KH Spirit Cranks - STRAIGHTNESS of disc mount.

My KH Spirit cranks shows unstreightness, the disc is moving left/right of about 2mm in one revolution.

I am using KH Spirit Cranks since they show up on the market many years ago and I have just spot some unstreightness to my new pair (I used 4 different pairs already). After investigating the problem, I found out that my older pair has simmilar problem but hopefully to only half extent.

How the story begun.

I bought new KH Spirit Cranks 117-136 as spares in May 2017. They were waiting in my store for use. Month ago I mounted them to 20inch uni and used for 3 sessions of hockey. Yesterday, the time come to put them on Schlumpf hub with brand new 180mm disc. I was very surprised seeing that disc does not seem to be straight. It was strange for the new disc not being straight (flat). I took the disc and checked it on the table, it looked OK. When eyes cannot see I had to proceed with some accurate measurements, so today I did.

I see the following problems that might cause that the disc is not rotating straight:

  1. Disc is bent
  2. Disc mount, flange, or in this case KH Spirit Cranks is not straight.
  3. Hub axle is bent.

I did several measurements to find out what part is causing the problem.

One set of measurements consit with distances between disc and frame at different angles of wheel rotation. The graphs of measured distances in function of angle are sinusoidal. I was changing relative positions between crank, disc and hub among sets.

Ad.1 “Bent disc”
I compared 2 sets of measurements, the second one with the disc rotated 180 degree in relation to crank. There is a good correlation between graphs which means that the disc must be straight.

Ad.3 “Bent hub axle”
I compared 2 sets of measurements, the second one with crank rotated 180 degree in relation to hub. There is a good correlation which means that the hub must be straight.

After all above the conlusion is that disc mount on KH Spirit Cranks is not straight (the axle of disc mount is not in line with ISIS axle).

In my particular case the distances between frame and disc varies of about 2mm in one full rotation.

I have never before thought that cranks could cause such a problem. In the past I had always disc going a bit to left / right and always blamed the disc straightness.

Does any of you had simmilar issues?

I don’t have a disc brake so I can’t really be of help.
But I wanna say: kudos for your thorough analysis to pinpoint the cause of the problem.

Hi there

I’ve got two KHs with the spirit cranks/disc brake combo and haven’t noticed anything wrong with them. They are great and work really well (no rubbing or humming). One of my KH frames was off by a little bit though (wheel not quite centered) but as it was minimal I didn’t bother to replace it.

I guess there is always going to be some slight imperfections…


Same problem

I had Same problem with 2014 29" frame And 2015 Spirit cranks (Also 180 Brand new rotor). After lot of tries I found out there was a bit too much color on my disc brake tab (mount? Dont know how to call it) I filed it of And then I bent the rotor a bit. works fine now.

I have the same issue. On my 29 Guni the disc rotor is very close to the frame and I can see it moving from side to side when I am spinning the wheel. I noticed it when I build it, but I was happy just to get it to work because it was a bit of a struggle. Both the cranks and the brake were brand new. I also had an issue with this when I installed disc brakes on my old KH 36. Once again I struggled with it so I was happy I got it to work at all. New cranks and brake on that one too. I don’t have my KH 36’er here at the moment, so I cant double check.

The discs on my Spirit cranks run true.

The diameter of at the mounting bolts is 45 mm. A runout of 2 mm at the 180 mm diameter represents an error of just 0.5 mm at the bolts.

Make sure the disc face on the crank is free of any ridges, especially where the threads are cut in the mounting holes. It is possible that some alloy has been upset during the tapping of these threads, causing a high spot around the hole.

This could be corrected with careful filing.

Disc on crank (KH) is much less precise than disc on hub (Nimbus)

All my KH with disc on crank have this issue even if less than in your case, all my Nimbus with disc on hub are perfect

180mm disc is more severe than 160mm

This issue is that crank is fitted on the hub with ISIS interface : 1° conical press fit which is by design less precise than direct machining on the hub

The only solution I found is to true the disc in order it goes well while fitted on the crank and crank on the hub

I prefer Nimbus design (disc on hub with 125mm bearing distance for large wheels) than KH one (disc on crank) even if it’s a nice idea but not perfect machining because very difficult to realize it properly

There are 0,1mm and 0,2mm shims for the rotor. With them, you shall at least be able to get it straight. The other Option would be to take a rotor bender and bend it whil it’s mounted.
With the ISIS and the rotor mount surface machine in the same jig, this would not be possible. so i assume, the use two different jigs which means unclamping and reclamping the crank. :frowning:

I’ve used the rotor shims to correct the problem with a Spirit-mounted rotor. They came with one rotor I got so it must happen to bikes, too, although I haven’t had the problem with internal disc hubs.

It is interesting to see that I am not only one who experiened simmilar problems. It ensures me that my theory is correct.

I understand that my pair of cranks (the fourth one) has some extreme defect which should be an alert for the maker to check production quality.

I will now try to put some shims to align the disc properly. Maybe it is also possible to lathe the crank in correct angle, but it is deffinitely more job, and expensive as I cannot do it on my own.

I used shims with 0.2 mm thickness. Putting 2 pcs on the place corresponding to biggest disc-frame spacing and by 1 pcs at sides. It works as expected. Now the disc to frame distances vary 0.6 mm in full rotation. Not perfect but nothing is perfect. Hope this will not cause some other problems like disc vibrations etc.

Thanks for the feedback and in-depth investigation of this - appreciated.
It is not something for which I’ve previously received feedback. Checking with UDC USA, they hadn’t either.
It is hard to say what is the culprit without inspection and even then, it may be quite subtle as others have pointed out. My suspicion is that there is slight variation at the rotor interface on the cranks, although given the fact it hasn’t come up I’m optimistic it isn’t a widespread issue. I could be polishing or tapping of the threads. Personally I haven’t had any problems with this while running 180 mm rotors. I’m glad that shims seem to have solved the issue.

My impression of tapered ISIS is that the issue of tolerances is overblown. It mostly comes up if there is a wide number of different original manufacturers and the manufacturer of the axle is not the same as that for the cranks. In unicycling, we currently have few brands and very few manufacturers. The same shop makes both KH hubs and cranks, which gives them strong control over matching tolerances. There are a thousand things that can go wrong and be headaches in manufacturing but the ISIS interface has not really been one of them, since we started using ISIS in 2005.


The actual ISIS interface was/is a very good spec - generally where problems occurred it wasn’t just because of different manufacturers, it was that one of the manufacturers involved wasn’t meeting the spec.

As for it being an issue here, I really can’t see it - the feature people seem to be unaware of is that ISIS isn’t just a taper fit, it’s a taper fit with a hard stop. The hard stop - which on unicycles is formed by the use of a spacer - should align everything.