#Focus – Ground Effect for Dummies.

Ground effect or the principle of using an entire car as a giant wing in order to increase downforce and minimize drag, basically that’s what it stands for. But in situ, how does it work ? That’s what we’re gonna try to explain.

Back in the early sixties, downforce wasn’t the point when designing a racecar, it was all about straightline speed in terms of shape and bodywork but it resulted in very unstable cars and of course accident which were not always lethal but almost. As in period, the first attempt to balance the cars were seen through the use of front spoilers in order to minimize drag and add front end downforce in order to help during high speed cornering. All these experimentation led to a major revolution in motorsport and car design more generally, the use of wings at both end of the cars in order to produce negative lift or downforce.

Actually, Jim Hall was the pioneer in terms of ground effect and application to motorsport. Back in 1961, he experimented it with his first design, the Chaparral 1, which was entirely shaped as an inverted aerofoil, nose and tail being upswept. The concept was to speed up airflow under the car but in situ, it resulted in the car lifting at high speed so a spoiler was fitted but then, the generated downforce proved too much and the idea was left aside.

1966 was a turning point and still with Jim Hall’s Chaparral but this time, the 2E which appeared at Bridgehampton that same year and featured a driver adjustable rear wing, in fact, the driver could adjust the wing incidence from the cockpit in order to lower or increase it and therefore help the cornering speed by increasing downforce or helping straightline speed by putting it in horizontal position in a same way that the DRS system is used in Formula One nowadays.

Jim Hall had a sort of unfinished business regarding ground effect and so he created the Chaparral 2J for Can-Am and the 1969 season, the concept was to create a low pressure zone underneath the car but this time using the principle of suction. The whole car, from the front-axle centre line to the back of it was isolated using skirts made of lexan and a 2-stroke engine fitted to the car in order to suck air from the underbody. It worked very well as the car featured no wing at all and generated the downforce from the underbody, but unreliability of the second engine proved to be the achille heel of the car which suffered many DNF because of its failure. But all in all, the whole idea was banned by the rulebook as the other teams were not very happy. That same concept was more or less later used by Gordon Murray through the Brabham BT46B which used the same concept but this time driven by the power-unit of the car itself, it raced once in 1978 at Anderstorp, Sweden and was then outlawed as considered an aerodynamic moving device…

But anyway, back to ground effect, it was another idea of Colin Chapman from Lotus through the Lotus 78 Formula One car. Peter Wright discovered it at the London Imperial College while experimenting with side-pods located radiators. As the wind tunnel featured a rolling road, the instrumentation showed an increase in downforce as it accelerated. It featured an underbody shaped as an inverted wing profile which generated downforce. The car debuted in 1977 and won four races with mario Andretti but the car suffered of reliability problems which helped hiding the idea actually and then came the Lotus 79, a better concept, better worked out which set the trend for the coming season..!

How does it exactly work, well it’s all about low pressure. To make it simple, by generating a low pressure zone underneath a car, the atmospheric pressure pushes it to the ground, the opposite of what an airplane generates and its the principle of the “Bernoulli Effect” which is the theory that if you accelerate a moving fluid or gas, its pressure will fall. Back to the cars, these were equipped with skirts as well in order to isolate the underbody and the low pressure zone, these skirts were at the beginning “suspended” from the sidepods with a vertical degree of freedom in order to work with the ride eight of the cars as well as featuring a ceramic insert known as the “rubbing strip” in order to cope with the distance of the Grand Prix. Later, these were banned and skirts had to be fixed which led to suspension work in order to make full use of the fixed skirt, more or less like having a “drop” system permitting the car to run at the perfect ride height at all time and preserving the flow volume.

Ground Effect Side Skirts and their evolution L-R.
Ground Effect Side Skirts and their evolution L-R.

Flow volume, what’s that ?! It is volume between the vehicle and the ground which is strongly dependent on the car’s attitude relative to the ground. Very small ground clearance results in positive lift, since there is almost no airflow between the underbody and the ground. With increasing ground clearance the airflow produces low pressures causing overall lift to be lowered to negative values and then to rise again as ground clearance continues to increase. This is due to the fact that the flow velocity under the car decreases as ground clearance increases.

Ground Effect in situ.
Ground Effect in situ.

In this case, more downforce can be generated using a diffuser between the rear wheels. The air enters the diffuser in a low-pressure, high-velocity state after accelerating under the car. By gradually increasing the cross-sectional area of the diffuser, the air gradually slows down and returns to its original free-stream speed and pressure. The diffuser’s aim is to decelerate the air without it separating from the tunnel walls, which would cause a stall, reducing the downforce and inducing a large drag force. By installing an inverted wing close to the diffuser exit it is possible to create a low pressure area, which essentially sucks the air from the diffuser and helping him do the job. The diffuser and wing combination permits a higher air mass flow rate through the diffuser, thus resulting in higher downforce, which you can see in situ with the sportscars such as the Peugeot 905 and Jaguar XJR14 as both design featured the double element rear wing which is nothing else than an extension to the diffuser.

But all in all, ground effect had a problem, miscalculations or failing to properly set-up the car would result in an unstable and undriveable car. Ground clearance being so important, it led to “rigidly sprung” cars with basically no suspension or ride height tolerance with no ability to handle bumps and curbs. The cars were getting faster and faster and proved too dangerous to be handled which led to its ban after severall accident and death at the end of 1982.

Today’s version of ground effect are the diffuser you can see on almost any modern racing cars, it uses the principle but in a much smaller way and the last cars to make full use of tunnels and diffusers were basically the IndyCars with Jim Hall again and its Chaparral 2K as well as Sportscars such as the Porsche 962, Intrepid RM-1 up to the Jaguar XJR14 or Peugeot 905 but many other examples such as the Williams FW07 can be found. And the final point being the active suspension which was in a way, the modern version of ground effect in Formula One but without skirts.

Below is a gallery detailing some of the cars that helped settle the “ground effect” concept.

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