Question about unicycle frames with a disc tab for internal rotor

My unmodified D’Brake broke on a ride yesterday:

I wasn’t completely surprised because I’m extra hard on my brakes (since I ride freewheel unicycles) and I’ve heard about this happening before, like reported in this thread by Jim Sowers on the facebook group. I have a backup ready but I should have been more proactive about checking it for cracks because I don’t know if it showed any before the break. I know a few people have welded on their own disc tabs but I was wondering if there are any stock unicycle frames with a disc tab for an internal rotor. I’m considering getting a custom drift trike front end.

Hi Waaalrus,

I hope it didn’t end up in a bad UPD :confused:

As for the unicycle frames with disc tab, there is the following ones (AFAIK):
=> QX-series disc frame: internal disc situation but not sold outside a complete unicycle
=> KH disc frame: external disc situation (but I think it should work with internal one as long as it is on the same side)
=> Nimbus Oregon frame: internal disc situation but with a bearing-to-bearing width of 125mm instead of 100mm
=> Triton frame: can be whatever you need as they are custom but you need the $$$$$

Basically, you have some choice even if the offering is still scarce because it is still getting momentum.

Thanks for the summary! As you already know, but for completion’s sake, I’ll add:

=> Jakob F.'s custom aluminum fat frames: in the prototype stage

The KH frame looks only good for external rotors and all the new Oregon frames seem to use the D’Brake although there may be some out there with the disc tab. QX looks really interesting. I may see if there’s anywhere to get a frame. That would be the quickest fix for me. There seems to be traction on Triton lately and maybe if they release a batch the price won’t be as high as for a custom frame. Titanium sounds really interesting to me because I’m concerned that aluminum is not a good fit for the kind of stresses I put on the frame. I also find that a 160mm rotor may be insufficient to my needs and a custom frame would let me try something bigger.

The UPD that resulted from the break was surprisingly mild. I ride around trees and cliffs and get up to decent speeds so I’d like to avoid a similar situation in the future where the results may not be as good.

Unfortunately I don’t think that QX disc frames can be purchased as parts anywhere, but if you do find a spot please let me know where. (I believe somewhere someone posted that they even email QX about it and they said that they will not sell the disc frames).

properly installed?

What I see in the photo is a gap between the d’Brake and the bearing housing. Did you loosen the screw before taking this photo or did you use the d’Brake with this gap? If you used the d’Brake with this gap, then this might probably have been the reason for it’s breaking. This would have terribly increased the tensile stress at the point where the crack started.
Does your frame have a plane surface at it’s lower side, respectively does it have machined bearing housings or such ones made of sheet metal? If they are made of sheet metal or otherwise offer no planar surface you’ll neet an adapter like Hugo or me did it here and here. Without the d’Brake being screwed gapless to a planar surface the brake force will not go into the frame as it should but will go over the whole d’Brake and into the bearing.

Thanks for pointing that out and the links to those other threads! I didn’t do a visual inspection before the ride so it’s possible the bolt somehow got loosened over time but it’s more likely that the gap showed up after the break since the bolt is mostly exposed now. The frame has machined bearing housings and that side was originally tightened down flush with the frame. It gives me something to check before a ride in addition to search for cracks.

Just curious why you’re wanting to go to a completely different frame with disc tab? IMO, that’s a D-brake working as designed.

If your really hard on your brakes I would stick with the D-brake adapter cause it’s a lot cheeper to replace than a frame mounted tab.

I’m not sure what you mean. It may be that it doesn’t work well for the forces I put on it and the frequency with which I apply them. I don’t fault the designers since I’m using it in a way that probably wasn’t anticipated. I’ve proven I can break it in a relatively short time so want to try something else.

I’m not concerned about the price but am more concerned about the material. If there was a steel D’Brake I would give that a try. I’m having someone weld a steel disc tab to a steel frame and we’ll see if that lasts more than 3 months. I’m also setting it up for a 203mm rotor which may extend its life.

Kool. I can get that, 3 months is a tad short. a steel d brake would be nice.

Hope the frame comes out nice.

I’m sure you could get a machinist to make a steel one for you. It might be a bit pricy though.

I’m not hard enough on mine to warrant that (my d-brake sees road duty, my muni’s tend to be born with Brycers maggies), but I do like steel stuff :).

For sure, producing this in steel shold not be a problem. And the price should not be the problem. It might even be cheaper than buying two original d’brakes. And you save costs in damaged rotors. So in the end, investing in one steel d’Brake is more economic than buying new d’Brakes and rotors every 3 months.

Heres a tip I have for you: a big part of the production costs will be programming costs for the cnc and set-up costs for jig and fixture. So, the more they produce, the cheaper it gets. Order 10 or 20 pieces of it and sell them here on the forum. I’m sure that there are enough riders that are interestet in this.

These are great ideas although I am mechanically-challenged. I broke a crank puller one time trying to get my cranks up and there’s always a chance I’ll pop a tube changing out a tire. In the short term I’m going to have someone weld a disc tab on a steel frame.

Jakob F. mentioned something about the properties of aluminum and I had a sudden thought about another thing that’s been troubling me related to this incident. My 26" was on an aluminum Oracle frame and I’ve ridden it at least as much as my 24". However I had never had as good braking as on my 24. I first thought it was worn out pads then pad composition. I got everything the same including the brake lever and still saw the difference. This morning I swapped out the Oracle frame for a steel Nimbus one and the braking was noticeably better. The aluminum flex may account for this and may also mean the D’brake is more likely (or maybe sooner) to break when used with a steel frame, at least when subjected to the large and repeated forces I exert on it.

The only plus side I can see from the d’brake failing is that it failed where it was designed to. If the bit that snapped was any stronger the weakest part of the d’brake would be right in the middle of the bearing housing.

As much as a steel d’brake would be different. Im not sure how much of a problem the natural flexibility of steel would pose. On a frame mount, the steel is completely buttressed against the frame. With the right angle bend, the steel would flex repeatedly making calibration a nightmare and possibly leading to fatigue failure.

If you have access to a machinist, i would suggest trying to get a brake tab retrofitted onto the frame. Unfortunately as Roger has said recently, setting up a frame tab is a nightmare even with the more accurate spacings provided by external bearings.

Steel is actually less flexible than aluminum, and has better fatigue resistance. People get confused by that fact that aluminum frames usually use oversized tubing - bigger diameter tubes are stiffer - and it makes for a stiffer frame despite the more flexible material. But for the same size piece, steel is stiffer.

I haven’t had any problems with my d-brake, but it looks like a fatigue failure waiting to happen, with that sharp, drilled-through elbow taking the braking force. I don’t see any reason that having it snugged up tight to the frame, or not, would change that. Having a gap would mean the bearing takes the braking force instead of the frame (bad for the bearing), but either way the braking force runs through that elbow.

But even cheap wal-mart bikes manage to do it.

Like I mentioned in a previous post I can attest that I can feel the effect of the flex difference between steel and aluminum on braking and much prefer (and require) the stiffness of steel.

I had a frame tab welded to my old Nimbus steel frame when I got mountainUni disc brake. And it was before they created UCM so welding on was the only option. Once you have your brake on, then correct positioning is not that hard.
I have an the old mountainUni welding instruction, so PM me if you want a look at it.