Part 2: Cars that suck

This follows on from my previous article on “cars that suck” – cars that create downforce by reducing the pressure beneath them so as to suck themselves against the road surface.

The first article discussed cars that utilise the Venturi effect, providing air flow paths under the car with a narrowed portion which speeds up the air, reducing its pressure so as to suck the car downward. The cars discussed in this article use a rather more simple premise – one or more fans extract air from beneath the car, again causing a low pressure zone that sucks the car onto the track. Such “fan cars” are essentially the polar opposites of hovercraft.

The first example of this concept was the extremely “distinctive” Chaparral 2J, which competed in the Can-Am series in 1970. The car had a pair of fans driven by a separate engine, which evacuated air from beneath the car and ejected it rearwards. A Lexan skirt ran around the perimeter of the low pressure zone to reduce air leakage into it, and was linked to the suspension system so as to remain a fixed distance from the track.

The 2J may have looked like the offspring of an airport pushback tug and a washing machine, but its suction system could hypothetically produce enough downforce for the car to drive on the ceiling. And with the suction effect not being dependent on air flow past the car as is the case with Venturi effect cars, the downforce produced was largely independent of the speed of the car. So the car could not only be driven on the ceiling, but could be parked there too (albeit with the ignition on)!

Mechanical problems prevented the 2J from being especially dominant when it raced, but the car nonetheless rattled its competition enough for them to lobby for it to be banned. The series’ governing body had originally approved the design, but reversed their decision after receiving complaints that the car’s rear facing fans sprayed gravel and dirt at the cars behind, and after hearing arguments that the articulated skirt constituted a banned “movable aerodynamic device”.

The concept of a fan car returned, this time in Formula 1, as part of the wave of cars that were spawned to compete with the Lotus 78 discussed in the previous article. The Brabham BT46B used a similar idea to the Chaparral 2J, with a rear-facing fan evacuating air from beneath the car. In this case though, the fan was powered by the same engine that drove the wheels, so while downforce was not linked to the speed of the car it was linked to the speed of the engine. Another interesting note on this car was the nod towards driver safety – a modified altimeter on the dashboard would notify the driver if the pressure beneath the car was unexpectedly high. Useful if on the approach to a corner, perhaps, but likely of little comfort when half way round!

Brabham argued that the sport’s “movable aerodynamic devices” ban did not apply to the BT46B because the main function of the fan was to draw cooling air over the engine, but the downforce it produced was clear for all to see. Indeed, when standing still the car could be made to hunch down by revving the engine. Following something of a theme, other teams complained and the car was swiftly banned, along with fan cars in general.

Until very recently, that seemed like it was going to be the end of the story. But last year the McMurtry Spéirling rocketed into the spotlight, almost literally, by breaking the all-time record for the Goodwood Hill Climb. Coming seemingly out of nowhere, built by a British start-up only founded in 2016, the Spéirling is deeply impressive on all fronts. Fully electric with a range of 300 miles, the car weighs less than a tonne, can corner at 3g, and accelerates from 0 to 60 mph in less than 1.5 seconds! But the downforce the Spéirling generates is even more impressive. The car follows the same basic idea as the Chaparral 2J, using a pair of independently-driven fans to suck air out from beneath the car and exhaust it to the rear. In this case the low pressure zone is bounded by ceramic sealing plates, and active height adjustment keeps the plates sealed well against the track.

McMurtry’s patent portfolio gives us more insight into the innovative technology behind the Spéirling. This patent and this pending application discuss numerous details of the layout of the car’s suction system, such as the skirt being “pre-loaded” downwards against the track, the suction path having a debris removal system, and the car having an auxiliary power store to ensure that the fans are always operational. This patent application focuses on a low-profile mechanism for adjusting the skirt to maintain a good seal with the track, and this one concentrates on the sealing surface of the skirt.

Thanks to technology such as this, the car produces 2,250kg of downforce when travelling at 150mph, and even at a standstill generates 2,000kg. So while the Chaparral 2J could park on the ceiling, the McMurtry Spéirling could do the same with an entire football team hanging from the roof!

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You can read Part 1: Cars that suck here.

You can read Part 3: Cars that suck here.