Physics/design question

There’ve been some design comments in various threads about whether to locate the pedalling axis below the wheel hub axis, as opposed to cranks normally attached at the hub.

I’m wondering if having the crank axis below the hub would really have any advantages. If you normally use your entire body above the hub to balance, wouldn’t having lower cranks take away from that efficiency? Wouldn’t leaning the upper part of the uni forward or backwards for balance be diminished by the fact that the lower cranks would move in the opposite direction?

I’m glad you are getting inventive again

:slight_smile: I really admire your craftsmanship. I worked in a bunch of shops welding and fitting about 25 years ago. I so wish I had access to the expensive migs and tools I used to get to play with.
I sort of assumed that the lower CG of the under axle crank design would improve balance. Especially considering the alternative of cranks above axle, which brings excess seat hight. The complication of the cranks pivoting the opposite way as the seat around the axle had not occurred to me.
Here’s an idea ! (note, I did not say good idea!)… 20ish years ago I got to ride a friends homemade recumbant ( low down seat, feet and cranks in front) tricycle. It was surprisingly easy to pedal, forward or reverse.
When I was commenting on the high mechanical friction of your jackshaft design, I came up with the laughable suggestion to connect each crank to the axle with a straight cut gear, not realizing at first the uni would go backwards.
However, perhaps by putting the cranks below and somewhat in front of the axle, this would make pedaling backwards not intolerably strange. Maybe.
There is four things I like about this idea.

  1. It’s 4-5 % frictional power loss would be the lowest of any geared uni. Perhaps making it the fastest.
  2. The chance of a chain coming off at speed :astonished: is eliminated.
  3. The distance from cranks to axle would be much less then required for sprockets, reducing the pivot problem you mentioned. Also, driveline slop would be much smaller.
  4. The sight of such a thing wizzing by with the rider pedaling backwards would make everyone stare with their mouth open !:smiley:
    Perhaps a simple test rig can be made of a regular 36 clamped up on something with a roller on the tire for resistance. Just to see if pedaling reverse is something one could get used to…

Hey Pete, a new idea !

I was reading stuff about gears, chains, sprockets, looking for references about power loss etc. then I found this. The internal gear. I was familiar with these from planetary gears in boat transmissions, but had forgot about them. The inside gear.
Now imagine a guni with an inside gear on each crank axle stub. This of course would mesh directly on a straight cut gear on the wheel axle.
All of the advantages of my pedal backwards design, in terms of low power loss and drive train slop, and safety. Except now we can pedal forward normally. Cool huh ?
OK, a pain to weld up, but you are good Pete. You could do this !:smiley:


ohhh sweet
but how will you keep “sludge” from building up in the internal gearing? (reasonably)
i guess I can come up w/ a way or 2
and you could make it an oil pan if you wish

i guess you can figure a way to connect the gears…
put one gear between the 2
make it solid to a frame extension
it will have a big Q factor tho

Worlds fastest uni

Maintanance ? It should last decades with just a wax and dust every few months. Sitting on the mantel, with the yellowing page of the Guiness book covered with yellowing plexiglass set out in front. Allow the curators in future centuries to worry about maintanance.
It is for us, the living now, to create the worlds fastest uni.
Few shall note , and none long remember what we may do here. The record, and the machine, craftsmen, and designers will not be mentioned.
Unless we really bust the record !:smiley: Then our you tube video of some super pedaler going 30 + mph on a uni will stand for all involved as being just so cool :sunglasses:
We are talking the record !:smiley:
The need for excellent gear allignment, combined with the fact that this design mandates a minimum overdrive ratio of 3.5 to one, says this will be a special machine. The bonneville uni will be about as practical an everyday ride as other bonneville machines. Not at all !

I don’t think frictional power loss is going to be your speed limiting factor for making the fastest uni. What you need is a stable unicycle with a lightweight wheel and a big enough gear to maintain the optimal cadence for the particular rider.

There are already unicycles like the 36" Schlumpfs which have a much bigger gear than people can push efficiently (or are happy to push)- evident in the fact that these machines tend to be fitted with longer cranks than the equivalent fixed Coker. ie- you’re not gearing up a unicycle much if you end up having to use longer cranks on it because it becomes too hard to push (unless you’re more efficient at lower cadences). The one’s I’ve seen have all been fitted with 170/175mm cranks. I doubt very much that a geared Coker would go very fast on the flat with 100mm cranks unless the wheel was lightened significantly. The geared 29"/110mm should be about right. I reckon that would be the fastest ‘on-the-flat’ Uni currently.

let’s pretend we have one horsepower

I want to talk winning by design here. Let’s pretend for a moment that the rider is generic. Mr. , Ms. , one horsepower.
Different gearing setup’s may reach the optimum mechanical advantage that we need to reach top speed with one horsepower.
The design that has the lowest % of power loss in the drivetrain will set top speed, because it will deliver the greatest % of one horsepower to the wheel.

That would make the fastest unicycle for a machine to ride, not a human being.

Besides, you have close to 100% efficiency now by using a fixed wheel. You can increase your speed to a large extent by shortening the cranks.

Ken, I’m with you buddy! After you actually ride a guni you get a wealth of knowledge about what works and what doesn’t. Growing up I was fortunate that my engineering Dad’s mantra was: “There’s no substitute for experience!”

This issue of a lower crank axle offsetting the upper body’s balancing inputs is still open. I agree that the issue is mitigated by having the cranks as close to the hub axle as possible, but even with the gearing now on both sides of the wheel (as opposed to a jackshaft), you need room for different size cogs/chainrings to vary the gear ratio.

I know there was an old penny farthing bike that used this setup, with both cranks set below the wheel axle, but I can’t find the pic on the web. Plus with a bike the offset issue is null because it’s not a uni.

I’m interested in building such a frame in order to have a different guni drivetrain, but I don’t want to waste my time either.

feel, how do you propose to change the gear ratios with your planetary system approach? The Schlumpf is based as a bottom bracket component, which is very different.

Hi Pete,

Sorry to go off-topic.

Back to your question- I wonder if it’s possible to somehow cross the chain or add some other gear to flip the forward/backward balance back in the correct direction (if that makes sense)?

If that was possible, then yes, I think having cranks below your wheel axis would be beneficial for speed because it adds to stability because you have a lower centre of gravity, allowing you to go faster.

I’m not sure how much the pedals below the axle will improve (or not) balance or stability when riding efficiently with all your weight on the saddle. It seems you would only get the theoretical improvement in stability when standing on the pedals or putting significant force on the pedals. At other times you should be spinning lightly on the pedals with as much of your weight on the seat as possible. When your weight is on the seat there would not be any significant improvement in stability.

Climbing on such a unicycle could be interesting. I don’t know what such a uni would be like to ride up a hill. My initial reaction is that it would be a very poor climber and react strangely to hill climbing. Might be the only way to find out is to build one and see. It would seem that the seat is going to want to pull back which would force the unicycle into a more upright position rather than letting you lean into the hill.

I was wrong

My assertion that all chain and sprocket sets have a min. power loss of 6 % was not supported by my recent internet research.
Here’s the best link I found.
Basically it says researchers measured bike chain power loss that varied from 1.4 % to 19 %
It does not give enough info to say what the loss on Pete’s guni chainset is.
However it may be closer to 3 % a sprocket pair, about the same as straight cut gears.
Given the cost, complexity, and perhaps no power savings with the gear system, I will give up on it.
I am sure Pete would save some power loss by going to a system of of a chain for each crank. It seems likely that this power savings will be less then the 6% I stated before.
Also, just switching to larger sprockets should help a bit.


I once had the opposit experience:

Opposit in 2 ways; cause as you can see… the gear is chosen to control the “big” “wheel” with less force than a 1:1 ratio.


Did you build that ? I think it deserves some kind of award. Both for building it and riding it !:slight_smile:
Is it your idea or did you steal your technical advancements from Dr. Zuess?

It’s made by Semcycle. It was also their idea to use this (in bicycling for many many years excisting) solution for the gearing of this unicycle.