Big Wheel vs. Geared Small for High-Speed; Performance

[magnustudios]

Can anyone here comment on riding, say, a 36 vs. a Schlumpf 24?

I'm very interested in high-performance high-speed, and I've been thinking a lot recently about possible advantages and disadvantages of big wheels. I've never gotten the chance to try anything geared or even a giraffe.

My thoughts have been that the differences are mainly between stability and energy.

Energy:

• Half the wheel size will mean half the moment of inertia for the same speed, ie, the energy going into making the wheel turn faster or slower (as opposed to moving you) is half. That assumes that the mass of the wheel is half, but I think it's probably much less (thinner tires, rim), so the difference would be even greater.

Does a 24 schlumpf feel more responsive than a standard 36?

Stability:

I'm seeing two main parts to this one

• Larger wheel diameter. As you fall forward on a wheel, the point of contact of the wheel with the ground moves forward. The actual distance the point moves forward would be proportional to the wheel-size. Does a 24 guni feel like it has a smaller balancing envelope?
  
• Angular momentum, wheel size over bumps. On my 36" I roll over bumps and usually I don't have to do anything, just loosen up and take them. On my 24 (non-geared), if I hit a bump it slows down the wheel and I have to recover. Obviously the wheel size has something to do with it, but I think it's also related to the angular momentum of the wheel. Since a smaller wheel at the same speed has much less momentum, would it be more prone to upsets from terrain?

This is not about geared vs ungeared. Big or small I'll be gearing up. Comparing the 24 geared with a standard 36 just seems like good way to test for differences in wheel sizes.

[skrobo]

24 will never handle bumps like a 36

36er will try to ease you into a bump, while a 24 won't

[skrobo]

24 will never handle bumps like a 36

36er will try to ease you into a bump, while a 24 won't

[corbin]

Originally Posted by skrobo

24 will never handle bumps like a 36

36er will try to ease you into a bump, while a 24 won't

I don't think that's true..

I'll try to write a comment soon. I have lots of miles on a geared 24, lots on an ungeared 36, and lots on a geared 36.

I am just as fast on a geared 24 as I was on an ungeared 36. However, I feel like i get tired faster on the geared 24, due to it taking slightly more effort.

corbin

[rob.northcott]

Originally Posted by magnustudios

Half the wheel size will mean half the moment of inertia for the same speed, ie, the energy going into making the wheel turn faster or slower (as opposed to moving you) is half.

I'm not sure that's true. A smaller wheel will be lighter, but it's got to spin faster to travel at the same speed, so is the energy used to accelerate it up to speed actually less? Of course a lighter overall weight would be an advantage in other ways, but the weight of the geared hub pretty much cancels out any savings. It is true though that the only available 36" tyres are really heavy ones, and you could use a lightweight 24" tyre.

As you fall forward on a wheel, the point of contact of the wheel with the ground moves forward.

No it doesn't. You move forward relative to the contact patch. The contact patch is always directly below the axle (assuming you're on smooth level ground).

I've not got very much geared unicycle experience, but I have ridden a geared 29 and a geared 24 briefly, and regularly ride an ungeared 36. I reckon a geared 24 is harder to ride (in both senses of the word - energy and concentration) than a normal 36. But it could be just down to technique - with more time on the geared uni it may become as easy to ride as a fixed 36 (I'm sure other people will confirm that). With everything else being equal though, a geared 24 has to be less efficient because of the friction of the gearing and the fact that larger wheels have lower rolling resistance.

On the plus side, a geared 24 is much more portable and (at least if it's a Schlumpf) can be ridden in direct-drive 24" mode on more technical trails.

Rob

[magnustudios]

24 will never handle bumps like a 36

Yeah I guess that makes sense. For my purposes this isn't much of an issue though.

Rob,

My calculations assume the whole wheel is just one ring of mass. It should be close.

E = m/2*v^2

E - energy
m - mass (assume wheel is a ring of mass w/ const. radius so we can use this)
v - linear velocity

The linear velocity of the outer edge of the tire will be identical for both sizes of wheel if you want to move at the same speed, so we can just cut that term out, so

E is proportional to m

Therefore there is double the energy stored in a larger wheel because it is much heavier, even though the small wheel has to spin at twice the rate.

Let me clarify what I meant by point of contact:

When you are upright on the unicycle, extending a straight line from the frame to the ground puts you very close to the wheel's contact point with the ground. Now lean forward so the frame is 45 degrees. Extending a line from the frame to the ground now lands nowhere near the contact point.
In relation to the frame and rider, the contact point with the ground has moved forward. If you do the same experiment with a very small, you'd find the contact point is very close to the same no matter what angle you lean.

[rob.northcott]

Originally Posted by magnustudios

Let me clarify what I meant by point of contact:

When you are upright on the unicycle, extending a straight line from the frame to the ground puts you very close to the wheel's contact point with the ground. Now lean forward so the frame is 45 degrees. Extending a line from the frame to the ground now lands nowhere near the contact point.
In relation to the frame and rider, the contact point with the ground has moved forward. If you do the same experiment with a very small, you'd find the contact point is very close to the same no matter what angle you lean.

I'm not going to argue about the stored energy - can't be bothered to work it out, so I'm prepared to be proved wrong.

But I really don't agree with the contact patch thing - what's a line drawn through the frame to the ground got to do with it? Gravity doesn't work in the direction the frame's pointing. If you fall forwards, your centre of gravity is in front of the axle, so in front of the contact point with the ground. The contact point has actually moved _back_ relative to the rider. The size of the wheel has nothing to do with this apart from putting the rider's c of g higher on a larger wheel, which is in theory easier to balance (same as it's easier to balance a long rod on your hand than a short one).

Rob

[johnfoss]

If the comparison is for overall performance, here are a couple of things that should hold true:

- Which will be faster? It depends on the terrain. On the flat, my money's on the 36. The more hills there are, the more advantage you may get from being able to shift, or just be lighter on the 24.

- Which will be more versatile? Duh.

- Which will be cheaper? Duh.

- Which will fit in the car better? Duh.

Does that help?

[magnustudios]

The 36 compared to 24 geared is just an example some people might be able to comment on. What I want to know is in terms of pure performance, forgetting about practicality and cost, which one can go faster?

1. small wheel at a very high gear
2. 36 geared
3. custom massive wheel ungeared.

In any case the gears would probably be a custom fixed-gear setup.


Rob: the attached diagram should clarify what I'm trying to say regarding the contact point. Since the wheel is round, leaning the frame forward means the contact point is somewhere in front of the frame (still directly below the axel). This has everything to do with balance, because the centre of mass shifts a smaller angle from the contact point for a given frame angle on a larger wheel.
The centre of mass still moves the same distance ahead of the contact point for a given angle.

[maestro8]

Originally Posted by magnustudios

My calculations assume the whole wheel is just one ring of mass. It should be close.

Oh dear! Give a man a handful of equations, and he thinks he has the solution to everything.

Don't get ahead of yourself, mag. First your professors teach you everything you need to know, then experience teaches you how little you can actually solve. You may feel very smart in the valley between the classroom and the real world, but don't fool yourself.

You're only learning gross approximations. There are many, many details that will bite you in the butt if you don't pay close attention to them.

[kb1jki]

Originally Posted by maestro8

Oh dear! Give a man a handful of equations, and he thinks he has the solution to everything.

That may be true, but there's no better way to learn so long as you're still listening to the experiences. I find this thread very beneficial and fun!

[magnustudios]

That's a great article, Sam, cool to hear from a muni perspective. I hadn't seen that website before, but there's some good stuff. I like the photos.

Corbin, I'm looking forward to it. Would these would be lots of long-distance road miles?

From The Woods. I personally don't doubt Dave's abilities on a 29er with 137s, after all, he did the Canadian Rockies in a few weeks on an ungeared 36 riding 110s last year. The crank to wheel ratio is about the same between those. Also, 10% grade is really steep for a long hill.

Dave, I'm surprised you're so taken by the 29, but what you said made sense. The fact that you don't feel unstable on it is exactly the type of observation I was looking for here. I'm definitely taking smaller wheels a lot more seriously now than I was before.

You say you like the acceleration characteristics better. Does that mean it's easier to get up to speed and slow down? I always find the first few strokes on the 36 to be sluggish, especially with short cranks.

Tholub, I'm definitely seeing things the same way. Maybe down to 26" even, but definitely a high gear and long cranks. I think the V-frames and such are very cool, and I've got some ideas of my own that I think could help in terms of good riding posture and aerodynamics.

Maestro8, thanks for your concerns. Gross approximations. That's all that we're dealing with here. I know that much. At the same time I think doing a grossly simplified bit of algebra to find out the approximate relationships between things like wheel mass, speed, and energy is useful. Those relationships certainly aren't intuitive to me.

[tholub]

Originally Posted by magnustudios

You say you like the acceleration characteristics better. Does that mean it's easier to get up to speed and slow down? I always find the first few strokes on the 36 to be sluggish, especially with short cranks.

A 29er is a lot more nimble than a Coker, just because of the tire/tube weight. You're saving probably a kilo of rotating weight with a 29er. That makes accelerating and turning a lot quicker on the 29er.

[From the Woods]

Originally Posted by magnustudios

I personally don't doubt Dave's abilities on a 29er with 137s, after all, he did the Canadian Rockies in a few weeks on an ungeared 36 riding 110s last year. The crank to wheel ratio is about the same between those. Also, 10% grade is really steep for a long hill.

10% is not much in Austria. It is more or less the average on the mountains here

Yeah, for sure it is possible to ride the mountains/hills I am thinking of on a 29" wheel with 137mm cranks (but it would not be an easy task) but he was talking about riding that in high gear. I even doubt that to be possible for a trained pro bicyclist with an equal gear ratio.

[corbin]

Originally Posted by magnustudios

Corbin, I'm looking forward to it. Would these would be lots of long-distance road miles?

hi mag,
Yes, I have lots of long distance on road, and offroad miles. My wife and I each rode the geared 24 through Africa on tour. There were some pretty long days. I didn't have much trouble keeping up with the geared 36'es (and ungeared 36'ers), although the geared 36'ers could easily pull away from me on the flat and downhill sections of pavement. On the bumpy stuff I was pretty fast, but I was also pretty confident at riding offroad at fast speeds. video: http://www.corbinstreehouse.com/blog...video-unimoon/

Also, do realize that I'm a fairly strong rider when I'm in shape (I'm the current marathon world champion from the last Unicon).

Comparing a geared 24 to an ungeared 36: In my opinion, a geared 24 is an adequate replacement for an ungeared 36. However, the guni 24 takes more energy to ride. No one has mentioned this, but the planetary system *does* consume more energy (say a few percent, due to the gearing), and it does start to effect you when you are riding long distances. This in particular affected my wife, and she had a hard time riding the guni24. Since the tour, she decided to not ride the guni 24 anymore, and just stick with an ungeared coker, which was faster for her. The thing she did like about the guni24 is the ability to easily mount it (she is 5'2", and mounting a coker is sometimes hard for her).

Your question was a "big wheel vs geared small wheel for High speed and performance". For high speed and performance, an ungeared 36 will be easier and slightly faster to ride than a geared 24.

For real high speed and performance, you want a geared 36. It will always be faster, unless you are doing a pure hill climb (like Mt Diablo in the Bay Area).

I've done the same 100km ride (Strawberry Fields in Santa Cruz, CA) on three different unicycles: geared 24, ungeared 36, and geared 36. Guess which one was the best?

corbin