First, I’ll explain what the “slipstream” is. A car punches a hole in the air when it moves. This hole is exploited by a car behind to reduce the drag of the following car and gain speed on the leading car.
During open-wheel races such as the Indianapolis 500 or some F1 races (DRS notwithstanding), the draft is so strong— it’s like the car behind gains an extra 200 horsepower!
But during NASCAR races at the unrestricted ovals or during sports car races like the Daytona 24, it’s not anywhere nearly as strong. It takes half of the Daytona oval’s back straight to get to the back of a leading GT3 car and pass him, and you’d be lucky to even get to the leader’s rear bumper at unrestricted ovals like Michigan. If anything, it looks like the leading car _gains_ speed. Why is that?
In: Physics
I am far from an expert, but my understanding is that it is less the car is punching a hole in the air as it is creating areas of low pressure due to drag from the car. Open wheel be cars are much less aerodynamic so there is a more significant low pressure area behind the car pulling the leading car back and pulling the trailing car forward.
As you get a chain of cars it’s just a bunch of turbulence the way down.
With more aerodynamic cars, there is a smaller low pressure zones behind the car. Additionally as another car moves into that spot it actually improves the aerodynamics as the cars are pretty aerodynamically clean.
So the lead car actually gets a reduction in drag as the other car moves into that low pressure zones, getting rid of it, and not it is forming behind the trailing car.
Whereas in open wheel cars you still get significant drag behind the leading car because the cars are not smooth and so you still get a lot of turbulence in that same area.
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