Ok, so physically the KH/Onza hub fits the frame, but as the bearing holders on the KH are bigger than those of the UDC hub, there’s a gap between the frame and the bearing holder, meaning that all the weight basily sits on two spots on the bearing holder. Has anyone had a problem with this, or used something to fill that empty space and spread the weight over the rest of the bearing? Crappy diagram follows (this is what happens when the artisticly untalented use a touchpad to draw).
Sweeet diagram.
can you maybe bend the holders? like put wieght on the frame real hard on the one side of it.
i think you’re doomed. the former hub = 40mm the KH = 42mm. if it were the other way you could shim it. as it i think you’re suck. or you could chop off the bearing holders and weld new ones on.
i’ve got a hub with 42mm od bearings in a frame with 40mm bearing casings and it works fine
I was actually thinking putting somethingint eh “empty space” to transfer the force into the frame… But what kind of easily installed material would work? I’m afraid wood would shatter, maybe I could get liquid steel or whatever it’s called to work, but if I messit up, how would I get it out? Doh-eth…
Worse comes to worse, I start saving for a better frame and upgrade it later 
That’s a problem with the frames that use the stamped steel bearing holders (the United Main Cap bearing holders). They aren’t perfectly round so even when using the proper size 40mm bearing the bearing ends up being supported by two or three points rather than being evenly supported all the way around. When you fit a 42mm bearing in there it’s going to be supported by two points.
The good news is that because of the slop in the bearing holder design it is possible to fit a 42mm OD bearing in there even though the bearing holders are designed for a 40mm OD bearing. It works. It’s not ideal, but it works.
The potential problems are premature bearing wear because the bearing gets deformed, or the bearing race actually cracking or splitting. If the bearing caps are tightened too much you could possibly split or crack the bearing. Just be careful of overtightening the bearing caps like the way the mechanics do at some bike shops.
I prefer the machined bearing holders that are nice and round. They’re more expensive though. A machined bearing holder designed for a 40mm bearing would not fit a 42mm bearing like the United main cap bearing holders can.
You could try using epoxy to fill the gap above the bearing. But once the epoxy is in there it becomes permanent. You aren’t going to be able to remove it all after it cures. You will be able to grind it to the right height and shape using a dremel wheel grinder or something similar. Just think ahead before going with the epoxy. Once it cures you’ll be able to grind it down to fit the bearing, but you won’t be able to remove it all.
Try squishing some aluminum foil in the gap to get an idea of how much of a gap there is. That’ll let you see how much of a gap you’ll need to fill and what the shape of the gap is. From there you can decide whether to use epoxy or some other material to fill or shim the gap.
I have used little strips of rubber (like what you use when mounting cycle computers or lights) as shims between the bearing and holders.
In my case I needed to increase the tyre clearance on the frame by a few mill but I think rubber would make a fairly good packing material. It should distribute the load, is deformable so it isn’t too hard to squeeze into a hole and won’t be nearly as messy as epoxy. You may need to cut and trim to get exactly what you want but I reckon it is worth trying.
Remember you don’t want to overtighten your bearing holders as that will destroy your bearings eventually.
I’m running an original KH hub in a Nimbus frame, the bearings are tight but do go in the holders without modification, you may find that doing the cap bolts up gently can be enough just to pull them in to place and seat them properly, also i found that the frame legs needed to be bent out slightly to allow them to drop over the bearings, this was also stopping them from seating properly.
I did a test fit before dropping the hub/rim off at the LBS, and the hub drops right into the Nimbus frame beautifully, aside from the oversize bearing holder (race was it called?). All these suggestions are good,the wheel is beginning to turn in my mind…