The vast majority of soapbox rules contain a requirement that all carties have effective brakes, and many specify a minimum force that they must be able to hold. This is commonly 50 KgF. The wording is typically "The brakes must be capable of holding the vehicle on a dry horizontal road surface, when laden with driver and any ballast, against a horizontal load of 50KgF exerted on the front towing eye.".
This has been adequate for most of the races up until now, but with racers and race organisers now increasingly looking for steeper, twistier and more challenging courses, the "50Kg static brake test" needs to be reviewed.
You are not Fred Flintstone. You cannot stop a cartie travelling at 30mph by putting your feet on the road.Fortunately, bike brakes are more than up to the job and can be fitted relatively easily. Many events have some form of brake test including in the pre-event inspection, so it is important to pay attention to your brakes.
The tricky part about brakes is fitting them. How you attach them is mainly controlled by how you mount your wheels. If you are using bike forks, the problem is pretty much solved for you as the forks will already have brake mounts fitted. If you are using stub axles, however, it can be rather more tricky.
Disk brakes would be the obvious choice for stub mounted wheels, but unfortunately it can be difficult to find 20" wheels that are disk compatible. The alternative is to make a bracket to hold rim brakes. This can be tricky as the bracket will need to be strong enough to not bend under braking. The other problem with using rim brakes on stub axles is that if the stub axles bend at all - not uncommon with the standard 10mm bike axle - the brake will start to rub on the rim.
Rim brakes work well in the dry but their effectiveness can be dramatically reduced in the wet.
In general, brakes on the front wheels are far more effective than brakes on the rear wheels. However, the down side is that they are harder to set up. To avoid "brake steer" effect which causes your cartie to veer off to one side or the other under heavy braking, you need to make sure that both sides are properly balanced and also that the king pin is aligned to pass through the tyre contact patch (see right).