Apparently, my rear wheels are out of balance. I notice what feels like bumps in the road surface, except, there are none. I’m driving through the Catesby Tunnel, a former railway tunnel in the English countryside that’s been converted into a proving ground unlike any other. Over its nearly 1.7 miles, the road is flat to within 2 millimeters. Jon Paton, managing director of Catesby Projects, sitting shotgun, reminds me of this fact. That slight vibration I feel: It’s certainly coming from the car.
The Catesby Tunnel is ostensibly a wind tunnel, but one that works opposite to all others (with one notable exception). Instead of rushing wind over a static model, within the Catesby Tunnel, the air is static, but you can drive a car at up to 160 mph, providing a real-world look at how air flows over the car. But, since opening in 2021, it’s also proven to be so much more than just an aerodynamic testing facility.
In what might seem like a bit of irony, the tunnel is owned and run by a company that specializes in Computational Fluid Dynamics (CFD), TotalSim.
“The question nearly always comes up is ‘Why is the simulation not right?’” says Paton. “People get judged at the race track for motorsport and they get judged on the road for automotive stuff. And when things effectively don't correlate, you come up with a whole load of excuses as to why your simulation doesn't work.”
There’s always a gap between computer simulation and reality, even as computing power increases reliably year to year.
“Simulations are always wrong,” Paton says. “But you're just trying to figure out which bits are wrong and whether it's wrong because of the reasons that you're happy with or whether it's wrong because of the actual tools being wrong.”
The tunnel helps bridge that gap.
TotalSim isn’t the first to build this sort of thing. About 20 years ago, Chip Ganassi Racing converted a disused Pennsylvania Turnpike tunnel outside of Pittsburgh into an aerodynamic testing facility. It quickly became legend, Ganassi playing coy about its existence for years. A symbol of racing’s extravagance, Ganassi’s tunnel seemed almost impossible, a classic “unfair advantage” that no doubt contributed to the team’s IndyCar and NASCAR success.
Ganassi and the team’s longtime aerodynamicist Ben Bowlby—also of DeltaWing and Nissan GT-R LM Nismo fame—hold a patent on using a tunnel for vehicle testing. Thumbing through the Companies House documents on the Catesby Tunnel, Bowlby was actually involved briefly, with the company.
It was TotalSim’s managing director, Rob Lewis, who initially pitched the idea of building something similar in the U.K. Paton says the shareholders thought he was “completely bonkers,” but he put together a list of disused railway tunnels in the U.K. and presented it to the group. Catesby, built by the Victorians in 1897, was the longest on the list and only a 25 minute drive from TotalSim’s headquarters in Brackley. (Not coincidentally right in the UK's motorsports valley.)
“Then next week, he took us on a hike through a farmer’s field because the tunnel was completely derelict and there were sheep around the place,” Paton recalls. Then came the long process of buying the land, and convincing the owners that, yes, they wanted to convert this derelict tunnel into a place where you could run race cars back and forth. Work on the conversion began in 2020, and the tunnel was opened in November 2021.
Paton talks about aerodynamic testing in terms of the compromise between repeatability versus reality. Out in the real world, you can test a real car on real tires at speed, but being outside throws up so many variables that make true repeatability basically impossible. A wind tunnel allows for tons of repeatability, but it’s not a great representation of reality.
“Things like the steel belt that goes around underneath the tire, the tires are probably the wrong temperature, the brakes probably aren't warm, the engine is not warm so your thermal gradient is all slightly wrong,” Paton says. He also notes that in a full-scale wind tunnel, you have to tether the car down, which has its negative effects.
“Catesby Tunnel comes at it a slightly different way in that you're trying to make a real condition, but repeatable… you basically run out of excuses as to why your simulation work doesn’t correlate,” Paton says.
The fact that the tunnel is buried underground means that the temperature remains between 50 and 51 degrees fahrenheit all year. And it’s easy to go back and forth for long stretches of time, with two remote-controlled turntables at either end. In theory, you can test at all hours of the day, though you might not need to. With traditional aero testing, you might need to make 10 repeats of a test to ensure something like statistical certainty. In Catesby, it’s enough to just drive back and forth once. So you can save time, which can then be used to test other things.
TotalSim is an offshoot of the old Honda Formula 1 team, which later became the Mercedes-AMG team. So, the firm is rooted firmly in motorsports. Yet, the tunnel has proven to be hugely beneficial in the development of road cars. Aerodynamics have always been important for road cars, even more so in the age of EVs, when efficiency has to be eked out anywhere it can be found.
Coast-down testing is a big part of Catesby’s business. Here, you drive a car up to a certain speed, put it in neutral and see how long it takes the car to slow down. This helps you determine drag coefficient and rolling resistance. Governments around the world require this sort of testing for homologating road cars. In the U.S., coast down work done by automakers is used as part of EPA fuel-economy testing.
But there’s a lot more happening than aerodynamic testing happening here in Catesby.
“As aerodynamicists, we were expecting the work to be 90% aero R&D, but I think that's just becoming less and less the bread and butter,” Paton says. “There’s the legislation, there's people testing headlights, there's people testing acoustics, people testing soiling, there's people testing vehicle communications… it’s a complete range of things.”
The reality is that this underground tunnel presents all sorts of possibilities. One auto engineer apparently left nearly in tears because the long, flat surface allowed him to feel a driveshaft vibration that never revealed itself anywhere else. And it showed that my wheels are slightly off.
I visited Catesby on a quiet day. It’s one of those classic “if you didn’t already know,” situations, with the tunnel a ways back from the main road, and no hints of its existence, even if you drive over it. You can’t hear cars from outside either, even the craziest of race cars. TotalSim opened a new building nearby, Catesby Projects, but even that was designed to look old. Paton says it’s patterned after a train station in Brackley across the street from TotalSim’s old headquarters.
Inside, the new building feels like a startup office. Modern, bright, with lots of glass. The tunnel is totally different. You enter a huge building just outside the mouth of the tunnel, where there’s a staging area to set up vehicles and equipment. There’s also a sort of construction-office trailer turned into a control room.
The tunnel operates almost autonomously from TotalSim, with a small operations team and safety personnel onsite for high-speed testing. This is to allow clients secrecy, if they choose (although plenty of Catesby clients have made their work in the tunnel public). Paton doesn’t necessarily want to know who’s in there and what they’re doing. All he has is data from the weather stations, which hardly matters since the temperature hardly changes.
A garage door opens to reveal the tunnel itself. It’s sort of what you’d expect from a long-disused railway tunnel. Cold, dark, and damp—though Catesby is working on mitigating moisture—yet there’s something of the uncanny valley too. You quite obviously can’t see the other end and the road surface is perfectly smooth and flat. Without a safety team present, we’re limited to driving at 30 mph, which really makes you appreciate just how long it is. At an average speed of 30, it takes a little over three minutes to get to the end. There are braking markers for those going much faster.
On the walls, there are tiny tags with targets printed on them, which, in concert with a high-speed camera system, measure speed. (There’s no GPS connection this far underground.) There are also small birdhouses for the bats that call Catesby home.
Those bats are a bit of a thorn in TotalSim’s side. A protected species in the UK, they built a “bat hotel” for them at the far end of the tunnel, but the bats decided they liked the near side better, hence the plywood birdhouses. It’s an apt metaphor for the challenge of turning Catesby into a vehicle-testing facility.
“Nobody ever laid a road that flat,” Paton says. “Nobody's ever tried to seal or stop drips from a 2.7-kilometer long Victorian railway tunnel. Lighting, you know, it's 2.7 kilometers and we only have power at one end. Yeah. How do we fix that? Next is a dehumidifier. It's all stuff that is just a colossal scale. How do you dehumidify a 42 meter cross section by 2.7 kilometers? It's a big challenge.”
But the seemingly endless possibilities are worth the challenges. A race team recently came in and measured how small changes in rear toe affect rolling resistance and aerodynamic drag. Normally, you’d need to test that outside and in a wind tunnel. Paton makes no bones about the fact that testing at Catesby is more expensive than alternatives, but you can do things there that you can’t do anywhere else.
Paton says Catesby and TotalSim are in the marginal-gains business. It seems like building and operating this facility was a lot of work for simply chasing tenths of a percent, but that is the automotive business, both in the road-car and race-car side. Just look at how many IndyCar titles Chip Ganassi has racked up in the 20 years since it opened its own tunnel.
At the end of the day, it’s yet another tool in the engineers arsenal. “We’re a simulation company,” Paton says, “so why go build an experimental facility? Because there’s reasons you want to use both. One’s quick and easy to iterate, and one’s the real answer.”
