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Comment by XCabbage | Hacker News Reader

At the heavy end, SUVs weigh about 3 tonnes, while at the light end buses weigh about 12, a 4x difference. 4^4 = 256. So if the claim about the fourth power is true, you'd need to replace 256 SUVs to break even on wear, which is obviously impossible.

(I don't really understand how the fourth power of axel weight thing can possibly be true, though. Why would joining two vehicles together into a mega vehicle with double the weight and double the wheel count suddenly cause the combined vehicle to inflict 16x more wear than before you joined the two together? It makes no sense.)

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Another example I worked out once

A Ford F-150 weighs about 2 tons and has two axles, for an axle weight of 1 ton. 1^4=1.

A garbage truck weighs maybe 30 tons and has three axles, for an axle weight of 10 tons. 10^4=10,000.

So if you drive an F-150, you’re doing as much road damage driving down the street 10,000 times as the garbage truck does once. Rural areas that don’t have garbage trucks and just expect everyone to haul their garbage to the dump in the back of their pickups are onto something.

Joining two vehicles together with double the weight and double the axle count does not change the load on each axle.

So, scenario A:

    4 ton vehicle, 2 axles
    load per axle is 2 tonnes.
    2^4 is 16
    2 axles - so load is 32.
    Another vehicle the same - also loading 32
    Total: 64

Scenario B:

    8 ton vehicle, 4 axles
    load per axle is 2 tonnes
    2^4 is 16
    4 axles - load load is 64
    Total: 64

Ah, right, sorry, I misunderstood the definition of "axle weight" I got when I hurriedly googled it even though the name should've made the meaning obvious.

The classic British Routemaster double decker weighs 7.5 tonne and can be configured with 72 seats. Newer double deckers weigh 12.5 tonne and have a capacity of 60 seated and 20 standing.

Doubling the weight and doubling the wheel count leaves the axle weight unchanged.

Plus the SUV is usually point-to-point, leave home, go to work, come back. Whereas the bus is going back and forth ten times per day.

In Europe, the numbers differ even more. Lighter weight cars typically 1.5-2 tons, a new London bus can be upto 18 tons when loaded - that's ~5-16 units of wear for the car to 104,976 units for the bus...

But this is all supposing we're optimising for road wear, which isn't really the point of a bus system.

I'm old. Back in the olden days - the 1900s - 2-ton cars were not lightweight, the so-called heavy Chevys.

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