> I noticed the other day that some of the cars in the showroom were parked 
> on little metal ramps.

You mean like these:

http://members.iinet.net.au/~mmckenzie/files/VX_CHARGED_S/ramps.JPG

> Now here's the puzzling thing. My car is only little, and that weighs 1.25 
> tonnes. Some of the bigger cars surely weigh even more than this. So these 
> ramps have multiple tonnes bearing down on them... and they're made from 
> 2mm steel.

The bigger cars are probably at most 2000 kg, worst case is say 70/30 
front/rear weight distribution, so I'd say max 700 kg on each ramp (if you 
put the front wheels on it).

> Now obviously somebody far, far smarter than me has carefully calculated 
> how thick the steel needs to be to hold a given amount of weight.

Well it seems to me that there are 4 vertical columns supporting the weight, 
so probably max 200 kg or a max force of 2000 N in each column.

There's a neat formula that Engineers use to determine the maximum load a 
column can take without buckling, find it here:

http://en.wikipedia.org/wiki/Buckling

A bit of rearranging gives us a formula for I (the required area moment of 
inertia):

I = F * (K*L)^2 / (pi^2 * E )

In this case we have:

F = 2000 N
K = 0.5
L = 0.3 m
E = 200 GPa

So I = 2.3 x 10^-11 m^4

This allows you to choose what shape and thickness to use.

For simplicity assume a cylindrical rod is used, the formula for the 2nd 
moment of area is:

I = pi/4*r^4

Rearranging:

r = (4*I/pi)^(1/4) = 2.3 mm

So there you go, make it out of diameter 5 mm steel rod and you'll be fine!

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