Adjustable crank idea

I wrote this post some weeks ago in a bout of enthusiasm at a unicycle idea I had. Since then I’ve been wondering if I’ve actually got the correct idea of what splines are because some recent posts seem to be saying that splined cranks aren’t particularly easy to take on and off.

However, here’s the post in its original form, it’s about an idea for adjustable cranks, all comments welcome.

The most obvious way to make adjustable cranks is to simply have extra holes for the pedal to screw into, this has the benefit of simplicity but also several drawbacks: -

  1. when in ‘long mode’ the inner hole is a weak area for the crank
  2. changing length necessitates unscrewing the pedal and rescrewing it into a different hole i.e. inconvenient if changing a lot and also, I suspect that there is a limit to how often they can be changed before the threads start taking damage

The other method, and one used in commercial models, is a sleeve arangement; it works and seems to be intended for tandems, but involves compromises between strength, ease of adjustability etc.

The few tests of available adjustable cranks by unicyclists seem to have not been successful, with the cranks breaking; though we should differentiate between cranks used for commuting/transport and those used for muni/trials (with big drops etc).

So here’s my idea, in theory it looks like it should work well, but, lacking the tools/knowledge necessary to make a prototype, I’m not able to make and test it out. (see attached image)

The idea is, rather than adjusting the actual length of the crank, the distance betweeen the crank and pedal is altered by a mechanism that effectivly creates a bend in the crank.

So, if it is set to ‘125 mode’, the pedal axis is 125mm from the wheel axle, creating the same leverage effect as if a 125mm crank is used.

The benefits of this system is: -

  1. Use of present unicycling technology; splined axle/cranks are becoming commonplace and the same tools could be used to create the elements of the adjustable crank.

  2. Strength. I’m no engineer, but, given that splined axles/cranks are used in top of the range muni/trials unis, and, as far as I can see there is no reason why the stress on pedal/crank splines should be greater than the present axle/crank splines; then this setup should be strong enough for all uses of uni.

  3. Ease of changing length. As all the stress of riding should be mainly in the direction of the circumference of the spline hole, rather than at right angles, it shouldn’t require a hefty mechanism to hold the pedal in the crank hole. This means that something as simple as a hole drilled through the protruding spline part with a retaining pin should sufice, enabling quick and easy changes.

Like I said, I’m no engineer so maybe there’s some flaws I’ve missed and it’d be great if someone with experience in making uni parts/engineering gave some feedback.

It’d be nice if it did work because one of the things I love about unicycles is their mechanical simplicity. As someone whose experienced the pros and cons of different crank lengths for a given wheel size, i know that a workable variable crank would be a great boon to unicycling.

Of course, it looks like geared unicycles will soon be on the market, but my feeling is that they will have issues of cost and lack simplicity.

While gears may have the edge in terms of a large range, I would be quite happy with a crank that adjusted from 150-125mm, and I see no reason why a range of 100-175mm should not be feasible with a splined angle crank mechanism.


This is the set up for 125mm mode: -


And 150mm mode: -


That is a really, really cool idea, and one that sounds as though it would work well. Why not take a couple of sets of cranks down to a local machine shop, show them your drawings, and see what they say?

That’s a great idea! I have never thought about doing it that way. One part i can’t understand or figure out is how the two parts of the cranks connect. I would assume that the pedal side would be on the outside of the axle side. right? Also, would there be something like a splined axel on the crank part that goes into a splined hole on the hub part?

Over all I think the idea is worth seriously looking into.

Just imagine how many crank lengths you could get out of a 48 spline setup. :smiley:

Keep the ideas coming.

Initially I was thinking that the short arm (with the pedal attached) would connect on the outside of the long arm (the one that connects with the axle).

This would mean that the pedal would be slightly further outwards from the axle than on a normal uni.

However, the short arm could connect on the inside of the long one, this would leave the pedal no further out from the axle than normal; it would slightly restrict the shortest effective crank length a little as, in the short mode the pedal axle would rest on top of the long arm.

Then again, as some people prefer the pedal further out, as it stops their heel accidently catching the axle, the first method may be ok after all.

That would be optional, but, as splining would be used on the long arm/short arm connection, it would probably be used to connect the whole crank to the axle as well.

That’s a really cool idea. The limitations are minor. One could be surprised by the piece of crank coming up and bumping into the bottom of their shoe when in short mode. If that piece were significantly long it could sneak into a pant leg also. It probably wouldn’t be able to take big drops but a device like this would most likely be used for distance riding anyway.

Make it. Good luck.

I am impressed with the idea. Now, who’s going to make the first prototype? --chirokid–

Sounds like a job for Steve!

Great idea! I might try making one for a machine shop project, but probably not for a while.

I’m also not an engineer, but I can say that one limit would be the range you could build with this. With a range of 125-150mm, the part attached to the pedal could be very short (pedal axis would be 12.5mm from the center of the spline axis), which would be a plus, because that would put less stress on the hinge. But, as you make the range larger, say 100-175mm, you encounter some serious issues with the strength of the splined hinge. To have a thin splined area 75mm from the pedal would be extremely weak. The strength of splined setups partially comes from the length (thereby contact area) of the splined area (actually, now that I think about it, the whole point of a splined steup is to maximize the contact area between the crank and axle with a minimal increase in axle diameter. That’s why profile is so strong. Massive contact area between crank and axle. Square taper cranks suck because they have a very spmall contact area). Onza and Profile cranks have 1" contact areas, I think. For contrast, square taper joints use about 1/2-3/4’’ of contact, which isn’t as good. So, the point is that in order to put one of these splined attachments on the middle of a crank would mean it’d need to be rather thin, which would weaken it. Also, it would be 97.5-175mm range. YOu could work a way around this, though. You could have a strengthening pin in it, so when you want to do extreme stuff, you just set the crank in it’s shortes setting, and put a clamp on each piece so the joint is reinforced closer to the pedal.

With that in mind, I think a modification of this design would be to include a crossbar that would be sleeve type of bar, which would only get direct straight forces. You have the hinge be a freemoving hinge (instead of splined), and have the adjustment be the length of the bar. You could use a bolt type of tightening system where you tighten the bolt on the outside of the bar, which would clamp down on the inside one. It’d work like a unicycle seatpost or a router collet. That might be a bit thinner and stronger.

My $0.02

If you want more material between the various pedal holes in the crank, just CURVE the crank arm forward, i.e. down from horizontal starting at the axle…the pedal holes will still be the desired length from the axle, but with the curve, the partial circumference of the curve will result in a longer path to the next outward pedal hole than a straight line. With this design, the crank can stay 1 piece and thin.

What about something like this. The big disk could be made out of steel for durability (the tapered hole will last longer) and the crank could be made from aluminum to save weight.

Re: Adjustable crank idea

from one non-engineer to another…

could this particular problem be overcome by using clip-on pedal type technology? ?
allowing to un- and reclip the pedal in different positions in a matter of seconds?
and if your onefooting is good enough…

The cool thing about John Child’s modification is that you could use the same wrench that you use for the bearing holders. Another advantage: it is cleaner for your feet.

Re: Adjustable crank idea

Cool idea. Seems workable.

RE: Stresses - Since the pedal sticks out from the crank it creates some pretty severe torques at right angles to the spline axis. These are resisted by the splines taking an up force on the outside of the crank and a down force on the inside of the crank. These stresses are far greater than the actual propulsive stress.

For example, assume a 160 lb rider is just standing on a regular pedal with the crank pointing straight down. The force is exerted about 3 inches out from the crank. This gives a torque of 480 inch-lbs (40 foot-lbs) that acts about the axis that is at right angles to both the crank and the pedal spindle.

On a regular threaded pedal the threads are about a half inch long. For simplicity, assume that the forces between the pedal and crank act at only two points - one at the inside face of the crank, at the end of the pedal threads, and the other at the outside faces of the crank where the pedal shoulder contacts the crank. (This assumption makes the forces act with the longest length which gives the smallest possible forces, which is unrealistic, however it’s good enough to make my point.)

Engineering statics say that these two forces must 1) carry the weight of the rider, and 2) resist the torqe of the offset pedal. If we call the upward reaction force on pedal thread at the outside crank face (pedal side) “Ro” and the downward reaction force on the pedal threads at the inside crank face (wheel side) “Ri” then the statics analysis can be described mathematically by the following two equations:

  1. Vertical forces must sum to zero:

(upward force on the pedal) + (upward force at the outside face + (upward force at the inside face) = 0

(-160 lbs) + (Ro) + (- Ri) = 0, or simply

Ro = 160 lbs + Ri

  1. Torques must sum to zero, so (think teeter-totter):

(3 inches160 lb) = (.5 inchRi), or simply

Ri = 960 lbs

Plugging this into the vertical forces equation means that

Ro = 1120 lbs

This was for an average rider just standing on the pedal. A heavy rider doing dynamic things would create reaction forces of up to a couple of tons. The contact area of these forces is going to be about the size of a pencil eraser, so we’re talking serious metal stresses. With forces like these it doesn’t take long for a little bit of slop to turn into a seriously worn fitting. Add a little road grit and the life expectancy and “feel” with the clevis pin setup won’t be very satisfactory even if it is perfectly machined at the beginning.

You’re going to have to consider some way to make the two parts really connect with a massive stress preload to keep them from moving against each other. Either a split outer spline with a quick-release pinch mechanism (borrowing from curent splined crank design), or tapered splines with a mechanism that creates a strong axial pull (borrowing from the square tapered spindle design) would do.

Best of luck. It’s a seriously good idea.

I’m almost sure I saw a picture of a unicycle with cranks like this once. On that unicycle the cranks where diamond shaped and could be flattened if you wanted shorter cranks.

Short cranks:

Long cranks:

I can’t remember where I (think I) saw this, and google fails me :frowning:
Does anyone know of this picture?

There was some discussion of adjustable cranks a few months back.

The ‘obvious’ way is multiple holes in conventional cranks. Yes, it involves removing and refitting the pedals, which might not be good for the threads, but every design involves tightening or loosening something threaded every time the assembly is adjusted. Also, it restricts you to a number of set positions (say 3 holes per crank) whereas other ideas involve ‘infinite’ adjustability within the limits o fhte size of the assembly. The weakness isn’t a problem, if you are prepared to sacrifice lightness (i.e. add weight).

The next most ‘obvious’ way is some sort of telescopic arrangement, with a barrel adjuster - a bit like the way that a chain is tensioned on a giraffe, or on a motorbike. This would allow ‘infinite’ adjustment within the limits available. It would need to be well engineered to avoid ‘slop’ creeping in, and I guess it would be vulnerable to wear. It would probably take approximately as long to adjust as the 3 hole design, would be more expensive, wear more easily, and be heavier.

Then there’s Dave’s hinged-crank idea. Clever to use the splines, providing a large number of fixed positions. It would be expensive to engineer, and either a bit weaker or a bit heavier. It might well be prone to wear and slop, unless the splined bit was tapered, so that the socket was pulled onto it - in which case the wear and tear on the nut and bolt would be as much of a problem as the one Dave mentions with the 3-hole system.

A variant of this would be with a simple square tapered peg and socket, rather than splines. This would offer 3 positions: straight, right angled, or short. I guess that the adjustability would be rather crude, with 2 very big steps.

Another variant would be the smoothly-moving hinged crank, with some sort of barrel adjuster across the inside angle, offering ‘infinite’ adjustment. It would be quite heavy, quite expensive to produce, but probably less expensive than the splined version.

Another option I floated once before is the disk, rather than the crank. A disc could have multiple sockets (as I suggested once before) in which case it would be no more than a variant of the 3-hole crank system.

However, imagine a disc (rather than a crank) with a slot in it. A socket (for the pedal) could slide along this slot, and then be locked in the desired position with a large lock but behind the disc. The pedal would remain in its socket at all times. The socket would be a sliding assembly with a large nut (less prone to wear) which would hold it in the desired position in the slot.

Now look at the slot: it could be simple a curve, offering various effective crank lengths. However, if the slot was ‘gated’ (imagine an ‘E’ with more than 3 prongs) then the socket could slide along the slot and drop into the ‘gate’ so that the walls of the ‘gate’ helped to hold it in place, rather than it relying on the grip of the clamp bolt.

In fact, with a gate system, it might even be possible to develop a version of the clamp with a cam-action quick release similar to a seat clamp or bicycle wheel QR.

Or we could all enjoy the simplicity and purity of the unicycle design, recognising that each set up is a compromise, and choosing the right compromise for the planned ride. ;0)

I saw this idea floated in this forum in relation to a suspension giraffe. It would work with the cranks thing, although, again, it would be heavy, expensive, and prone to wear. Possibly, as it would be triangulated, it would be the best of the various hinged versions.

this it the only decent idea ive ever seen for adjustable cranks.

although i personaly have no need for adjustable cranks, i can see that people who ride 29ers would like them verry much.

some ramblings on the idea:

this would actuale be not imposible to make form bits and bobs.

a splined bmx/mtb botom braket axle

and some aluminium cranks

you could esentialy just cut the cranks in half,

use part of the splined axle to make a tool like steve howards tool for splined aluminium cranks.

and stick the two ends together with bits of splined axle.

and modify one end to have a pinch bolt.

I’m not sure that 29er riders would be the only people interested. Perhaps you meant people who ride distances on varied terrain - as opposed to people who only do hard MUni, or hard trials, or freestyle. This group would include people with 26, 28/29 and Cokers, as well as the occasional 24. (I have done 24 miles on a 24, and 20 on a 20, so those rides included sections where a change of cranks might have been nice.)

The idea of ‘knocking together’ a set of cranks from odds ‘n’ sods is nice, but simplistic. It is absolutely vital that the pedal spindle axis is exactly parallel to the axis of the hub. Even a tiny error is detectable by the rider, as the pedal appears to ‘seesaw’.

A friend of mine runs an engineering business, and I set him the challenge of making me some 75 mm cranks (in the end, I didn’t go ahead) and he agreed to do it, but only after he had worked out how to make some sort of jig to ensure the two axes would be perfectly parallel.

Back to my old theme, though: to me, the unicycle is the purest form of pedal cycle. It has the minimum number of components: seat, wheel, cranks, pedals, and a frame to hold it all together. Part of the beauty of the unicycle is the interaction of the two main variables (wheel size and crank length) and the compromises which result. (Can I manage this difficult steep diversion on these short cranks? Can I go far and fast on these big cranks? What is the right set up for today’s planned ride?)

Adding the complexity, weight and expense of adjustable cranks to such a beautifully simple machine seems a pity.

(I make two exceptions here: the geared hub which introduces a genuinely new element to a unicycle; and the ultimate wheel, which is ‘more pure’ in a sense, but is less of a vehicle and more of a toy. (I have one on order, so don’t think I’m belittling them!)