Motorsport – Understanding Downforce
Downforce is essentially a force pressing down on the tyres of a car, increasing grip. Whether that be gravity (the weight of the cars mass) or via the air pressing down while in motion. Today I will discuss the effects of air over a car and how it can be manipulated to increase downward pressure and force the wheels onto the road/track surface.
Aerodynamic downforce can come from either wings/spoilers or under car (ground effect). Wings and spoilers are precisely the same as what we get on aeroplanes, only turned upside down. Ground effect is essentially when we turn the entirety of the car into a wing.
Now, how do wings work?
Wings work by splitting air across two surfaces, the upper surface of the wing and the lower. In doing this the air begins to travel at different speeds according to the shape of the two surfaces. When one side is traveling at a faster pace to the other, the other side of the wing is sucked by the faster air, pulling it into a low pressure area (the gap induced by the air being split). This is the suction that pulls the wing up or down. (NB: in the picture below I’ve tried to convey the idea of a pressure change under the wing, the airflow would actual stick to the surface of the under wing throughout, the vacuum being created at that point by the different speeds of the air over and under, toward the edge).
A spoiler essentially spoils the flow of the air, pushing the spoiler away from the flow (be that up or down). Here is a drawing of a spoiler:
As you can see the spoiler is a simple system of the air physically forcing the surface down via the angle, and by this pressure creates a lot of drag. Here is an example of a spoiler on a Ford RS200 rally car (and half of my fiance):
Ground effect is when the whole of the bottom of the car is utilized to form a vacuum underneath the car, this sucks the whole car to the ground. If you’ve ever wondered why racing cars are so low to the ground, not only is it for a lower centre of gravity, it is to ensure a small amount of air goes beneath the car which when stretched by the diffuser at the back (or throughout in the case of ground effect cars) creates a greater vacuum. Too little air though could cause the airflow to stall.
As you can see in the above quick sketch, the air enters from the front and is split by the splitter (this manages the amount of air flowing beneath), some goes up over the car and some under. As it meets the diffuser the air is forced away from the road by the angle of the diffuser. the airflow wants to follow the bodywork and thus, pull away from the road creating an area of low pressure (nothing there), creating a vacuum sucking the rear of the car down. As this air leaves the diffuser it will interact with the air traveling over the back of the car which is often manipulated to work in conjunction with the rear wing. Some cars, particularly ground effect cars of the 80’s essentially had the front of the diffuser at the front of the car, this created a suction throughout the bottom of the car.
So I hope this has been a useful introduction to rudimentary car aerodynamics. In later blog posts we will discuss the minute details of each wing, look at aero innovations in motorsport specifically, and explain how drag and downforce works as a whole package in motorsport to create the perfect racing car.