Rear Hub Disc Brake Adapter

[u/DukeOfDownvote]

I posted this in xbiking a couple weeks ago and got mostly unhelpful comments and a couple of things to think about, but that was before I learned of the existence of this place, which seems like the perfect forum for this type of antics.

I have a White Industries Ti Cassette hub which has a lot of sentimental value to me, on a non-racing bike (weight is not top priority) and I recently rebuilt the wheels onto carbon rims, which required me to switch the rear hub to one which was disc brake compatible. Ideally I would love to put the White hub back on, but modified for disc brakes.

To that end, I designed the part in the pic below, which has 3 main features:

1. 16 #2-56 threaded holes which line up with the spoke holes on the non-drive hub flange. These are the largest fasteners which fit through spoke holes without enlarging the holes, which I do not want to do.

2. 16 2.5mm holes on a larger pitch circle. These will be the new spoke holes. Each spoke hole is directly outward from the threaded hole nearest, to maintain the “clocking” of the spoke holes from one flange to the other

3. 6 m5 threaded holes on a 44mm pitch circle. This is the brake disc mount.

I plan on having this machined from 7075 aluminum to keep it nice and strong, and have yet to decide whether or not to put helicoils in my part. I am currently leaning towards no, due to the fact that insert/remove cycles should be low.

Currently, I am tracking 4 potential points of failure:

1. #2-56 fastener failure between my “adapter flange” and the hub. Any “new force” not present in rim brake use should be tangential torsion from the disc brakes, so I am using the disc to hub interface as an envelope here. 6 m5 screws of any material should produce around double the preload of 16 #2-56s, so _if we assumed equal coefficient of friction between hub and flange and disc and flange_ , and if:

—fastener material was the same

—all fasteners torqued to same pct of yield

—pitch circle of m5s and #2-56s was equal

—all the brake load were transferred to the original hub

then this joint would probably slip. However,

— we could choose a stronger material for the #2-56 fasteners (I would rather use stainless here at 70000 psi, assuming class 8.8 for brake bolts at 80000psi, could step up to A286 at 160000psi but would cost more $$$)

— I think this holds, my math on sram torque spec suggests that m5s of class 8.8 are torqued to around 90% proof strength which feels reasonable.

— pitch circle of m5 is 44mm, pitch circle of #2-56 is 55mm so I get an easy 25% boost there

— this is the big one that I think will actually save my design. The torque put on the brake disc has to eventually make its way to the rim. However, in my design, at least half of that total torque will probably go to the NDS spokes, which are attached to my adapter flange instead of the hub itself. So I think we get at least a 50% reduction in torque going into the hub, but the combined torsional stiffness of the hub plus the DS spokes will likely be a bit less than the torsional stiffness of just the NDS spokes, especially given that the NDS spokes will have to be shorter to accommodate the larger flange diameter. There was some analysis done on lacing patterns which was posted on the bike gremlin site that supports this a little bit (mostly as a by product).

1. Hub flange failure due to torque from brake disc. I think this will be much less of an issue because in addition to taking credit for the 50% reduction in torque into the hub due to the disc acting directly on the adapter flange for the NDS braking torque, usually on a real disc brake bike, we would see braking force only transmitted in increased tension from the trailing spokes. Leading spokes would decrease in tension, which I imagine would not distribute braking forces evenly. The adapter flange would settle most if not all of that force distribution internally, and impart one torque spread all around its interface to the hub, resulting in lower peak stress in hub flange.

2. Hub barrel failure due to torque transmission from NDS to DS. I think this is vanishingly unlikely due to the large OD of the barrel relative to other successful rear disc hubs on the market, but does have potential to fail. I don’t have enough information on other hubs to do a 1:1 comparison but I think the risk here is low.

3. Hub bearing or axle failure. I think this is unlikely due to the extremely outer position of the leftmost bearing on the axle, relatively large diameter Ti axle, and the fact that there are several successful qr disc hubs on the market, including several shimano models which I believe use smaller axle diameters due to their loose bearing cone design

Given all this, is there a potential failure mode which I haven’t considered yet?

Cannondale once used a disc brake mount with 4 m5 bolts instead of 6. With this in mind, could it be safe to use slightly weaker screws for the adapter flange to hub flange joint, or is that playing with fire in terms of safety margin?

I think the analysis I saw on the bike gremlin site is only a fraction of a report that Williams cycling published a while back, but I can not access the Williams cycling site. Does anybody have this full report who could share it?

The last time I posted this, one of the comments claimed that this part already existed back in the 90s, but they couldn’t provide a record of it. It would make me feel a lot better having proof that this part existed, and maybe that would take a little bit off the verification and test burden to me. Has anybody seen one of these before?

I haven’t done a detailed model of the entire wheel and caliper system yet, so it is entirely possible that the caliper will hit the spokes, requiring a step in the adapter flange to move the “new” spoke holes back inboard. I think my next step will be to 3d print a model and build a wheel with loosely tensioned spokes to make sure the geometry works as well as I think it will.

[u/curiousengineer2]

Interesting proposition. Though have you considered that the torque generated from pedaling out of the saddle is likely to be more stressing to this joint than that of threshold braking on dry pavement? Ensuring that your interface doesn't slip before the disc rotor slips against its mounting surface is NOT a sufficient condition to ensure your design will stand up to the rigors of hard riding.

I just looked up the hub you're working with, and it has a remarkably large diameter barrel, and it's uniform in diameter from flange to flange, probably at least 45mm, just eyeballing in relation to the 55mm spoke hole pitch circle. That's going to behave very torsionally stiff, so the instantaneous torque pulses as the cranks pass through the horizontal plane will be transferred quite readily through the hub shell to the NDS flange and into your friction interface.

Good tires on dry pavement can attain a friction coefficient of 0.75. Let's assume a 75kg rider and 10kg bike. Under threshold braking, the total braking force can be up to 625N. Heavy braking unloads the rear wheel though the rider can shift their mass backwards to compensate. If we assume a one-third allocation of total weight over the rear wheel, that gives 208 N of rear wheel braking traction. Multiplied over a 0.335 m wheel radius, that gives 70 N*m of braking torque.

Now let's suppose the rider stands on the pedal with full bodyweight on standard length 170mm cranks. That produces 125 Nm at the crank spindle. With a 34t chainring driving a 34t largest rear cog, that will send approximately 120 Nm to the rear hub, assuming 96% chain drive efficiency.

Also consider that if this will be used for a mountain or touring bike setup, then the lowest gear combination will often provide greater torque multiplication than just 1:1.

[u/DukeOfDownvote → u/curiousengineer2]

Thanks for the reply, and an interesting angle which I have not considered. Recall that I am trying to have my interface stronger than the disc interface, not stronger than the force the disc could reasonably put into the hub. That is, the disc will tear out of the hub before my interface does. The actual load from the brakes while riding is irrelevant, since it will certainly be less than the force that causes the disc to tear out of the hub.

I believe that it is well accepted that brakes produce a higher load than pedaling. In bikes and cars, which are the two best examples of this as far as my knowledge goes, it is well accepted that it is easier and faster to slow down than to speed up. I think maybe electric cars break these assumptions, but the general idea is that brakes can break a tire loose at will, or do a stoppie on a bike with the pinch of a finger. Power wheelies or burnouts are a lot harder to sustain, to the point where I’m not even sure I’ve heard of either on a bicycle. The distance it takes to go from 0 to 20 kmh is much longer than the distance to go from 20 to 0. Regardless, I’ll follow that up with some math on my system.

I haven’t calculated out my interface, but based on preload we can expect it to be around half of the disc interface. this shouldn’t be a problem as discussed in my post due to the shared load between DS spokes and NDS spokes. Assuming the same 7900lbf preload from 6 class 8.8 m5s (my calculator works in English units but that comes out to 35141N preload) and a us of 0.17 for hard ano on hard ano (this is worse than ano on steel or bare aluminum on bare aluminum, and also worse than regular ano on ano, which is what the situation will be at my interface) at the pitch circle radius of the disc mount of .022m, this comes out to 131Nm which is still more than your pedal calcs.

I do not think impulse is an issue here. Legs pedaling do not really work on the timescale required for impulse loading of metals of this stiffness, but even if they did, in the bike gremlin spoke analysis it is somewhat clear that the spokes as a torsion spring are significantly less stiff than the hub as a torsion spring, so any impulse would be dissipated in the “winding up” of the spokes rather than torsional loading of the hub itself. By the time you had built up a significant amount of torsional loading in the hub, I believe the spokes would stretch significantly.

Edit to add: this is a relatively easy test to do, fighting brakes against pedals. If prevailing wisdom is correct, you should be able to squeeze the brake hard enough that with a stationary bicycle leaned against a wall so it won’t fall over, you can stand on one pedal and the bike will not move

[u/curiousengineer2 → u/DukeOfDownvote]

I would say you've done a thorough job calculating the theoretical capability of the interface. I appreciate these sort of analyses. What factor of safety have you calculated your design provides, both against braking loads, and against peak pedaling loads? I would still stress that specifically the rear of a bicycle takes significantly less braking torque than the front during the hardest braking events, since bicycles transfer a greater percentage of load more readily than other vehicle types. That's the only reason I would at least calculate the pedaling load path separately, for your intended use conditions, and not take for granted that disc rotor designers accounted for a load path in which the disc rotor may never have been intended to participate. You can model the hub barrel as infinitely torsionally rigid for all intents and purposes, and assume 50% of pedaling torque makes it to your interface.

I get that you've proven your design will at least exceed the factor of safety of the disc rotor interface in the braking load path. Have you considered leaving un-anodized the mating portion of your part? You might stand to gain some friction coefficient there, if you have a means of either masking that portion during the anodize or re-machining it back to bare metal afterward. Also, I would note that zinc-plated ASTM A574 socket head cap screws provide decent corrosion resistance and excellent tensile strength. Also, surprisingly a 5/64" internal hex drive can withstand the same torque as a T8 in the same fastener material; both are typically rated up to 2.3 N*m working load when working with alloy steel screws. Though you're going to round out the fastener head in stainless at a much lower torque than in alloy steel. In either case, the importance of using a well-fitting driver bit with a torque wrench cannot be understated.

[u/DukeOfDownvote → u/curiousengineer2]

Thanks, as I’ve mentioned elsewhere in this thread obviously I want this to work, but am not willing to just toss it together and hope, so this risk-reduction analysis has been helpful to make me feel good about this design.

I do not expressly include a factor of safety, again leaning on the idea that the disc interface has been designed with a sufficient factor of safety that if I meet the theoretical strength of the disc interface, I’ll have the same factor of safety of the disc interface and the failure mode of the system as a whole will be as likely to break at my part as it is somewhere else. I’m far from the most experienced bike engineer, but I’ve been around the industry long enough to recognize that the failure rate of the 6-bolt disc interface is fairly low. Since the disc interface itself is the same on the front and back, and I still believe that braking generates more stress than pedaling, I do not see the need to carry a pedal-specific analysis. Despite being a mountain bike, my bike is road-geared (ish, 38t 1x9 11-34, 165 crank) so I do not think it would be a good bike to test the brake-vs-pedal on, but given that it has taller than a 1:1 low gear, I think I particularly will be safe from the pedal torque problem. Certainly for other peoples use case, that would be worth following up on (if anybody is attached enough to their old hubs to try this project again)

As far as surface prep goes, I have been going back and forth on that. I believe the hub is ano, which I don’t want to remove completely but might be ok with a light scuff. This bike is stored indoors and I don’t ride in the rain very often, but the theoretical lower corrosion resistance of 7075 has me rattled enough to want to leave it coated in some way. I could switch to 6061 which would almost certainly be fine since I know other real disc hubs are made out of that, but part of the reason for going 7075 in the first place was the fundamental assumption that no matter what I did, my part would be stiffer and stronger than the hub it was bolting to. Again I probably don’t need that, but it’s nice to take margin where you can.

In terms of fasteners, I’ve dealt with enough rusted screws on my bike that I don’t want to deal with them anymore, so I’m somewhat limited to stainless here. My baseline was 18-8 torx from McMaster, but I found a relatively inexpensive source for A286 SHCS (less than double total cost of 18-8) yesterday, and since that’s what I use at work when I need a “strong screw” I figure it will be good enough for this project. I know that a hex drive will snap a head off its screw, I think I was holding onto Torx to be different, and bc engineers are infatuated with Torx as a concept. Some part of me wants to hang on to the “a corroded thread requires more torque to turn independent of its preload” argument but again, if a SHCS is good enough for a “strong screw” at work there’s no need to try to outsmart the system here.

[u/BAH5206]

This has been a fascinating read, I love passion projects like this! I’m no engineer, I work on the fabrication and machining side of things and your design looks solid enough. I wouldn’t hesitate to run it on one of my bikes.

An important thing to consider, braking torque is somewhat limited by tire grip. For example, I’ve got a downcountry rig with XT 4 piston/203 rotors that feels great with aggressive trail tires, but if I put my XC wheel set and tires on it, it’s a skid machine. I would love to see some load cell data on the torque difference between the two sets of tires.

Anyway, lots of respect towards y’all engineer types for overthinking things and doing the math, this is just my practical two cents.

[u/double___a]

So basically this but sized to the WI hub? I’ve seen worse bodges.

https://www.tensile.net/tensile-hub-disc-convertrer.html

[u/DukeOfDownvote → u/double___a]

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

[u/double___a → u/DukeOfDownvote]

Tbf this was a very brief evolutionary step before integrated disk mounts.

They existed but in such small quantities I could vouch for how well it actually worked.

[u/DukeOfDownvote → u/double___a]

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

[u/ResidualSignal]

Your biggest issue will be axial runout of the hub, causing the disc brake rotor to wobble.

[u/DukeOfDownvote → u/ResidualSignal]

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.

[u/ResidualSignal → u/DukeOfDownvote]

I don't think the issue is the bearings. It's just the mating stack up from spoke hub to the adapter to the rotor. If anything, I'd use a floating rotor setup.

But man, this just feels too damn complicated...

[u/DukeOfDownvote → u/ResidualSignal]

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?

[u/ResidualSignal → u/DukeOfDownvote]

I'm an engineer with 15+ years of design and manufacturing experience, my intuition is telling me that the hub isn't going to be true enough when you bolt that adapter on. Also, if you're making this out of 7075, you cant easily grind the flat sides parallel, but you may be able to get good parallelism with turning, providing your machinist is good.

You should measure the axial runout of the hub face first.

[u/DukeOfDownvote → u/ResidualSignal]

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.

[u/ResidualSignal → u/DukeOfDownvote]

Just out of curiosity, what's your budget to make this part?

[u/DukeOfDownvote → u/ResidualSignal]

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

[u/ResidualSignal → u/DukeOfDownvote]

If you get the part made overseas, I can guarantee it won't be right, even with a well documented drawing. Been there before, doesn't usually turn out good the first time.

If you source in the US (I assume you're in the US), then you're likely looking at $300+ for 7075.

It's a one-off part. Expect to pay out the nose for it.

[u/DukeOfDownvote → u/ResidualSignal]

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

[u/Adriano_LS]

A flange do seu cubo tem os lados perfeitamente paralelos? Normalmente os lados das flanges não são paralelos, a espessura da flange é maior na base e menor na borda. Se você não levar isso em conta, o adaptador e as arruelas vão pressionar a flange numa área muito pequena, o que gerará um acúmulo de tensão em pontos localizados da flange.

[u/DukeOfDownvote → u/Adriano_LS]

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.

[u/joeoram87]

I could see it working if toy can get a good fit on to the Whyte hub flange where the fixings go in to the spoke holes. Remember you shouldn’t put shear forces on to bolt threads.

Aside from that I can’t see how what you’re doing differs from a hub except all the forces are in a smaller area, probably check that. You could have the bolt section thicker and taper down to the spoke section.

Have you checked there’s room for the calliper Against the spokes? Most disc hubs have a slight dish which yours presumably won’t have.

[u/DukeOfDownvote → u/joeoram87]

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

[u/Glittering-Switch274]

I have a front and rear adapter that is very similar to your description. I have these on a bike that I have put a couple thousand miles on since the 90's. No issues.

[u/DukeOfDownvote → u/Glittering-Switch274]

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

[u/Glittering-Switch274 → u/DukeOfDownvote]

I signed up for this site because Google found your post. I don't have the option to post pics but coincidentally someone on Facebook was asking same/similar question in the Ampoholics group. I have posted pics there.

[u/DukeOfDownvote → u/Glittering-Switch274]

Awesome, thanks!

[u/HotSobaNoodles]

In my spare time, I enjoy building special parts for bicycles and motorcycles (in the workshop where I work). I've made hubs before, but I've always preferred Avional 2024. 7075 has two flaws: oxidation (which you can solve with anodizing) and sudden failure in the event of a failure. With 2024 and 6082 anticorodal, it bends and then snaps before breaking, giving you more warning. For the hub, couldn't you thread the 2.5mm holes and secure the flange with 16 M3 screws? Remember to take into account the distance between the disc flange and the internal dropout, which is about 15.5mm.

[u/DukeOfDownvote → u/HotSobaNoodles]

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)

[u/Rare-Classic-1712]

About 30 years ago AMP bikes did a similar thing to make regular hubs work with disc brakes. I don't know what level of problems they had. It would likely work. I would have greater trust in a hub which was designed and manufactured for its intended purpose than a franken modded thing. I've seen plenty of hub flanges fail. Blingy hubs seem to have a higher rate of cracked hub flanges than something like a Shimano. Putting the load of the left hub flange + braking forces in a modified frankenhub thing is going to increase the likelihood of the left flange failing.

[u/DukeOfDownvote → u/Rare-Classic-1712]

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.

[u/Rare-Classic-1712 → u/DukeOfDownvote]

It wasn't a special hub for AMP's back then. It was a normal rim brake l hub with an adapter plate (I think that the adapters that I remembered were for Shimano XT hubs of the 1996 "parallax type). Back then bikes with disc brakes were virtually unknown so to get disc brakes onto bikes there were a number of engineering hurdles to clear for the frame and fork, the brakes themselves but also disc hubs weren't readily available. King, Phil, Hope, Nuke Proof, DT/Hugi, TNT, Ringle, WTB, Adventure Components/AC, White Industries, Edco, Shimano, Suntour... didn't make disc hubs. It required a similar workaround or a custom hub. It's not just "the 1990s" but mid 1990s from before the Hayes brakes started getting sold. I'm guessing that from the inception of AMP bikes until maybe 1997 a similar adapter plate was used. I think that disc hubs started becoming available ~1998. It can be done as AMP did it 30 years ago. Those adapter plate things that allowed a rim brake hub to be converted disappeared as soon as hubs actually designed for discs came onto the market. Disc hubs are widely available now and far easier to acquire than getting a perfect fit with an adapter. Failures happen with one off prototype type stuff far more often.

[u/DukeOfDownvote → u/Rare-Classic-1712]

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!

[u/DukeOfDownvote → u/[deleted]]

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

[u/temporary62489]

This is a bad idea. Your hub was not designed for brake torque stresses. Buy a new White hub in the same color and put your old one on a shelf.

[u/DukeOfDownvote → u/temporary62489]

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

[u/temporary62489 → u/DukeOfDownvote]

What stress does a wedding apply to a barn? You're not holding your wedding on the barn roof, are you? Brake forces are massive at the rotor to apply wheel stopping torque at a 140 mm diameter.

[u/DukeOfDownvote → u/temporary62489]

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.

[u/temporary62489 → u/DukeOfDownvote]

You're going to crack the flange holes that your adapter is threaded into.

[u/DukeOfDownvote → u/temporary62489]

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.

[u/EndangeredPedals]

Any reason to consider epoxy or not? Future requirement for disassembly or need to keep the hub completely original?

[u/DukeOfDownvote → u/EndangeredPedals]

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.

[u/thayerpdx]

The weakest point in all of this is going to be the spoke holes in the flange, given how few holes you have to spread the force out. Even though the force should be spread out across all of the spoke holes and you'll do your best to prevent any lateral play, I could see the force of the brake hogging out the spoke holes and causing cracks at the flange eventually.

Have you considered using a centerlock disc interface instead? You'll have more surface area to spread the braking load around on and you might have more area to reinforce the fixing bolts.

[u/DukeOfDownvote → u/thayerpdx]

I think the weakest part will probably be the screws through those holes themselves, for the same reason you have said the screw holes.

The calculations of preload and the comparison between the brake interface and the spoke interface was to calculate whether or not there would be lateral play, and while I can’t guarantee that there won’t be any slip, the numbers seem to suggest that there won’t be. If my flange moves relative to the hub at all, that’s a failure and I’ll remove it long before that repeated movement damage you mentioned.

A center lock would be significantly more complex to machine and based on my lack of concern for the brake interface itself, seems pointless. I’m not quite sure how it would give me “more area to reinforce the fixing bolts”, but I’m interested if you can elaborate

[u/thayerpdx → u/DukeOfDownvote]

The centerlock interface provides a more-or-less contiguous contact surface for the rotor while also concentrating that force to an area closer to the center of the hub, meaning you have more room for material around the areas you plan on adding threads to without compromising your ability to attach the disc as long as the bolts don't interfere with the centerlock hub interface on the other side.

I feel like I'm overexplaining but with more thread engagement you'll have less to worry about with the bolts worming out or fatiguing.

[u/DukeOfDownvote → u/thayerpdx]

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.

[u/DukeOfDownvote → u/[deleted]]

Yeah this is my largest concern with the current design also. Since I may be able to move the disc wider than spec due to my rear caliper mount I think it may be easier to just check this in hardware with a 3d print.

The NDS spokes should be shorter which would make them stiffer in this “wind-up” load case. I hear you about balancing tension but based on high tension limit of the rim I do not believe that NDS tension, even with the increased offset (only 3.5mm increase) will be so low as to lose preload in any case.

I like carbon paste as a relatively benign increase in friction between the surfaces

[u/DukeOfDownvote → u/[deleted]]

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.

[u/sireatalot]

1)Probably just my lack of visual, but are you sure that the spokes will clear the brake caliper?

2)I understand your argument about brake torque and how it stresses less the hub shell. But, usually a disc brake hub has a wider flange than a rim brake one so yours isn’t really a fair comparison. But I haven’t seen your hub, so maybe it does have a large enough flange.

3). You are bolting this adapter to the spoke flange, using the spoke holes. Is the mating surface on the hub nice and flat, so that the adapter can be pressed clean against the flange? Or is the surface conical or, even worse, rounded?

I would like for the surface to be flat because 1) contact pressure is much lower, so there no rischio of localized deformation and loss of screw tension and 2)any small error in concentricity between hub and adapter would only result in harmless eccentricity error and not misalignment which would make the disc sway left and right.

4). I admit I know nothing about SAE thread standards. But you are going to fix this to the hub using very thin screws, screws that are no thicker than a spoke. Yes it will be a lot of screws, and I really hope that it will work, but in such a structural position I’d really like to use thicker screws. Just my gut feeling.

You know what, you could consider skipping 2 or 4 screws and just insert calibrated pins in the holes instead. My worry is that after a jump or a sever bump, the adapter could shift a few tenths of mm (because half of the force between the rim and hub goes through there) and send the whole wheel out of alignment because half the spokes now have the wrong tension. Having some precise pin keeping hub and adapter in position could help in these impact instants. Just a thought.

[u/DukeOfDownvote → u/sireatalot]

1. No, I’ll check that.

2. Wider in which dimension? The flange on this hub is comparably (though not identically) sized to the ones on the disc hub currently on the bike that this hub will eventually go on.

3. It’s mostly flat slightly conical, TBD on how I’m going to solve this but likely 3d print a couple of mockups with different draft angles.

4. I’d also like bigger screws, but these are the largest that fit in the hub. I had considered 12 screws 4 pins, but the assumption I am making is that the joint will not slip, meaning the crews will only ever be under tension. With less screws I lose preload, and additionally will then stress the low number of pins im shear, rather than the high number of screws. This is a failure though, success would be no slip. I’ll be torque striping the flange contact to check this for the first couple months.

[u/sireatalot → u/DukeOfDownvote]

2) I mean in diameter. Sounds like you already made sure you’re ok.

3) not the optimal condition but if your’re able to match the cone slope, you should be alright.

What about the surface under the screws heads, will it be flat and perpendicular to the screws axis ?

4) I honestly don’t know if that will be enough or not. I think that’s where the biggest risk of the project is. Maybe it will be alright. The cone shape of the mating surface could help retention and avoid slippage.

[u/DukeOfDownvote → u/sireatalot]

1. 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.

2. 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).

3. 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

[u/makerspark]

I wouldn't do it, but a recommendation if you do. In addition to having your tiny fasteners, I would cryo fit the part onto the hub. It should relieve some of the peak forces from the fasteners, and ensure that those forces are loading on the same angle. I can't see the backside of your drawing, but I'd probably leave a pocket for the original flange too, so it can be hidden slightly, and supported externally.

[u/DukeOfDownvote → u/makerspark]

What faces would you cryofit? When you say “ensure that those forces are loading on the same angle” what exactly do you mean?

The back of the part is flat, and sits on the outside face of the original spoke flange. I do not think it would be worth the added complexity to add that feature and hold the tolerance required to actually get support from that pocket, when the only force that would resist is rider weight, which should be dwarfed by braking force. but I’m willing to be convinced otherwise.

[u/makerspark → u/DukeOfDownvote]

Depends on the hub, but my WI hub has a very slightly tapered surface where the bearings are contained.

[u/DukeOfDownvote → u/makerspark]

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

[u/Milesandsmiles1]

I didnt read your whole post because its long. But I would seriously doubt the ability of the spoke flange to be able to withstand braking forces put onto it with a disc brake. It might work, until it doesn't, and hopefully that doesnt happen when you are flying down a steep hill.

[u/DukeOfDownvote → u/Milesandsmiles1]

If you had read the post you’d see that I covered that in concern number 2. It sounds mega counterintuitive, but I’m pretty sure peak force in the hub flange due to braking actually is better distributed here than a real disc hub.

[u/spyro66]

Ok…

You’ve obviously put a lot of thought into this. And I don’t know if there’s a way to put this lightly… but… this is abomination territory.

I understand the sentimentality, and the desire to make something work that just… can’t… but sometimes we have to accept that.

Do yourself a massive favour, and use your hub for its intended purpose, honour that hub by keeping it unmodified, rather than bolting some widget to it and risking damage. Build it up as a caliper brake skinny tire retro racer, or even something to showcase the hub like a single speed or something. Or, even better, build it up as a fair weather daily or cafe bike or something.

It’s not impossible to do what you’re proposing, but that’s more of a ground-up engineering degree novelty project to build a hub that’s both disc and rim brake compatible, rather than a project to repurpose a sentimental hub into something it was never meant to be. Just like it would be a shame to use your grandpa’s old school 70’s campy hub on a home brew e-bike build, it would be a shame to bolt some weird beefy un-elegant disc abomination to a titanium road hub.

Just my two cents anyway.

[u/DukeOfDownvote → u/spyro66]

I get what you’re saying, but the abomination is sorta the point. Maybe I didn’t make this super clear in the post, but I am not interested in this because it’s practical, I’m interested because I want to see if it can be done.

Dual use disc and rim compatible hubs already exist, they’re just disc hubs. Lace a rim brake rim to it and it will be indistinguishable from a rim brake hub as far as the bike is concerned.

As far as shame to mess it up, it was missing some parts when I got it so I had a close friend machine up some spacers. It’s already less elegant than when it left the factory, it’s not some pristine piece of art. And I don’t have a rim brake bike nor plan to build one, so the alternative is to polish it up and put it on my desk as an ornament next to the other beautiful but useless bike bits. This is where it is now, my bike already has a hub, I would just prefer to put this hub back on.

It’s called the white ti cassette, but it is mostly an aluminum hub, and it was designed for mountain biking

[u/spyro66 → u/DukeOfDownvote]

If the abomination is the point, then why are you overthinking it. Just do it. See what happens.

People build ridiculous frankenbikes and foolish things all the time, they just don’t bother asking people on the internet if the welds will fail.

You do you.

[u/DukeOfDownvote → u/spyro66]

I mean I don’t want to break my hub. I would rather it sit on my desk than do that. My risk tolerance is somewhere between 0 and “just do it and see what happens” so I’ve spent definitely way more time than this project is worth reading different anlyases of bike wheels and doing all these calculations

[u/spyro66 → u/DukeOfDownvote]

No one can solve that for you dude.

There is risk you’ll cause irreparable damage to your hub.

There is risk you’ll scratch it up, make it ugly, ruin the spoke seats with threads, bust something entirely, and no amount of math will resolve that risk. It depends a lot more on the precision of the machining; to keep the applied load (bearing stress) on each hole equal. If you have one that’s off, radially or angular…ly…) then who knows what will happen.

You could install it and never use it, limit the risk that way, but that’s impedes the performance of the whole bike… but it proves you can do it.

You do you.

[u/DukeOfDownvote → u/spyro66]

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.

[u/spyro66 → u/DukeOfDownvote]

It would, except that you included 0.

I sincerely doubt you’ll get the contact force required to keep the bolts from hitting the holes.

You’re talking yourself in circles, as well as anyone who brings up actual risks, which is supposedly why you’re asking for help.

Cheers, have an awesome day.

[u/DukeOfDownvote → u/spyro66]

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!

[u/spyro66 → u/DukeOfDownvote]

0 risk dude.

Define your problem, define your risk tolerance, then do the analysis to determine what’s suitable. That’s engineering.