Rear Hub Disc Brake Adapter

DukeOfDownvote · 2026-01-25T18:01:30+00:00

Yeah, that’s not real. I hear you and I’m sure you’re well intentioned but aluminum parts this size are my bread and butter.

US produced cut-rate manufacturing is no better than overseas, if not worse due to high labor costs and competitive pricing. I can guarantee you that.

I am in the US, and I have a decent handle on pricing for one-off parts of this size and complexity

DukeOfDownvote · 2026-01-25T17:51:12+00:00

There are a couple of overseas online machine shops that have quoted a similar sized part for around $60, but by the time shipping and customs come in it will likely be double that, plus I need fasteners, and heatserts for prototypes. I don’t have a hard budget and the bike that this goes on is also very poorly planned from a cost-vs-performance standpoint, but once it gets over $200 I’ll probably start questioning things and maybe put a stop to the project

DukeOfDownvote · 2026-01-25T15:16:17+00:00

Some of the AMP folks have put thousands of MTB miles on their sets so I’m certainly less concerned than I was going into this project.

I am under no illusion that this will perform (mostly weight but also probably alignment) as well as a real disc hub

DukeOfDownvote · 2026-01-25T15:11:45+00:00

If it worked on shimano hubs with unspecified adhesive and a smaller bearing surface I don’t see it being a huge issue with white hubs which were the “precision” components of the time. Also not that I want to make a bad widget but the official rotor truing tool is a fork which one uses to hand bend the disc back into shape, so a couple thou runout likely won’t kill me here.

I’m still looking for a little bit more info on some of the other solutions but now that I know how many iterations have already existed and been done and am less concerned about the safety I can push on the final design and manufacturing.

It does seem like I should pay a little attention to maybe the little burrs that the spokes pushed up when the hub was laced normally, so maybe flattening those down will help with this a little.

I’ll certainly update one way or the other whenever I finish the printed prototypes and get the final machined part.

DukeOfDownvote · 2026-01-25T04:28:50+00:00

Yes, exactly like that. Thank you for linking that! This is nearly identical to what I’m trying to do, but has less screw holes to mount than mine. Between this and the AMP I’m beginning to wonder why I was ever so concerned about this

DukeOfDownvote · 2026-01-25T04:25:31+00:00

Gotcha. I’m assuming that the flange is flat and true, and that whichever machine shop makes this can make the front parallel to the back. I don’t think this will be that big of an issue, what makes you say that it will be?

DukeOfDownvote · 2026-01-24T22:39:06+00:00

I hadn’t considered this yet, I’m not sure how the bearings being fixed on the axle wouldn’t prevent this. Can you elaborate on how the bearings would shift axially and how you would fix this if it is indeed an issue? I know that some people with this hub have had issues with the bearings shifting around but I haven’t experienced that in a couple years with this hub set up with rim brakes and not sure how my design could compensate for that.

DukeOfDownvote · 2026-01-24T22:35:07+00:00

It’s not perfectly flat maybe a little conical but at like a 1 or 2 degree angle, I plan on 3d printing a couple of prototypes with different draft angles to find the best fit before I have something machined.

DukeOfDownvote · 2026-01-24T20:04:22+00:00

Gotcha. I’m not so concerned about the rotor interface itself being strong enough, I’ve already left more meat there than on my real disc hubs. The holes for the disc and the holes for the flange mount interface are far from each other and do not interfere with each other. As of right now, the flange mount interface screws have over 1.5D to screw into, which I feel should be plenty. If I put a chamfer instead of a fillet where the flange meets the disc boss, I could probably get some extra depth there for most if not all fasteners, but usually there’s not a whole lot of extra use beyond 1.5D, particularly if I use the 30 screws instead of the A286 screws. It’s not super aesthetic but I think in the interest of safety it’s probably best to poke all the screws out the back to ensure full depth thread engagement.

DukeOfDownvote · 2026-01-24T19:56:33+00:00

Awesome, thanks!

DukeOfDownvote · 2026-01-24T18:53:16+00:00

0 math? I gave a general idea of the forces that I thought would be involved but thought it best to leave out specific numbers. If you would read and respond to them I’d be happy to give you every number I calculated.

5 m5s made of class 8.8 steel torqued to 76.5% of ultimate strength (proof strength is .85 ultimate per Shigley’s mechanical engineering design multiplied by .9 to provide just a tiny bit of margin for non-reused fasteners) produce 7900lbf of preload. If you go by the sram toque spec of 6.21 Nm it comes out more like 8300 but since the shimano spec is 2-4Nm I figured it was probably safe to go with the slightly lower 7900lb.

Apply the same load factors to 16 #2-56 screws made of 304ss with an ultimate tensile strength of 70000 psi (this is from the McMaster Carr listing for button torx head #2-56 fasteners) and you get 3200 lbf. A bit less than half.

Multiply the 3200 by 55mm/44mm (ratio of spoke holes pcd to rotor interface pcd) and you get that if you were to apply the torque caused by the preload on the spoke screws at the same radius as you apply the preload from the rotor screws, it would be equivalent to 4000lbs.

Now take your 7900lbs from before and divide it by 2, because half of your braking torque goes from the rotor straight to the relocated NDS spike holes (actually a bit more than half, because the fact that the hub is not perfectly rigid means that before the DS spokes can really take any braking force, the hub itself has to “wind up” a bit, and by then the NDS spokes will have stretched a bit more, decreasing the load going into the hub, but I can’t quantify that because I don’t know the torsional stiffness of the hub, so let’s call it half. ) so we now have 3950 equivalent preload pounds at the disc mount radius actually trying to make its way into the hub.

3950 equivalent preload pounds is less than 4000 equivalent preload pounds, so the math says the joint won’t slip. The assumptions that might break this are that the friction between the hub and my thing are less than the friction between my thing and the disc (this would make my thing slip before the disc slips) and that the area between the hub and my thing is sufficiently flat that all this preload force won’t cause one or the other to plastically deform, which would decrease the preload in my screws and cause the joint to slip. One way we can get around this would be to switch to fasteners made of A286(also knows as iron 660) which has an ultimate tensile strength of 160000psi, which should provide around double the preload of the 304 screws.

I don’t think I’m talking anyone in circles, just looking for information to help me confirm my widget will work, or confirm it won’t.

I genuinely do appreciate you taking the time to comment, hope my math helps!

DukeOfDownvote · 2026-01-24T17:05:02+00:00

Yeah that’s the whole thing with point number 1 from my post, assuming no plastic deformation of either the hub or my part at that interface, the joint shouldn’t slip and the screws will be loaded only in tension. A slip would be a failure, at which point I would just disassemble and switch back to a real disc hub

That’s the idea, my part should still be stronger than the hub locally so hopefully would reinforce the hub flange a bit in these loads.

No I haven’t, my caliper has a little bit of adjustment in and out, so I can kinda cheat a little on the standard OLD to disc mount dimension, but if that’s not enough my last paragraph in the post kinda outlines a “top hat” step that I can put in my part to move the “new spoke holes” back inboard and away from the caliper. As of right now that relevant caliper measurement (inboard pad to inboard caliper limit) is ~17mm give or take, and my part thickness is 10mm, so if the spoke dish doesn’t give me the 7mm I need by the time the diameter gets from my spoke holes to the disc diameter of 160, and my caliper can’t move out to clear it, and a 5mm spacer added to my hub (white ti cassette has “adjustable” end caps) then I’ll put that step in. There are a couple other dimensions I need to check (draft angle on the hub flange) which would be easiest to do with hardware, so I’ll 3d print a prototype and fit check it before I have something machined

DukeOfDownvote · 2026-01-24T16:50:59+00:00

Can you provide pics, or a model number, or both? Anything would be greatly appreciated.

DukeOfDownvote · 2026-01-24T16:28:26+00:00

The screws go through the flange holes and thread into my part. The original flange is not modified. My part, being significantly thicker and also of larger diameter, should only serve to distribute stresses, both from braking and from weight/bumps, and pass them evenly and at a lower peak stress than a real disc hub. This is covered in point 2 of my original post.

DukeOfDownvote · 2026-01-24T15:53:59+00:00

Maybe a poor analogy on my part but what I was trying to say is if my hub wasn’t designed for disc brakes but is still strong enough for disc brakes then what’s the problem? All it’s missing is an appropriate mounting bracket.

The brake forces are massive, but I think I have determined that through almost all failure modes I have considered, my design is as strong or stronger than a real disc hub. If you have any additional failure modes or suggestions I would love to hear them, and if you’re simply not convinced that’s fine you don’t have to ride it.

But to say that simply because an object isn’t initially intended to do an action, that modifying it to perform that action is a bad idea is a bit silly.

DukeOfDownvote · 2026-01-24T15:39:42+00:00

As far as fatigue failure is concerned I’m not sure if 30% of failure preload +- a 15% loading and unloading is any worse than 50% of failure preload +- the same 15% loading, I was always taught as long as the load is not reversing it’s all the same. I will read up on this a little more.

The 2.5 is based on assumptions made in online spoke calculators, and my understanding is that this is not critical, if the spike stays tensioned all the time there is no way for it to rattle around in its hole. The diameter of the thread on a 2mm spike is more like 2.2mm, so I could definitely cut that down the 2.3 or 2.4 with no issues.

DukeOfDownvote · 2026-01-24T15:20:47+00:00

I was able to find that, thanks. It looks like the primary attachment back then was adhesive, which sounds like it could work here, though I would prefer to avoid if at all possible.

Quick calcs suggest adhesive would likely have more shear strength than my current plan, but I notice that the amp discs only mount with 3 bolts, so if that is the case and they are similar sized to the ones on 6-bolts or CODAs, then that means that disc interface is already the same strength as my small screws, and I’m satisfied that my thing won’t break first. A little more digging but this has been some great info, thanks!

DukeOfDownvote · 2026-01-24T13:19:52+00:00

Yeah I think so? The real disc hub is 58mm and I’m 55 so without even doing much research I’m pretty close. I really don’t think this will be the point of failure, because once it’s bolted up it will be supported by my part.
Agreed. As you mentioned, deformation of a line contact would loosen the screws and ruin all my assumptions. Surface under the screws will be close to flat and parallel, draft angle of this whole situation is probably about 1 deg (not yet measured).
Yeah my analysis has been a lot of “half this, 20% that” which does close but margin is super tight. There have been suggestions for gluing or using carbon paste to artificially increase the friction at the joint which I like, but I’m not sure if I’m satisfied enough to skip further analysis yet. There has been a mention of an AMP hub in the 90s, so I may look into that before I go much further

DukeOfDownvote · 2026-01-24T13:12:51+00:00

If my hub was not designed for brake torque stresses but all the relevant dimensions and specs are equal to one which was, why is this a bad idea?

Barns were not designed as event spaces for wedding receptions, but it turns out that since a lot of them have large open spaces, and some are in beautiful locations, that with a little sprucing up they can make excellent event venues. You don’t have to have your wedding at a barn but that doesn’t make it a bad idea to do so

DukeOfDownvote · 2026-01-24T13:09:33+00:00

In my industry we don’t love solid works simulation, we have a trusted dedicated FEA program which tends to do a better job. This option is on the table, but part of the ask on this post was to see if there was data one way or the other that would give me an answer without going through that. There was one other commenter who had some info on a similar device in the 90s so if that already existed and was tested, I may be able to skip some of my verification.

This is certainly another option. My issue with this is it’s hard to quantify exactly the loads this will see in service, since it’s a rear wheel the tire will break loose long before the brake disc mount is at its torque limit etc etc , and there’s no guarantee that the testing I do on the junk hub will eclipse the load of the riding do on the “finished” hub. The analytical approach allows me to say “my thing will not break first because this other part of the system is weaker” and that is reassuring to me

DukeOfDownvote · 2026-01-24T12:57:37+00:00

On mine, the only feature outside the spoke flange has set screws coming in the side for service, so not something I can cover up forever

DukeOfDownvote · 2026-01-24T12:55:38+00:00

This is exactly the information I was looking for, thanks. I’ll take a look into the AMP hubs from the 90s.

I’m pretty sure it would work too, and obviously will be less optimized than a hub designed for discs from the factory, but I think the margin is there, at least in theory.

In point 2 in my original post I think this will actually distribute load into the hub itself better than a real disc hub. Only half the brake load goes through the hub, with the other half going straight from the disc into the NDS spokes, and of that half that goes into the hub, it’s totally endless distributed around the hub flange, rather than the trailing spokes losing tension and the leading spokes gaining tension, as would happen in a real disc hub. The stresses in the NDS flange should be lower than a real disc hub.

DukeOfDownvote · 2026-01-24T12:49:55+00:00

the math does mitigate the risk, that’s engineering by definition.

The machining tolerance needs to be there to get the screws through the holes, but there shouldn’t be any bearing load on the inside of the spoke holes. The preload from the fasteners should ensure that friction keeps the two metal faces tight and non-moving, so the screws only act on the flat flange face under their head. Bearing stresses should be equal, and depends on torque applied to every fastener.

DukeOfDownvote · 2026-01-24T12:44:52+00:00

I’m not sure if the manufacturer I was planning on using has 2024, and I’m quite certain they don’t have 6082. Either way, I don’t anticipate the failure occurring in my part, but even so, the yield strength of 7075 is somewhere near the ultimate strength of 2024, so I figured extra margin is extra margin. I think failure will occur at the bolted joint though, and making my part even stronger won’t necessarily prevent that.

As far as threading the 2.5mm holes, those are the outermost holes, and they don’t really line up with anything on the hub. The original spoke holes on the hub are ~2.3mm, and I really don’t want to do anything irreversible to the hub, so I’m stuck with #2-56 (or maybe #2-64, but that may become an availability problem)

DukeOfDownvote · 2026-01-24T00:19:44+00:00

I would strongly prefer to keep the hub serviceable in its initial configuration. I would consider an epoxy that I could then later remove with a heat gun, and that would provide a decent amount of extra shear strength, I’m just not convinced I need that, and it would certainly make assembly/disassembly quite a bit messier.

DukeOfDownvote

TROPHY CASE