To The TunnelLocated about 100 miles north of London, MIRA is a giant test facility for what's left of the British motor Industry. Nowadays, that's mostly Jaguar and Land Rover, even though they are now owned by Ford. Used to seeing mostly production cars, the staff was excited by the sight of the roadster, especially when it was fired up and driven (with open headers) into the huge, hangar-like building.
At one end there's a big mesh screen, and at the other are four enormous propellers-it looks like a B-52 parked up against the building. The car itself was carefully positioned on top of steel plates by the MIRA staff. Beneath the turntable is a very complicated set of scales known as a six-point aerodynamic balance, the six components being lift, drag, side force, rolling moment, pitching moment, and yawing moment. The movement is measured around the X, Y, and Z axis. The balance is set up to accommodate the wheelbase and track of the vehicle to which the only part that is actually "live" is a 10-inch square pad upon which each tire sits. The pads are connected via a large H-frame beneath the turntable. All forces are measured at ground level through the contact patch of the tire.
Prior to the vehicle going onto the balance and after it has been set for wheelbase and track, a zero measurement is taken. Other measurements are taken to ascertain the vehicle weight and each axle weight (not corner weights). The center of gravity is also determined relative to the center of the wheelbase to be used later to establish pitching moment. The balance is then zeroed.
Once the car is correctly positioned and held in place with the parking brake (of course we didn't have one, so a piece of wood was jammed between the seat and the brake pedal) the fans are turned on and wound up until the required velocity is reached, usually 100 kph (62 mph). Once the wind is steady at the required speed, measurements can be taken. It is the difference between the "wind on" and "wind off" measurements which are of interest. Incidentally, measurements are averaged over one minute with 300 samples being taken every minute.
From the forces measured, calculations can be made by extrapolation to establish vmax (maximum velocity) of the test vehicle and also the power required to overcome the aerodynamic drag. The wind speed is measured using a "pitot static " system. A pitot tube is used to measure total air pressure and a ring of static vents around the test section in line measure the static pressure. It is the difference between these two that give the dynamic pressure that is then converted to give wind speed.
BaselineWe began by taking a baseline of the car as it most recently competed with its 7-inch So-Cal Speed Shop headlights and Pro Stock-style scoop. The only thing on the car not usually run on the Salt are classic-style T-bucket headers, but they were there to allow running the car on the street with mufflers. Also, the figures were extrapolated to provide results as if the car were running 200 mph.
That said, the drag coefficient was 0.521, and the drag horsepower was 551, meaning that it took 551 hp to push the car through the wind at 200 mph, not taking into account anything to do with traction, track surface, or other variables not associated with aerodynamics. The front lift was 460 pounds and the rear lift was 148 pounds. Peter considered both the drag horsepower and the front lift to be quite high, but the rear lift was low. Clearly, the rollcage causes a lever-arm effect that tends to unload the front axle while loading the rear axle. In Peter's opinion, that creates a stable situation.
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