> Does that really work? I mean, can you really just say "oh, this thing has 
> 4 wheels, so each one only takes 1/4th of the load"?

Well from experience most cars are pretty equal for left/right weight 
distribution, and as I said worst case is probably 70% on the front wheels. 
So yes, probably 35% of the total weight is the maximum on any single wheel.

> Also, something like a Prius is 3042 Kg (unloaded). I'm presuming they 
> make these ramps with a damned wide margin for safety.

Usually when you buy a pair of those ramps they will be marked as supporting 
a certain load.  2 ton per pair of ramps is common.  If you go over that 
then obviously there is a large risk they will break.

> But can you really do that? Can you really just say "there's 4 columns, so 
> divide the load by 4"? Wouldn't it depend on the angle of the force being 
> applied?

Of course, I was simplifying to get a rough estimate.  In reality you would 
have to take the worst case loading condition, which is probably with the 
tyre directly on top of a single column.

> And what about the horizontal elements? They need to not bend at the 
> points where they're unsupported as well.

You would obviously check for this if you were designing it, but I assume 
this wouldn't happen as a tyre usually spreads out the load across an area. 
The loading would be concentrated at the tops of the support columns as 
these won't budge.

> Looks thinner tham 5mm to me - but then again, it isn't cylindrical...

Yes, something like an I beam or a hollow cylinder is a more efficient use 
of the metal.

> Just how strong is steel?

It varies from about 250 - 750 MPa.  That's the stress you need to apply to 
it before it starts to deform plastically.  As an example, my steel ruler 
has a cross section of 1mm x 25mm, you'd need to pull on it with between 
6-18 kN of force, that's the weight of a car.

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