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

[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?

[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

[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.

[muniing sam]

Heres an article that might interest you :

http://unplannedismounts.com/2009/09...-virtual-36er/

The guy who wrote it is called lunicycle on these forums I think.

[Dave Cox]

I have some real-world experience with changing wheel sizes on a geared unicycle and the impact that will have on average touring speed..

While riding with Sam Wakeling through New Zealand this January, I started breaking spokes on my schlumpf 36, and the only realistic option was to rebuild the wheel on a 700c rim and keep going, which I did. Prior to the wheel swap we were pretty evenly matched - it wasn't a significant effort to ride at Sam's speed, averaging perhaps 22-25km/hr on the flats.

After switching to the new wheel size however, I had to put in quite a bit of effort to keep up with Sam (or watch him slowly pull in front of me). A geared 36 has a tendency to float along at a higher cruising speed - you could well keep up on a smaller or ungeared wheel, but the effort required is much greater.

In addition, without changing crank sizes (i.e. at my most comfortable crank length - 137mm), pushing as hard as possible on the 29, I could not outdo my top speed on a geared 36. I can come quite close, say 34-35k/hr, but I can't hit the 38-40 mark that is doable on the 36.

All of that said, I like the acceleration characteristics and the size of the 29 much better. I've put off ordering new spokes for my 36 rim because I like how portable and comfortable the 29 is. For the 29 to be a viable top-speed machine though, it will require taller gears. As it is now, I would be happier with a 29 if it was by default 1:1.5, and geared up to 1:2.25 or so. These ratios are rough, and 'ideal' is of course different between riders, but I can climb up most anything in high gear, so for me that means it could stand to be a bit bigger.

And of course, the technology has to catch up with that. If the hubs ever become available (it doesn't look likely in the near future), we won't have to keep pushing huge 36" wheels around, because a 29 just makes so much more sense, assuming you can get the same virtual wheel sizes as on larger wheels.

I could see a 24 wheel working the same way. When I first tried a geared 29 it felt quite finicky, but it's rare that I come off from a bump or feel unstable. I imagine this translates downward as long as your wheel doesn't get overly small or hard (a high geared 20 would probably be a huge pain, for example.)

I will continue to bring a geared 36 to races until that option is available though!

[From the Woods]

Originally Posted by Dave Cox

These ratios are rough, and 'ideal' is of course different between riders, but I can climb up most anything in high gear, so for me that means it could stand to be a bit bigger.

I can't really believe that or are you just talking about VERY small hills. If I tried riding up about 1km with a 10% grade in high gear I would have to ride faster than I could on a bike to not loose momentum (and I have 165mm cranks). That simply is too exhausting (at least I can't even imagine to do that some time).

[tholub]

Originally Posted by 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.

The major contributors to speed are crank length and effective wheel size (wheel size * gear ratio). An ungeared 36 with 150mm cranks will be pretty similar in speed to a Schlumpf 24 with 150mm cranks. The 36" will be heavier, the 24" sloppier; the 24" might be faster on climbs, the 36" on downhills, but those are marginal effects compared to the questions of leverage.

Another thing which comes into play is that larger tires absorb bumps better, which reduces the amount of energy you need to put into corrections. I personally think the sweet spot for performance uni tires is around 1.75"-2" width; narrower than that and it's twitchy, wider and it's heavier than it needs to be.

Up until some point of diminishing returns which we have yet to reach, increasing effective wheel size will increase cruising speed and top speed. 54 gear-inches (Schlumpf 36") is clearly not the top of the curve for uni speed on flats.

A mythical 54" pneumatic tire unicycle would perform similarly to a geared 36", except that it would weigh a ton, and short people wouldn't be able to reach the pedals. We've probably reached diminishing returns on increasing actual wheel size.

If I were building the ultimate speed machine unicycle (for an hour record attempt or something like that), I would use a 29er wheel, gear ratio of something like 2.5:1 or 3:1, 170mm cranks, and 2.0" tire (probably the Schwalbe Big Apple or Marathon Supreme). With that kind of speed you'd also have to consider aerodynamics, so a V-frame or aero handlebar setup would be part of the mix.

[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!