In the TV programme Knight Rider, lead actor David Hasselhoff drives his talking car, Kit, up and onto a moving lorry via a ramp projecting from the back onto the road, often at high speed. The same thing happened in the original The Italian Job movie starring Michael Caine. But is this possible? As soon as the car hits the ramp, it would be moving relative to the lorry and therefore would only have the length of the inside of the lorry in which to brake, probably ending up crashing into the cab. Am I correct?
A couple of corrections from the purists. The car in Knight Rider is not Kit, but KITT, Knight Industries Two Thousand. Second, in The Italian Job, the Minis drive into the back of a converted bus, not a truck 鈥� Ed
鈥� This stunt is possible. It鈥檚 all to do with relativity. The bus moves at slightly less than the speed of the Minis. If the Mini approaches at 41 kilometres per hour and the bus is travelling at 40 km/h, relative speed is 1 km/h. The result is almost the same as driving the car into a garage.
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I say, 鈥渁lmost鈥�, because at the moment of entry the Mini鈥檚 engine, transmission and wheels are running far too fast for driving at 1 km/h. So in the transition from road to ramp, the front wheels driving the Mini are suddenly spinning at the wrong relative speed. As the wheels touch the ramp, a similar effect occurs as that produced by revving an engine at standstill and then suddenly letting out the clutch: a lot of wheelspin and burnt rubber. In this case, of course, it is not the wheels trying to burn up the road, but the ramp trying to slow down the wheels.
As soon as the drive wheels touch the ramp, the car will leap forward. A dab of the clutch can stop that, and the driver would quickly have to drop down in gear before driving the car into the bus as if it were a stationary garage, except for a little initial acceleration from the rear wheels.
Terence Hollingworth, Blagnac, France
鈥� At a relative speed of about 10 km/h 鈥� about jogging pace 鈥� a car can easily stop in a short distance, although there will be plenty of screeching as the fast-spinning wheels come to a halt, which is possibly where the confusion lies. Perhaps it is easier to imagine instead a small aircraft landing on the back of the truck: as long as the difference in speed between the aircraft and the truck was small, there would be no problem landing in the limited space.
鈥淭he car can stop in a short distance, although there will be screeching as it comes to a halt鈥�
Moving travelators in airports offer another perspective. In order to avoid being jerked when we roll our luggage trolley onto the moving walkway, we make sure that we鈥檙e walking at about the same speed as the travelator. Then as soon as we鈥檙e on board, we stop pushing. Having made sure that the trolley is moving at just the right speed, there鈥檚 no more to do once the wheels of the trolley have stopped turning, and friction takes care of that.
Hugh Hunt, Cambridge, UK
鈥� Perhaps the most interesting aspect is the consequence of not getting this stunt quite right.
With a manual transmission, any attempt to drive straight from the road slowly up the ramp would be doomed to failure. The engine would stall as the driven wheels hit the ramp. This would leave a front-wheel-drive car such as a Mini with its front wheels stationary and rear wheels spinning at road speed. Using the brakes to hold the car on the ramp while restarting the engine and engaging first gear would slow the rear wheels and pull the car off the ramp dumping it onto the road at speed while still in first gear. A rear-wheel-drive car like KITT would fare better, but there would still be the danger of slipping back as the engine stalled.
It is hard to predict what a car with automatic transmission would do. Normally these cars do not stall, but who can predict what would happen as the driven wheels stopped almost instantaneously from high speed?
Using manual transmission, the lorry should be approached with enough relative speed to roll up the ramp after declutching before hitting it. The car should then coast up the ramp with a brief squeal from the tyres.
A driver who wanted to increase entry speed for dramatic effect should use a heavy car and a lightweight lorry. As the car brakes, the dumped momentum would accelerate the lorry. The effective braking distance available for the car would then exceed the internal length of the lorry, giving more time for the car to stop.
Trying this with a four-wheel-drive vehicle might be unwise. Many have differential locks to limit speed differences between front and rear wheels. If the transmission survived, the lock would stop the rear wheels as the front mounted the ramp, or more likely keep the front wheels spinning at road speed. Neither would be conducive to the fine control needed.
Mark A. Jones, Gloucester, Massachusetts, US
鈥� I have seen interviews with the production team on The Italian Job. The stunt people really did drive the Minis onto a moving bus. The bus driver鈥檚 cab was reinforced to protect him as the cars ran into the back of it. As each Mini came in it smashed the preceding one further along the bus until all were in. However, the force of the cars slamming into each other and the cab caused the driver to be rammed against the steering wheel, and he had to be cut free.
Susanna Sherwin, Salisbury, Wiltshire, UK