Based on what I can see in the picture, by eyeball examination, it looks like there is enough “extra” metal so that the axle diameter could be reduced by 2mm without taking any metal away from the axle’s ISIS connection fitting, so apparently that consideration could be ignored. Reducing the axle diameter from 22mm to 20mm is somewhat of a strength gamble but engineering also includes what is known as a safety factor, that in this situation an engineer might have considered the strength and weight of a human gorilla and then have based their design on that, so that removal of 2mm here might not be too much of a gamble, but please note my use of the words “might not”.
The out side diameter of the spline is 21.2mm, so turning the spline and shaft down to 20mm would sacrifice some strength in the spline as well as the shaft.
That last 2 mm has a radius where it comes off the spline. Without it there would be a sharp corner which would focus the stress. The result would be dramatically weaker.
The ultimate strength isn’t just about the amount of material. The geometry of transitions can be more important.
OK, I was interested in that but relative to the inside diameter of the bearing the spline outside diameter just looked smaller than it actually is.
I can see that and for the same reason as you mentioned I would not change it but it just looked like there was more diameter metal available. Apparently pictures have limits for me.
Ah, time out for coffee.
What I find curious is why the need for an expensive to develop non-standard bearing. A designer could know what size of bearing they need but then as is commonly done search for a standard bearing of the same or slightly larger size and then make adjustments to their design if need be. Sure, there are situations where nothing but an exact design match will do, but for a unicycle? ISIS is a great system but consumers now have only one source for bearings. Oh well, Schwinn used to routinely do that sort of thing with its bicycle tires.
Probably because it’s the cheapest method overall as a OEM. It’s just having the bearing supplier turn 1mm more of the radius of a 6004 series bearing. If you go with a 6005, your outside diameter changes 5mm, so you might not be able to use the same machined bearing cap parts as with square taper bearings, the use of shims to convert between the two is made impossible too. I’d guess that previous onza and other outdated splines used 42 mm outside diameter, and 12 mm wide bearings too, so you would want to keep that compatability.
If you go large on the inner diameter, you are throwing compatibility to the previous frames away. You also have to get larger round stock, and machine more of it to get your Isis spline, and to keep similar weight. Machine time is very expensive, for the bearing manufacturer turning 2mm more of the inner diameter is a lot less machine time that for the hub manufacturer to turn the 30mm stock you need to get a 6005 bearing on there down to 21.2 mm for the Isis spline.
Most unicycles never get a bearing replaced, so the aftermarket supply being limited is pretty irrelevant.
That’s my analysis at least.
Nothing compared to why anyone would be concerned about whether a bearing that doesn’t often need to be replaced costs $10 or $20 on a $500+ machine. 
Also remember that the ISIS standard was developed for bicycles. The engineers who put it into unicycles didn’t have carte blank as the starting point.
I think you all are getting a little carried away with your talk about machining spindles down because UDC bearings might disappear. I see them on the QU-AX web site, and probably any manufacturer of ISIS hubs sells them as replacement parts.
And they might be “non-standard” but that doesn’t mean you would have to find somebody to custom make one for you. Alibaba showed several Chinese manufacturers selling them cheap. No idea what the shipping would be but you could probably get a lifetime supply for all your unis for the price of having an axle machined. For example here and here.
I suspect that we’ll all die due to climate change before ISIS spindle bearings become completely unavailable.
I think I’ve replaced two sets of bearings on all of my unicycles in around 9 years, so with the number I’ve got spare, I’m probably set for life!
You assume such a bearing could be available if needed but non-standard bearings are not guaranteed to be available. Then what to do? All I did was to ask myself that same question and then spend a few moments here on my keyboard. You are suggesting such a question could be unfit to discuss?
Well, apparently the ISIS standard for bicycles does not include standard bearings. Or if they are available as standard bearings for bicycles then why do they not fit unicycles?
Are you rationing thought and ideas around here? It took me all of a few seconds to have a thought, then not many seconds to put it here and then I have not lost a single second of sleep about the matter.
And that was followed by discussion about the quality of those same bearings. The idea here is to be progressive thinkers, right?
Hehehe! You likely are quite right! And yes, most bearings can take a lot of use and moderate abuse before they bugger out.
My background is mechanical engineering and I am trained to consider extreme possibilities and situations. That is what I professionally do.
Actually, until this bearing topic thread got posted here last week I was aware there are larger and smaller unicycle bearings and I never gave bearings any more thought than that.
ISIS hub bearings on bikes is very different then unicycles. Unicycles have to have a separate bearing on the end of each fork leg where a bike has bearings in a single tube in the bike frame. Some do use sealed bearings but the size/configuration is much different then required on a uni.
Take a 6004 bearing. Find a friend with a lathe, and have them turn of 2mm of the inside diameter.
Buy whatever hub is the new standard in that very distant future.
Assuming it’s probably the outer ring that gave away (as it’s usually the case) take bearing apart, and rebuild it with parts of a 6004 series bearing.
Take a 61905 bearing, and machine two spacers (one for the inside ring, one on the outside so it doesn’t move side to side in the frame)
Buy 3 sets of bearings when I notice ISIS hubs on unicycles are dissappearing, and have enough of them for the rest of my unicycle career.
I’ve never experienced a bearing failure that was so fatal that it kept me from being able to ride, it’s only the noise that gets annoying usually. (It has apparently happened to others, but i’m pretty sure they overtightened the bearing caps at some point.) I’ve ridden with my tire taped to the rim, since the bead broke, and a JB welded seatpost for a short time, since I was travelling and couldn’t order parts, I think I’ll find a solution if I need to.
I’ve wore out one set in a 1 1/2 year, the noise is driving me nuts. I don’t think we’ll have a problem getting bearings. We have unicycle. Com. They can always get a source to make more.
I have a lathe. You can think of me as a friend if you’d like.
But that bearing race is hardened and it would have to be ground.
Keep this in mind: The inside dimension of the bearing was made bigger to give more strength to the axle that passes through it. The bearings are rated for loads many times higher than we put on them and usually fail because of rust, grit, or salt getting in, not from load.
Before ISIS, aggressive unicyclists used to break axles all the time. See the George Peck video where he puts “just a few of” the broken hubs he’d collected on the table. IIRC (been a while) stress at the outside and most heavily loaded part of a beam-loaded circular shaft varies with the 4th power of the diameter, so every mm counts.
But I don’t see a problem getting “6004/22” bearings at present. I have an eBay “Schwinn style” square-taper hub that takes 6203-2RS-5/8 bearings because the bearing surface on the axle is 5/8" instead of the usual 16 mm. It turned out that they’re cheaper and more plentiful than the “standard” size because they’re used on a popular piece of farm equipment or something. It’s all down to how many distributors want to place orders for that size. Bearing manufacturers don’t forget how to make them!
Call it standard, non-standard, or a non-standard standard but one search for “6004/22” returned dozens of sources for me. Stock up now if you’re worried about it I guess.
For sure! You’d have to unlace the wheel to do any hub machining to make another bearing fit anyway. You’d do better to lace in a new hub that came with the new standard bearings already installed if it came to that.