Front Dropout with Integrated Flat Mount Boss

If you build - or are interested in building - metal forks with plate style dropouts and flat mount brakes, please read on and comment as I’d like your input.

I’ve been using a Long Shen cast dropout (pictured) which has an integrated lower flat mount boss and a second boss that the builder must locate. This idea is so appealing to me as a builder with a low-tech shop, as it makes me much more confident in my placement of the brake mount - much like the various rear dropouts available with self-locating brake mounts (PMW’s many thoughtful designs, for example). But this is the only front dropout I’ve found with this type of functionality. I’d like to see something better, or at least more options.

My issues with the Long Shen product:

  • You have to cut a lot off of the tapered end of a traditional fork blade to get to the 17.2 mm diameter. A plate style dropout allows the builder to decide where to cut and to finish the blade end in their own style.
  • The plug is a good fit on heavier blades but loose on a lighter blade, making alignment difficult. Again, this would not be an issue with plate dropouts.
    -The finish is a bit rough and tolerances not as precise as a machined dropout.

I’ve been in touch with @Calvin_pmw @mark_pmw @Coco_PMW and they are interested in pursuing a machined plate style dropout with the integrated flat mount boss, at least to the preliminary design phase. It would be cool if there were a version for 160/180 as well as the common 140/160. I’m not sure yet whether I’m the only builder in the world who would want this, so please comment if you would like to see this product - or something like it.


If anyone else starts making something like this, make sure the upper brake boss is included with the dropouts!


I’d love to see a plate option for flat mount!

Random thoughts:

  • These are the blades I’d use for this type of fork. There aren’t a lot of unicrown blades out there anymore!?

  • I like the idea of having two bosses on the DO for rack and fender options but so that the bosses can be removed by the builder if the customer doesn’t need them.

  • The plate part of the DO should be wide enough to accommodate at least an 18mm blade width.

  • Probably good to go with 12mm thru axle since that’s the current preference for gravel/road bikes.

  • I don’t see the point in acommodating a 140mm rotor. 160/180 is the way to go.


Thanks for chiming in, Nick! All good thoughts. Maybe there could be a couple different tab widths like the 13 and 18 mm PMW currently offers in their 12mm TA dropouts. I, for one, would prefer a daintier fork tip.

Agree about the 140mm rotor being kind of pointless. I had a back and forth with Todd Farr a year ago or so about adding a 160/180 front arm to his Le Disco tool and he said he won’t do it unless there is more interest. I was the only one who had even inquired about it! What tool do you use for front flat mount?

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I have the same Le Disco tool.

The only flat mount fork I’ve built used bosses I turned on my lathe. It worked fine but it’s more work than I want to do so I don’t offer flat mount forks currently.

Are you able to get away with skinny blades and disc brakes? I thought that was a recipe for cracked blades. But maybe that’s only a problem with IS mounts. And maybe an integrated mount on the DO would help with the braking forces.


Nice execution! I totally understand your not wanting to fabricate bosses for every fork and I imagine other builders are in the same boat. I used some modified rack mounts and was not thrilled with the result on my first fork, although it did work.

I don’t have enough experience to really answer that. I’ve only put a year on my first disc fork (not yet broken) and I like it quite a bit, though I do experience some brake chatter. I used a set of oval blades that are 1/.7 butted and utilized most of the tapered end. The blades move a lot, softening out small bumps. I was worried about going too thin, so I consulted Rob English - my go to for engineering type questions. He said he has about 80k miles on a personal fork that he built with .9/.6 tapered blades and post mount. So I guess you can do skinny/thin blades and disc if you know what you are doing.


@mark_pmw please for the love of all things don’t make these for 140/160.
160/180 makes way more sense, especially with shimano and sram including stickers and notes with their brakes specifically saying that 140 should only be used for closed courses with no descents!
From the design end of things the lower boss crowds the fender mount, so the extra 10mm solves that.
I made a bunch of bosses. It’s a simple lathe part. Just a 13mm rod with a tapped hole on center. Go find your local machine shop.
Hahn Rossman


Rob is also built like a Tour de France climber. :wink:

I hear you wanting to keep the blades skinny for more bump absorption. But would the blades be durable enough for the average American male (200lbs these days) and 180mm rotors?

Hopefully we can get some feedback from the engineers lurking on this forum.


From now on, it’s all 160/180 for flat mount.


@mark_pmw that’s music to my ears!
Hahn Rossman


Won’t larger rotors put less stress on the fork?

It’s my understanding that larger rotors apply more torque. Since the braze-on for FM160/180 is at the same location on the fork blade (it’s the adapter that moves the caliper), I would imagine that a larger rotor would apply more stress on the blade.

That makes sense. I would then argue for native FM180 and FM203 for forks!


Fully integrated into the body of the dropout, or shipped in the same bag as two pieces? Fully integrating the upper boss would increase cycle time, tool wear, and material costs and decrease yield per 12’ bar. Might make it prohibitively expensive. But if you mean two pieces, yes, we would ship both together.


As a 210 lb American male, I think we need to introduce a new standard process for rotor design. 1 mm of rotor/ lb. (I’m being sarcastic).


For sure, a separate upper boss makes the most sense. As for the second boss, I’m thinking something with a T-shaped profile - like a bottle boss with a big, flat head. It would be nice not to drill into the blade at all but I think it’s necessary if you want to keep the assembly flat (as the name suggests).

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Not the worst idea! Kinetic energy is linearly scaled by mass, so 200lb = 200mm rotor makes sense haha!

I don’t build forks, but here is some out-of-the-box thinking: It is VERY common for bike companies to make their own front (drop bar) and rear (mountain bikes) brake adapters. Normally if a single builder makes an adapter, it’s not sustainable (not enough traction for a new standard). However, if Paragon makes the dropout and the adapter, it’s an easy-to-consume package.

It’s clear FM is not the best standard for metal bikes. Mainstream manufacturers like Open, BMC, and specialized don’t even use the standard front FM.

This is Specialized custom adapter for their Aethos fork.

For a road/gravel fork maybe moving the bosses up and 3mm outboard better interfaces with the fork blade? This can easily be done with a custom adapter.

From a manufacturing perspective, the position of the flat mount boss close to the dropout is a good thing. We can make a more compact dropout, so there’s less material cost and less cycle time for a less expensive part. The other good thing is that the rider can get a brake adapter almost anywhere; they don’t have to come back to PMW to buy a replacement.


Definitely still in two pieces (three when you count the right dropout). But having to buy the upper boss separately means a) people will forget it/miss it, and b) makers and suppliers need to keep track of two item numbers for one set of parts - you can’t use either without the other (maybe), so just keep them together.

Personally I think keeping things as universal as possible is almost always best. Using the rear FM160 standard on a fork makes perfect sense in my eyes, especially for steel/ti. But making a whole new adaptor for that one thing, that only that one place sells, seems a bit crazy.

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@Calvin_pmw Do you think there’s potential for developing a dropout like this? Or is the idea dead in the water? I can’t tell if there was enough interest on here…