A small space where the axle is visible

I’m very sorry if this problem has been addressed before, but after wading through the forums I haven’t found quite the same thing.

Yesterday I received my new 20" Nimbus trials from UDC. However, after a few minutes of very minor fooling around, I noticed something. There is a small space (maybe 1 or 2 millimetres) between the left crank arm and the bearing spacer, and through the space I can see the axle. When I ride the unicycle, it feels perfectly fine; there are no wobbles or creaks or anything of that nature.

I just got this unicycle. Did UDC not put the crank on all the way? Should I remove the crank bolt, cover the crank with a piece of wood, and try to hammer it on further? I’ve actually tried doing that, but I was admittedly shy with the hammer. I didn’t want to damage anything.

Once again, I’m sorry if this type of question is wearing your patience thin.

I have the same thing on my nimbus. What kind of cranks do you have? I’ve only had that problem with KH moments. I’ve ridden pretty hard on mine like that and they’re fine. As long as you make sure they’re tight you won’t have any problems.

I don’t understand why UDC puts a spacer on there; a space seems to me like a violation of ISIS standards. If there is a spacer, there should be space between it and the crank–if the crank is jammed up against a spacer, it’s not installed properly.

I believe the spacer is purely to stop the berrings drifting along the axle if they are loose fitting. It is not for the crank to come up against. Both ISIS and square taper hubs/cranks are tapered so as long as they are fitted corectly, drifted on far enough and the crank bolt is tight then it doesnt matter if there is a gap. Like tholub said, its better to see a small gap then to have no gap atall.

After changing my cranks from k1’s to moments (it’s temporary :wink: ), I realised the spacer does more than that. If you don’t have the right spacer (of course k1 have different spacers to kh) - then your frame wobbles like hell.

That could be cause my hub is a bit **** and the bearings come off by just pulling on them abit.

Tighten the crank bolt as far as you can with a longish allen key, and then have a look again. The space shouldn’t be a problem, but if you turn the key far enough - it’ll dissapear :wink:


The spacer is very important, it keeps the bearing from sliding on the axle. I was not running a spacer on my Isis equipped muni and the bearing slipped so far that the crank hit the frame :frowning:

The problem is that for each crank there is a different spacer specification:

Nimbus/KH Isis + KH Moments = 6mm
Nimbus/KH Isis + QuAx = 4mm
Nimbus/KH Isis + K1 = 2-4mm

Now the KH Ti hub, in my opinion, is a tab bit smaller at the splines, so it did better with a 2mm spacer when I ran the K1, but when I mounted Moments I tried a 4mm and there was too much spacer between the bearing and cranks, so I went back to a 6mm.

Something to keep in mind is that there is also a spacer between the bearing and hub, so be sure your bearing and hub spacer is tight as well, esp if you’re not running a spacer between the bearing and crank.

UDC sells spacers.

Regardless of whether it “violates” any specifications, in practice the spacers seem to work perfectly well with ISIS. The only contact I’ve had with ISIS is on unicycles (all my bikes are square taper apart from a weird Truvative proprietory splined thing), but both my ISIS unicycle setups came with spacers, so I didn’t question it, and have had no problems. They’re both Nimbus hubs, one with QuAx light cranks and one with Moments. It takes a fair torque to do the bolt up until the crank hits the spacer (I haven’t measured it, but nowhere near as much as I normally use on a square taper crank) and I’ve never had one come loose, so it seems to be working well. Whatever the official ISIS spec says, I don’t reckon ISIS cranks need to be forced onto the axle as tightly as sq-taper.

Wasn’t there somebody who did some actual research into what happens when square taper cranks deform under riding torque, and they reckoned they actually tend to move ONTO the axle momentarily, allowing the bolt to slacken? I’m sure I’ve seen links to that article somewhere, but I can’t find it now. If that’s the case, then the spacer would seem a good idea after all.

Anyway, in reply to the OP’s question, in my experience of ISIS cranks (not huge, as I mentioned) the cranks are always tightened up enough to hit the spacers - never needed a hammer, just used a normal 8mm allen key in the crank bolt (greased splines first). I’m not a big dropper though, but I do ride miles and miles of very rough rocky xc.


Yes, I have the moments as well.

Ah, I see. Now I feel stupid, as I knew I probably would when I made this thread. I assumed that the left crank should look like the right one, which doesn’t seem to have a space between the crank and the spacer. So then, what (if anything) should be done about the right crank? Thank you very much for your help so far.

That shouldn’t happen, and it represents a problem for which a spacer is a bad fix. The bearing should be press-fit onto the axle, and it should stay where it’s put; if you need a spacer to keep it from moving, your axles or your bearings are not properly machined.

You shouldn’t need one, but it can’t hurt at all. My frames wobble, using kh hubs. The one 07 hub I got close to two years back had the bearing fall right off when I got it, you can slide them on and off with no force at all. Spacers should ‘fix’ that.

I’d say try tightening it with an allen key to get it as tight as the right, if it will not budge I’d say ride it as is.

A wobbling bearing will damage your axle though. The axle on my hub is kind of screwed up, the hub is all beat up and kind of rusted. I’d fix it, but it’s an 07kh and the flanges look like they are about to pop off any day.

The new hubs seem better, my 08 worked fine. I’m picking up an 09 soon.

Now I see the real problem. The problem is that I’m an idiot who is way too paranoid about breaking his new unicycle. Please, everyone, disregard this thread. Let my embarrassment die along with it.

Caterpillar you should tighten the crank bolts on your new uni after every ride. the crank will gradually settle in until it hits the spacer, then it should be OK. Also it is ok to be paranoid about your most prized possesion. it shows you care about it.

Now I am about to come across as a prick so I would like to apologise in advance.

This statement is wrong. The ISIS specs call for a “crank stop” as part of the interface. In fact this is the datum for the whole system. ISIS is not like the square taper interface at all. it is more of a parallell spline drive but with a slight taper to remove the movement between the splines. Technically without the crank stop (spacer) it is not ISIS at all, which leads to problems like most schlumpf hub owners seem to be having (loose cranks, Cranks creeping and hitting frames,etc)

If you are interested in the ISIS drive please check here:

ISIS Drive

The entire spec is avaliable to download from the “documentation” section of the website. There is most of the reasoning to do with the crank stop in that section

It’s not you who looks like a prick mate.

Thanks for that - I’d not read the official ISIS spec before, and it confirms most of what I suspected from my limited exposure to ISIS (and extensive exposure to sq-taper). I assumed the spacers were there to provide a stop for the crank because the taper seems much shallower than on a sq-taper axle - it seems like it would be far too easy to wind the crank way too far onto the splines otherwise. Good to see that’s what they’re supposed to be for (and obviously have a nice side-effect of stopping the bearings creeping on the axle), otherwise it’d be a bit silly for all ISIS unicycle hubs to be sold with these spacers.


You are spot on Rob. The square taper is 2 degrees (per side) and ISIS is 1 degree. Square taper also has a much larger surface area to try to compress, but on the ISIS spline only the the bottom of the ten flutes are actually tapered, the rest is parallell. This makes it much easier to deform and hence loosen up.

The crank stop also increases the strength of the interface by effectively increasing the diameter of the axle and supporting the crank, reducing the chance of bending the axle. Add the ability to clamp your bearings (requiring less interference on the inner race) which clamps your frame into position (holding the whole system together = stiffer frames) I personally think that ISIS is nearly perfect for unicycles!

Also, you should never need to hammer on either type of interface, althought it may take a few rides to settle sometimes. I see no reason to be shy with the allen key though, I am renowned for stripping and breaking bolts at work, but i have never had any problem with crank bolts (sq taper or ISIS)


You are not supposed to tighten the bearing holders very tight onto the bearings. Kris himself warns about this. therefore with time the bearings will move.

When I bought my KH24 I got the non standard 165mm cranks. I guess that when they replaced the 150’s they forgot to put the spacers back on. I rode fine for a year and then in a MUni ride the bearing fell right out of the holder. I put it back in and finished the ride. Later I wrote to the seller and they sent me spacers and new cranks to rectify this problem.


Sorry if I wasnt clear. I was talking about the crank bolt being tight not the bearing holders. The bearing holders don’t need to be very tight as they completely enclose the bearing anyway. I only ment that the spacers stop the frame from being able to spread apart, since the bearings cant slide along the axle.

Right On!



Not true, you should call UDC and ask them, they’ll repeat to you what I wrote, the bearings are not press fit, they shoudl be snug, but without some form of retainer (spacer), they can slide on the axle and cause problems.

But hey, I leraned the hard way, so there’s no reason you can’t learn the same way :roll_eyes:

Pay attention to what another poster mentioned about avoiding overtightening of the crank against the bering. A little space (1mm) is better than having the crank press too tightly against the spacer/bearing, which could cause premature bearing failure.

The spacer/crank/hub specs I posted are accurate.

I think you’re misunderstanding the term. The “crank stop” is the part of the spline which widens and stops the crank from going any further onto the axle. It’s not a spacer which pushes against the bearings or the frame. The KH hub doesn’t use a spacer, my bike’s bottom bracket doesn’t use a spacer.

See this from the spec:

The crank stop shoulder is the place on the spindle where the crank stops–it’s not a spacer. See the diagram on page 6 of the spec. Bearings are not part of the spec, so there’s no way that a spacer between the crank and the bearings could be part of the spec.

Nurse Ben: It may be the case that the bearings on the Nimbus hub aren’t press-fit; if so, that’s a design or manufacturing defect that uni.com is covering up by using a spacer.

I’m sorry. I think you are close but are mis-interperating the drawing and the understanding of the crank stop.

The crank stop is not a widening of the axle. It is a 90 degree shoulder that the crank pushes against. It does not matter how this is achieved, only that is is. The ISIS spec only refers to the actual interface itself not the rest of the axle. I have included the picture from page 6 of the spec. You can clearly see the shoulder as a pressed on fitting (ie spacer) marked as “male crank stop” in this drawing. Please note it can be machined as one piece, this is an option.

Keep in mind that this drive system was designed for bikes, not unicycles.

The reason that bearings are not included in the spec is: (from the ISIS FAQ)

[I]" The spline ends are so large, what kind of bearings should I use?

It is up to you. The outside and inside diameter of the spline shape was determined so as to make use of as much existing tooling as possible in the industry. Bearings for this bottom bracket may take any number of forms, including custom cartridge bearings, needle bearings, bushings, or loose balls. The purpose of this standard is not to standardize one particular bottom bracket design, but rather to standardize the interface between the bottom bracket and crank. It is up to the individual designer to determine the type and configuration of bearings in the bottom bracket."[/I]

Just because bearings are not part of the spec, doesn’t mean a clever engineer can’t use them as part of the system, providing the geomety of the interface is still correct.

The bearings are only relevant on unicycle hubs because whoever designed these hubs decided to use a 22mm diameter inner race as part of the system, with the spacer to combine to create the shoulder. They could have used much larger (heavier) bearings and machined the crank stop into the axle. The current spacers could also have been pressed on tight, but that would make it difficult or impossible to change the bearings.

Finally, since the current design clamps the bearings into position, there is no need to have a high level of interference fit, as long as they are not loose on the shaft. It is also surprisingly easy to stretch the inner ring of a bearing and make the bearing tight, if there is too much interference.

One last thing. Both my KH hubs have this spacer. Your bike bottom bracket wouldn’t but i bet they use larger bearings. I also bet it has this shoulder that the cranks should push up against.