Adjustable crank idea

All we need is an axle-less disk-wheel with a slot running across it. The seat post could go down onto a set of ball races that attach to the top of the rim and the effecttive crank length would be almost infilitly adjustable.

Imagine if we could adjust the length on the move: gearing!


Interesting replies. I find it difficult to understand some of them, i find diagrams more useful than words in stuff like this.

Concerning worries about stresses on the cranks, one of the ideas behind the design was that, as splined hubs/cranks exist because they can take high stress, then the same splined set up on the adjustable crank should be ok?

If the problem is that the dimensions would be smaller than the ones on the splined hub/crank, then couldn’t it be overcome by simply making the cranks parts thinner, so the splined components are the same size as on the plined hub/crank?

Admittedly the cranks would be heavier, but I’ve never understood why lighter cranks are so sought after, does crank weight really make much difference?

John childs, on your diagram, does the square taper fit on the hub axle, or is it representing the short crank part so the square taper fits on the end of the long crank part? What is the prime advantage of it?

Yes, I think it could

An axle-less wheel, with a disc, rather than spokes, and the frame perched on top, held in place with a set of bearings which somehow grip the rim?

Yes, it could be done. The disc of the wheel would need a ridge on each side running parallel to the rim, so that one set of ball races/bearings could rest on the underside - otherwise the frame would only be sitting there under the influence of gravity. The frame would also need to extend some distance down onto the surface of the disk to gain enough leverage to stop the frame wobbling from side to side in relation to the wheel.

So I estimate that you would need beartings as follows:

2 being 1 each side to provide lateral stability
2 being one each side to ‘grip’ the inner edge of the ridge to stop the wheel falling off when you hop.
4 being two each side (one pair forwards, one pair backwards) to provide fore and aft stability of the frame, relative to the wheel.

Then assuming that the rider could cope with the gyroscopic forces of the heavy disc wheel, and that there was no gusty cross wind, it would work really well.

But I agree, it would look soooo cool. :sunglasses:

Re: Re: Adjustable crank idea

I had difficulty grasping most of what you said (not your fault, like I said just above I find diagrams easier to grasp than verbal descriptions).
Am I right in thinking that you’re saying one of the difficulties with the design is that fact that there’s effectively an angle in the crank, rather than it being straight? i.e. that it’s not the fact that there’s a splined join, but that the two parts are at an angle.

As for the contact area being the size of a pencil eraser, can this be solved as I suggest above by just making it bigger?

I totally agree with the purity/simplicity of the unicycle being a big part of its appeal. I’m not so keen on the challenge of the crank/wheel size compromise.

The main appeal of adjustable cranks for me is that there is a possibility of a design which would remove the compromises of the wheel/crank size limitation, whilst retaining a purity/simplicicty of design.

i.e. an adjustable crank of this type, assuming it’s workable, would be something I can understand and adjust/maintain myself, unlike a geared hub which would be beyond me.

Re: Re: Re: Adjustable crank idea

Hi Dave,

No, I think the bent crank idea is fine. It’s actually kind of elegant, as there really isn’t much difference between a bent crank and a straight one as long as the holes for the crank and pedal are parallel. Symmetry is just an aesthetic. The bent crank is going to be heavier, but otherwise it should function the same.

All I was responding to was one paragraph that suggested that the spline didn’t have to be tightly clamped and could be held in place with a clevis pin. A regular spline with pinch bolts works just fine; and with a little work the pinch bolts could be replaced by some form of quick release mechanism.

I also like John Child’s design. There is a fixed gear hub for vertical dropout road bikes that uses an excentric axis method to adjust the chain tension.

But the splined joint in the middle of the bent crank is the first that I’ve seen that is simple, strong, and with a discrete number of settings. It’s easy to make the left and right cranks the same length.

You understand exactly what I mean :smiley:

I looked on google for some pictures and found that they are called orbital wheels.

Looking at the bike 6 down on the right, it gives me a great idea for a short giraffe…
If a giraffe of this sort was made, would it be easy to gear it?


It looks like Ed Mosimann already built something similar in concept to for Bronsons custom coker. I don’t believe Bronson has that coker anymore, but it would be interesting to hear how that system worked out. It looks similar to John Childs picture, except the rotating wheel is attached to the pedal rather than the hub.

I love the idea, Dave. I just want to point out that as far as I can see, all of cyberbellums objections apply equally to the regular crank/axle connection. Since we already know the splined connection works, it shouldn’t be a problem for the same thing to be applied to making a bend in the crank. As a few people have mentioned, it will add weight, but it’s a trade-off. It probably doesn’t have to be built super beefy anyway since the main application for this is road riding and cross country.

Someone should certainly build a prototype of this so we have something to talk about other than theory.


Agree. If there are any problems they won’t be obvious until there is a working prototype. I can’t think of any reason why it can’t be made to work mechanically.

These are some great ideas! Actually, John Child’s eccentric hub crank is something I’ve thought of before but Dave’s idea is one I’d never considered.

I got so excited about the two ideas, I decided to do a little design work. The attached .jpg file shows Dave’s idea on the top with John’s idea below - both mated to my extra wide hub.

Dave’s crank is adjustable to five positions: 170mm, 163mm, 147mm (shown), 129mm and 120mm. When made from aluminum with a short steel splined shaft between the two crank parts the weight of one cranks is 1.02 pounds. The spline pattern is the 8 spline KH.

John’s crank is adjustable to six positions: 170mm, 165mm, 156mm, 142mm, 130mm and 125mm. With the eccentric hub and crank made from aluminum (and the lightening holes as shown) one crank weighs 1.11 pounds. By contrast a 170mm Profile crank weighs about .64 pounds.

I think cyberbellum is right on about the splined joints being something with no slop. A pinch type arrangement should work pretty well in both designs.

I know what the next question is: When am I going to make a set?

Steve Howard

John’s design has two advantages - easier ankle clearance and less clamping force. The huge bearing area might make a ski-buckle type quick release possible. Since you probably have access to a CNC machine it might not add much effort to add some internal keys on either spline system to eliminate the possibility of it sliding off the end. This helps reduce the required clamping force too. On John’s original design this could be a simple band around the core with a corresponding slot in the hoop.

Very fast work, btw… and very clean drawings.

So, when are you going to make a set? :slight_smile:

BTW, you three (Onewheel Dave for the bent-crank spline idea, John for a smooth-surface eccentric interpretation of the idea, and Steve for combining the two) have 1 year from today to submit a patent application otherwise you abandon any claims. Might be a good idea to talk to a lawyer and submit a provisional writeup. For an idea like this that would cost about $400.00. The actual patent would run about another $4K. Don’t know how marketable the idea is, so the patent idea is a business decision. Unicyclists are a niche market - you’ll probably never see a return on your investment.

I’ve gone through the process a few times. It’s tedious and expensive. Message me if you want any advice.

On Dave’s cranks, why not have the joint a little closer to the center? Or, make the part that attach’s to the axle shorter. In my opinion the main draw of an adjustable crank would be to let you have something really short and fast, and 120mm isn’t that short. Incidentally, If you do build those and need someone to put them through there paces, I’d be happy to help out.


Steve, you rock! :slight_smile:

I don’t know which of the two would be easier to manufacture. My gut feeling is that the eccentric disk design might be easier machine on a CNC mill because the splines are bigger and more widely spaced and there is more room for the machining tool in the bigger hole.

Yes, Ed Mosimann’s adjustable cranks were the inspiration for my design.

Steve Howard posted a nice 3D rendering of both designs. Hopefully the 3D version makes my design more clear. I used a simple flowcharting tool to sketch mine. Steve used a professional CAD program to draw his.

Advantages for the eccentric disk design:
Lower Q-factor
No protrusion that might knock your foot off the pedal
Possibly a stiffer design (less flex in the crank)
Larger contact area around the disk which means less clamping force is needed
Probably better handles the pedaling forces (less flex and torque on the crank)

The big disk will get in the way when adjusting the bearing clamps on the frame
Making a crank with more range of adjustability would require a larger disk. To handle the goes from 110 mm to 180 mm (a good range for a Coker) you’d need around a 3.5" diameter disk. That’s kind of big.

The big question is which one is easier to make.



Backup to the bottom of page 2 in the thread. Steve’s post was posted at 4:13 PM PST today.

Here’s another adjustable crank idea. I snapped this picture at the Seattle Bike Expo last year. Since that adjustable crank design is on a commercial product there might be patent issues.

The manufacturing of this style would be easier and therefor less expensive. They’re from Power Cranks. I mentioned them in this thread last year.

Power Cranks are really weird. Check out the videos here to get an idea what they are. The text descriptions of how the cranks work don’t give you the full idea.

Well there you have it! Nice research John!
Maybe Power Cranks would make a set just for unis??
Heck, they’d have almost the entire crank market with 1 product!