# Driving a car on to the back of a moving truck

1. Jan 1, 2004

### phatmonky

Okay, and odd title, but let me continue.

I recently read a 15+ page argument on another board about what would happen in the following scenario.....

You are traveling 62mph(~100kph). There is another vehicle (let's say, a car carrier truck as pictured below, with a ramp to the roadway :p ) moving at another, slightly slower speed.

http://www.midamericawrecker.com/images/trucks/000002.jpg [Broken]

If the car were to drive up a ramp into the back of the truck, what would happen?
A. The car would drive up and be able to hit the breaks to stop on the truck.
B. The car would traveling at 62mph on the back of truck and thus unable to stop before flying off the other end.

I've made up my mind, and am frankly astonished at the amount of conversation this derived on another board. However, I believe the PF crowd to be slightly of higher intelligence, and thus don't expect such a large deviation in answers

Last edited by a moderator: May 1, 2017
2. Jan 1, 2004

Staff Emeritus
There's no need to invoke relativity for 62 mph, so Gallilean relativity is sufficient. Your speed relative to the truck is just the difference of your speed and its speed relative to the road (say 10 mph) so you could drive up and stop with no problem, provided the truck doesn't accelerate some way while you are doing this manoevre.

3. Jan 1, 2004

### Dissident Dan

Well, the front tires are spinning so quickly when they hit the truck that you could run into some problems...

If you replaced cars with people in the question, then A would definitely be the answer. Motion is all relative, and relative to the ramp, the person would not have such a great speed.

4. Jan 6, 2004

### TonyG

Interesting problem. You have to consider the critical moment that the driving wheels roll onto the truck. The car won't retain its 62 mph speed on the bed of the truck, because the car will lose a lot of its energy in making the transition from the static road, to the bed of truck thats moving away from the road at 60 mph.

To get an intuitive feel for this, have you ever used those moving walkways in airports? If so, I'm sure you felt the sensation of being "pulled backwards" the moment that you step onto the walkway (or being "thrown forward" when you step off). So if you're moving at a speed v before you step on the moving walkway, you'll move with a speed v' < v, once you're on the walkway.

Realistically, a lot of energy will also be lost in wheelspin during that critical moment, and the truck may actually pull away from the car. I get the feeling that the car may need to be drive much faster than the truck to overcome this barrier.

5. Jan 6, 2004

### krab

The first part of this sentence is correct, everything after "because" is wrong. An object travelling at speed has the same amount of kinetic energy whether it is travelling under its own power or whether it is being carried by another vehicle. The only energy lost in the case of a car is the extra energy of the spinning wheels. In a typical car, this constitutes less than 1% of the total energy.

6. Jan 6, 2004

### TonyG

Yeah, bad wording. Replace, "energy" with "speed within the driver's frame of reference". Main point is that the MPH reading on the car's speedometer will drop tremendously. Conservation of E (although this is sort of an inelastic collision) precludes the car going 62 mph relative to the bed of the truck.

Last edited: Jan 6, 2004
7. Jan 6, 2004

### Molydood

I think in the real world the car would most likely just stall (depending on it's characterstics)

My assumptions
1. the truck and car have mass and are at constant velocity before the touch down
2. the car does not have infinte torque
3. all velocoties discussed below are absolute (ie not relative)
4. The car accelerates until the moment the driver senses acceleration (ie when the drive wheels touch down), and then he hits the brakes
5. The truck driver does not depress the clutch or release throttle at any point

I see two main things happening once the cars drive wheels hit the ramp:
1. the truck will be decelerated
2. the car will accelerate
3. losses occur (wheelspin, engine damage etc.)

the amount by which 1. and 2. occur is dependant on the truck and cars characteristics (engine power, weight, aerodynamics etc), but you couldn't simply add the two velocities together to see the car suddenly go from 100 kph to 200 kph; this would require infinte torque, or zero mass etc. and this of course is not reality

Anyway, real world scenario would not see the car off the end of the ramp in my opinion, BUT...

IF the car had a huge amount of torque, and the ramp angle was low, and the truck was very light, and the cars engine did not expode when subjected to the loading etc. etc. then yeah, maybe, but not in the real world IMO.

my first post by the way,

Martin

8. Jan 7, 2004

### TonyG

Molydood -I agree. The moment the driving tires hit the bed of the truck, there will be a tremendous load on the engine, causing it to bog down.

But if there is wheelspin (i.e. loss of traction), which is likely, it may help prevent the engine from bogging by reducing the engine load.

I would think that this would be an easier move for a rwd car to pull off rather than a fwd. Can't imagine an awd car doing it (without blowing it's differential).

9. Jan 8, 2004

### mighty2000

?/

Oh, come on now, we've all seen this stunt performed on Knight Rider. Granted it was a RWD firebird, but I believe that little or no damage would be done to the vehicle given the velocity of the car. I think this could also be done with a FWD vehicle because it's momentum is the same whether it's FWD or RWD. Plus the cooeficient of friction would have to be considered. Regular asphalt would have a greater cooeficient than a steel ramp, but not if the ramp were specifically designed to have a greater Fs than the road.

The fact that the vehicle's velocity is so much greater than the truck's velocity could conclude that the truck is basically static, or insignificantly moving.

This is more like an elastic collision. However the angle of the ramp would cut down on the forces to push the truck foward so much. This is of course why the truck needs to be moving at some velocity. If it wasn't then the car would make a hole in the top of the trailer and sail over the cab.

Anyway, My two cents

M2k

10. Jan 8, 2004

### krab

Re: ?/

You did not read the question carefully enough. The case considered is where the truck is moving slightly slower than the car.

11. Jan 8, 2004

### mighty2000

Sorry about that. I guess I just wanted to spew in so fast I thought I had the question down. Then Yes I agree with the previous post to mine that the car would definately stall out, and thus creating an inelastic collision.

Thanks Krab.

12. Jan 8, 2004

### Dissident Dan

The only things that should prevent the car from going up smoothly would be the gap between the truckbed ramp and the gound and the tires of the car spinning out, similar to when street racers press the brakes while also pressing the accelerator to spin the wheels while not moving. I would think that the tirespin effect should be negligible, if you are expecting to just cruise onto the truck, without turning or pressing the gas.

13. Jan 9, 2004

### Molydood

I can't imagine it being problem free; the way I look at it its the equivelent of driving at 100kph, then suddenly the tarmac is hauled from beneath the car in the direction of travel, in zero time, simulating the tyres touching the moving ramp (apart from the change in co-efficient of friction)

the fact the the ramp is probably made of metal helps towards absorbing the forces less dramatically, I just wonder if the stuff we see on knight rider, and in the movies (italian job etc), is quite as it looks or has been helped a little, I'd be interested to know

14. Jan 9, 2004

### mighty2000

Just imagine the opposite situation. If the car were backing down the ramp and was pulled onto the road with static friction. It doesn't matter how fast the car was moving, unless it jumped out right onto the road, it would flip over and over until coming to rest 200 meters down the road. Unless the transmission slipped into neutral and it skidded to a stop.

M2k

15. Feb 7, 2004

### Gara

omg this is so simple i cant understand why no ones said it before.

drive towards the truck at say, 10 mph faster than the truck is going. put it in nutrual. roll onto the truck, apply breaks. no danage to engine at all.

16. Feb 7, 2004

### LURCH

This has been done as a driving stunt in a movie. I believe the title was "Self-presrvation Society". Lining up a car's tires with the type of ramp used on a car-carrying truck is very difficult, and the cars approached the truck at a speed that was only slightly faster than the truck's speed (relative to the road, the speed on the speedometer). I'm not certain if that was done due to the need for precission in lining up the front tires, or for dramatic effect. A bit of both, I suspect.

At any rate, the cars were rear-wheel drive (c. '65 Jaguire XKE's; reeeaaally nice ), and the results were rather a combination of A snd B. At such slow closing speeds ( <1 kph relative to the truck), stepping no the clutch and coasting was not an option, so power was still being sent to the rear tires when they made contact with the ramp. Jags are so light and pwoerfull that each vehicle lept foward when the drive wheels hit the ramp, and the breaks needed to be applied quickly and with considerable force. I'm sure this breaking force was accentuated for film, but nevertheless a real need.

17. Jul 24, 2007

### chrispc_nyc

Physics of driving car onto moving truck

I can't believe how much incorrect speculation, false information and general confusion there is on the web regarding driving a car onto a truck while both are moving at high speed. Not that I understand the physics behind it, which is why I am posting here and hoping that since this is the Physics Forum, that there is someone here who can CORRECTLY explain the physics behind how it works. I have not found any complete or correct explanations in searching the web. (I did find a lot of people saying it can't be done, and one plausible but incomplete explanation of how it is done.)

Bear in mind it has been clearly proven that the Knight Rider like scenario of driving into the truck and backing off the truck at speed both work perfectly. See the MythBusters episode of earlier this year, where they did it at 55 mph. There was no tire squealing or excessive crazy braking or crazy skidding when backing off the ramp.

The most sensible sounding explanation I've found so far regarding driving onto the truck, is that the fast spinning tire slows dramatically once it hits the ramp due to the car being in high gear and not having enough torque to keep spinning the wheel. But the logical follow-on question is why doesn't the car stall then?

I've found no explanation of how backing off the trailer works, intuitively it seems like backing off while truck is going 55mph and your back wheels hitting the pavement would really buck the car out of control, but in practice it seemed very smooth indeed.

So, anyone? We've seen it done. Can someone explain how it works? Pretty please?

thx

18. Jul 24, 2007

### Staff: Mentor

Perhaps to back off the truck, they don't go in reverse but coast off in neutral? I haven't seen that mythbusters episode, though...

19. Jul 24, 2007

### Danger

That's certainly the way that I'd do it if I were the driver. Going the other way, I'd certainly get off of the throttle as soon as the driving wheels got onto the ramp. I also never saw that episode.
As for seeing the Knight Rider do it repeatedly with no complications whatsoever... anyone ever hear of 'editing'? We've also all seen the General Lee jump 50' gaps repeatedly with no complications whatsoeverâ€”except that they needed 20 cars in constant rotation to maintain that illusion.

20. Jul 24, 2007

### Staff: Mentor

Ok, I've watched the video now and the rain clearly helped things. The powered wheels can be seen spinning as it goes up the ramp, perhaps 6 feet. They don't say anything about it, but I can't see how the car would not stall as the RPM drops going up the ramp (have you ever tried driving at 5mph in 5th gear?).