A record-breaker's body shape not only needs to be highly aerodynamically efficient but the underside of the vehicle is also of critical importance, because the air flowing under the car accounts for about a half of the total aerodynamic drag.
Ron Ayers believes that the interaction between tyre and salt can significantly affect the aerodynamic efficiency; the effect of salt and debris thrown up by the cars passage slows the car down. To minimise this drag he has very carefully shaped not just the spats around the lower section of the wheels, but also the flow of air through the choke points between the wheels. Spray beneath the front of the car is deflected out by the sides of it behind the front wheels, to ensure that the rear wheels and tyres run on as clean a surface as possible.
All of the aerodynamics study was done using computational fluid dynamics, not in a wind tunnel, for two reasons.
Even at the speeds we envisage, Ayers explained, compressibility effects are beginning to become significant. Indeed, in the region near the wheel/ground contact points the local airflow actually goes supersonic! We could not simulate such effects in a low-speed wind tunnel with a rolling road.
The second reason is one of scale. To fit our long slender vehicle into a tunnel with a rolling road would mean restricting ourselves to a model scale of about one sixth and the errors would have been too great.
The main changes as the shape evolved were to lengthen the nose and round it off, to lengthen the tail and minimise the frontal area. At every stage Ayers had to achieve the optimal balance between aerodynamic drag, skin drag (the larger the surface area the higher the skin drag) and downforce. If the car is envisaged as an arrow or a dart, it is the tail fin that acts as the flights to maintain stability at maximum speed.
