Comment by el_nahual
8 hours ago
For people that don't watch the video (I don't even know if this is in the video): road wear is a function of axle weight to the fourth power. [0]
That means a 6,000lb escalade creates 3x the road wear than a 4,500 wagoneer from 1990.
The other key takeaway from the video is that carmakers are highly incentivized to sell SUVs because they're still classified as "non-passenger work vehicles", which have looser emission requirements making them cheaper to produce.
The bug in the law really seems to be that cars that really aren't intended as work vehicles are being treated like them.
This model is the basis of the 1993 AASHTO guide on a flexible pavement design, which is not the state of the art, but is still commonly used. This is why pavement design is mostly controlled by commercial traffic. For estimation purposes, I would not even consider the load of passenger vehicles in a flexible, pavement design.
You are incorrectly assuming the Esclade isn't on 32+" tires with 285+mm width and the Wagoneer isn't on pizza cutters. Tire size has increased greatly on SUV and light trucks, which exerts less ground pressure.
It's not realistic to do this on a heavy truck, which run 110+ PSI on heavy wall tires and why they cause the power law damage to roads.
No way does the Escalade exert less ground pressure!
Firstly, a Wagoneer is never on pizza cutters. You can't put a 4500lb car on pizza cutters even in 1990! It came with 235/75R15 tires. They are big sidewall donuts, but no pizza cutters.
The Escalade runs 285/40R24 tires, that's wide and low-profile.
Widening a tire increases ground pressure, because low-profile tires have massive amount of reinforcement to prevent that wheel from cracking. This stiffness adds to the pressure the road feels.
Tire contact patch is a function of weight and tire pressure. A 205mm width tire has the same contact patch as a 285mm tire, given same weight and pressure. The only thing that changes is the shape of the contact patch, which becomes wide and short instead of narrow and long.
The 6000lb Escalade runs its 285/40R24's on 35 psi, the Wagoneer runs its 235's at 30 psi.
So assuming even weight distribution, the contact patch per tire is 6000lbs/4/35psi=42.8in^2 inches for the Escalade, and 4500lbs/4/30psi=37.5in^2. So the contact patch is only 14% larger on the Escalade, yet it carries 33% more weight!
If you look at the road wear formula, it's entirely a function of weight. So the width of the tires only impacts surface-level abrasion. And with the power law, that's still 3.16x of Wagoneer's wear (or 216% increase).
So the wider tires do virtually nothing to protect the road from the extra 1500 lbs weight.
In fact, the dynamic load when hitting potholes is greatly exacerbated by the 285/40R24 low profile tires, because instead of of absorbing the bumps within the tire, the stiff sidewall low-profile tires absorb way less.
The spring rate of the Wagoneer tires is ~1200-1500 lbs/in, the spring rate of the Escalade tires is ~2500-3500 lbs/in, so that's a 2x stiffer tire! As a result, it transmits twice as much force when hitting the same bump.
So as a result, an Escalade accelerates road cracking considerably worse than the Wagoneer, not even in the same league.
Yes, the heavy trucks wear the road outsizely, incomparably to the SUVs we are discussing. However, we have roads that do not allow trucks (parkways) or see little heavy truck traffic.
The tire spring rate is not what is "felt" by the road every bump, there is forward motion and coilover suspensions in modern cars running low profile tires versus solid axles and leafs on the Wagoneer, but yes nearly every modern vehicle ships on low profile tires unfortunately.
We are talking a few hundred pounds of weight difference per tire, it simply doesn't matter in the real world. I maintain private roads and even if your parkway is placarded everything is subjected to vocational vehicles and HDTs whether signed or not, because work needs to be done and local deliveries are exempt from transit restrictions.
The road wear model is obviously simplified versus the real world, the power law accurately enough extrapolates to HDT axle loads. You can drive a vehicle above snow, or move million pound super loads without causing excessive wear by thinking in additional dimension.
Keep this in mind next time some crank on Nextdoor dot com goes off about taxing bicycles. "Sure, as long as we're both paying according to the road wear and tear we cause".
one garbage truck - 40,000 pounds wears road 2000x than escalade.
We can't do without garbage trucks though
- have less garbage
- collect garbage more frequently in smaller trucks
taiwan has very cute small garbage trucks and they have a ice-cream-truck like song signalling for people to bring whatever trashbags they have out to the truck, so you don't even have piles of garbage outside for days waiting for the weekly truck. quite nice.
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I only need the garbage truck to do a run for me and a couple hundred others once a week. My Escalade is transporting me and maybe 1 other person on average 7 days a week.