Misunderstood that, sorry.
But, there is also something called a "super single" that's a double wide single wheel/tire, with one on the end of an axle rather than two. I'm told they're cheaper and lighter total, as opposed to the more common two per end. I've pulled a few trailers that had them. I don't really know why one or the other, I just drive them.
Let's not forget the context of this discussion, however. Fourth power expressions are still fourth power expressions after multiplication by a constant factor. 4/18 is a constant factor.
I don't know if doubling tire counts really does this, but it's hard to see why twice as many axles would help but tires per axle would not. It's possible (the pressure underneath the tires is reduced, but the tires exerting pressure on the road are closer together) but counter intuitive to say the least.
And there's no proof in this thread that it doesn't help except for masklinn's misinterpretation of the original study.
You're absolutely quibbling though.
> So you should include in your calculations that the load is spread out over eighteen tires, as above.
The fourth power rule is specifically about axle load, not about wheel load. It's also an estimate / rule of thumb (which is why it's so useful), not a precision instrument.
But hey if you want more precision than the FPR we can do more precision: https://pavementinteractive.org/reference-desk/design/design...
As the link notes, road engineers usually work in LEF/ESAL (ratio to a standardised 18000lbs axle load), and provides some conversion tables.
Assuming a relatively large 4000lbs sedan this is 2000 lbs per axle, or 2x0.0003 LEF = 0.0006 LEF.
A fully loaded truck would be ~0.3 for the 12000lbs front axle, and 1.11 for each of the rear tandem axles, for a total of 2.52 LEF.
2.52 / 0.0006 = 4200. Which shows that the fourth power rule's 4096 was pretty close but if anything underselling it.