Cars - For Car Enthusiasts

Cheating With Physics

The dilemma is that our car is limited by the frictional force at the start. You can raise the coefficient with drag-car tires, but you won't be driving home on those. Alternatively, you can boost the friction by increasing the mass of a car. But then you also have more mass that needs to accelerate, so you don't gain anything. The key is to increase the frictional force without increasing the mass.

Now, there are “super cars” that can do the quarter mile in a crazy 10 seconds. (The sticker prices are crazy too.) But forget that. What if I told you there's a road-legal “hypercar” that can do it in under 8 seconds? True fact. The McMurtry Spéirling clocked a time of 7.97 seconds, even on a slightly wet road. The secret? It has fans that pull air out from underneath, creating a pocket of low air pressure that sucks the car downward.

Let me just go ahead and draw a force diagram for a car with a fan.

Even though there's an extra downward pushing force (Ffan), the car stays on the ground and still has a zero acceleration in the vertical direction. In this case, Newton's second law looks like this:

With the extra downward force, the only way for the forces to add up to zero is if the normal force increases. This means the frictional force is greater, so the new acceleration is greater.

Now instead of getting a maximum acceleration of 6 to 7 meters per second squared, it's possible to get much higher values—maybe 15 or even 20 m/s2. The McMurtry Spéirling in the video went from 0 to 60 mph in … wait for it … 1.4 seconds. Just thinking about that will pin your ears back.

The fan idea isn't new. In 1978, the Brabham BT46B used it to win the Formula 1 Swedish Grand Prix, but it was quickly banned. The idea of increasing downward force lives on, however. F1 cars today channel airflow through the body in clever ways to achieve some of the same “ground effect”—justified by saying the purpose is to cool the engine.

While these airflow systems do help cool the engine, everyone knows the real purpose is to generate low pressure under the car to suction it closer to the road. In fact, the new McLaren W1 that we recently reviewed is a road car that bases its sales pitch on this. (You can buy one for $2.6 million—or you could have if you'd signed up in time. Only 399 are being made, and they're all spoken for.)

The cool part is that this higher acceleration isn't just for increasing your speed. It also allows the car to slow down faster and even make sharper turns, since these are also types of acceleration. The downward thrust can turn a fast car into a crazy-fast car—if that's what you want. For me, I'm happy as long as it's a red car and it drives.

Rhett Allain is an associate professor of physics at Southeastern Louisiana University. He enjoys teaching and talking about physics. Sometimes he takes things apart and can't put them back together.