Aerodynamics

The aero model is three forces from the same dynamic pressure: a drag force against the velocity and a lift force at each axle, with the drag on a line that may sit above the centre of mass (ADR-0015). Every coefficient in aero is referenced to the one frontalArea, as wind tunnels quote them, and q = ½ · airDensity · frontalArea is the common factor.

Drag

F_drag = q · dragCoefficient · V², against the velocity of the centre of mass. On its own it only slows the car. dragHeightAboveCg puts its line of action that far above the centre of mass along the body's up axis, which is where a real car's drag acts: the body is above the wheels and the wake is behind the roof. Drag high on the body pitches the nose up and moves load rearward at speed. The single-track model has no pitch, so it moves the load directly: F_drag · dragHeightAboveCg / wheelbase from the front axle to the rear.

Lift

liftCoefficientFront and liftCoefficientRear give a force at each axle, L = q · C_l · V_x² with V_x the body-frame forward speed, so sideways motion makes no lift. Positive is lift, negative is downforce. A road car has a little lift at both ends (a few hundredths), a winged car a lot of downforce.

On the four-wheel model each lift force is applied to the body along its up axis at the axle's position. It reaches the tires through the springs, as it does on a real car: downforce compresses the suspension, the ride height drops with speed, and the tire load rises by what the spring now carries. The raycast suspension needs no special case for it, and the consequences come for free: a car with downforce needs stiffer springs or less travel than its mass alone would ask for, or it sits on its bump stops at speed, and the SuspTravel_* channels show it happening. The single-track model has no springs, so the lift is added to each axle's load directly.

Because the load sits on the tire, downforce raises the grip of that axle through the tire's load sensitivity: not in proportion, since friction per unit load falls as load rises (slip), but a car that corners at 1.6 g on its tires alone corners harder still with half its weight again pressing down on it. The balance of front to rear lift sets the aero balance, which shifts the handling balance with speed in the same way the static weight distribution sets it at low speed.

The open-wheeler preset

openWheeler is a junior single-seater in the 600 kg class with lift coefficients of −1.2 front and −1.8 rear on a 1.0 m² frontal area and a drag coefficient of 0.9. At 180 km/h that is about 4.6 kN of downforce, near 0.8 of its weight, with the drag acting 0.15 m above the centre of mass at the wing heights. Its ride frequencies are around 3.5 Hz for that reason. Below about 100 km/h the wings add little, and full throttle in the lower gears is more than the rear slicks can take, so the preset ships with traction and stability control on (and ABS); turn them off under assists for the unassisted car. The road-car presets carry small positive lift coefficients from their data sheets; the kart has none.

Telemetry

DragForce is the drag magnitude in newtons. AeroLift_F and AeroLift_R are the lift forces at each axle, positive up, so a car with downforce shows negative values that grow with the square of speed.

Tuning

• More negative liftCoefficientRear adds rear grip at speed, which is stability: the car that is loose only on fast corners usually wants rear downforce, or less front, rather than a tire change.
• Keep the two coefficients in proportion to the static weight distribution for an aero balance that does not move with speed; a front-heavy aero balance is a car that gets nervous as it goes faster.
• dragHeightAboveCg is a small effect on a road car (a tenth of a metre moves a couple of kilograms of load rearward at motorway speed) and a real one on a tall vehicle or a winged car with the rear wing high up.
• Downforce needs springs: check SuspTravel_* at top speed against travelBump, and raise springRate before touching the dampers.