Some Basic Tire Physics

[Flanders]

The beadlocks are 1” wider, the tire pressure is the same. ??‍ the economy difference is basically 98% highway running down hwy 395. Vehicle weight/on vehicle weight is only changing with the spare on the back.

Chalked this up to increase in wheel weight before this discussion.

Driving patterns and patience are pretty well set. Fill up in Inyokern and running north to Lone pine/Bishop and refilling. Or running south to Big Bear.
Both directions have minimal stops, and fairly consistent traffic…. Lots of cruise control action.

One should expect the wider rim to put more rubber on the pavement at the same pressure, and more rolling losses as a result. I don't know about 15% worth though.

Here's a question: Coasting down a steep hill, foot off the gas, can you tell any difference between the two wheelsets? (Maybe put the transfer case in neutral to get the engine out of the equation.) It should coast a bit faster with the heaver wheels - unless the rolling losses are greater.

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[Volt0]

It’s outside my wheelhouse but I doubt that contact patch is useful in predicting fuel economy.

It boils down to estimating energy losses due to deformation of the rolling tire and friction with the pavement.

And losses in energy to go back to shape. Seems like there would be something to explain why a laden wheel barrel pushes easier with more tire pressure. I like your formulas, I really do. All other things equal, I would expect a softer sidewall to get lower mpgs, and to your point, I agree that it wouldn’t be a 10% difference ( definition of significant? ), but 5% might be reasonable. Thoughts?

 

[WestwallNF104A]

Ouch. Indeed, I would have mentioned it.



It’s outside my wheelhouse but I doubt that contact patch is useful in predicting fuel economy.

It boils down to estimating energy losses due to deformation of the rolling tire and friction with the pavement. Contact patch doesn't tell you anything about the unloaded shape of the tire, stiffness, sidewall deformation under load, distribution of mass or distribution of contact pressure.

Contact pressure varies over the contact patch. It can be more than the absolute tire pressure where the carcass provides support. It can be less than atmospheric pressure in places (i.e., a tread block pulling away from the pavement creating a bit of vacuum). All of this depends on the tire structure and varies non-linearly with speed, load and tire pressure.

Ouch. Indeed, I would have mentioned it.



It’s outside my wheelhouse but I doubt that contact patch is useful in predicting fuel economy.

It boils down to estimating energy losses due to deformation of the rolling tire and friction with the pavement. Contact patch doesn't tell you anything about the unloaded shape of the tire, stiffness, sidewall deformation under load, distribution of mass or distribution of contact pressure.

Contact pressure varies over the contact patch. It can be more than the absolute tire pressure where the carcass provides support. It can be less than atmospheric pressure in places (i.e., a tread block pulling away from the pavement creating a bit of vacuum). All of this depends on the tire structure and varies non-linearly with speed, load and tire pressure.

I don't see any reference to the coefficient of friction between the tire, and the road surface. Obviously that changes with the type of roadway. Left out also is aerodynamic drag, and the base construction of the tire, which also has an effect.

Tire compound too has an effect as does the degradation of the tire over time.

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