soapbox wheels

Soapbox Wheels - Does Size Matter?

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.

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Soapbox Wheels

DSC_6257 Bentley Soap Box Cart disk brakesThe de-facto standard wheels for soapbox racers and gravity sidecars in the UK are 20 inch BMX wheels. This is because they are strong enough to withstand the side loads experienced when cornering and readily available. There are also a good selection of tyres for them. Other smaller bike wheels are also commonly used, although tyre options can be somewhat limited. 20" wheels from children's bikes should also be avoided as these are often not as strong as BMX wheels of the same size. BMX wheels are more rugged and often have higher spoke counts than ordinary bike wheels.

Other wheels used include go-kart wheels, pit bike, scooter and mini moto wheels. These are very strong, and give good grip when cornering, but have high drag on long fast straights. 

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