Soapbox Wheels - Does Size Matter?

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I'm building a soapbox racer and I've read lots of stuff on the internet about what is the best kind of wheel. Now I'm hopelessly confused. Which is best - large diameter or small?

OK - this is complicated and there is no single right answer. It depends on what you want to achieve and what kind of course you will be racing on. There are several factors to consider;

  • Rolling resistance
  • Moment of inertia / mass
  • Braking and cornering grip
  • Aerodynamic drag

OK - so what about rolling resistance? Larger diameter wheels give lower rolling resistance right?

In theory yes, but it's not as simple as that. Rolling resistance comes from deformation of the rubber in the tyres, and (all other things being equal) smaller tyres are made proportionally "less round" than larger tyres so have higher rolling resistance. However, a bigger factor on rolling resistance is the tyre pressure, and if you can get high pressure tyres for your small wheel then you will be able to more than offset the effect of the smaller diameter.

Wider tyres have lower rolling resistance than narrow ones for the same reason - the deformation of the tyre is proportionally less for a wide tyre inflated to the same pressure. However, narrower tyres tend to support higher pressures, which can more than compensate for this. Aerodynamic drag is a bigger factor though. Wider wheels have more drag than an equivalent narrow wheel, and at high speeds that will more than cancel out any gains from lower rolling resistance.

Check out this page on tyre manufacturer Schwalbe's web site for further information on rolling resistance.

Cairngorm Soap Box Extreme Challenge 2010 Aviemore Highland Spey Valley Editorial Images DSC_6296A more important factor when considering wheel diameter is the wheels strength and ability to support the sideways loads when cornering. Large diameter bike wheels are simply not strong enough and will fail if you take a corner too fast. Wheels larger than 20" should be avoided.

What about weight? Do I want light or heavy wheels?

The overall weight is not as important as where the weight is. When you start to trundle down the hill, part of the force due to gravity that is moving you forward goes into accelerating your wheels. How much force is needed depends on the wheel's moment of inertia, and the lower the moment of inertia, the less force is required. Because of some maths, the diameter of the wheel doesn't actually matter in this instance, since the extra moment of inertia required to accelerate a larger diameter wheel is cancelled out by the larger torque that is applied to it by the cartie. Consequently, only the mass of the wheel and how that mass is distributed actually matters. Lighter wheels will accelerate faster so you will get to your top speed quicker with a lighter wheel. The majority of the moment of inertia of a bike wheel comes from the rim, the tube and the tyre. The effect of the hub, spokes and (where fitted) brake disk is very small in comparison.

The flip side of this is that lighter wheels are also easier to decelerate. If any sections of the course are shallower gradient then a wheel with a greater moment of inertia will decelerate slower and so will help you maintain your speed for longer. It is possible that heaver wheels might actually be an advantage on some courses.

The other interesting thing about moment of inertia is that it is only affects acceleration. Once you have reached your top speed you are no longer accelerating and so the force used to accelerate the wheel is zero too. Moment of inertia does NOT affect your top speed. It only affects the time it take you to get there.

If you are using bike wheels, the tyres, rim and inner tube account for about 95% of the moment of inertia. The efffect of your hubs, spokes and (if fitted) brake rotors on the total moment of inertia is negligible.

So if I fit lighter tyres and tubes I'll accelerate faster?

Probably, although it might not help you get to the bottom of the course any quicker. If you reduce the mass of your tyres you'll reduce the moment of inertia of your wheels, but you'll also decrease the overall mass of your cartie by the same amount. The wheels will take less energy to spin up to speed, but you'll also have less mass to create the speed in the first place, so the benefits might not be as much as you'd hope for. Your top speed might actually be slightly lower unless you put the weight you saved on your wheels back as ballast in your cartie.

What about grip?

Good question. On some courses, grip is not very important. If there are no corners that you need to brake for and you have plenty of room to slow down and stop at the end then it's not really important. However, if you have to brake hard or carry speed around corners then you will want good grip and resilient tyres so you can brake later and corner harder. On some courses, you may find that gokart or scooter wheels will be better than bike wheels.

Got it. So - light skinny wheels for downhill drag races. Fat wheels for low speed twisty courses.

Yup - that's pretty much it. Or maybe not. It depends on the course really.

Wheels and tyres are an area where diminishing returns on your work and expenditure kick in pretty quickly. You can make a large improvement to your acceleration and top speed if you spend an hour or so cleaning and oiling the bearings, pumping the tyres up as hard as they will go and making sure they are pointing in the same direction. Spending a second hour on it is probably not going to yield anywhere near as much an improvement, and spending huge amounts of money on ceramic bearings is going to make only a very marginal difference. If you've got that much time and money available, you'd be better off putting it into making your cartie as aerodynamic as possible.

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