Building your Soapbox Cart

A view from the pits by Richard Smith

If you’re Wacky enough to contemplate building a cart to enter the event then we’d encourage you to do so, since it’s a lot of fun. But even before you start, there’s a decision to be made. Are you in it to win it? Are you going to be a Gravity Racer or a Soap-Box Racer?

The difference is big – and the approach if you’re serious about challenging the all-conquering Steve Thomas of C12,  in speed terms, is quite different than joining in to snatch the “Wackiest” prize away from entrants such as the fabulous Parry’s. So what’s the first thing, then?

The Chassis

Some racers enter with a “bespoke” chassis and C12 is probably one of those, with a chassis manufactured specifically for speed racing: low, strong and heavy. Why heavy? Well, Newton’s Laws of motion tell us that an object (a cart,) competing in the race (moving down an incline,) is enhanced toward terminal velocity by its mass (gets faster more quickly if it’s heavier.)

Air resistance then plays a part (so it‘s better if it’s streamlined) and also assists in keeping it from flying off the track. (With some exceptions.) Noticeably in 2014, even without a push, C12 lead the field by over a second before the last race….

A low, strong, heavy chassis then, if you’re going to go fast. If you’re going to look Wacky then quite possibly a taller, wider, chassis. For the former, there are specialist component providers. Go Google them to drool and to laugh at the prices.

For a more reasonable solution here, you could do worse than buy an ex- go-kart chassis. Such a solution should arrive with brakes already in place – which is handy – and a steering system which fits the rules. (No steering with your feet, no prone positions and so on. The rules are comprehensive.) With a go-kart chassis you will have bought safety and stability too. Several cart solutions at Wacky usually employ this approach. Except you’re going to have to have a think about those wheels….

Another way? Build a chassis with bicycle-like components. This is both a cheaper and perhaps more personally rewarding option, but you’ll need to be able to weld, or know someone who does, and you’ll need to pay some attention to the rules, again, to get the dimensions right. There’s absolutely no reason why you can’t go very fast with a bike-style chassis.If you look in the pits, you’ll see there are other choices, too. Recycled ride-on mowers aren’t popular with us because they are inherently unstable. The size of the wheelbase and the high centre of gravity don’t make for an easy machine to ride, but some entrants insist on using their favourite creations year after year. Bon Voyage!

And there are also “trolley-on-caster” approaches and various other devices which should probably be referred to as “contraptions.” We don’t know quite what the “Westonbirt Whizzbangs” entry was based on, fundamentally, but it had casters up front and by crikey it was quick.

The corner we name “Druids” is another thing which should influence your choices and your design. Especially when you think about axles (do you want some?) and wheels. Fully-fledged Gravity racers do not usually face obstacles like this near-270-degree corner, mid-course. Soap-box racers don’t either!



The lateral forces imposed here are a test for all the vehicles entered whatever their aspirations. If you have bespoke wheels, there will probably be no problem – and you’ll get your money back if there is. If you keep wider go-kart-style wheels you won’t be fast, but you’ll be safer… (too much rolling resistance.) If you choose bike wheels, think about the size, think about the potential fragility when cornering.

Druids likes to break bike-type wheels, so if you want to go fast then you’re going to need wheels which have less friction (narrower,) but are very strong (perhaps not spokes, then,) and quite tall (which is that rolling radius thing I keep meaning to Google…..). If you’re going to go more slowly in style and safety, get some decent weighty wheels with a good width of tyre.

The Centre of Gravity and the Brakes.

Once again, Druids can and does undo the finest. The combination of speed generated by being relatively heavy, having a decent shape and good low-resistance wheels can be a nuisance if you’re going too fast and you’re sitting a little high. Have you seen the videos? No-one beats the physics. Go lower if you want to be faster, but don’t forget you may well need to brake just around the area of that last sleeping policeman – and having good brakes is just important as being fast. Maybe MORE important! We’ve had very fast carts with brakes which were difficult to use – and …. Calipers and disks, like a car? Bicycle-style and cable operated? Something else? This needs some VERY careful thought.

The Bodywork

It’s important for three reasons. First – you will be faster if you’re more aerodynamic. Duh! Second, it would be nice if there was something around you to absorb some impact since you never know, the laws of motion may again be in play. Last, “naked” cart chassis’ are dull!

Glass fibre’s nice if you’re a professional outfit and want to bring that air of competence AND looks – the yellow Car Care vehicle looks fabulous – but plastic buckets and wood are the Wacky Racer’s favourite go-to materials and there’s nothing wrong with ply, especially if you don’t plan to run into anything. Or there’s always using a re-worked garden shed.

The Parry tank, Penelope Pitstop’s pink racer (I am un-biased) and the Sheriff’s car have all been highlights in recent years – and let’s hope that this year Wacky bodywork abounds. And why not also bells, whistles, lights (if we’re sensible about using glass on the cart), sound effects and even water pistols? Why not?