Quote:
Originally Posted by GretchenGotGrowl
Different color calipers, so I am guessing they are different cars. GM did say the A8 would be faster than the M7 and that the Z07 produced more downforce than any car they had ever tested. If the Z07 package has 200 lbs of downforce (who knows what it really is) at 100 MPH, that's like having another full-size male occupant...not to mention the drag that must go along with it. That can significantly impact acceleration. So, maybe what we are seeing is that A8s with less aero accelerate faster. They won't handle the twisties as well, but they may be really good in a straight line.
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As far as grip is concerned? Yes. The downforce will increase the friction between the tires & the road (thats why its there in the first place). That increased rolling resistance might help (if the tires were in danger of spinning) or it might hurt (if the car already has enough grip to prevent the tires from spinning). But hurting high-speed acceleration because of apparent weight? No.
Lets go back to Newton. Net (or sum or total) force = mass x acceleration
In the case of a car, our mass is the car + everything in it. For forces, lets break this down into horizontal & vertical. In the vertical direction we have the cars weight + any aerodynamic downforce pointing down. This is met by the normal force of the earth pushing back up resulting in 0 vertical forces (otherwise the car would either lift off the ground or bury itself).
Along the horizontal, you've got the force of the tires (driven by the engine) pushing the car forward. You have the friction from the air (drag) & the road (rolling resistance) holding it back. You should notice that downforce does not factor into the horizontal acceleration, not directly. It increases both forms of drag as a side effect, but it does nothing by itself with regard to forward acceleration. Subtract the frictional forces from the propulsive force & you get the net force thats trying to accelerate the car (or truck or train or boat or plane or anything really).
The major impact will be felt in increased drag. I'm not sure on the ratios for lip spoilers, but on wing-types its usually a few pounds of downforce for every pound of drag.