So, we all know (or should, at least) that it’s actually fairly easy to drive up a moving ramp, like we often saw in Knight Rider.
My question is whether that sudden shift in wheel speed (different from vehicle speed) would tear up the car’s differential (or transmission, or whatever parts responsible for that).
One second, the wheels are spinning at 50 mph*, the next they’re spinning at 3-4 mph* (or whatever the speed difference is between Kitt and the trailer).
*Actually, it’s whatever wheel rpm produces 50 mph, which I suspect is going to be 50 miles divided by the tires circumference and adjusted for the mpH vs. rpM
Well, assuming the car is left in gear, it’s really going to stress the entire drive train. Engine, transmission, torque converter, drive shaft, diff, axles, even the tires.
True and regardless of what I just said above, your car slowing down rapidly from 50 to 3 isn’t much different then jamming on the brakes. The car doesn’t know why it stopped moving, just that it stopped moving. I supposed the trick is to change from gas to brake or gas to neutral/clutch in to brake at just the right times so you don’t rocket up and into the semi, but Adam sure made it look easy.
Also recall that, in-universe, the Knight Industries Two Thousand was powered by a turbine, and would presumably have some pretty seriously beefed up transmission/power train parts. It wasn’t just a stock Trans-Am–it only looked like one from the outside, kind of like how Airwolf damn sure wasn’t a stock Bell 222A.
The slow motion playback of the mythbusters in the video link above makes it look pretty easy on the transmission. The wheel doesn’t suddenly stop violently, it looks like it slows down over a few seconds. It looks to me like they can just take their foot off the accelerator at the moment the rear wheel touches the ramp. The speed of the wheels is enough to propel it a meter or so up the ramp before the cars inertia slows it down.
Watch at 4:30 - it looks like he takes the right wing mirror off on the side of the truck. I don;t have sound on this computer so I don’t know if that was mentioned…
No, it’s not especially hard on the car. Same thing as driving on ice with a non-ABS car and hitting the brakes hard. All 4 wheels lock up. Doesn’t cause any damage.
The MB’s both were trained by a stunt guy. I got the impression it wasn’t particularly hard if the big rig truck was driven at the right speed. It looked like the big rig was driven by a pro stunt driver.
During the second, high speed attempt there was significant wheel spin (due to the higher speed difference). I believe this is what caused the car to pull to the right.
It is quite possible that in the Knight Rider series they filmed the scene at a slower speed and then increased the video speed to make the car look like it is going faster.
Also in the series, the interior of the semi looked quite roomy, and there was enough room for David Haselhog to open the car’s door and get out. But the semi looked like standard width on the outside scenes. Unless there was some L-space bending taking place, the interior scenes were shot in a studio somewhere.
A semi-trailer can be up to 102 inches wide, and the third generation Trans Am was 72.4" wide. Even if only 92" of the trailer was usable space you could probably open a door inside. That’s not to say that they didn’t shoot those scenes in a studio; I’m sure they did.
I’m sure it would still work, but I wonder if it would make it trickier to perform the stunt if you did it with a front wheel drive car, like most modern cars are. With RWD, when you are making the transition from the road to the ramp, with the front wheels on the ramp and the rear on the road, it’s the powered wheels that are on the road. You don’t actually have to let off the gas until the rear end of the car hits the ramp, at which point the unpowered front wheels have already slowed. With FWD, the powered wheels hit the ramp first.
If it was judged to be “particularly hard” I’m sure the Mythbusters insurance folks surely wouldn’t have allowed Adam to attempt the stunt. They don’t film those segments when they go over their plans with their insurance agents, but I bet they are sometimes entertaining. “You want to do WHAT with WHAT in a WHAT?”
On a rear wheel drive car, the tires act as sort of a circuit breaker against shock. The tire traction should be the weak link. 4wd/awd cars don’t have this luxury, so the clutch should be the weak link. There just isn’t enough traction to break anything.
The villain here would be a sudden change in momentum as the drivetrain/tires slow from road speed to relative momentum speed, which is only 10mph or so. That’s like a small burnout. . I don’t think the diff would be the weak link as both tires are turning at the same speed. Probably a shaft in the transmission.
The bigger concern would be the momentary instability on the ramp.
I’ve seen the mythbusters episode and watching those wheels go from spinning blurs to I-can-actually-read-the-text-on-the-tire is what prompted this question.
But Joey P. does have a good point. It’s mechanically no different from stomping the brake on a non-abs car. In the stunt the ramp and the weight of the car provide the increased drag, while brakes increase drag by applying friction to the wheel (through an attached disk in modern cars, or directly to the inside of the wheel in old cars).
