Crank length

Hi,

I’ve read on Brett Bymaster’s page, that he uses cranks with a length of
152mm (6.") on a 26" unicycle. I wonder if this is the length from end to end or
between the center of the holes. I have a 24" with cranks that have a
total length of 152mm and I have already problems when climbin uphill.
Well, maybe Brett is in a much better shape than I am , but looking at
the pics of his MUni, the cranks seem to be longer than mine.

BTW, after all the responses to my “shoes and pedals”-question (thanks a lot), I
was at my local bike store this saturday and ordered a pair of BMX pedals: big
platform and steel pins.

lars


Lars Burgstahler Institute of Communication Networks and Computer Engineering
University of Stuttgart, Pfaffenwaldring 47, 70569 Stuttgart, Germany Tel: (+49)
711 685 7966 Fax: (+49) 711 685 7983 E-Mail: burgstahler@ind.uni-stuttgart.de

When was the last time you were flabbergasted? CIRCOMIQUE

Re: Crank length

Lars,

> I’ve read on Brett Bymaster’s page, that he uses cranks with a length of
> 152mm (6.") on a 26" unicycle. I wonder if this is the length from end to end
> or between the center of the holes. I have a 24" with cranks that
> have a total length of 152mm and I have already problems when climbin
> uphill. Well, maybe Brett is in a much better shape than I am , but
> looking at the pics of his MUni, the cranks seem to be longer than
> mine.

You made me laugh… the thought of 76mm cranks off road! Brett does ride fast,
is this his secret?

I ride with Dotek 152 cranks and can ride up any hill that I need to, they also
lets me spin to get good speed up off road.

Here is a little maths:

with an applied force of “F” using a 36" 26" 24" 20" 178mm (7") force at wheel =
0.40F 0.51F 0.57F 0.68F 152mm (6") force at wheel = 0.33F 0.43F 0.48F 0.58F
125mm (5") force at wheel = 0.28F 0.36F 0.40F 0.48F 100mm (4") force at wheel =
0.22F 0.29F 0.31F 0.38F

These figures are only really important when you are doing things on one wheel
that require high force like idling or riding up hills. But you can see
(ignoring factors like wheel weight etc.) that it should be as easy to ride up a
hill on a 36" with 7" cranks as it is with 4" cranks on a 20".

When you look at the speed, comparison between cranks it is not the force that
can be applied because with a 1:1 gearing generally we don’t need all the forces
that our legs can exert on the pedals, in fact too much and wobble is created.
What is more important is how far your foot travels. Some more figures: with a
ratio of tyre travel to foot travel: using a 36" 26" 24" 20" 178mm (7") gives a
ratio of: 2.53 1.96 1.77 1.46 152mm (6") gives a ratio of: 2.96 2.30 2.07 1.71
125mm (5") gives a ratio of: 3.60 2.80 2.52 2.08 100mm (4") gives a ratio of:
4.50 3.50 3.15 2.60

I know that there are other factors that effect the speed of a unicycle but this
is the main factor and the figures ring true, a 26" with 6" cranks is about the
same speed as a 24" with 4". Wow do I want to try 4" cranks on my coker. Can
someone with a 28" see if they can calculate where it comes in the table? I
don’t have one here.

Cheers

Roger

Re: Crank length

> Wow do I want to try 4" cranks on my coker.

What size cranks do you have on your Coker? Mine came with 165 mm cranks, but
they seemed way too long for me (my legs started to hurt), I guess I am used to
shorter ones from doing freestyle riding. When I first got my Coker, I tried
putting 125 mm cranks on it and that was way too small for that big of a wheel.
I found that 152 mm cranks work perfect on it, and I can even idle on it
(although it’s not easy).

If you try 4" cranks on yours, which is probably pretty dangerous, you might
want to have a brake on it too. Short cranks make it really hard to slow down.
I’ve ridden a 28" with 4" cranks, and when trying to stop or slow down, the
unicycle will keep going and thus leave you behind (there’s nothing like being
thrown off a unicycle). There just isn’t enough torque (or moment) with short
cranks like that. I found it hard to maneuver on the 28" with 4" cranks,
especially when comparing to my Coker or my 28" (with 125 mm cranks) unicycles.

> Can someone with a 28" see if they can calculate where it comes in the table?
> I don’t have one here.

I calculated my 28", and the results were very close to that of the 26" one you
have, just .02 to .06 more than yours for the ratio. I have a 700x35B tire on it
and the circumference is 223 cm (about 88").

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Re: Crank length

Kevin Gilbertson wrote:

> > Wow do I want to try 4" cranks on my coker.
> What size cranks do you have on your Coker? Mine came with 165 mm cranks, but
> they seemed way too long for me (my legs started to hurt), I guess I am used
> to shorter ones from doing freestyle riding. When I first got my Coker, I
> tried putting 125 mm cranks on it and that was way too small for that big of a
> wheel. I found that 152 mm cranks work perfect on it, and I can even idle on
> it (although it’s not easy).

Mine came with 152mm cranks so it looks as if they have learnt.

> > Can someone with a 28" see if they can calculate where it comes in the
> > table? I don’t have one here.
> I calculated my 28", and the results were very close to that of the 26" one
> you have, just .02 to .06 more than yours for the ratio. I have a 700x35B tire
> on it and the circumference is 223 cm (about 88").

thanks, I didn’t realize (or had forgotten) that the 28" Sem was a 700c. On my
measurements it is, as you have noticed, very close to the 26" tyre; in fact I
have a 26x2.5 tyre that measure bigger than my 700x35.

Cheers

Roger