Latest Soapbox Racing News

Soapbox Speed Calculator

Gradient %
Total Mass kg
Mass of Wheels kg
CdA m2
Air Density Kg/m3
Top Speedmph

Terminal velocity of a gravity racer is the theoretical maximum speed at which the drag forces exactly equal the force due to gravity. It is theoretical because it is an asymptotic limiting value that cannot ever be reached, although you can get very very very close.

The table on the right will allow you to calculate the approximate top speed of a cartie on a given slope, and the distance needed to reach that speed. Just use the "up" and "down" arrows to adjust the parameters and see how the top speed changes.

Because terminal velocity is is never achieved, the figure quoted is the distance needed to reach 99.9% of the terminal velocity.

For a more accurate estimation of speeds on any given course, grab yourself a copy of SCA Cartie Sim.

"Power to Weight" for Soapbox Racers

One of the key indicators of a car's performance is its power to weight ratio. The better ratio is, the faster the car will accelerate.

Obviously this can't work for a gravity powered soapbox racer as it has no engine, but there is an equivalent figure.

Aerodynamics of Soapbox Racers

soapbox racer drag v. speed Aerodynamic drag is by far the largest factor affecting a carties top speed, but how big an effect is it? I've been doing some maths and have come up with some interesting typical figures. The three sources of drag on a cartie are;

  • Aerodynamic drag
  • Rolling resistance
  • Wheel inertia

soapbox drag versus speed Rolling resistance is almost constant at all speeds, and wheel inertia actually decreases as speed increases (or - more correctly - as acceleration decreases), but aerodynamic drag increases as the square of the speed. It starts off very low, but soon becomes the dominant factor.

Posted In

Windmill Hill Soap Box Derby, West Yorkshire

Saturday 13th September 2014
Saturday 13th September 2014

Windmill Hill Soap Box DerbyWindmill Hill Soap Box Derby has been created to help raise money for Clifford AFC Development Fund, whilst aiming to be a fun day for the teams, supporters and spectators.

‘Fun’ or ‘Themed’ Soap Boxes are especially encouraged since they provide the most interest to watch! Check out youtube for inspiration.

The first race, which will be the Junior Soap Box Derby will commence at 10.30am and the adult races will start from 12.30pm. In addition to the races there will be food, drink and light entertainment.

For further details, check out the  Windmill Hill Soap Box Derby web site.

Posted In

Scottish Team Set New Gravity Speed Record

An unnofficial world gravity speed record has been set during testing of new gravity racer in Scotland.

Speeds in excess of 80mph were achieved during testing last weekend, with a maximum speed of 87.4mph being recorded by the team from the University of Caledonia.

Taking advantage of pleasant spring weather, the team had headed up to the Highlands to do a spot of testing and originally planned to just do a few test runs to prepare for the official attempt that is scheduled for the beginning of June.

"We did a couple of 'sighter' runs to get used to the conditions", said team leader Rolf Lapio. "The racer handled really well so we decided to push a bit and see what we could get out of her. We knew she'd be fast, but we didn't realise quite how fast! It's a brilliant start. Obviously we need to repeat this under controlled conditions and with independent verification to claim a record, and we're really looking forward to the official attempt in June. Our calculations show that we should be able to do well over 90mph".

Video verification of the fastest run has been uploaded to youtube.


Posted In