Soapbox Wheels

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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. 

Cairngorm Soap Box Extreme Challenge 2010 Aviemore Highland Spey Valley Editorial Images DSC_6296Novice teams often make the mistake of fitting larger bicycle wheels under the belief that they have lower rolling resistance. This may or may not be true, but unfortunately they are not strong enough to withstand cornering loads and will can easily buckle, with disasterous results. Many races do not allow wheels larger than 20 inches for this very reason. Similarly, plastic spoked BMX wheels can often fail under cornering and should be avoided.

Check out this video to see the disasterous result of combining large, flexible wheels, sensitive steering, a high centre of gravity and enthusiatic cornering [clicky].

BMX wheels with 14mm spindles can be single side or "stub axle" mounted. If the spindles are smaller than this the wheel will need to be fitted in forks or swing arms. Make sure your wheels are firmly mounted and that neither the wheel nor the chassis flexes when cornering loads are applied, otherwise it will be extremely unstable.

Further Reading :
Notes on wheel sources for recumbent trikes, which are solving basically the same problem as for a cartie - i.e. coping with high side loading during cornering.


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wheel mounting

Where is the best place to find instructions on how too mount bike wheels for a cartie.This is the first time i've tried to build one.
matty11 | June 23, 2014 - 12:23

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