40mm Frame on 42mm Bearings

I know there are a few threads on this but I wanted a fresh opinion on this. I am looking at this http://www.unicycle.com/unicycle-hardware/frames-and-frame-parts/trainer-20-inch-frame.html to put onto a trials wheel-set with 42mm bearings. I know that the frame isn’t meant to be put on a trials uni but would it work or would I have crushed bearings from too few points of impact?


I have been riding an ISIS hub (42mm bearings) in a frame desighned for 40mm bearings for over two years with no issues so far. In-fact i found the larger bearings to be a better fit in the pressed bearing holders.

My concern with that frame is seat-post diameter. I don’t see it listed but if it is 22.2mm stay away. Believe me even a bit of weak beginner trials will quickly wreck a 22.2mm post. You can get some pretty good posts in 25.4 though.

I think you’d be better off spending a little bit more for a compatable frame i.e. with 42mm bearing housings. Is $20-$30 more going to kill you? Unlike saskatchewanian’s success with mixing a 42mm hub with a 40mm frame I’ve had very poor success the times I’ve tried. I have an old 26" Yuni frame with 40mm housings and non machined bearing covers, the simple bolt and nut and pressed covers. I could never get the hub to fit in the frame quite correctly, it was always a bit crooked and even with shimming it just wasn’t quite right. Even worse there was almost no clearance between the outer frame and my cranks so that a lot of paint/metal was initially scraped off of the frame. Not the best ride IMHO. Get a compatable frame, you’ll be much happier, oh and make sure it has the machined bearing housings like:

You’ll be much happier in the long run.

My two cents…

Interesting. I have the same frame and find it fits great. I did have to shim the one side since it is not perfectly even (had to do this with the old wheel as well). Tolerances could have been pretty loose for those frames though and I might have just gotten lucky.

Before we start suggesting products we should probably know why Gene wants that frame. Sure the trials frame is a safer bet but maybe he just wants a round crown.

Yeah you might have gotten lucky, or else I was a bit unlucky? Either way, I didn’t mean to suggest a particular product, just that they make 40mm and 42mm compatable frames for a reason and one would be best served getting a compatable frame. That isn’t to say you can’t try and make it work, but I think that in the long run one would be better off with compatable components; just my humble opinion. (I’m all for tinkering, I’ve got a few Franken-Uni’s that I’ve developed in the past) :slight_smile:

Yes, eventually it will effect the bearings. They will wear out prematurely, and it will possibly affect how well the wheel rotates.


I dont really know about the bearings. but if you want to put a trials wheel in that frame you’ll need a Try-all tire, because it’s rounder than any other trials tire. My Luna didn’t fit in my round crown frame I had from my learner uni but my Try-all did.

I’ve used one of those frames for a while doing hard trials on it. and I was useing a nimbus wheel set ISIS. still going strong. but now I have KH long neck. bearing are still good tho

I’m using a Nimbus II frame from a couple years ago when the holders were 40mm and not machined. The wheel has 42mm bearings, but I haven’t ridden the setup long enough to comment on how much it will wear out the bearings. There is not slop in the setup and it seems pretty solid though.

I’m only using this setup because I already had the frame, and it wasn’t worth $65 for a new Nimbus II. In your case, I would go ahead and spend a little more for the right frame.

Well, I was going to get then Nimbus round-crown but Unicycle.com US stopped carrying them and the are a b to find.