For both the Challenger class and the Cruiser class, solar car racing is to a large extent about aerodynamic drag. That’s overwhelmingly what the hard-earned solar energy is being wasted on, and therefore it’s what teams need to concentrate on minimising. The drag force on a car is given by the equation:
F = ½ Cd A ρ v2
Breaking that down, v is the speed of the car, ρ is the density of air (about 1.2), A is the frontal area of the car, and Cd is the drag coefficient, a number which indicates how aerodynamic (and therefore, how energy-efficient) the shape of the car is. For Challengers, minimising Cd allows the speed v to be increased, while for Cruisers (for which the average v is essentially given), minimising Cd allows non-solar energy use to be minimised. Of course, minimising frontal area is important too (and that is the motivation behind asymmetric Challenger cars).
To give a feeling for the all-important Cd, here are some vehicles with values ranging from 0.19 to 0.57:
Drag coefficients for a selection of vehicles. Clockwise from top left: 0.57 – Hummer H2 (photo: Thomas Doerfer); 0.30 – Saab 92 (photo: “Liftarn”); 0.26 – BMW i8 (photo: “youkeys”); 0.19 – General Motors EV1 (photo: Rick Rowen)
Because Cd is so all-important, it is the one thing that solar car teams are really secretive about. Challengers generally have values under 0.1. With no need for practicality, they chase their way towards the impossible goal of Cd = 0, trying to come up with the perfect race car, which will slice through air like a hot knife through butter:
Cruisers, on the other hand, have to balance aerodynamics with practicality. Bochum’s early SolarWorld GT had Cd = 0.137:
Bochum’s 2011 car, SolarWorld GT, with Cd = 0.137 (photo: “SolarLabor”)
Eindhoven’s recent Stella Vie, with its sleek aerodynamic shape, does much better than that (but they won’t say how much better):
I understand that Sunswift’s 2013–2015 car eVe had Cd = 0.16. Appalachian State (Sunergy) have stated that their newly-built ROSE has Cd = 0.17. PrISUm’s Penumbra has a higher value (Cd = 0.2), because of the blunt end which they chose for practicality reasons (although they did do a few clever things to reduce the impact of that blunt end). I’m not aware of the Cd values for other ASC cars.
Appalachian State’s beautiful ROSE, with Cd = 0.17 (image credit)