Lets see they carry about 20,000 lbs * sin(15) = is about 25 percent of the weight is what the force pushing it down the hill. Next we need to find out the frontal area of a semi. . .

 

ok

do this, get infront of a semi on a downhill road off of a mountain pass in a vehicle that has a speedometer thats capable of going to 160. if you you've got the guts to do something like that then by all means cause i'm telling you right now, Semi's can, will and do go that fast because they CAN't slow down. they can't even use thier jagg break because it will shatter thier transmission.

hell i remember i wasgoing over to eastern washignton to Moses Lake and we were doing 120mph down the road and a Semi started catching up to us and started flasshing us to get us out of the way. when it passed us doing 120mph i looked as if someone was passing us at 35-40mph if we were sitting still on the side of the road.

just think about it, nearly 40,000lbs of metal goind DOWNHILL with hardly any resisting wind force being that the weight is distributed through a 50 foot giant metal block with the wind only hitting a small 5-10% of it's surface area not to mention the fact that it's nowhere near enough resisting force to hold it back as you think. the TGV train in france is designed for 160mph continouesly no matter if it's flat, downhill or even uphill. a semi *when going over passes* is generaly going to be a full load and you can expect anywhere from 10-20tons of weight behind it and it CAN and WILL hit those speeds. being that it's a long nose semi and not a flat face, i wouldn't be suprised, given there was a long enough straight, to see one hit 180!

i don't feel up to doing the math right now, but figure it out your self if you'd like, the terminal velocity of a semi IS higher then 145. 10-20 tons *20,000-40,000lbs* with a frontal resistance surface area of about 160ft^2, no breaks or jagg break being they will blow out and a hill thats about 10-20% downfall, it will and does hit those speeds.

 

Sounds like it is definately possible to me - steel brick on wheels . . . I know the brakes are pretty much useless at higher speeds anyway. Heck my xB which is a box will coast at 50 down a 4-5 degree incline - increase the weight and the angle and it will go a lot faster. Terminal speed for a human body is 120mph and that is prone position . . . go head first and you go a lot faster.

 

exactly! 120 prone position, 180 +/-5mph when head first.

a semi is limited to 75mph through the tranny, but in a free coast down hill... ya right, 145+ not possible my a**. i will not sit here and say i agree with whats been said when i've seen it my self.

plus this has sort of gone WAY off topic in the first place.