Deriving a unicycle crank length formula?

rob.northcott wrote:
*crank length is a very personal choice, based on leg length,
strength, stamina, preferred riding style and, possibly,
skill/technique*

This is absolutely right.

Unfortunately being right about this means that you are wrong when you
say not everyone can be right, because it is possible to imagine a world
with no crank zealots, where everybody realises that crank length is a
matter of personal preference.

Also, it's time to knock another silly belief on the head - that
shortening your cranks will always make you go faster, at the expense of
control and acceleration. (You do lose control and acceleration, but
your maximum speed does not increase without limit.)

Most people appreciate the potential for an increase in speed due to the
decrease in the circumference of the circles that your feet have to move
through when you put on shorter cranks.

Relatively few people seem to appreciate that the maximum torque you can
exert also decreases. The maximum torque dictates your maximum speed as
you will reach* a speed at which the resistive forces (friction, rolling
resistance and air resistance) match the torque and you can no longer
accelerate past this speed.

I hope that the diagram is visible when I submit my reply, because it's
nowhere to be seen in the preview and that it clarifies what I've said
above.




(* You won't actually reach this speed, but get asymptotically close to
it in my idealised, mathematical world.)


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Please excuse that hideous spelling mistake in the diagram.


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Rowan wrote:
*How long does it take to reach max speed on short muni cranks (short
in this discussion means 140-150mm)? Only a few pedal revolutions at
most.*

This is why your post is so full of rubbish! You're failing to engage
with the limits that I'm talking about. When I'm talking about short
cranks, I'm talking about arbitrarily short cranks - possibly as long as
89mm on a 28" uni but probably not. Seeing as this is this is your first
foray into this discussion you have no business telling me what short in
this discussion means!

The particular steepness of the gradients involved in muni means that
everybody rides with longer cranks than they would do otherwise.

For the time being, just forget about gradients and consider two 24"
munis racing on the flat. One has 89mm cranks on, the other has 140s.
You say that the 140s accelerate quickly. Well, they do! That's because
140mm cranks are actually fairly long when you're on the flat. I
wouldn't put 140s on a 28" wheel for a hilly route, never mind a 24" on
the flat. In the end though, the 89s will cream the 140s as their
maximum speed is so much higher.
*Losing control? Yeah right. Go back and practise against a fence
until you figure out how to balance.*

Don't patronise me. My balance is sufficient for riding a unicycle,
thank you very much. If you go and ride down a long, steep slope on a
unicycle with short cranks, such that the slope is steep enough and the
cranks are short enough that the maximum reverse torque you can apply is
less than the torque due to the weight of the wheel, you will lose
control as you get faster and faster. No matter how good your balance
is, unless you can glide, you will reach a point where you fall off
because you won't be able to pedal fast enough. You can practise against
a fence all you want, but that's not going to help you in this case.
*Maximum torque means maximum energy spent keeping your top speed, so
unless your mountains only have downhill and no up- then short rules!*

I don't understand what you mean here.


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Rowan wrote:
*You are wrong, cos not everyone can be right regardless of the crank
zealots and coming to terms with it being personal preference. *

Lol! Did it take you 35 minutes to think of that?


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Rowan wrote:
*For someone who doesn't like to be patronised, you are pretty dumb in
assuming that the time a message was posted determines how long that
person has been pondering their reply. It may be some indication,
especially in fast replies (an indication that they didn't think long)
but it is not an accurate measure. I can tell by your reply that you
didn't think for 56 minutes before coming up with that lame reply.
Unless you really are stupid! *

Ok, that's a fair point.


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rupert wrote:

Rowan wrote:

*How long does it take to reach max speed on short muni cranks (short
in this discussion means 140-150mm)? Only a few pedal revolutions at
most.*

[snip]

Seeing as this is this is your first foray into this discussion you
have no business telling me what short in this discussion means!

And you have no business being so condescending. Relax a little and
remember that it is perfectly possible to read the entire discussion
many times without actually saying anything and so understanding what is
being discussed.

I think if you go back and have a look at the previous posts, you'll
find that with the exception of tholub mentioning 125mm cranks,
everybody else has mentioned 150mm as "short".

Either way, there is no call for rudeness.

Roger

Rowan wrote:
*Ok, you're not stupid if you got that so quick. Back to the topic eh?
(Sorry I am guilty of thread jacking for my selfish argumentative
purposes). Crank length-
Determining a formula is an interesting idea, but the number of
variables would make it a very complex formula. Someone mentioned
cadence. It's a concept I vaguely understand, but I looked it up in
the dictionary and couldn't find a cycling definition for it-
something about rythym was about the best description for it. Q-factor
is another thing to consider too- cos the length of the cranks does
not include any distance it sticks out- wide Q-factor can cause more
wobbling and slower top speeds. As for determining your own
preference- I think one of the best ways to find out is to go to a
Unicycle weekend gathering and try everyone elses unicycles out- and
hopefully they will have variations on crank lengths. Otherwise it
could be an expensive business trying out the different lengths-
especially if the ones you like get bent! *

Yeah, I know when I'm beaten.

If I had to guess, I'd think cadence is the wiggle from side to side.


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Roger Light wrote:
*rupert wrote:

Rowan wrote:

*How long does it take to reach max speed on short muni cranks
(short
in this discussion means 140-150mm)? Only a few pedal revolutions
at
most.*

[snip]

Seeing as this is this is your first foray into this discussion
you
have no business telling me what short in this discussion means!

And you have no business being so condescending. Relax a little and
remember that it is perfectly possible to read the entire discussion
many times without actually saying anything and so understanding what
is
being discussed.

I think if you go back and have a look at the previous posts, you'll
find that with the exception of tholub mentioning 125mm cranks,
everybody else has mentioned 150mm as "short".

Either way, there is no call for rudeness.

Roger *

Those posts are on a different thread, and hence a different discussion!
In my post, as I have tried to explain, it didn't matter how short it
actually was, just that it was very short, not in absolute terms but
relative to its previous value.


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Rowan wrote:
*Someone mentioned cadence. It's a concept I vaguely understand, but I
looked it up in the dictionary and couldn't find a cycling definition
for it- something about rythym was about the best description for it.*

"Cadence" in the context of cycling is just pedalling speed in rpm.
Interestingly, inexperienced cyclists will in general use a lower
pedalling speed (i.e. ride a higher gear at a given speed). Most more
experienced cyclists find a higher rpm to be more efficient. There are
exeptions of course, but as a general rule it is often true. This can't
really be transferred to unicycling where _shorter_ cranks (analogous to
a "higher gear") are used to allow a higher pedalling speed. On a bike,
the rider picks the crank length to suit, then varies the gearing of the
bike to give the most efficient/comfortable cadence at a given speed -
to apply this to unicycles we shouldn't be talking about crank length in
isolation from wheel size (gearing).


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Some interesting input even if there was a short side step, but hey it's
a discussion

John mentioned BMI which is something I hadn't thought of but ought to
have as I've lost quite a bit just lately . Whether it'd affect crank
choice I don't know but I'd just be interested what BMI's unicyclist
have

Q-factor I'd not thought of and could make quite a difference though
you'd probably have to take hip width into consideration.

Short? Long? Whatever size you consider as short or long doesn't matter
with regards to a formula.

Speed? Cadence? These may be determining factors for crank length choice
but I believe one size does not fit all people in a particular
situation, which is what the formula idea is about. Rider 1 may be as
fast as rider 2 with the same wheel size and the same situation but have
different crank sizes and different body sizes. So have their body sizes
contributed to their preferred crank size? Thus does it work the other
way, know the body size choose the cranks?

Preferred crank size after experimentation is the key for the data. As
has been pointed out it wouldn't be very informative if someone had
ridden with nothing but what came with their unicycle (even if it turned
out to be the ideal length). Which leads to - wouldn't it be better if
you had an idea what crank length would suit you when you first bought a
particular unicycle? One seat tube length doesn't fit all, and although
more complex, why should crank length?

Muni terrain could well be very difficult to quantify and categorise,
any ideas?

Oh and am I thinking too much


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