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mrbuddylee
09-04-2002, 07:48 AM
Here is a question that's been bugging me for a while.
Lets say I'm cruising down the highway at 70 mph. I notice a semi truck in front of me towing an open trailer, going about 65 mph. This trailer happens to have a ramp down so I could drive right up onto the trailer (think Spy Hunter).

My question is this. What would happen if I drove up the ramp? Would I immediately slam into the cab, would it be like driving on the roadway, nothing changed, if I slammed on the brakes would I immediately slide off the back on the trailer onto the road again?

Anyone have any ideas?
Thanks!

ski
09-04-2002, 07:55 AM
If you've ever seen the TV show Knight Rider from the 80's, they have this very scene in every show. However, I understand they only did it a few times, just filmed it from a lot of different angles because it was very hard on the drivetrain of the car (no cite).

You can't accelerate instantaneously, but the moment your driven wheels hit the ramp, they're going to try to accelerate you to 60mph relative to the truck. However, you'll back off the gas at the right instant and coast up the ramp. You will probably have some significant axle damage as the wheels go from 60 mph to almost 0 in a very very short time. You'd probably get some tire smoke as well.

Sure it could be done. But I don't recommend it!

Philster
09-04-2002, 08:32 AM
Done correctly, you would be cruising about 15 mph faster than the truck and would enter the truck w/out power being applied to the driven wheels.

When you hit the ramps, the momentum would get you onto the trailer.

You'll scrub off speed by climbing the trailer and you will have the 25 or so feet needed to stop the vehicle. Plenty of room for a car now going about 5-10 mph.

Applying power to the wheels make things a bit more hairy.

barking frog
09-04-2002, 09:11 AM
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=30575&highlight=Knight+Rider
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=30873&highlight=Knight+Rider

That should give you some reading material.

mrbuddylee
09-04-2002, 09:21 AM
Thanks for the info and the links. That question has driven me nuts. I'll read over the links and post any additional questions.

mrbuddylee
09-04-2002, 09:41 AM
Very interesting reads! The second thread was very interesting
Well, before the whole thread turned into a debate about Kitt and Air Wolf fighting, there were a few good points, but I really don't know who is right.

Part of me thinks that if a truck is moving at 60 mph relative to the road, and a car is moving 61 mph relative to the road when the car drives up on the truck, the car is driving at 1 mph compared to the truck, so stopping would not be an issues. However, the energy used by the car has to be somewhere. The car can't magically go from 61 mph on one surface to 1 mph on another surface. I would think the car would be going 61 mph regardless of the surface. In this case, the car would be going 121 mph relative to the road, and would crash into the cab of the truck.

another part of my brain however argues this point (which I don't know the answer to). Lets assume we have a super fast jet sled in the desert going about 500 mph. Lets say it's 100 ft long. On that sled, we have another model sled that's 6 in. long. It can go 300 mph (We're in theory world, not the real one). Now, does the second smaller sled break the sound barrior on the back of the larger sled while the first one doesn't? I would think so, but I don't know...

Now, after thinking about that, it doesn't even really relate to the car/truck question, but gets me even more confused...

Anyone have a couple remote controlled cars?

Skippman
09-04-2002, 10:01 AM
It's a "Relative". :)

Philster
09-04-2002, 10:47 AM
BuddyLee:

You have to consider the car is applying power to the wheels....not just moving forward.

If you stop applying power to the wheels, the relative speeds make it work as I outlined in my first post.

If you keep applying power to the wheels, you have more than just two objects that are just differing by a few mph.

Stop applying power: relative speeds make it more manageable

Keep applying power: when the wheels hit the truck something has to give(the car thrusts foward and accelerates versus the truck...and given the short distance...when decelleration is needed, any acceleration is brutal)

RM Mentock
09-04-2002, 11:52 AM
Originally posted by mrbuddylee
Lets assume we have a super fast jet sled in the desert going about 500 mph. Lets say it's 100 ft long. On that sled, we have another model sled that's 6 in. long. It can go 300 mph (We're in theory world, not the real one). Now, does the second smaller sled break the sound barrior on the back of the larger sled while the first one doesn't?

INJOLTBMI (I need just one little teensy bit of more information): is the little sled inside the larger one, or outside? No, yes, respectively--although in the second case, it will be harder for it to go fast when the larger sled is moving, so...maybe.

mrbuddylee
09-04-2002, 12:44 PM
Well, first, thanks for the help with my first question. We were able to have a nice discussion about this at lunch.

In regards to the sled issue:
I would say the little sled is riding on top of the larger sled. One of my friends brought up a very similar question. A fighter jet is flying at 1200 mph chasing another jet. He fires a missle.
How fast is the missle going? Instant acceleration from 0 to 1200 or did he just get a push from the plane and while the missle is now going 1300 mph, it started out at 1200? Don't know if that's a good comparison to the sled question, but basically:

Big sled goes 500 mph.
Little sled on top of big sled goes 300 mph. Does little sled on the back (outside) of the big sled break the sound barrier? I guess the little sled would break the sound barrier, and the big sled is just a helper.

kpm
09-04-2002, 12:57 PM
The missle is never going 0 MPH, it's going the speed of the plane 1200 when it is released.

Some missles fall for a second or 2 before their engine is started but others do not. I think the 1st case is so the missle engine does not burn the attachment point on the plane.

Philster
09-04-2002, 02:01 PM
You're in the back of a see-thru trailer cruising the highway at 60 mph.

You throw a normal pitch for someone your size towards the front of the trailer....say about 60 mph.

The pitch is going 120 mph when you release it.

Relative to you, it's going 60.
Relative to people on the side of the road looking into your see through trailer, it's going 120.

Anachronism
09-04-2002, 04:06 PM
I used to work as a mover and ocasionally we would move a car on the trailer. Even with a stationary truck it was a PITA, particularly with a sports car. The ramps we used were IIRC 14' long and with a sports car you had to be very carful not to scrape the front end because of the steep angle. Often you would have to build up the bottom part of the ramp to prevent bottoming out. If the car was at speed this would obviously be much worse as the suspention soaked up the impact of hitting the ramp.

A moving van is about one foot lower than a standard tractor trailer, so with a regular trailer the ramps would be a lot steeper. I belive on the show the 'ramp' was the back door of the trailer, that would be only about 10' long. There is no way that would work, the angle would be way to steep. In addition when you are going up the ramp as soon as you cut the power you start going backwards due to the steep incline. If you are only going 1-5 MPH faster than the truck and trying to cut the power when the rear tires hit the ramp it is going to require a very skilled driver, the ramp is so steep that any forward speed would be lost almost instantly when you cut the power.

The trailer is only about 1-2' wider than the car, so you don't have a lot of room to spare. For this to work in real life it would require a very long (20'+) ramp and a very skilled driver. A front wheel drive car would probably be easier becuase the drive tires would be on the ramp before you had to worry about the car rolling back because of the steep angle.

The car would not instantly go from 61 MPH to 121MPH when it made contact with the truck obviously. But I wonder what would happen with the transmission in gear for 61 one second, and 1 mph the next? At a very slow speed difference (1-5mph) you might roll off the back of the ramp before the transmission found the right gear (auto) or the driver did. I guess if you were willing to have some tire spin when you hit the ramp it might work.

robby
09-04-2002, 04:20 PM
Originally posted by mrbuddylee
The car can't magically go from 61 mph on one surface to 1 mph on another surface. I would think the car would be going 61 mph regardless of the surface. In this case, the car would be going 121 mph relative to the road, and would crash into the cab of the truck.

I wanted to address this misconception directly (though others have answered much far more succinctly--as in "it's relative.")

You certainly can be going different velocities, relative to diferent objects or reference frames. For instance, relative to the computer chair you are sitting in, you are at rest. Relative to the Sun, you are moving at a speed in excess of 60,000 mph as the Earth revolves about the Sun. Relative to the center of the Galaxy, you are moving even faster.

Also, in no instance could the car instantly go from traveling at 61 mph (relative to the road) to 121 mph (relative to the road). The car would have to undergo an infinite acceleration for this to happen.