Friends of mine could be forgiven for thinking that the topic of this article series is my 15-year-old Skoda Fabia, in which I spent many hours of last summer’s heatwave with neither functioning air conditioning nor opening windows. Although there would be much that I could say on that subject, this is not the case. In fact, the articles summarise some of the efforts in the automotive industry to create downforce by reducing the pressure beneath a car so as to suck it against the road surface, rather than pressing it down from above as is currently the norm.
One way in which this can be achieved is using the Venturi effect. When an air flow passes through a constriction it speeds up, in much the same way as the water exiting a hosepipe can be sped up into a narrow jet by partially covering the end of the hosepipe with a thumb. As set out in Bernoilli’s principle, when an air flow is sped up, its pressure decreases. Thus by providing a constriction in the flow of air under the car (or more accurately the passage of the car over relatively stationary air), the pressure beneath the car can be lowered and the car will therefore be sucked downward.
This effect has seen some recent limelight in Formula 1 racing due to the emergence of the problem of “porpoising”. However, the technology has its roots nearly half a century ago in the Lotus 78, used in the 1977 and 1978 F1 seasons. The car had large sidepods between the front and rear wheels, the undersides of which were shaped like the top surfaces of aircraft wings. These surfaces provided the constriction required for the Venturi effect to occur. Side plates with flexible ‘skirts’ extended all the way down to the ground to prevent air from rushing into the low pressure zones from the side. The extra downforce generated led the great Mario Andretti to comment, after his first drive of the 78, that it was as if the car was “painted to the road”.
Generating downforce in this way creates much less drag in comparison to the more conventional approach of using wings to create downforce from above. The “free” extra grip that was provided to the Lotus cars was clear for all to see, but how they obtained this advantage was initially a mystery for the other teams – another effect of the side skirts was to block the unusually-shaped underside from inquisitive eyes!
By 1979 the cat was completely out of the bag and the entire sport moved towards favouring this “ground effect” approach over conventional wings. The technology was very much in its infancy, however, and some of the smaller teams could manage little more than copying the visuals of the Lotus cars.
Even when implemented well, a key drawback of using the venturi effect was that it was very reliant on the side skirts functioning correctly. Damage to a skirt, or lifting of a skirt due to the car hitting a bump or a curb, could allow air to rush into the low pressure zones inside the car’s “Venturi tunnels” and eliminate the downforce produced. Sudden loss of downforce half way through a fast corner could and did have catastrophic results, and the FIA felt compelled to act. At first a minimum ground clearance was implemented, but teams found work-arounds (some rather ingenious, some utterly blatant) to allow their cars to squat down when racing but raise up for post-race measurement. Ultimately, in 1983 the FIA decreed that cars must be flat-bottomed and the venturi effect could be used no more.
That was until the update of the Formula 1 technical regulations governing the 2022 season. A problem with conventional wings is that they leave a turbulent wake behind them, which reduces the amount of downforce that can be generated by the wing of a car following close behind and thereby makes overtaking more difficult. In an effort to make racing more close-packed and exciting the FIA has allowed the use of ground effect again.
Thankfully, the return of ground effect downforce has not meant a return to the danger it posed in the past. As well as the general incremental improvements in safety that have come into play in the intervening decades, the sealing of the Venturi tunnels under the car is now much less temperamental. Whereas the cars of the 70s and 80s used skirts extending downward from the bodywork, contemporary cars use fins attached to the rear brake ducts. Whilst the bodywork can move relative to the wheels of a car due to its suspension, the rear brake ducts are in fixed positions relative to the wheels. Thus, the fins extending from the rear brake ducts stay a fixed distance from the track as long as the wheel remains in contact with it, and sudden loss of downforce is therefore unlikely.
That being said, since the Venturi effect was banned in Formula 1 for such a long time it has not received the same amount of research attention as conventional wings, and teething troubles still remain. Chief among these is the issue of porpoising mentioned above. In general, if a car is low to the ground then the constriction providing the Venturi effect will be narrower, air will be accelerated more and greater downforce will be produced. However, there is a point at which the constriction becomes too tight and air flow stalls, and current F1 cars can sometimes reach that point on fast straights where the downforce is highest. When that point is reached, no acceleration of air means no downforce. When the downforce disappears the car raises up on its suspension (albeit with the sealing fins staying in place in the case of modern cars), widening the constriction and allowing air to flow again. With air flow restored the car is sucked downward again, which narrows the constriction and increases the downforce, which pulls the car further down until (potentially) the air flow stalls again, and so on. The car then bounces up and down, in a manner reminiscent of a porpoise on the sea bed.
The Venturi effect also sees considerable use outside of purpose-built racing cars. These days plenty of contemporary road cars, even those relatively far from the performance end of the market, can be seen with rear diffusers. A diffuser is an air flow channel which gradually widens from a narrow underfloor area to a wide mouth at the rear of the car, often with vertical ribs or ‘strakes’ projecting down from its top surface. The underside of any car will form a rudimentary constriction and accelerate the air underneath it to some extent, and a rear diffuser provides a gentle expansion area for that air, minimising turbulence and thereby allowing the air under the car to travel faster (and thus drop to a lower pressure). The smoother return of air passing under the car can also reduce drag by minimising the size of the ‘bubble’ of low pressure air that forms behind the car.
Whereas the relatively high ground clearance required of most road cars means that there is little that can be done to stop air leaking into the low pressure zones under them, the same does not apply to track day cars and more dedicated supercars, and the Venturi effect can be used to greater effect here. A particularly noteworthy example of such a car is the Elemental RP1. As well as an extremely pronounced pair of rear diffusers, this car has a pair of front diffusers which run under the nose of the car and out to the sides behind the front wheels. Together, the diffusers can produce over 200kg of downforce at 100mph and 400kg at 150mph. For the sake of comparison, the Bugatti Veyron can generate a maximum of 180kg of downforce. The Aston Martin Valkyrie takes the same general layout as the RP1 to even greater extremes, with front and rear diffusers that look large enough to climb through. Together the diffusers generate a downforce of over 1,800kg, allowing the car to corner at up to 3.3g.
A quick glance at recent patent activity shows that the Venturi effect continues to be an area of research interest to Automotive manufacturers. As one example, in January this year Porsche obtained a granted patent directed towards a rear diffuser with a portion that is actively movable so that the trade-off between downforce and drag can be adjusted to fit different circumstances. Building on previous filings in this area, the patent describes articulation mechanisms that allow better air flow through the diffuser than a simple hinge can provide. Along slightly different lines, a patent grant to Honda and a recently-published patent application filed by Volkswagen are both directed towards surface shaping in diffusers which aims to provide better air flow characteristics.
A final example of recent IP in this area has a slight lean towards the technology that will be discussed in the next article. This 2021 patent grant to McLaren protects the idea of a Venturi tunnel with a bleed duct through which some of the air inside the tunnel is drawn away. Reducing the amount of air in the tunnel reduces the pressure under the car and thus increases the downforce acting on the car.
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You can read Part 2: Cars that suck here.
You can read Part 3: Cars that suck here.
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