20" vs 24" for sloping trail, and crank length question

Unicycles and Equipment
1

I have only one unicycle (still…) which is Semcycle 24". I have
borrowed a 20" Sem for a few weeks. Both have 125 mm cranks. Now I
have noticed that on the 20" I can negotiate significantly steeper
sloping trails both up and down. That seems not so strange as the
leverage ratio is better on the 20".

If I would have 150 mm cranks on my 24" it would theoretically equate
the 20" with 125’s for slope ability. Would this be correct in
practice?

Klaas Bil

(Who thinks he should buy a good MUni.)

“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“CISCO, SGDN, Cap-Stun”

2

i belive so,

3

Klaas,

Into muni, huh. Glad to hear it. What hills are there in The Netherlands - forgive me if I am wrong about your local.

I ride a Sem XLW( a wide Torker frame) with a Gazzaloddi 24 x 2.6. Originally, the Muni came with 150mm cranks which were fine to train and learn on. I had very little wobble with that set up and I could get up gradual slopes.

However, I recently replaced them with 170mm cranks. This is the ticket for steeper terrain. It creates a little more wobble when riding flat or downhill, but it is not too bad. I find it acceptable. The great thing about the 170mm cranks is the torque you get from them and you can microadjust when climbing or working between dips and rocks in the trail. I would never go back.

I think there is more to just comparing the ratios to the two unis you describe. The 20" just seems like you would be spinning way to much over any distance like a short mile or 2km. While the ratios are compatible, the distance covered per revolution is quite different.

Go with the 150’s on the 24 to get the hang of stuff, then buy a real muni. I think you will be really happy with a 2.6 tire. The Gazz 2.6 is a great tire if your frame can’t take the 3.0.
Work the maze.

4

Geoffrey Faraghan has done some calculations for comparing the mechanical leverage for different wheel size and crank length combinations. He doesn’t have the numbers for a 20" wheel, but it would be easy to calculate those. He bases his calculations on the ratio of the circumference of the wheel to the circumference of the pedal circle. Wheel and crank combinations with the same ratio will give you the same mechanical advantage.

His web pages seem to be down right now.
The page with the calculations is:
<http://www.telford-design.com/wheeland.htm>
That page is still in Google’s cache so you can read it there
<http://www.google.com/search?q=cache:EpuvqIeNlEMC:www.telford-design.com/wheeland.htm+telford+crank+length&hl=en>

A 24" wheel with 150mm cranks has a Ct/Cp ratio of 2
A 20" wheel with 125mm cranks has a Ct/Cp ratio of 2

So a 24" wheel with 150mm cranks will feel about the same as a 20" wheel with 125mm cranks.

john_childs

5

Yuppers - John Childs is right on. That’s the best site I’ve found so far for those calculations. If you’re just comparing wheels and cranks, you don’t even have to match measurement systems!

150/24 = 125/20

(= 6.25, a temporary meaningless number; meaningless by itself because it mixes metric and English systems)

On a Coker, then, you’d have 225mm cranks (whew!) to get the same leverage (36x6.25), and on your 20" you’d have 125mm cranks.

Also on the slopes, the 20" will not handle irregularities in the slope nearly as well as the larger wheels, even if the leverage is the same. A bump on the hill may be twice as steep locally as the surrounding slope. A larger wheel with lower pressure has a smaller sense of “local”.

6

RE: 20" vs 24" for sloping trail, and crank length question

> Wheel and crank combinations with the same ratio will give
> you the same mechanical advantage.

That covers the math. Then you step out the door into the real world. As one
writer already mentioned, the larger wheel covers more ground with each
pedal turn. What was also mentioned was the relative way different wheel
sizes react to bumps. What nobody mentioned yet was that there is also a
weight difference.

So it’s like temperature vs. wind chill factor. The temperature reading is
accurate, but it’s only part of the story. Equal leverage will not give you
an equal ride. The best way to find the ride that works for you is to try
various things out “in the field.”

And have fun doing it.

Stay on top,
John Foss, the Uni-Cyclone
jfoss@unicycling.com

“If electricity comes from electrons, does that mean that morality comes
from morons?”

7

Re: 20" vs 24" for sloping trail, and crank length question

“John Foss” <john_foss@asinet.com> wrote in message
news:mailman.1020269583.29272.rsu@unicycling.org…
> That covers the math. Then you step out the door into the real world. As
one
> writer already mentioned, the larger wheel covers more ground with each
> pedal turn.

Yes, but if you have the same crank length to wheel size ratio you should be
able to go just as fast, as your legs have to move at the same speed to go
the same distance. Hence why a 24" wheel with 90mm cranks is much faster
than a 26" wheel with 150mm cranks. Spinning very fast on short cranks is
just as easy as spinning normal speed on normal length cranks.

That’s how the mechanical advantage relates to the real world as far as flat
stuff goes.

When you’re doing very steep or technical stuff with single pedal turns with
gaps in between then wheel size makes a difference because the single pedal
turns will move you a different distance, but it won’t make a difference in
how hard it is. For rolling over stuff a big wheel is easier, but when
you’re just riding along generally it doesn’t make much difference if you
have the same mechanical advantage.

Joe

8

Re: 20" vs 24" for sloping trail, and crank length question

In a message dated 01/05/2002 08:55:12 GMT Daylight Time,
john_childs.3yrla@timelimit.unicyclist.com writes:

> He bases his calculations on the ratio of
> the circumference of the wheel to the circumference of the pedal circle.
>

This is identical to the ratio of the wheel radius to the length of the
crank. And as the radius is simply half the diameter, the calculation is
pretty simple. Bringing in circumferences introduces Pi to the equation
which makes it appear more complex and daunting, whilst not changing the
result. :0)

9

Re: 20" vs 24" for sloping trail, and crank length question

On Tue, 30 Apr 2002 23:37:35 -0500, teachndad
<teachndad.3yj9a@timelimit.unicyclist.com> wrote:

>Into muni, huh. Glad to hear it. What hills are there in The
>Netherlands - forgive me if I am wrong about your local.
We have some decent hills in the Netherlands, our highest point is 321
m (just over 1000 ft - don’t laugh). I’m in the flatter western part
of the country but still we have some park areas within unicycling
distance which are technical, at least for me. I plan to make some
photos this weekend (weather permitting) and put them up.

>I think there is more to just comparing the ratios to the two unis you
>describe. The 20" just seems like you would be spinning way to much
>over any distance like a short mile or 2km. While the ratios are
>compatible, the distance covered per revolution is quite different.
I realise that, and I wouldn’t want a 20" muni. I was just wondering
if 150’s on my 24" would permit equally technical stuff. I think to
just try 150’s on my Sem 24" is a good idea, thanks for the hint.

>Go with the 150’s on the 24 to get the hang of stuff, then buy a real
>muni. I think you will be really happy with a 2.6 tire. The Gazz 2.6
>is a great tire if your frame can’t take the 3.0.
Would the Sem XLW take a 3.0 tyre? I love Semcycle generally.

On Wed, 1 May 2002 02:35:30 -0500, john_childs
<john_childs.3yrla@timelimit.unicyclist.com> wrote:

>A 24" wheel with 150mm cranks has a Ct/Cp ratio of 2
>A 20" wheel with 125mm cranks has a Ct/Cp ratio of 2
>
>So a 24" wheel with 150mm cranks will feel about the same as a 20" wheel
>with 125mm cranks.
That was my calculation too (can do it without spreadsheet, just my
grey cells), and from the answers I gather it is approximately true
also in practice.

On Wed, 1 May 2002 07:53:31 -0500, U-Turn
<U-Turn.3z6em@timelimit.unicyclist.com> wrote:

>Also on the slopes, the 20" will not handle irregularities in the slope
>nearly as well as the larger wheels, even if the leverage is the same. A
>bump on the hill may be twice as steep locally as the surrounding slope.
>A larger wheel with lower pressure has a smaller sense of “local”.
Good point but almost ambiguously phrased. You mean the sensitivity of
the 24" to locality is smaller. Conversely, the length scale with
which the 24" looks at locality etc is larger.

On Wed, 1 May 2002 09:31:13 -0700, John Foss <john_foss@asinet.com>
wrote:

>> Wheel and crank combinations with the same ratio will give
>> you the same mechanical advantage.
>
>That covers the math.
I have a mathematical argument that a 20" with 125’s would give a bit
more leverage than 24" with 150’s. I would need a lot of words to
explain. I’m happy to do so if anyone wonders.

On Wed, 01 May 2002 17:21:35 GMT, “Joe Marshall”
<news@joemarshall.org.uk> wrote:

>Yes, but if you have the same crank length to wheel size ratio you should be
>able to go just as fast, as your legs have to move at the same speed to go
>the same distance. Hence why a 24" wheel with 90mm cranks is much faster
>than a 26" wheel with 150mm cranks. Spinning very fast on short cranks is
>just as easy as spinning normal speed on normal length cranks.
I believe your example, and in general your assessment that shorter
cranks give increased speed. However, I would say that this would not
be the same ratio, i.e. cranks shorter by 20% would give less than a
20% speed gain because you still have to muster the rotational speed.
And I believe wheel larger by 20% would also give less than 20% speed
gain, but closer to 20% than in the crank length example.

Thanks for all the responses!

Klaas Bil

“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“Enaam Arnaout, Suha al-Taweel, JERTO”

10

>That covers the math.

>I have a mathematical argument that a 20" with 125’s would >give a bit
>more leverage than 24" with 150’s. I would need a lot of words >to explain. I’m happy to do so if anyone wonders.

I’m interested. Is your argument angular momentum? Of course you said leverage . So is it a tire size thing? Actually, don’t tell me if it isn’t these. Just give me a clue.
-gauss

11

Re: Re: 20" vs 24" for sloping trail, and crank length question

I think you are right that this is unclear. Let me fill in a little:

A smaller, higher pressure tire deforms relatively little, so that on a tilted plane (the hill), small bumps really affect the path of the entire wheel. So the 20" wheel’s performance has a great deal of the “local” in it.

The larger, lower pressure tire deforms a lot, so it absorbs (integrates) the local variations; its overall behavior is determined more by the plane of the hill rather than the small bumps. So the larger wheel’s performance has a great deal of the “global” and very little of the “local” in it.

And, as you mentioned, Klaas, the senses of local, global, and even small and large, are relative to one another as far a wheel size; a given path is absolute as far as perturbations go.

To go any further, we’d have to distinguish small and large wheels from the high/low pressure effects, which seems as though it would be going too far.

One additional comment: Using different cranks on the same wheel size (110, 150, and 170mm on a 26" wheel), I’m beginning to believe that the additional leverage of the longer cranks also increases the angle spread of the power position. That is, that the power position widens to cover more of the wheel’s 360 degrees. This makes sense to me when I consider that my leg strength is fixed. So the additional leverage of longer cranks not only gives one more mechanical advantage for powering over obstacles, micro-adjusting balance, or climbing hills, but also gives one a greater range of wheel positions where power can be applied meaningfully.

12

Re: 20" vs 24" for sloping trail, and crank length question

Hmm, how to give just a clue. It has to do with the ratio between the
angle through which the upper leg travels and the change of distance
between hip and foot. This ratio is larger when the leg is almost
stretched.

Does that help?
Klaas Bil

On Wed, 1 May 2002 18:31:34 -0500, gauss
<gauss.3zzsz@timelimit.unicyclist.com> wrote:

>
>>That covers the math.
>
>>I have a mathematical argument that a 20" with 125’s would >give a
>bit
>>more leverage than 24" with 150’s. I would need a lot of words >to
>explain. I’m happy to do so if anyone wonders.
>
>I’m interested. Is your argument angular momentum? Of course you said
>-leverage- . So is it a tire size thing? Actually, don’t tell me if it
>isn’t these. Just give me a clue.
>-gauss
>
>
>–
>gauss - memory fault (coredump)
>------------------------------------------------------------------------
>gauss’s Profile: http://www.unicyclist.com/profile/651
>View this thread: http://www.unicyclist.com/thread/17818
>

–
“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“Samford Road, Lacrosse, EuroFed”

13

Re: 20" vs 24" for sloping trail, and crank length question

Your use of “larger” seems to refer both to wheel diameter and tyre
width though the two need not be coupled. Originally I read your
comments as referring to wheel diameter only. But you’re right: a
wider tyre is generally used at lower pressure and hence absorbs local
variations much better.

About your widened power position with longer cranks: makes sense.
There is more power along the total 360 revolution, so that more of
the 360 sticks out above the threshold required for calling it
“power”. That’s good.

Klaas Bil

On Thu, 2 May 2002 09:10:01 -0500, U-Turn
<U-Turn.414jm@timelimit.unicyclist.com> wrote:
[color=darkred]
>> >A bump on the hill may be twice as steep locally as the surrounding
>> slope.
>> >A larger wheel with lower pressure has a smaller sense of “local”.
>>
>> Good point but almost ambiguously phrased. You mean the sensitivity
>> of
>> the 24" to locality is smaller. Conversely, the length scale with
>> which the 24" looks at locality etc is larger.
>>
>
>I think you are right that this is unclear. Let me fill in a little:
>
>A smaller, higher pressure tire deforms relatively little, so that on a
>tilted plane (the hill), small bumps really affect the path of the
>entire wheel. So the 20" wheel’s performance has a great deal of the
>“local” in it.
>
>The larger, lower pressure tire deforms a lot, so it absorbs
>(integrates) the local variations; its overall behavior is determined
>more by the plane of the hill rather than the small bumps. So the
>larger wheel’s performance has a great deal of the “global” and very
>little of the “local” in it.
>
>And, as you mentioned, Klaas, the senses of local, global, and even
>small and large, are relative to one another as far a wheel size; a
>given path is absolute as far as perturbations go.
>
>To go any further, we’d have to distinguish small and large wheels from
>the high/low pressure effects, which seems as though it would be going
>too far.
>
>One additional comment: Using different cranks on the same wheel size
>(110, 150, and 170mm on a 26" wheel), I’m beginning to believe that the
>additional leverage of the longer cranks also increases the angle spread
>of the power position. That is, that the power position widens to cover
>more of the wheel’s 360 degrees. This makes sense to me when I consider
>that my leg strength is fixed. So the additional leverage of longer
>cranks not only gives one more mechanical advantage for powering over
>obstacles, micro-adjusting balance, or climbing hills, but also gives
>one a greater range of wheel positions where power can be applied
>meaningfully.
>
>
>–
>U-Turn
>------------------------------------------------------------------------
>U-Turn’s Profile: http://www.unicyclist.com/profile/691
>View this thread: http://www.unicyclist.com/thread/17818
>[/color]

–
“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“Samford Road, Lacrosse, EuroFed”

14

Re: 20" vs 24" for sloping trail, and crank length question

Your use of “larger” seems to refer both to wheel diameter and tyre
width though the two need not be coupled. Originally I read your
comments as referring to wheel diameter only. But you’re right: a
wider tyre is generally used at lower pressure and hence absorbs local
variations much better.

About your widened power position with longer cranks: makes sense.
There is more power along the total 360 revolution, so that more of
the 360 sticks out above the threshold required for calling it
“power”. That’s good.

Klaas Bil

On Thu, 2 May 2002 09:10:01 -0500, U-Turn
<U-Turn.414jm@timelimit.unicyclist.com> wrote:
[color=darkred]
>> >A bump on the hill may be twice as steep locally as the surrounding
>> slope.
>> >A larger wheel with lower pressure has a smaller sense of “local”.
>>
>> Good point but almost ambiguously phrased. You mean the sensitivity
>> of
>> the 24" to locality is smaller. Conversely, the length scale with
>> which the 24" looks at locality etc is larger.
>>
>
>I think you are right that this is unclear. Let me fill in a little:
>
>A smaller, higher pressure tire deforms relatively little, so that on a
>tilted plane (the hill), small bumps really affect the path of the
>entire wheel. So the 20" wheel’s performance has a great deal of the
>“local” in it.
>
>The larger, lower pressure tire deforms a lot, so it absorbs
>(integrates) the local variations; its overall behavior is determined
>more by the plane of the hill rather than the small bumps. So the
>larger wheel’s performance has a great deal of the “global” and very
>little of the “local” in it.
>
>And, as you mentioned, Klaas, the senses of local, global, and even
>small and large, are relative to one another as far a wheel size; a
>given path is absolute as far as perturbations go.
>
>To go any further, we’d have to distinguish small and large wheels from
>the high/low pressure effects, which seems as though it would be going
>too far.
>
>One additional comment: Using different cranks on the same wheel size
>(110, 150, and 170mm on a 26" wheel), I’m beginning to believe that the
>additional leverage of the longer cranks also increases the angle spread
>of the power position. That is, that the power position widens to cover
>more of the wheel’s 360 degrees. This makes sense to me when I consider
>that my leg strength is fixed. So the additional leverage of longer
>cranks not only gives one more mechanical advantage for powering over
>obstacles, micro-adjusting balance, or climbing hills, but also gives
>one a greater range of wheel positions where power can be applied
>meaningfully.
>
>
>–
>U-Turn
>------------------------------------------------------------------------
>U-Turn’s Profile: http://www.unicyclist.com/profile/691
>View this thread: http://www.unicyclist.com/thread/17818
>[/color]

–
“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“Samford Road, Lacrosse, EuroFed”

15

Re: 20" vs 24" for sloping trail, and crank length question

On Tue, 30 Apr 2002 23:37:35 -0500, teachndad
<teachndad.3yj9a@timelimit.unicyclist.com> wrote:

>Originally, the Muni came with 150mm cranks which were fine to train and
>learn on. I had very little wobble with that set up and I could get up
>gradual slopes.
>
>However, I recently replaced them with 170mm cranks. This is the ticket
>for steeper terrain. It creates a little more wobble when riding flat
>or downhill, but it is not too bad. I find it acceptable. The great
>thing about the 170mm cranks is the torque you get from them and you can
>microadjust when climbing or working between dips and rocks in the
>trail. I would never go back.

Q to all: are any longer cranks than 170 mm available? Not that I
would want to purchase them rightaway coming from 125’s but I’m just
curious. And if so: are they practical for MUni or anything?

Klaas Bil

“To trigger/fool/saturate/overload Echelon, the following has been picked automagically from a database:”
“TA SAS, burhop, GEOS”

16

175 mm cranks are common. 180 mm and 185mm are available as well.
The longer cranks are great for going up and down hills but harder to hop with because your feet are so far apart. I guess it would depend on how long your legs are.