I’m learning on a Qu-Ax Profi 20" that I got used on ebay and it had no spacer between crank and bearing when it was delivered.
I’ve change the cranks to Qu-ax 125 mm myself and I’ve had to tighten them once during my training sessions as they became loose but I didn’t really think much about it until I read some random post that noted something about spacers… it seems I’m missing something like Spacer for ISIS Hubs / Cranks 28x22x6 mm - QU-AX, 1,40 € (qu-ax.de) right ?
All ISIS cranks should be used with spacers, although if you always do your cranks up with a torque wrench and check them regularly you can get away without.
The spacer ensures that the crank is tight, without deforming the interface. The spacers also ensure that the bearings cannot move on the axle.
The spacers you linked may be correct, but which spacers you need depends on the specific hub and cranks. As you’re currently not using spacers you can probably get away with just measuring the current gap between the cranks and the bearings and getting some based on that.
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My qu-ax Profi also came without spacers, but since I’ve never had any issues with it, I haven’t looked into adding any…(my freestyle unicycle isn’t exactly loved)
But normally, Isis cranks on unicycles need spacers, and adding some might help your issues with them coming loose.
I agree with mowcius, measure the gap between bearing and cranks you have now, and get the closest spacer you can find.
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Spacers are not absolutely necessary, but they offer two advantages:
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The crank has a clear stop and can no longer slide inwards
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The bearings are hold in position, they cannot come loose
Every unicycle manufaturer is using spacers with ISIS hub, except QU-AX with red hub (but they use spacers with ISIS yellow hub !)
If adhering to the standard, all cranks and hubs should be designed to have a spacer as a stop 16mm in from the axle end. I doubt that this is how the standard is applied on unicycles though, or even that a standard stop distance could be applied to a specific model of crank - I suspect hubs vary too.
I’ve had numerous unicycles from brand new where the cranks have not been tight enough on the axle due to spacers being too large, or either the axle or crank interface not being correctly formed/positioning, which would lead to premature wear. I won’t name names, but they weren’t cheap unicycles, or ones likely to be subjected to limited crank forces.
The spec seems to be down on the official website, but there’s a copy here:
http://my-sport.spb.ru/manual_1/ISIS%20Drive%20Complete%20Rev%20D.pdf