Aero testing the new Ariel Atom 4R in Catesby tunnel

The idea was that Prior and I – plus Ariel’s own technical expert, Kelvin Marshall – would do a series of coastdown tests in the tunnel, with and without screen, accelerating to 60mph, then coasting in neutral to 20mph and doing it again. The loggers would record variations in speed and time, assessing the effects by computer.

Prior and I both knew from experience how effective Ariel’s modest air dam ahead of the driver has always been at cutting turbulence around the driver’s head. Back in the 1990s, we were among the first to learn that a fist-sized flyscreen just ahead of the steering wheel made a surprising difference. But that was in an Atom 2 and now we’re up to the Atom 4R.

The first lesson at Catesby, which took minutes to learn, was about the effectiveness of the Atom’s latest cockpit screen. Without it, the 60mph-plus blast was hard to bear for more than a few hundred yards, even for bespectacled drivers like us. With the screen in place, though it looked small, Prior and I could each have driven 100 miles home.

And there were more lessons ahead. Catesby’s main boast is that it caters to the industry-wide need for repeatable, real-world aero testing.

Such conditions are difficult to achieve in conventional wind tunnels (where tethering systems intrude, wheels don’t spin, tyres don’t achieve their loaded shapes and engine exhausts and cooling systems aren’t active) or on open test tracks (where temperatures, pressures, gradients, variable road surfaces and weather intrude).

It’s true that designers nowadays confidently use CFD (computational fluid dynamics) to predict with great reliability how air flows over cars. Indeed, Catesby’s leading backer is Totalsim, a well-regarded CFD company. But car designers still have a strong demand for physical testing – which is where Catesby wins.

Which other venue could offer a straight and flat test track protected 365 days a year from wind, weather and prying eyes, with a perfect surface and no noise regulations to interfere?

Catesby provides all of this. It is a perfectly straight, 2.7km (1.7-mile) former Victorian railway tunnel, running through a confluence of rural hillsides south-west of Daventry in Northamptonshire.

Opened in 1897 as part of the Great Central Railway’s double-tracked London Extension, it was ‘retired’ by Dr Beeching in 1966, then sealed and fenced off until the early 2000s, when it came to the notice of Totalsim managing director Rob Lewis and his colleagues as they searched for a suitable tunnel.

They already had experience of aero testing in the world’s only other converted railway tunnel, Chip Ganassi’s shorter and semi-secret facility at Laurel Hill in Pennsylvania, although even before that they had intended to open a bigger, longer and flatter facility available to all comers.

“If you take away the usual variables in an aero test,” says Lewis, “you’re inevitably going to get a better result. From an engineering point of view, it’s common sense.”

Work began in 2017 on what was to become the world’s biggest aero test facility, comprising around 30 million Staffordshire Blue bricks but now relined for its entire length with plastic sheeting.

Its 8.2m width was bounded by Armco on both sides, CCTV cameras were installed all over, and a network of overhead fire extinguishers and lights was fitted. The whole project took over four years and unspecified millions to build (including £6.2 million from the government’s Local Growth Fund) and it also included resurfacing with Tarmac of the same quality as Silverstone’s grand prix circuit. 

One of Catesby’s best claims to fame is that beginning an experimentation programme is quick, and so it proved. Ariel’s blokes arrived with the car’s data collection system already working. All we had to do was remove the screen and start driving.