Top motoring innovation: Why Catesby tunnel is all but boring

Vision, nous and persistence have turned an old railway tunnel into a game-changing aero facility

The winner of Autocar’s Innovation Award for 2024 is Catesby Tunnel, the 1.7-mile hole through the Northamptonshire countryside that once formed part of the Great Central Railway and has recently been repurposed as a globally unique aerodynamic testing facility.

“Fourteen years ago, I was thinking about how we test our designs,” says Rob Lewis, the brilliant aerodynamicist once of BAR and Honda Formula 1 teams and now managing director of Totalsim, which specialises in computational fluid dynamics (CFD), explaining how today’s facility came into being.

“We would optimise things in CFD, but in the UK our wind tunnel choices were MIRA or a scale tunnel, and we didn’t have a full-scale moving-ground automotive tunnel. I was Googling and found Catesby Tunnel, just up the road from our office in Brackley, on the same train line. So I started kicking the idea around,” he tells us.

Lewis had been working with Prodrive and discovered that some of its engineers had been testing race cars using a tunnel called Laurel Hill in the US, run by Chip Ganassi Racing.

“That proved it works,” says Lewis. “But how could it not? You’ve got a controlled environment and a car moving along a body of air.” 

Catesby was even more enticing than Laurel Hill.

It’s arrow-straight and longer, which is important because “you can get up to speed, run a number of tests and get down from speed”, says Lewis. 

“In a shorter tunnel, the getting up to and down from speed takes the same distance, so you lose test time in the middle; 1.7 miles is really good.”

Catesby is wide too – “it has a cross-sectional area of 40 square metres”, says Lewis – and that’s desirable because of an aerodynamic effect called ‘blockage’. 

"As a vehicle moves, it displaces the air around it, and the smaller the area it’s passing through, the less space there is for displaced air so it will move more quickly. 

"In a confined space, a car at 100mph might have air passing over and around it at 110mph or 120mph, making for noisy results. “Catesby is a lot bigger than even a very large wind tunnel, which makes blockage effects small,” says Lewis.

Even Catesby’s constant 1-in-172 gradient is “handy”, says Lewis, “because it’s small enough to be in the limits of certified testing like WLTP, but it’s big enough that when you test in two directions, you get an offset, which is ‘mg sin θ’, and that helps you cancel things out. 

So if you’re trying to work out what the rotational inertia of parts of the vehicle is, then you tweak the number from the up and the down until they line up and sit on top of each other. We’d rather have a little slope than no slope.”

Knowing it was the ideal spot, the painstaking work of making Catesby a reality began. “The inspiration was 10 minutes,” says Lewis. 

“The rest took forever. You go to a bank or an investor and they say: ‘It’s never been done before and you’ve got no purchase orders.

“So the thing that really unlocked it was the local growth fund from Semlep [South East Midlands Local Enterprise Partnership]. They gave us £4.2 million and did it with a lot of faith in us, and that then unlocked a deal with Subaru [which Totalsim was working with via Prodrive] to sell them some shares and some time in the tunnel. Right at the end, we managed to get a small bank loan from NatWest.”

Part of the challenge is that aerodynamics, despite how much more efficient it can make cars, is not perceived as a key area for investment. “In the UK automotive tech world, aerodynamics isn’t well funded,” says Lewis. 

“If we were doing things to do with lightweighting electrics and batteries, smart connectivity, or autonomy, there would have been many more grants available.

"The one we took was for economic development rather than tech. If you look at the Automotive Council and what they fund in the propulsion centre, aero development barely gets a look-in.”

And yet the advantages are key. “In a typical road car, half the petrol is burnt by overcoming the aerodynamics and you’d think that would be a priority but it’s not yet,” says Lewis. 

Meanwhile, in an electric car, “if you can take 30% off the drag, then the car will go further for the same battery, so you can reduce the battery and the weight, and by doing that make it more efficient and go further again”.

There have been issues. Bats have needed looking after and it took longer to certify fire regulations than was hoped, but Catesby is now up and running, with a £4m innovation centre in development near the tunnel entrance, and the tunnel is getting busier and proving its worth.

“Like most things, when you invent something new it takes the market time to realise it and adopt it,” says Lewis. 

“OEMs are interested and are trying it, but they take a long time to change their processes. We’ll get there.”

Launching and proving new ventures is what Autocar’s Innovation Award sets out to acknowledge and that’s why we’re proud to name Catesby Tunnel as our 2024 winner.