Flat mount steel fork

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  • My next build is going to be a French randonneur inspired touring/audax bike. 650B fat tyres, lugs, chromed racks - retro but with a modern twist as it'll have hydro discs and Di2. Given the retro theme, and because I've never done one before, I'm going to make a steel fork for it.

    I'm aware of the potential pitfalls of make disc brake forks, and I'm planning to use 853 disc specific fork blades which should be up to the job. I could, of course, go down the tried and tested route of using an ISO mount, a PM adaptor and then using PM calipers, but I want to have flat mount calipers.

    This means making a steel flat mount caliper fork. Rather than trying to copy most carbon forks, and fitting threaded inserts into the fork legs than an adaptor bolts onto, I'd like to try and make it so that the flat mount caliper mounts directly onto the fork legs (without the adaptor) and then the bolts go in from the front of the fork.

    This will require some top-hat shaped inserts with an oval-ish slotted hole to allow side-to-side adjustment of the caliper, and I'm still rather doubtful that there will be enough space inside the fork leg for both the top-hat caliper mount and the dynamo hub wire. Any views on why this is otherwise a terrible idea and will result in me having to wear dentures for the rest of my life? I'll try and do some quick sketches of what I'm trying to achieve, on the Picture=1000 words basis.

  • Maybe look at how the bmc flatmount fork solves this. They made it for a 160mm rotor without an adapter.

  • Ah, I was sure I'd seen somewhere else that had done it in carbon, but I couldn't remember where. Basically what I'm thinking of doing is exactly that ^ but in steel. Anyone got any thought on feasibility/need for dental insurance?

  • Just drill some notches into the fork blade and weld a couple of these in?

  • That's what I've done on chainstays in the past, but I think they'd be a bit big for forks. I suspect that you wouldn't end up with any of the fork blades left once you'd taken off enough material for the bosses. Also, the way they're designed means that you'd have the hole for the bolt exposed on the front of the fork, which I think would look a bit fugly. What I'm thinking of is something similar, but rather than an open tube at the rear having a top-hat shaped insert to go into the fork leg.

  • Yeah, I found those instagram pictures. Unfortunately, I think he's totally fucked up making that fork. He's put the caliper mounts at the offset which is correct for the rear caliper, not the front, so the disc is never going to fit in the caliper.

    The centreline of the mounting bolts on a front flat caliper mount is 1.7mm outside the inside face of the dropout - pretty much in line with the centreline of the dropouts on most forks. The rear is 3mm inside the inside face of the dropout, so there's a 4.7mm offset between the two. He's used the rear offset. The mounting points should be much further to the left in the second photo.

  • yes the front and rear are different offset. I thought they had thought of that ... :)

  • open tube at the rear having a top-hat shaped insert to go into the fork leg

    I'm struggling to visualise this. Where are the drawings you promised?

  • Where are the drawings you promised?

    In my head. I've spent all day travelling to and from Canterbury, and Southeastern trains are not conducive to careful CAD work.

  • I thought they had thought of that ... :)

    I thought they had thought of it, but the more I looked at it, the more I thought they hadn't thought of it. If you compare the steel fork to the carbon BMC one the difference is pretty obvious. It's possible the steel one is boost spacing or something weird which affects the offset, but I have a horrible feeling it's a fuckup.

  • yes, also the bmc fork above has been recalled because of failures. But it is the steerer tube cracking and not the fork legs so I guess the method used is ok :)

  • Another idea! Keep the flat mount with adapter layout as normal, but use replaceable barrel nuts like this fork:

    so still big gaping holes in the fork but side ways instead....

  • are they replaceable threaded inserts?

  • "No-Thread" stainless steel thru-bolt cargo mounting system damaging
    threads mounted permanently in the carbon

  • ok maybe not but on danstuff's future steel fork they could be

  • ah i think they are just barrel nuts like this:

  • yeah just saw that picture! looks like a neat design.

  • so still big gaping holes in the fork but side ways instead

    Other than big gaping holes in the fork legs (not something I'd have by inclination) I'm not sure what that wins me though. Brazing in some threaded doodads to bolt an adaptor to is relatively easy. That's how it's done on the Genesis steel FM fork. I'm trying to get rid of the adaptor.

  • ok I thought you didnt like permanent unreplaceable threads. I actually think the adapter is a neat idea. (switch around for different rotor side)

  • In carbon, with soft alloy threads, I'm not a massive fan due to the risk of cross-threading. In steel, not a problem. If you cross-thread it, you can just drill it out and braze in a new one.

  • yeah yeah, IF you know how to glue metal (not envious at all...)

  • potential pitfalls of make disc brake forks

    In terms of safety, or in terms of the compromise(s) to comfort/compliance/weight?

    Regardless, new age fun with vintage feel randonneuse sounds like an exciting if challenging project.

  • Looks like someone got there ahead of me. This is what I was thinking of, albeit with rather slimmer fork blades:

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Flat mount steel fork

Posted by Avatar for Brommers @Brommers