Get on your soapbox: how to build a gravity racer

The basics

Have a digital brainstorm

Sketchup is a free and very useful tool for visualising your soapbox design. For inspiration, check out the All American Soapbox Derby and Gravity Racing.

Go aero

Even at relatively low speeds, aerodynamics are important. Make sure your design interacts as little as possible with airflow. If you don’t interact with the air, it isn’t going to know you’re there. This will reduce drag and increase your speed.

Budget wisely

The costs of building a basic soapbox car can quickly add up. We saved some money by using bicycle wheels and a go-kart steering wheel, but we still spent £250 buying raw materials and getting the body panels cut out.

Start with the wheels

The general consensus is that 20in wheels offer the best balance of speed and stability for gravity racers. Also consider how you want it to look, and what you can afford.

Wood or metal?

Beginners should choose wood as their main material. The basic tools are in most people’s garage, and sheet timber is pretty cheap. All-metal cars offer greater strength and longevity, but demand more specialist skills.

Keep it real

eBay and Gumtree are useful sources for parts, but it’s worth spending some time perusing second-hand bike shops and skips. We got the wheels for our prototype from a kid’s bike that had been thrown out.

Next steps

Get onside

We started with the side panels and built the chassis to suit. If you don’t have access to a CNC (Computer Numerical Control) machine, a good quality electric Jigsaw will make light work of sheet timber. Set it to a slow speed and go steady.

Use bike brakes

The ‘direct friction’ route (homemade pads that rub against your wheels) might be cheaper, but buying bike brakes is the easier option. Some pram wheels even come with very handy disc brakes.

Weigh up your course

Every Soapbox competition has a weight limit and the ideal weight will depend on the course. Flatter, straighter courses tend to favour heavier builds, while ones with corners and chicanes are better suited to lighter, more nimble designs.

The Build

Tool up

Beyond the toolbox regulars (hammer, screwdriver, drill), a circular saw is handy for bulldozing through wood, while a Mitre saw will accurately cut across the grain. We also used an angle grinder, impact driver and electric orbital sander.

Avoid rope steering

This becomes unstable at high speeds, so adopt a go-kart’s ‘pitman arm’ steering. You can pick up a strong axle, steering column, track rods and hubs online at the likes of eBay.

Customise the cockpit

Before bolting in your seat, get your driver to sit in the car and move it around until they’re comfortable. Tape in some padding too, they’ll appreciate it when landing a jump.

Finishing touches

Tweak the steering

If you have overly responsive steering, try shortening the length between your steering column and the track rods. Test and re-test your car until you’re happy with the sensitivity.

Plan your paintjob

Soak up inspiration from magazines such as Hot Rod or Red Bull’s Soapbox gallery. Use fast-drying, water-based acrylic paints and aerosol spray paint.

Prepare to fail

Make sure your driver wears a motorcycle crash helmet and gloves. If they’re really crash-phobic, think about incorporating a rollcage into your chassis design. This should include lots of triangles to boost its strength.

Driving tips

Brake wisely

Going fast is easy; making sure you don’t lose speed is a real skill. Practice taking the racing line, going slower into corners and only braking when necessary – not too much, and not too little.

Don’t oversteer

Lots of small corrections, particularly oversteering, will slow you down. If you feel yourself moving to the outside of the track, try to make one steering adjustment and don’t overcorrect.

Practice jumps

If your race includes a jump, it’s best to make sure beforehand that your car won’t collapse in on itself. If it’s strong enough, get some big air for the cameras – you only get one chance.