Watching Top Gear from Britain, I’ve seen them jump limousines, buses, and so on. I notice that they tend to plow into the the motorcycles/caravans etc inside about 20 yards, whereas Evel cleared something like 15 buses at a clip. If gravity is pulling on everything equally, why do the Top Gear jumpers fall so far short?
I can think of a few possibilities:
They’re not going as fast as Evel.
Their ramp isn’t as steep.
Somehow being attached to a long jumping vehicle does in fact pull the front down faster.
There are only about three variables in how far a projectile will go. One is speed, another is launch angle and the third is wind resistance. Probably the biggest factor here is speed. There may be variations in the other two also.
Unless a vehicle is balanced right on the center of pressure from air resistence, it will either tilt forward or backwards, depending whether the center of gravity is ahead of or behind the center of pressure.
Hold a hammer parallel to the ground and drop it. The heavy end will drop faster, which means the hammer always falls heavy end down.
Evel had the advantage of being able to shift his weight or chance his body shape to keep the bike in balance in the air. Can’t do that in a car.
Even if so, this shouldn’t really matter. Unless the vehicle has a rocket engine on it, the direction it’s pointing while in mid-air should have almost no effect on how far it gets before hitting the ground. It might have an effect on the driver’s chance of survival, but it won’t magically pull you into the busses you’re jumping over.
I would say the main factor here would be weight. Evel Knievel was usually jumping with specially built lightweight vehicles, especially motorcycles. A limo is many times heavier than that, and would need much more speed than it is capable of to jump the distances Knievel was reaching.
No, it could have an effect. The direction it’s pointing (particularily if it’s changing) will affect the wind resistance. (Bullets ‘tumbling’ and how the increased wind resistance from that slows them down has been discussed on several SMDB threads.)
Also, a trajectory that would clear an obstacle by five feet for a motorcycle whose lowest point is three feet below its center of gravity, will not clear that obstacle if it’s an RV that’s tilted down to where its lowest point is ten feet below its center of gravity.
Incorrect. All else being equal, a heavier projectile will travel further than a light one. Imagine a polystyrene ball and a lead ball, each the same size. Imagine we fire both using a device that will launch each at the same angle and speed. Which will go further?
There was nothing at all lightweight about Evel Knievel’s motorcycles. He used stock Norton motorcycles, and while they weren’t exactly Harley Wide Glide big they certainly were not comparable to the types of bikes jumpers use now. I’d say that they were comparable to a stock Harley Sportster in size. They were not these 250cc ultralight dirt bike jobs that his son uses in his jumps.
Which is why I said almost no effect. I don’t think that, on the scales we’re considering, wind resistance would be particularly significant, and it would be less significant for a larger vehicle than it would be for a motorcycle. By a back-of-the-envelope estimation, an RV going completely sideways would still have less acceleration from wind resistance than would a motorcycle going straight in, and considerably less yet if its aspect isn’t so pessimal.
I hadn’t considered the clearance issue, though, which depending on where the obstacles are and how the vehicle gets into its bad orientation, could be a problem.
OK, I admit that I was almost totally wrong in my previous post, I was entirely guessing that weight would be a factor.
However, the Smithsonian says that he rode a 300 pound Harley-Davidson XR-750 for “some of his most spectacular stunts.” It is described as lightweight and customized.
OK, so you’ve watched Top Gear and not picked up on their absolute hatred of caravans? You’ve never seen them toast one with a jet powered dragster? You’ve never seen them play “darts” using cars and a caravan as the bullseye? Never seen the special trip they took with a caravan where they burned it down at the end of the show?
You don’t think there is a reason for them to fall short and obliterate a few caravans?
For a rear-wheel-drive vehicle, if the front wheels are not in contact with the ground (say, after leaving a jump ramp), the front of the car will begin moving downward in reaction to the turning of the rear drive wheels (in addition to the normal pull of gravity). This will cause the vehicle to tumble forward. For a very long vehicle, there is a longer time when the drive wheels are on the ramp while the front wheels are unsupported, so I’d imagine the tumbling effect would be greater. And since the vehicles are so long (and so non-aerodynamic when vertical), tumbling would probably make them hit the caravans much sooner than Evel on a motorcycle would.
That, plus the fact that Evel was trying to clear the obstacle, while Top Gear is trying to produce an entertaining-looking crash.
I thought the idea was that the rear wheels would cause a rotation of the car in the opposite direction, i.e. nose upwards, under acceleration. There was an Evel night on the BBC ages ago, they showed how professional stunt drivers managed their cycles in flight, revving to pull the nose up a little.
