MMD:
fritz:
If you are really interested, downforce is in fact negative lift.
When aerodynamic lift is reduced (by using such sensible measures as spoilers) to a value below zero, then it starts to distance itself from its nasty reputation for destabilizing cars at high speed by calling itself downforce.
Definitely interested! although my bad habit of adding humor at all times might not indicate that.
I have always amateurishly equated downforce with “spoiled” lift. I wonder about it this way:
If you could weigh the rear axel when the car is at top speed would it show an additional (approx) 136 kg.? Or would it show the same weight as when the car is at rest because the airfoil (lift) has been merely neutralized?
Seems like the scale would indicate a gain of 136 kg.
Oops..., if it read +136kg that would be a "subzero" lift right?
Now I'm betting the scale at top speed would show the same value as when the car's at rest.
Actually, I am not adverse to humor myself.
It might be better to think of lift as being "spoiled downforce", since downforce is the desirable quantity in a car whereas lift is more useful in a plane.
Using that logic though, I should have written above that lift is negative downforce.
To use figures to illustrate it:
If the static weight acting on your car's rear axle is 800 kg at standstill and the car has efficient aerodynamics which generate a downforce of 100 kg on the rear axle at 200 km/h and you were to drive over your notional scales at 200 km/h, then the scales reading when just the rear axle is bearing on them would be 900 kg.
If, OTOH, you have sawed off the rear wing and the resultant configuration results in rear-end lift of 100 kg at 200 km/h in the wind tunnel, then the scales reading would be 700 kg under the conditions described above.
The additional 200 kg acting on your rear axle, aided by an appropriate force acting on your front axle to give balanced handling, would make the car that much more stable at speed. So lock that saw away in case you go sleep-walking some night.
A wind tunnel just has load cells under all the wheels of the car measuring its net weight (static weight minus lift or plus downforce, as the case may be), as well as measuring the horizontal force imposed on the car due to aerodynamic drag.
--
fritz