raymondu999 wrote:Is it just the perspective of the shot, or does it look like the floor is actually scraping the rear tyres?
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It's not scraping the tyres, but it's almost touching. That's another way to get downforce.
The reasoning behind it is that the tyre has a stagnation point in front of it, where the pressure is very high. This pressure can be used to create down-force when coupled with the suction under the floor.
In another sense it is a way of scavenging off the drag created by the wheels and making down force out of it. This improves lift to drag ratio.
Here is a pressure plot i did of a moving wheel, notice the high pressure infront:
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