Those threads in the fork are the express design from Shimano for flatmount brakes. Every flatmount fork following the Shimano design will have alloy threaded inserts, the Fango is no exception. And in fact, every postmount fork also has threads like that, so they face the same issue with repeated use. Maybe I should have been clearer, but the only thing different on the Fango fork is that the area where the adaptor sits is countersunk, so the adaptor sits a bit inside the fork leg instead of being stuck outside of it.

      Reply
      1. R

        Rasmus says:

        30 October 2019

        Also, there are shims that can be but between the brake disc and the wheels so that the brakes don’t have to be adjusted on every wheels change.

        BTW, after having adjusted the flat mount calipers on my Specialized Sequoia I’ll never again complain about brakes being hard to adjust on my post mount Exploro. 🙂

        Reply
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