Downforce/Upforce (street racing)

[NDR008]

Oh, and one final note - on most road cars, spoilers and 'aero' devices are stuck on for the wrong reasons.

Also some cars like the Audi TT have a stock spoiler at the back for different reasons. Besides drag and downforce you have pitching moments. The Audi TT is by nature unstable, when you go over a bump the car prefers lifting its nose up, and this in turn further increases the pitching up effect. The spoiler at the back is to bring back the stability by causing a restoring moment in such a scenario.

It has less to do with moments and more to do with overall lift. Most cars produce a small amount of lift at speed. Certain cars like the VW New Beetle and the Audi TT have a rather airfoil like side profile. They produce measureable amounts of lift at speed and cornering capabilities suffer as velocity increases. The purpose of the spoiler is to disrupt the flow so that the car isn't an efficient "wing" anymore. I'm not sure pitch and dive have much to do with it.

These devices do not produce downforce, they merely decrease lift.

I am actually sure because my aero lecturer was part of the aero-test team of that car

[Jersey Tom]

Slimjim does CFD / CFD Software work for a living. He's a sharp guy. Think I spoke with him briefly in person at a FSAE competition.. though don't have his card anymore.

You are correct NDR in that the TT design is inherently unstable at high speed.. because its profile is a wing. Or something that will generate an appreciable amount of lift anyway. Spoilers are there to spoil the airflow and decrease lift, just like the guy said.

That, is what helps stability. Particularly in the rear.

Given the differential in velocity and moreso the time involved in a brief pitch or dive maneuver.. I'd also have a hard time believing its generating much stabilising effect based on pitch inputs about the X axis. If the chassis is somehow jacking over bumps and the underbody is designed for an amount of downforce though.. I'd imagine that chassis heave would drop downforce fairly abruptly and lead to issues.