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Flat mount versus standard tab mounts for disc?

Old 10-07-20, 10:58 PM
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sannerbikes700
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Flat mount versus standard tab mounts for disc?

Hi all!


I've been out of the frame game for a while now, so I was hoping for little help. I am building a CX frameset (apparently they are now called gravel bikes??), and I see that many of the disc brakes are now flat mount. Is there any reason I should, or shouldn't (or can't) build the frameset to accommodate flat mount disc brakes versus the traditional tab mounts? Perhaps there is some reason flat mounts are a challenge or don't make sense.


Tab mounts would be easier for me since I've done it a million times. But the flat mounts do look sleeker, and maybe there are other advantages?


Anyway, any input would be much appreciated!
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Old 10-08-20, 02:12 AM
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guy153
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I made an MTB with flat mounts and just got cast dropouts with them built-in. This was very easy.

Cast Dropouts

DR06 on there.
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Old 10-08-20, 04:02 AM
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I'd build it with flat mounts. You can adapt a post mount caliper to a flat mount frame, but its not so easy to adapt a flat mount caliper to an IS or post mount frame. Flat mount is much easier with a dropout that has the mount built in. Otherwise, you'll need a fixture to locate the mounts. If you use the weld in round mounts, it is a bit of a challenge to fixture the stay and cut the holes in the right location.
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Old 10-08-20, 06:50 AM
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production bikes are still coming with IS mounts because they can't control the location of flat mounts. And the facing tools available stink, including the vanishingly rare VAR.

I don't see IS going away soon, even though nobody actually mounts anything but an adapter to it.

The difference between a CX bike and a gravel bike is that CX bikes always had ridiculously small tires to make the sport more difficult. Can't recall too many CX bikes that would clear a 40mm tire, much less the bigger tires now commonly used on gravel bikes. Gravel riders aren't particularly interested in having flats, so they use bigger tires.
The industry took 5 or more years to actually understand the difference, I guess some people still don't get it.

Last edited by unterhausen; 10-08-20 at 06:54 AM.
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Old 10-08-20, 10:44 AM
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Wow, thank you so much for the input guys. Flat mount it is. And thanks for clarifying gravel vs cx. Also I see now most disc framesets are built with the through axle dropout. Man, a lot has changed since my framebuilding days. I assume through axle dropouts are the way to go?
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Old 10-08-20, 11:36 AM
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I'm not a huge fan of thru axle dropouts but I think the main thing is to check what hubs are available for the rear spacing you want. Thru axle tends to be associated with enormous rear spacings like 147mm or so.

There's a horrendous variety of different diameters , lengths and even thead pitches for the axles but you will probably find the one you need in the end. Or could consider making it if you have a lathe.
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Old 10-08-20, 11:54 AM
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Tim, If you are still building with a Bringheli or one of your jigs, thru-axle will be tough. I haven't found a good way to convert the dummy axles to thru-axle. If you find one, please report back.
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Old 10-08-20, 11:54 AM
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Ah ok. I should be OK using a traditional breezer dropout front and back with the flat mount hydro disc system? Hub spacings are 100 and 135.
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Old 10-08-20, 11:59 AM
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You can use one of these from Paragon. How are you going to handle the fork?
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Old 10-08-20, 12:17 PM
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I was planning to use either of these guys from Paragon for front and rear. For fork I'm doing a straight blade fork. But maybe there is something I am not thinking of that would make using these mounts a challenge for both front and rear?




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Old 10-08-20, 12:39 PM
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Originally Posted by Cynikal View Post
Tim, If you are still building with a Bringheli or one of your jigs, thru-axle will be tough. I haven't found a good way to convert the dummy axles to thru-axle. If you find one, please report back.
If you have a mill, you can make adapters to use Anvil Dummy Axles. I made these for a friend's Bringheli frame and fork jigs
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Old 10-08-20, 12:46 PM
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Originally Posted by guy153 View Post
I'm not a huge fan of thru axle dropouts but I think the main thing is to check what hubs are available for the rear spacing you want. Thru axle tends to be associated with enormous rear spacings like 147mm or so.

There's a horrendous variety of different diameters , lengths and even thead pitches for the axles but you will probably find the one you need in the end. Or could consider making it if you have a lathe.
142 Thru axle is the same as 135QR in terms of the spacing between the inside faces of the dropouts. The dropouts just have a 3.5mm recess on each side that makes them 142mm. In terms of the axles, I would agree that there are too many thread pitches, thread lengths and overall lengths.
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Old 10-08-20, 12:56 PM
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Originally Posted by sannerbikes700 View Post
I was planning to use either of these guys from Paragon for front and rear. For fork I'm doing a straight blade fork. But maybe there is something I am not thinking of that would make using these mounts a challenge for both front and rear?





Just be aware that the front and rear are completely different for flat mount. The front mounts use an adapter plate that bolts to the fork with M5 bolts spaced 70mm apart. They can probably be done with large flange water bottle bosses, if the fork leg is straight enough to allow the bosses to fit in the proper locations.

That rear mounting plate will require you to mill away a section of the chainstay in order to weld it in the proper location.
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Old 10-08-20, 04:13 PM
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Originally Posted by dsaul View Post
If you have a mill, you can make adapters to use Anvil Dummy Axles. I made these for a friend's Bringheli frame and fork jigs
That's awesome. Thanks I'll PM you about those.
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Old 10-08-20, 05:18 PM
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I very much appreciate the input everyone. Iíll forget about the through axle dropouts, thatís beyond my wherewithal. So for anyone who has done the flat mount on a straight blade fork with a wide crown... challenging? Shall I just go with a tab iso mount??

Last edited by sannerbikes700; 10-08-20 at 05:24 PM.
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Old 10-08-20, 06:39 PM
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I always thought the case for flat mount was better on forks than on the back. I'm going to make one soon. It's a pain though, it's not easy to get bosses or fixtures. I plan on using a flat mount to post mount adapter on my Post Punk fixture.

I really don't think there is any problem going with IS mounts. Bike mechanics will like you better, but your frame might not be featured in Pinkbike
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Old 10-08-20, 07:27 PM
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Ok. Iíll flip a coin and make a decision. Iíll post some pictures here of my way, way out of practice work.
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Old 10-09-20, 05:36 AM
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Originally Posted by unterhausen View Post
I always thought the case for flat mount was better on forks than on the back. I'm going to make one soon. It's a pain though, it's not easy to get bosses or fixtures. I plan on using a flat mount to post mount adapter on my Post Punk fixture.

I really don't think there is any problem going with IS mounts. Bike mechanics will like you better, but your frame might not be featured in Pinkbike
The problem with IS mounts is that Shimano and SRAM make all of their drop bar hydro brakes in flat mount only. You can still swap them out and use an MTB post mount caliper, but they are starting to transition those as well. There is only one company making an adapter to fit flat mount calipers on an IS tab and they are usually out of stock. I'm not a huge fan of flat mount, but it is being forced on us.
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Old 10-09-20, 06:39 AM
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I'm familiar with the issue of putting a flat mount on a IS tab. Additionally, the tabs we can get in steel don't accommodate the adapters without modification.

I thought they were sticking with post mount on mtb. Is it possible to put a flat mount on a suspension fork?
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Old 10-09-20, 08:32 AM
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Shimano currently has flat mount versions of the XTR, XT and SLX brake calipers. It is certainly possible to make a flat mount suspension fork, but I'm not aware of any being produced right now.
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Old 10-09-20, 11:32 AM
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I have been told by a respected frame maker (in early conversations about a custom) that you need to build the fork beefier to handle disk
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Old 10-09-20, 02:25 PM
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Old 10-09-20, 05:53 PM
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If you are building the gravel bike for sale or for a customer, flat mount is what is now expected. Also 12mm thru axles front and rear with 142 rear spacing. This has become standard and it's already challenging to find wheelsets with other configurations. There is very little interest in a disc bike with QR. Obviously if this is for your own use, roll however you like.
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Old 10-10-20, 03:02 PM
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I certainly agree about TA being expected nowadays. Bad news is I finally gave in and ordered a set of 12mm TA front dummy axles, so sometime in the next month the industry is going to announce they are going to 13.5mm TA.
When I finally gave in and got 15mm TA front dummy axles, it was the same month the industry announced 12mm TA front.
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Old 10-10-20, 04:34 PM
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The flat mounts you show in the above pics will require some kind of jig to make sure the axle to caliper position is correct, there is no a lot of room for error. If you are set up with a machine shop should be easy but if not try using some Paragon mounts that incorporate the flat mount with the drop out. Most decent wheel manufactures make their wheels so they can be adapted to 15mm, 12mm or QR by changing the end caps DT Swiss is one. Also, consider buying a carbon flat mount fork, a steel fork will be very heavy in comparison and as you noted with the change in hub standards most fork steerer tubes/headsets are tapered.
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