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Agree, the damage to the road (and the tyres, presumably) is proportional to weight^4.

https://en.wikipedia.org/wiki/Fourth_power_law

> The fourth power law (also known as the fourth power rule) states that the stress on the road caused by a motor vehicle increases in proportion to the fourth power of its axle load

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missinglugnut 15 hours ago | parent | next [-]

I hate that the 4th power law is called a law. It's not a law of nature, it's a lazy curve fit.

Think about this...if car does 'x' damage to the road, 2 cars does 2x damage. 2 cars welded together side by side (axle to axle so the axle count stays the same) would also do 2x damage, but the 4th power law says it does 16x damage.

If it's wrong by a factor of 8 in the simplest thought experiment it's not a law. You can obviously make a heavy load act like many small ones, or concentrate a light load so it does a lot of damage.

Constant * X^4 just coincidentally went through the data in a single 1950s dataset...and for some reason we're calling it a law 70 years later, when it's really just a loose trend that we could easily break with a little engineering. And we probably have broken it...tires, roads, and vehicles have changed a fair bit in 7 decades.

	▲	vel0city 14 hours ago \| parent \| next [-]
		If you're welding the two cars together connecting the axles you're still having 8 contact patches instead of 4, so the axle load is the same as 2 separate vehicles they're just moving in tandem. You'd need to stack the two cars on top of each other to increase the axle load. In which case I'd say it's not obvious how much more the road wear would be without looking at data. I'm not saying the 4th power law is absolute truth, I truly don't know what the wear patterns would look like on a modern surface. But your example isn't proving it wrong at all.
	▲	icehawk 14 hours ago \| parent \| prev [-]
		> Think about this...if car does 'x' damage to the road, 2 cars does 2x damage. 2 cars welded together side by side (axle to axle so the axle count stays the same) would also do 2x damage, but the 4th power law says it does 16x damage. Are you removing the two inner wheels from the axle? Those would also support weight

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rapjr9 14 hours ago | parent | prev [-]

A little off topic, but what are the roads releasing into the environment as they wear down? Asphalt is often somewhat radioactive since it's made from oil? Is there benzene in there? What is the scale of asphalt nanoparticles compared to tire nanoparticles?