Mythbusters Bus Jump Small Scale

[diazona]

Great writeup. Thanks diazona. One thing I noticed in the episode was the short run to accelerate the model bus to 23 mph, and no clear reading on the radar gun. The radar gun could be off by +/- 1 mph according to some specs I've read. Also, once the front of the bus starts downward it is possible an aerodynamic downforce develops as well.

The takeoff drag force on the front of the full scale bus is > 1,000 lbs and on the scale bus at least 1/2 pound force (rough numbers). This doesn't slow the bus much over the short jump but I neglected the possibility of aero downforce.

Yeah, well... if you do any quantitative analysis, post it here, I'd be interested to see if you can make the numbers work out. I actually considered running a simulation of the drag force on the bus, but what I wrote went on long enough as it was. What I'm aiming for is to give people* a taste of how physics applies in real situations, not to do a full accurate analysis.

*if anyone actually read my blog, which is doubtful ;-)

 

[dr dodge]

*if anyone actually read my blog, which is doubtful ;-)

i did
nice job

dr

 

[dr dodge]

system, pm'd the sensor info
(tried to refrain from shameless plug)

dr

 

[SystemTheory]

Tonight the Mythbusters tried to skip a sports car across 100 feet of water. The cars at 1/12 scale and full scale worked best with high velocity and no jump ramp.

The jump ramp caused cars at 1/1 and 1/12 to flip and land roughly nose first, what we used to call an "endo" when I raced motocross. One thing that contributes to the endo motion is the impulse imparted by compression and rebound of the rear spring at the lip of the launch ramp. I think this is what is causing the cars to rotate so much, and it may be a function of ramp angle and length being at the harmonic of the spring rebound. In other words the ramp length might need to be "tuned" to the suspension in addition to considering the weight, balance, and drive torque of the vehicle to get a machine to fly properly. If so some fairly complex physics are involved. In motocross the rider will preload the springs on a jump and use the rebound energy to best advantage. This video (maybe fake? looks real) has a car jumping a short fence successfully using a ramp a bit shorter than the length of the sedan.

http://clipaday.com/videos/redneck-car-jumping

 

[rcgldr]

Highways are not designed with launch ramp angles. The launch ramp and landing ramp point virtually right at each other.

Dave has the answer, perhaps it would be more simply stated that these aren't ramps, just 'level' sections of road with a gap in between. Imagine that the ramp was completed. No ramp would be designed to send a vehicle airborne for the distance covered by the gap in the movie, at least not at the speed the bus was going. The ramp sections are level, and given sufficient grip, a vehicle could probably take the completed version of ramp at 120 mph without going airborne. With the missing gap, the vehicle would start falling as soon as it entered the gap.

 

[SystemTheory]

Well, I have caught air on my motorcycle unexpectedly when crossing elevated train tracks at ordinary road speeds! But I agree a road should not have a 50 foot jump built in at 78 mph, so that part of the Myth/stunt is totally busted. In regard to getting a bus to jump 50 feet I watched the episode again last night. Grant says on the DVD release of the movie the out-takes describe (or show) the use of a pneumatic lift at the end of the launch ramp. I suspect it would impart an upward torque impulse on the front wheels to compensate for the downward torque impulse about the rear axle over the estimated time of flight.

 

[diazona]

i did
nice job

dr

cool, thanks Ideally I try to write something all the cool physics-y myths they do, but I've been missing a lot of them lately because of schoolwork. (Just got one up about "Unarmed and Unharmed" from last week, though, and sometime between now and this weekend I'll do the car skipping)