My Way on the Highway - Slipstream Surfing

Ever been standing by the side of the road, while traffic rushed by ? Noticed how each passing vehicule, especially large trucks, seemed to create a draught ? Slipstream.

Slipstream. Every car has it, but it's more noticable with big trucks. It is caused by the movement of the truck. Consider the folllowing drawning. The truck occupies space. As it moves foreward, it creates a void behind it : there is, for an extremely brief moment, no air there, because the truck is/was there. Of course, air rushes in to that void almost instantanously, thus creating a flow of air with roughly the seem speed as the truck.

Now, imaging that you try to use that slipstream : if you move your car in behind the truck, relatively close, you not only take advantage of the fact that the truck keeps you out of the wind, you also suddenly get a 90 km/h tail wind - 90 km/h (56 mph) is the typical speed for a truck on a European highway.

Using another car's slipstream - slipstream surfing - is done a lot in races, and by drivers of cheap, not so powerful cars (such as the 2CV). It also helps to save gas.

It is easy to understand that a car, driven by a 90 km/h tail wind, will need less gas to maintain its speed than a car that does not have this advantage. (If you don't get this, buy a bicycle).

There is also a law in physics that says that objects tend to maintain their 'state of movement' : if an object is standing still, it wants to remain standing still, and you'll need to apply force to get it to move. This is called inertia (Latin for 'laziness'). It also applies to moving objects : an object that is moving in a given direction will continue to move in that direction untill a force makes it change its direction. And, equally : an object moving at a given speed, will continue to move at that speed until some force slows it down or makes it stop.

Thios works well in 'ideal circumstances', but in reality there will always be forces at play : gravity, friction, air resistance, ... Nontheless, once you're car is going 90 km/h, it will tend to keep going. That's the reason that cars have brakes. They'd stop eventually without brakes as well, but it would take a while.

Now, if your speed is, for a big part, maintained by that slipstream, that 90km/h tail wind, your engine still has some power to spare ; It is therefore possible to accelerate out of the slipstream. 2CV drivers use this to overtake trucks and lorries. In races it is used as well : the car in the slipstream has a reserve of power that the car in fromt of it does not have. The car in the slipstream will keep, for a while, the speed picked up froom the slipstream (inertia), and add his acceleration to it, so for a brief while, it has en advantage over the car in front.

To effectively surf the slipstream, you need to move in quite close to the car in front of you. A few yards. This can be tricky : what if the driver in front of you suddenly hits the brakes ?

If you hit the brakes at the exact same moment, there is no problem : you will slow down as well end maintain your distance. When you're behind a truck, it's even better : because of its mass, a truck has more inertia : it needs more time/effort to slow down, so you probably won't hit is.

Keyword here is 'simultanously'. Unfortunately, you will not be able to hit your bnrakes the same moment the driver in front of you does : you'll need first to notice it (see brake lights come on), then react to it, move your foot to the brake, start braking. This whole process takes about 1.5 to 2 seconds. If you have good reflexes and are extremely concentrated, you may be able to bring it down to 0.8 seconds or so. But watching the rear of a truck does not really help to keep your concentration up.

The absolute minimum distance you need to keep is the distance you will cover during the 1-2 seconds that the truck slows down while you don't. Assuming a truck slows down 5 m/s per second, you'd then need to keep a 5 to 10 meter distance. Assuming ...

Anyway, 90 km/h equals 25 m/s, so to stay on the save side, you'd need to keep a distance of 35 - 50 meters, which is way to far for slipstream surfing. Hence, slipstream surfing can and will rightfully be considered reckless driving by most people, including police officers. This information is therefore for educational purposes only. It should not be applied in the real world. If you do, you do it at your own risk (and that of the people around you).

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