Deriving a unicycle crank length formula?

There have been lots of threads on crank length with various suggestions
on what you should use for various size unicycles in various riding
situations resulting in information that is probably not all that
helpful, particularly for new riders. Yet the data from all these
experienced riders is helpful if only it could be collected and used in
a more unified manner.

This got me thinking (a little bit dangerous but still) that if this
empirical data could be collected some form of useful crank length
formula may be able to be derived. My line of thinking is that the crank
length a rider prefers (after experimentation) is dependant on the
individuals height, leg length (maybe upper/lower proportion as well),
wheel size and the terrain/style of riding. The awkward one is defining
the terrain/style of riding to be understandable and relatively
consistent.

It may be that with this data collected there is no defining correlation
between them but knowing that would be better than the situation is now.
If there was a correlation or the data pointed to further ideas a useful
general formula might be derived that would help unicyclists, especially
new ones, choose an approximately appropriate crank length. Whilst I was
searching I came upon 'this site '
(http://www.nettally.com/palmk/crderiva.html) about a bike crank length
formula that I found interesting.

So what do you think? How would it be best to collect this data? Is
there any other data that would be useful like seat height above
pedals?

BTW I’m not asking for the data to be posted here just a discussion on
the merits of collecting data to derive a general crank length formula.


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unicus wrote:
*My line of thinking is that the crank length a rider prefers (after
experimentation) is dependant on the individuals height, leg length
(maybe upper/lower proportion as well), wheel size and the
terrain/style of riding. The awkward one is defining the terrain/style
of riding to be understandable and relatively consistent.*

Firstly, you'd need to survey everyone on the forum and find out all
these measurements, for all their unicycles. The main thing is to do a
good questionnaire, or the following analysis will be made harder and
its conclusions weaker. Also, getting lots of respondents will be
particularly important, as many people will ride with suboptimal crank
lengths and you will need as large numbers as possible to "average out"
this effect.

Then you'd have to analyse the data with regression analysis, keeping
data from different terrain/style of riding separate
(muni/freestyle/distance etc.) as this is discrete and cannot be
quantified, which I think you were touching on. Right now, I can't think
of the best way for allowing for people who use a unicycle for two
different sorts of riding. I think you might have to exclude them. Also,
I think the data will be overwhelmed by "stock" unicycles with their
default crank lengths, and you might need to exclude people who haven't
“thought� about crank length or something like that.

During analysis of this number of pairs of variables, you'd be very
likely to find several statistically significant correlations, a high
proportion of which would be false positives.

You could certainly make a formula for crank length, but I doubt it
would have any predictive value, although it could be a good starting
point for beginners, as you suggested.

All this said, if you want to investigate it and you would like some
help, I'd quite enjoy helping.


--
rupert
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That would be a very interesting excercise, but possibly quite difficult
to carry out usefully. It would be good to see how the theories people
spend so long arguing about actually fit into reality. But, as has been
suggested already, it would only really make any sense if the data came
from people who have tried a few crank lengths and settled on a
favourite, rather than people who have just got used to what they have.
I've got reasonably quick on my 26x3 with the default 165 cranks, and my
maximum speed (i.e. top speed in a short sprint, not very sustainable)
is up to about 13mph, cruising at 9 or 10ish on easy trails. When I
first had it, I could only manage about 9mph max, but I haven't changed
anything, just got used to it.

I've been thinking about trying some 150s to see what it's like, but
there are some hills on my usual route to work that I can only just
climb (and only just descend without killing my knees) on 165s, so I'm a
bit undecided. Lots of people rave about shorter cranks, but I don't
want to have to walk half way home just for the sake of a higher top
speed on the flat bits. I'm quite tall and I've always ridden a
relatively high cadence on a bike compared with some cyclists, so
perhaps the 165s happen to be a good length for me. It would certainly
be interesting to collect some hard data. I'll probably try some 150s
anyway though, just to see what happens. If Mike's theory on the other
thread applies (crank length change proportional to speed change for
small differences) then I'll only gain about 1mph in speed, and may have
to walk up a few hills that I used to be able to ride. But then again,
if the riding feels smoother with shorter cranks that may compensate
slightly... hmmm.... looks like I'll have to get hold of some cranks
and try it for myself.

Well that got a bit waffly - and half of it probably belongs in the
other thread...


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rob.northcott - Speed Freak (apparently)

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Rider weight or body mass index may also play a role. Probably more of
a factor for muni than road riding.

I've been losing some weight and I've noticed that I'm having an easier
time pedaling now and climbing. Could be because I'm also getting more
fit at the same time. Slightly shorter cranks now may feel better to
me. I should give them a try again and see.


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If you're riding to work on roads, you should try out 150s. I recently
switched from 150mm to 125mm on my road uni, and I haven't yet found a
hill I could make it up on the 150mm that I couldn't on the 125mm, on
road. Off-road is a different story. It's worth a try.


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tholub - Kinetic Sculptor
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I think that personal preference has more of a role than any formula or
application. For example, I like long cranks. For everything. I think
the 6 inch (150s) on my 28 feel small.
I love the 175s on my Guni. I wouldn't mind longer cranks, say, 175s on
my 24" Muni.
I'd probably actually like 175s on everything, regardless of type of
uni. So, I don't think a forumula will really work--at least not for me.


--
Skippii - Kingpin, One for the Road

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rob.northcott wrote:
*It would be good to see how the theories people spend so long arguing
about actually fit into reality.*


Infidel! You must have faith in the unifying power of the scientific
method!

Seriously though. Is it really worth that much effort? Isn't it easier
for newbies to just try some different lengths than to receive a
mathematical explanation of which crank length best suits them? Since
most newbies buy complete unicycles complete with cranks, isn't the
following enough?

If you want to go faster and give up control, get shorter cranks. If
you want more control and don't mind giving up speed, get longer cranks.


--
phlegm - skill level e
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you forgot to add-

'If you want more control but don't want to sacrifice speed, just
practice! You'll become better in time and wonder why you were ever
thinking about using longer cranks in the first place'

:P

Often, barring certain things like limits of the equipment (splined vs.
square taper hubs going off huge drops), it's all in the mind and fine
motor control of the rider. What I could do three months ago vs. what I
can do now is quite a bit different, but I'm still on the same uni.


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flyer - Level 5 Unicyclist
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phlegm wrote:
*

If you want to go faster *

Get a bigger wheel.


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tholub:
My route is mainly off-road (non-technical though) but with a couple of
miles of road. The steep hills I mentioned are on tarmac.

phlegm:
I wasn't hoping for an alternative to actually testing different cranks,
just suggesting that it would be an interesting survey. People seem to
have very strong views on crank lengths, but different people have quite
different views, and similarly different arguments to back up their
beliefs. It's almost like arguing about religion - everybody strongly
believes they are right but they can't all be. I feel perfectly
comfortable with my 165s, but reading all the crank length agruments has
made me curious to try some different ones for myself. At the moment,
and with my limited experience with unicycles (but considerable
experience with bicycling) it seems to me that crank length is a very
personal choice, based on leg length, strength, stamina, preferred
riding style and, possibly, skill/technique.

Rob


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