How an old railway tunnel is becoming a top-secret test track

The best ideas often come from lateral thinking. Wind tunnels make the air move at speed around the object being studied. The obvious, easier alternative is to make the object itself move – normally pretty easy for a powered vehicle with wheels – while keeping the air still. At a basic level, this is easily done, and much aerodynamic testing does take place in the real world. But the consistency required when chasing fractional gains has always been a challenge given the influence of prevailing wind, weather and variable temperatures. What’s needed is a carefully controlled environment – one sealed from the influence of the elements. The idea of using an existing tunnel isn’t a new one. In the US, Chip Ganassi Racing took over an abandoned one in Pennsylvania in 2003, and this has since been used extensively for motorsport development (including, reputedly, by Formula 1 teams). Here in the UK, aerodynamicist Rob Lewis had a very similar idea. Having worked for the BAR and Honda F1 teams, Lewis founded his own company, Totalsim, specialising in computational fluid dynamics (CFD) modelling. As well as automotive clients, Totalsim has worked closely with athletes seeking to improve aerodynamic performance, especially cyclists; so much so that Lewis has won an OBE for services to Olympians.

In 2013, he decided to look into the possibility of repurposing an abandoned railway tunnel in Britain for testing. And by happy coincidence, the longest, straightest and tallest of these proved to be in Catesby, four miles south-west of Daventry, so pretty much next door to ‘motorsport valley’.

“There was lots of climbing through nettles when we first came to have a look,” remembers Totalsim’s Jon Paton, the aerodynamicist who shows us around the site. “Then we got to the small matter of finding out who it belonged to and negotiating to use it.”

Once inside, they found the tunnel was partially flooded by blocked drains and the vertical vents that once carried smoke away was full of dead birds and animal bones. A collection of recovered relics now sits in the covered workshop. Protecting the habitat of the bats living within the abandoned structure required it to be narrowed at each end, so they would still have space close to the portals. Eight years later and with around £20 million spent so far, the tunnel is close to its rebirth.

Entering the clammy darkness of Catesby Tunnel’s southern portal, it’s clear that none of this would have been possible without the engineering prowess of the late Victorian period. Even after six decades of neglect, the structure is in remarkably good condition. The roof has been lined to help reduce drips from the damp brickwork, but the exposed section is still heavily coated with thick soot deposits from nearly seven decades of hard-working steam locomotives.

Although it rises on a steady 1:176 (0.006%) gradient, the tunnel is both flat and almost perfectly straight. “We were amazed when we actually scanned it,” says Paton. “It’s within a couple of centimetres of being perfect.” A new concrete base is being laid, and this will be covered with an ultra-precise layer of Tarmac to create a uniform surface with no gaps or expansion joints.

“The most important thing for testing is consistency and repeatability,” explains Paton. “That’s where high-quality data comes from.”

The finished tunnel will be linked to a preparation area by a covered passageway, ensuring confidentiality. It will also have turntables at each end so that cars can return to base without an Austin Powers-style shuffle.

The expectation is that much of the early use will be for what’s known as coastdown testing: recording the speed profile as a car coasts in neutral to work out drag. But Catesby Tunnel can also be used for more ambitious testing from the off. For example, Chip Ganassi’s tunnel has reportedly been used to study the aerodynamics of Nascar racers drafting each other at speed.

The team behind Catesby Tunnel is already thinking of future upgrades. These are likely to include ultra-sensitive pressure-sensing floor plates, an internal GPS antenna system (signals can’t get through the ground above), fans to allow the effect of sidewinds to be studied and even a splash area so that spray patterns can be created.