Design problems with Nimbus ISIS hub

I just bought a Nimbus X 20" freestyle unicycle, which is the first unicycle I’ve owned with the Nimbus ISIS hub. I mentioned back in this thread that the Nimbus design seems to be in violation of the ISIS spec. After looking at the overall hub design, it seems to be only one of several extremely questionable design and/or manufacturing issues with this hub.

In this video, I demonstrate that my wheel is not spinning freely, even after I’ve totally removed the bearing holders. The spacer between the cranks and the bearings is causing friction in the system; the wheel spins more freely when the cranks are removed (which should never be the case). In addition, the Kris Holm-branded bearing shims appear to be of a particularly soft aluminum; they are already deforming and producing aluminum shavings. One of the bearings, I could not get out of its bearing holder, yanking on the frame as hard as I could; I had to chisel it out. Also, on one side, the bearing shim is loose on the bearing itself, creating slop in the system. needs to come out with a hub which:

  • has a real ISIS crank stop
  • has bearings press-fit to the axle so as not to require spacers
  • uses bearings with steel outer races and/or shims

And while you’re at it, do it with 36 holes.

This is really interesting. I’ve also noticed a bit of difficulty in removing my frame. However, not nearly as much difficulty as yours!

Would it be easier to flip if the wheel spun freely.

Oh, and I may be wrong, but maybe you clamped the wheel in to tight with the bearing holders.

I screwed up my bearings by doing this.

I personally didn’t clamp the wheel in tight; in fact, when I got the thing and saw that it wasn’t rotating freely, I loosened the bearing holders, thinking they must have been too tight at, but they weren’t particularly tight. As I demonstrate in the video, the bearings themselves are working just fine, once you stop rubbing things against them.

Your thread got me interested because I have the Nimbus ISIS hub, on the Nimbus 29" (stock) I bought a little over a year ago. I, too, have to use the spacers as the bearings are not press-fit and were sliding off when the uni first arrived to me. Spacers were sent, and I haven’t had issue since, and the wheel spins freely. My spacers contact the inner metal race of the bearings, thus creating the correct motion of crank-spacer-inner race moving together. I shot a quick video to demonstrate the rotation with cranks and spacers removed,

It appears that the inner race of my bearing is wider than yours; the spacer does NOT contact the black area at all during rotation. Upon close inspection, I see no wear on the spacers, or any other evidence of friction.

Secondly, my bearings never get stuck in the frame. They sit snugly, but come out with minimal force.

This leads me to conclude that the problem may lie in your bearings, which are not the same as mine. Mine are not branded Kris Holm.

my KH has qu-ax bearings? i don’t think KH brands his bearings. at least not on my older model.

Very interesting video … way to get to the root.
Always get in a hack when something comes and it isn’t quite right. Hope they do you right at UDC.

This is all certainly interesting. I am not sure that it is a “design problem with Nimbus ISIS hub” is really a correct thread description. Your problems seam to be associated with the KH 40mm double bearings.

Now, let’s have a look at this.

Bearing shields and spacers…
The spacers for the bearings are supplied to match the bearings (by KH, not UDC or Nimbus). They should not touch the moisture shield at all. I have attached an illustration of how bearing shields work, they are always recessed. This is so that the clamping surface can be larger than the minute edge of the bearing. The design is not at fault here… I have a couple of suggestions for the touching though. The writing on the bearing shield is proportionally higher than on larger bearings and it is this that is touching. The other thing is that the spacer could have been deformed on contact with the thin bearing edge when it is clamped. Both are solvable, a bit of wet and dry on the shield (the problem will be inside and out I guess) and a flattening off of the leading edge of the bearing shield.

Deformation of the bearing outers. The original design was for a steel outer and I am not sure where it got changed to an aluminium one or what specification it is. If the damage to the shield has just been caused by normal use then this is not good, it is odd that it has primarily happened on one side not the other.

It is worth realizing that these bearings are tiny, they are very delicate. Their design advantages are to give a more rigid frame and allow their fitting in to 40mm frames. They will of course not spin as freely as single row bearings, especially when new.


bearing shield.jpg

I am pretty sure that UDC US has decided to get rid of theyre old (40mm) frames by using the new KH bearings to allow them to use the ISIS hub. I would be fairy annoyed with this too if I was not aware of this when I bought it. It appears to me that all of your problems are associated with the bearings, not the hub, as Roger Mentioned. I have 1 nimbus hub and 2 KH hubs (that are basically the same) and they all have the 22x42x12 “ISIS” bearings, without any shims.

I have some issues with some of your other statements though.

As I stated in this thread there is no reason not to use the spacers as crank stops, and they are a very real crank stop. You could machine the axle with solid crank stops, but this would require larger bearings and hence another frame type, just when they are fairly standard.

I agree that the bearing shims should be made of steel, better is to not use a shim at all (ie supply the frame that is designed to work with the hub)

The nimbus ISIS hub is avaliable in 36 Hole, but the Nimbus X (and most freestyle uni’s) come standard with 48 hole wheels. That is quite clear on their website specs, so I think you have to wear that one if you are unhappy.

I think you may be missing the point a little with these bearings… they are considered an upgrade! They are lighter (a lot!) and are twin row of bearings so create a more rigid system. I use them on my 36".

As for the bearing fit on the ISIS hubs, they are all non standard and are outside the normal criteria for bearings. To get the ISIS to work on unicycles at all a lot of work has been done to specify and manufacture bearings that will do… (the nearest standard bearing is the 6005-2rs which is 25x47x12) there has of course had to be compromises here. Basically the outershells on our bearings have to be thinner.

Normal bearings can be fitted on the shaft using a H7/s6 press fit, with our bearings this would not work as they would distort due to the thinner shells. We have to work with a slacker tollerance, the theoretical tollerance (from memory) is H7/n6 which is transition fit. These are tight tollerances and once we have paint/finishes on this does vary. This is something we of course keep an eye on.

On a sideline… the ISIS complience of the current hub and KH cranks is correct as far as we can determine. There are some ISIS cranks that do not meet 100% of the criteria due to clearances, but work. That is overall… looking in detail at tollerances, the ISIS standard is actually rather slack and the unicycle manufacturers have agreed a tighter standard to work to amoung themselves to help with compatibility. We have also received different advice about the requirement for the locking surface for the cranks, we use our spacer to produce this. There are definately 2 camps, one says don’t need, one says do need. So we now supply them with them (unless you have old stock and it slipped us by) so you can take them off if you want.


yeah…on a different note i have 3 nimbus trials and have not had these problems.

the problems I do have on 2 of 3 of the nimbus are just with the frames…they were manufactured incorrectly or something. one side of the frame is longer than the other, so i have to cut some aluminum strips from cans and shim it…but I don’t mind this too much…they’re only 45$ and didn’t snap after 3 months like my kh frame did.

Bearings: The damage to the bearing shields appears to have happened really before “normal use” even started. I’ve barely ridden the thing. There is damage on both sides. The bearings actually spin perfectly well; it’s only friction in the overall system which keeps them from spinning properly.

Thanks for the response.

What’s wrong with the bearings used on the KH hub?

This sounds like you’re saying two different things; are the spacers required or not? It would certainly be possible to install the cranks without them, but you say the bearings aren’t press fit, so it sounds like the spacer is needed to keep the bearings from moving on the axle. I would be glad to ditch the spacer if I could.

The ISIS spec clearly requires a crank stop, though it will probably basically work without one. The spec isn’t explicit about whether the stop can be a loose spacer or not; my reading is that a loose spacer is not sufficient to meet the standard. In any case, the use of a loose spacer, which also appears to be required to hold the bearings in place on the axle, makes the whole system seem pretty hokey and subject to a number of potential problems which are not problematic in other designs. You’ve added dependencies to the system; the proper functioning of the bearings is dependent on the tolerances and installation of the cranks, which is not the case in other designs.

Nothing (they’re the same as usually supplied on the Nimbus hub AFAIK), except they’re 42mm outside diameter and wouldn’t fit that frame.

As far as I see it, the bearing inner race and the spacer together ARE the crank stop. As long as the spacers supplied are the correct width to put the stop in the right place relative to the splines then what’s the difference between that and having a stop machined into the axle? And it has the bonus of holding the bearings still on the axle so avoiding having to have a really tight interference fit and making changing bearings easier. Fitting the cranks without the stop/spacer would just rely on them bottoming out on the splines, which would be far less secure and lead to crank creep like people seem to be finding on Schlumpf hubs.

If the spacers supplied are interfering with the KH 40mm ISIS bearings though then that is a problem (they’re fine with the normal 42mm bearings), so is a wobbly bearing after so little use (could be just unlucky with the particular bearing) and the soft shims getting stuck in the frame. Out of interest, how tight did you do up the bearing holders? When I got my new Nimbus muni frame with machined bearing holders I sort of expected the two halves of the holder to touch at the ends like on a KH frame, but they don’t, so you still have to be careful with tightening up the screws the same as with the cheap pressed bearing holders, otherwise it squeezes the bearing (which would be even more of a problem if the shims are a bit soft).

For the record, my two Nimbus ISIS hubs have performed perfectly. They’re both running on 42mm bearings, with spacers, 36 spokes. One of them is in a Nimbus frame with machined bearing clamps like yours (but obviously a 42mm version) and the other is crammed into an old QuAx Coker-copy frame with 40mm pressed clamps.


The bearings fitted to the KH hub are the same as those fitted to the Nimbus and Qu-Ax (although different factory manufacturing them now). These are 22x42x12 single row bearings. They have the same problems.

The specification is for transition fit, not sliding fit. This is a fit that does not allow the bearings to slide off, but is not as much an interference as a press fit.

One of the primary reason for the stop in the ISIS design is simple… it is so that the sprockets line up correctly every time on a bike. This is not a criteria that is of primary importance on a unicycle!
Since the bearings needs to have a small a shaft dimension as possible we can not include a stop on the shaft as on bikes, so the use of spacers is the only option I can see. It also holds the bearing in place (good side effect).
Do not think that the problems we have with ISIS are new or specific to them. They may be slightly different since we have the spacer locking the bearing in place for once… this is an advantage (remember the old coker hub, ahhhh!).

Here are my thoughts on Spacer or No Spacer.

No spacer:

  • allows for looser tolerances
  • can be tightened until tight on taper
  • less components
  • creep in material over time means it can go on too far
  • crank could go on so far that it catches the frame
  • potential for different offsets on each side of unicycle
  • does not hold bearings/frame in place
    With spacer:
  • cranks always to same place
  • bearings held in place
  • system can not be over tightened (within reason!)
  • tolerances could stop crank from locking tight on spline
  • at opposite tolerance… they wont get to stop (leaves loose spacer)
  • extra components
  • if movement does appear in system there is no solution (you can’t just tighten more, that would not really solve the problem).


It’s more likely that one or both of the frame legs are a bit bent sideways. It doesn’t take much to put the wheel noticeably off-centre. You can correct it with shims, but better to bend the frame legs back (pretty easy by leaning on it without the wheel in). There’s a tutorial here.


Personally, I don’t think stops would be helpful as they might stop the crank before it’s tight, esp after they have been taken on and off a few times so that the crank splines have worn.

Spacers are necessary, you’d know that if you’d ever had a bearing slip. The spacers work fine as long as you are using the correct thickness. I tend to err on being too thin as I’d perfer so movement and tolderance for worn cranks vs having my bearings damaged or the cranks being loose.

The bicycle ISIS standard is fine on paper, but doesn’t really apply to unicycling. The current system ain’t broke, you just need to be aware of how things work.

Changes I’d like to see:
Spacers made in increments of one mm (1,2,3,4,5,6) which would allow for the wearing out of components. My biggest concern is having my cranks either too loose or crushing the bearing. If you’re running tight, that’s likely the problem, crushing the bearings that is.

May also need to redish your wheel, which is just as likely as the frame being built wrong. You could start by measuring the frame legs or better yet, flip the rim over and see if the clearance difference stays the same or switches side.

The ISIS standard is that the crank is tightened against the stop, not that it’s tightened against the splines in the way that a square-taper crank would be.

Spacers aren’t necessary for press-fit bearings. My current basketball uni, an old-school Miyata Deluxe, has press-fit bearings that haven’t moved at all. In fact, I have 9 unicycles without bearing spacers, and I’ve never had a bearing move on the axle.

If you don’t fit the standard, you can’t call it ISIS.

I reckon you’d have to do the crank bolt up amazingly tight to actually squash the inner bearing race from the side - in fact I’d suspect the (aluminium I think) spacer would almost certainly give first, if not the crank bolt thread. There’s no need to do the bolt up that tightly on an ISIS axle anyway - once the crank hits the stop there’s no point reaming it up much tighter (just enough to put a bit of tension in the bolt to stop it wiggling undone). It’s relatively easy to deform the outer bearing race by squeezing it too hard with the bearing clamps.

The bearings on square-taper unicycle hubs (at least the ones I’ve had) were usually a tighter fit, but I did have one on my old muni hub that could be pulled off by hand. I was always advised to set the frame legs very slightly narrower than the bearing width so if anything they were exerting a very slight inward force on the bearings, stoping them drifting. I don’t know if this was necessary or not, but I never had a problem with moving bearings. I have seen and heard of people having trouble though. It seems from what Roger said that the ISIS bearings have to be made with unusually thin inner and outer walls to accomodate the thicker ISIS axle and still fit in the standard frames (presumably otherwise you’d need to use custom-size balls which would cost silly money), and that this dictates that the fit must be slightly less tight to avoid risking stretching the thin walls on fitting (and messing up the bearing). The spacer is there because ISIS needs an end stop and a side effect of this is that the slacker-fitting bearing can’t move - everyone’s a winner. As long as the spacer doesn’t rub on the bearing seal (as yours appears to) I can’t see a problem with it. The splined interface has a lot more contact than a square taper so wouldn’t need as much pressure to make it secure. Both my hubs take a fair amount of cranking on the bolt to push the crank up to the spacer anyway - it’s hardly loose. After many fitting/removing cycles the cranks would obviously wear and may become too loose, but without the silly forces needed to fit a square taper securely I’d guess it would take quite a few repetitions to get that bad (although I rarely swap my cranks so I can’t say from experience). Then you would need a narrower spacer, or new cranks.

I don’t even reckon the width of the spacer is all that critical within reason. The spacers that came with the hub on my 36er are not quite the same size, so at least one of them must be strictly “wrong”. The cranks both seemed to seat nice and tightly before hitting the stops, so I didn’t worry too much. I’ve ridden it like that for hundreds of miles, about half of that off road, with no problems whatsoever - I’ve only ever touched the bolts twice (once to change to shorter cranks and once again to swap them back when I didn’t get on with them). And I’m not light (just under 12 stone), and like hills!


EDIT: Sorry - that got a bit long!

I’m not an expert on the ISIS hardware and I don’t own any of it, but if all the uni manufacturers are having to deviate, or otherwise make extra specs on their ISIS parts, I wonder if it might be easier/stronger to work with the other manufacturers to develop a spec that’s unique to unicycles?

I assume this would only work if it still essentially used the same parts, but if you guys are having the parts specially made for you anyway, maybe it isn’t that big of an issue. So in other words, does it make more sense to keep customizing the ISIS spec., or would it make sense to develop a similar-but-different spec specifically for unicycles?

The specified torque on ISIS crank bolts is 25-35 foot-pounds (33-47 N-m). That’s a lot of force. Not all the force will be taken by the spacer, but the amount which will be taken by the splines is not defined; there are plausible scenarios (such as if you’re using the light aluminum Qu-Ax cranks which easily deform) where torquing the bolts to their specified rating will cause a significant percentage of that load to be transferred to the spacer.