Help with rebuilding my 36er.

I have a blue Nimbus Impulse 36" frame. I want to build a new wheel set for this and I was hoping to be able to run disk breaks as well as ISIS cranks. Anyone have a suggestion for a wide hub that will fit? I was told the bearings for ISIS hubs will simply not fit this frame (40mm vs 42mm). Anyone else built something like this up before?

Can’t you just put 40mm bearings onto a new hub?

A little more reading it looks like this might be the solution to my problem.

Those look like they’ll work to solve your problem, but I’m wondering why there’d be a shim if the bearing size is getting smaller.

Yup, that’s exactly what you need.

The description on the Canadian UDC is kind of weird, the UK site has a much clearer explanation. (The UK site generally has the best and most accurate product info, esp. anything Nimbus)

"ISIS hubs normally have 42mm bearings and can only fit in frames with 42mm bearing cups, but if you swap the 42mm bearings to these 40mm bearings the ISIS hubs can then fit into frames with 40mm bearing cups.

Each bearing consists of two smaller sealed cartridge bearings held in a 40mm shim.
Sold in pairs, complete with pair of 6mm spacers."

Before Ride the Lobster, some people tried machining out the bearing holders to accept 42mm bearings on the Nimbus frames, but they ended up being too thin.

I’m probably confused here but this solution doesn’t make much sense to me…

The frame has 40mm bearing holders right? (as the original bearings on it are 40mm). The isis hub uses 42mm bearings. 42 is bigger than 40 right? How will a shim on the 42mm bearing make it smaller to fit the 40mm frame? (shims make something fit into a bigger space, not a smaller space) Seems like this will not work, but maybe I’m misinterpreting the question…

click the link above.

The shim is to fit a 32x22x14mm bearing (the standard for impact unicycles) into a 40mm frame. The standard 40mm bearing is 40x17x12, which will not fit around the 22mm axle of an ISIS hub. Note that it comes with smaller ISIS spacers (4mm vs 6mm) to get this to work properly and still be able to tighten the cranks down enough. The shim is slightly thinner than the bearing itself.

Good to know as well, once you have these, you could just buy impact size bearings, which isn’t really mentioned on the product page.

Edit: not really sure what’s preventing the frame from slipping off the bearings here. It seems like the frame wouldn’t actually lock the bearings into place, and that the shims are just press fit on top of the bearings. Any lateral force would then kick the frame off to one side. Anybody care to chime in here?

Frame attaches to the shims like it would on a regular 42 mm bearing, with the shoulders taking the axial load, and I guess they have a pretty tight press fit on the bearings, which will take the axial load, too. The lateral forces seem to be taken care of to me. This will prevent you from (easily) swaping out the bearings without buying the whole assembly, but bearings on unicycles do not have to be replaced often anyway.

sorry, I’m using lateral here to mean any forces in the direction of the axle. Yes, normally these are taken care of by those shoulders that hold the bearing in place, but those will now ONLY hold on to the shim, and despite being a press fit, if you have repetitive motion, eventually that seems like the bearing would work itself out to the side. The interface between the shim and the bearing is smooth. For a 40-42mm bearing shim, the shim is so small that those shoulders still work to hold the bearing in place.

If there was a lip on the shim that you could put towards the outside of the bearing to hold everything in place, that would probably take care of it, but the pictures don’t really show the full product.

Just looking at the picture on the site, the pair of bearings in the shim have a bit of space between them; I would guess there’s a ridge running around the middle of the sleeve on the inside, and that the bearings slide in from either side.

So that would hold things in place. But I’m just looking at the picture and guessing…