this post was submitted on 09 Oct 2023
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Specifically, wear and tear on the road surface scales with the fourth power of vehicle weight.
As a worked example, this means that if we compare a 3-ton cargo van and a 1½-ton sedan, the cargo van weighs twice as much as the sedan, but it does sixteen times as much damage to the road.
https://en.wikipedia.org/wiki/Fourth_power_law
Axle load, actually. In theory a 1.5-ton car with two axles and a 3-ton truck with four equally loaded axles would cause the same amount of damage. A 1-ton unicycle would cause more damage than the truck.
Note, though, that this is a rule of thumb. A 50-ton tank is still a 50-ton tank even if you manage to make it have fifty tiny axles. But for fairly average motor vehicles under fairly average conditions it's close enough to be useful for planning.
Surely that's damage-per-axle? So it'd be two 1.5-ton cars to match the trucks 4 axles
It could be, but I wouldn’t assume that it is.
I remember when calculating bridge wear, we were discussing the truck arriving on the bridge in terms of impact. It could be the damage somehow comes mostly from the first wheels to encounter the material.
As with all equations in civil engineering, it’s not really derived from physics so much as it’s the curve that best fits the data.
It’s kind of interesting in that way, because it’s an implicit declaration that when shit gets really heavy (pun intended), we trust empirical observations more than we trust our own theories of how things work.
I think it’s a great example of coming back to the roots of science in measurement, as a practical humility we must take seriously because everything civil engineers do is high stakes.
Wow, it makes sense but definitely not good at all