THE REALLY COMPLICATED BIT
Nothing is straight-forward in the land of the GT-R LM
Ben Bowlby likes harmony, and his holy trinity is weight distribution, aerodynamic grip and tyre grip. Add more weight to one end of the car, and it’ll need more aero and tyre grip in order to be balanced. This equation was the glue that held both the DeltaWing and Zeod together, with a distribution of around 20:80 front to rear. Here, due to the opportunity that the Le Mans regs have to create front-end downforce, he’s reversed that thinking. The proportions of the GT-R LM are close to 65:35, “about the same as most front-engine, front-drive road cars,” says Bowlby.
If you’re still not sure, divide nine by 14 for the tyre width proportions and you arrive at 64.2:35.8. Close enough. What this means is that, aside from sitting further back in the frame, the car should feel perfectly balanced to the driver. Aero grip through high-speed corners means it will be tricky to even tell the GT-R LM is front-drive. Just don’t go expecting any spectacular drifts out of slow corners. Understeer ain’t pretty
What’s new is the hybrid system – which, unlike its rivals, contains no electrics at all. Le Mans regs demand hybrid technology, but that doesn’t have to mean electric. This system is purely mechanical. I’ll hand back to Bowlby to explain how it works.
“Say you’ve got a 900kg car travelling at 220mph down the Mulsanne Straight, that’s a heck of a lot of energy, and with caliper brakes all you’re doing is turning it into heat, so instead we take the torque that’s required to slow the mass down and we use it to spin up a fywheel. The fywheel has a lot of inertia, so we’ve got some clutches and gears that we mash together in sequence to cause the fywheel to spin up, and the opposing torque reaction slows the car down. Then, when you come out of the corner, you do the reverse – slow the fywheel down, and that speeds the car back up. It’s an elastic efect.”
What Bowlby didn’t touch on are the numbers involved in this. Used to its maximum potential, the 8kg steel-cored/carbon outer fywheel (it’s about 10cm tall, with a 24cm diameter) gains 19,000rpm each second, so a braking event of a little over three seconds can spin it up to its 60,000rpm maximum speed. At that speed, the outer edge of the fywheel is subject to 47,000g and is spinning at Mach 2, about 1,500mph. If it weren’t in a vacuum, you’d hear a sonic boom. That’s right, the GT-R LM could theoretically create its own sonic boom. Awesome. On a more mundane level, the fywheel can send those forces back to the wheels just as quick as it harvested them, at an equivalent of about 700bhp.
The fywheel sits deep down behind the V6, connected to the front transaxle gearbox via a shaft that runs forwards through the vee of the engine. It also has a shaft that runs to the rear wheels, but at the moment, it’s not connected. So, 1,250bhp through the front wheels. And it might stay that way: “We may well run this season as front-wheeldrive, but we have to declare our intention and stick to it for the whole season.”
This is the frst time the car has ever run with the fywheel hybrid. The system is four months behind schedule, and they’re being cautious. French driver Olivier Pla confesses to not being able to see over the front wheel pontoons, but says he’s otherwise comfortable with the driving. But he’s not racing right now. What they’re doing is checking the hybrid system is harvesting and deploying as expected. So the car goes out, does three laps, comes in, fingers point at laptops, the car goes back out and repeats the process again. And again. And again. Three hundred miles over two days. The weather is sunny, the perfect tarmac sparkles – and there are smiles from the team. They’re getting the right numbers, and the driver is happy that he can’t detect whether the caliper brakes or the fywheel are doing the work.
How does Bowlby think this season will pan out? “Racing is very difcult. If you think you’re going to rock up and do well, dream on. Audi has owned that place [Le Mans] for a decade. Our ambition is to do the best we can possibly do and to showcase a diferent way of doing it, and to interest people and, well, racing is meant to be fun, and we’re all motivated as hell to try to stick it to them!”
“THE OUTER EDGE OF THE FLYWHEEL IS SUBJECT TO 47,000g”