This is the final article in the series on “cars that suck” – cars that create downforce by reducing the pressure beneath them so as to suck themselves against the road surface.
I would have loved to have been able to finish the series with the mental picture of a McMurtry Spéirling parked on the ceiling with a football team hanging from the roof. However, there are a pair of cars which cannot be glossed over. These essentially bridge the gap between the Venturi effect cars discussed in the first article, and the fan cars discussed in the second.
Both these cars were designed by the man behind the Brabham BT46B, Gordon Murray. They have rear diffusers to make use of the Venturi effect, but use fans to assist the flow of air over the diffusers. Generally speaking, in an automotive diffuser if the upper wall is positioned at a steeper angle relative to the ground then the diffuser will accelerate the air to a greater extent and produce more downforce. However, in conventional designs there is a limit beyond which the air is unable to follow such a steep angle and instead stalls and becomes turbulent, dramatically reducing the downforce produced. In these two cars, the diffuser has a very steep angle and fans are used to suck air towards the top surface of the diffuser to prevent it from separating.
This patent, listing Gordon Murray as the inventor, gives a brief explanation of the general idea and includes a handy schematic illustration in Figure 2. The owner of the patent (McLaren) and its priority date (1992) also give us a hint as to where we first saw the fan-assisted diffuser concept. Much to my surprise when researching for this article, the idea first appeared in the iconic McLaren F1! I’m surprised I didn’t know this, given the amount of car geekery that went on in my house at around that time. But then my heart really belonged to the Ferrari F40 and yet I never really noticed that the entire rear of the car was one large diffuser, with gaping channels either side of the exhausts. I suppose aerodynamic performance took a back seat (if you’ll pardon the pun) to things like engine noise and 0 ‑ 60 times.
In the Mclaren F1 two narrow but steep diffuser passages, each with a small electric fan to keep air adhered to it, generate a 5% increase in downforce and also a 2% reduction in drag. Lack of available time in the wind tunnel prevented this idea from being capitalised on more fully, but Murray made a mental note that were he ever to design another supercar he would make use of this effect across the full width of the car.
Mr Murray kept his word, and there is no missing the 40cm, 8500W fan on the Gordon Murray Automotive T.50! Not only is the downforce much higher thanks to the low pressure zone extending across a much wider area, but the multi-mode fan setup of the T.50 has other tricks up its sleeve as well. A problem with most Venturi effect cars is that downforce increases with speed, so when on the straight of a track or on a motorway, the car experiences a lot of downforce which is unnecessary and actually counterproductive – it pushes the car down towards the bottom of its suspension travel, giving a less comfortable ride and reducing fuel economy. In these circumstances the T.50 can take air from the roof of the car rather than the bottom, reducing downforce by allowing air to stall in the diffuser. This also acts to clean up the airflow over the top of the car and eject air into the wake of the car to form a “virtual longtail”, thereby reducing drag by 12.5% (as well as providing an extra 15kg of thrust, for good measure). Alternatively, in a hard braking mode the fan can take air from the diffuser and an active rear wing can raise up to steep angle, doubling the downforce to reduce skidding and thus decrease the stopping distance at 150mph by 10m.
This patent, granted to Murray in 2021, gives some more explanation around some of the different modes in which the fan can operate. This will be of little immediate use to competitors, however. The patent gives Murray a broad monopoly on the idea of a vehicle with a fan that is controllable to draw air from either an upper or a lower surface of the car. It will be interesting to see if Mr Murray can be tempted to license the idea to other manufacturers, but my suspicion is that they will have to find alternative ways forward.
Whilst the T.50 and the Brabham BT46B differ hugely (Murray himself describes the Brabham as “a crude device…. a blunt instrument”), in my mind the T.50 includes some interesting nods to Murray’s experience in the “ground effect era” in Formula 1. In both cases the fan also acts to draw cooling air through the engine bay, and in both cars the driver has a readout to indicate the level of downforce being provided. One notable difference is that while Murray still dismisses the idea that the Brabham ever ejected gravel at other drivers, the air exiting through the fan of the T.50 is filtered first. Murray notes that this was something of a disappointment to one customer! Perhaps afterburners or Goldfinger-style oil sprayers will be added to the options list in due course.
Looking to the future, it seems safe to assume that other manufacturers and racing teams will be taking note of the renewed success of cars that suck. Whilst full-on fan cars like the Spéirling may well remain in their high-performance niche, in general it seems likely that innovation in the performance sector will continue to trickle down to the mass market. In the meantime, I’ll count my blessings that my old Skoda doesn’t have any of that technology. At this point, if its undercarriage was sucked down towards the road, I’m not entirely sure that the top half would follow. Perhaps it needs another layer of duct tape.
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You can read Part 1: Cars that suck here.
You can read Part 2: Cars that suck here.
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