Inreased speed, tire deformation, increased friction?
 

A tire/wheel assembly is basically a flywheel, and as the speed increases, so does the energy stored in it. A tire is moderately flexible, and at some point this energy has to be stretching the tire away from the axle. In one scenario you could imagine that this is pushing harder on the ground, thus adding to the resistance of friction. But in another, you could claim that it causes the tire to form a pointed shape, with less contact with the road, thus reducing friction. I do know that tires look lower when they are parked, but temperature plays a role. Unfortunately, it is not feasible for me to examine a tire moving at 65MPH with a great deal of accuracy.
Anyone got the physics answer or personal observation? Just trying to better understand this. I practiced and practiced driving a manual. Stalled it every over stop, maybe more. I read about how it worked, and RARELY stall it. Only in heavy traffic, or up a really steep hill where slipping the clutch feels bad.

First, a tire's rolling resistance is not caused by friction to the road surface. It is caused by the internal friction of the material.

There are primarily three things that affect RR. The amount of deflection (movement of a portion of the tread as it moves in and out of the footprint - which is basically a load vs inflation pressure thing), the amount of material being deflected (basically the amount of tread rubber), and the nature of the material (basically the properties of the tread rubber).

Here's a more through explanation:

Barry's Tire Tech

Now the speed effect is because there is a thing called a standing wave that generates more movement - and hence more energy consumption. The faster you go the greater the energy consumption.

What is your goal here? :confused:

I can't imagine tires pushing harder on the ground due to deformation from speed- it is the mass of the vehicle pushing on the ground and if we ignore aero lift or downforce (as it is a variable independent of tires) that mass should be the sole determinant of how hard the tires push against the ground (assuming, of course, a straight and level road condition).

As far as a tire getting a pointed shape, I think the tire would blow up first due to centrifugal forces beyond it's design and construction limits, and it'd be going a hella lot faster than it's speed rating, especially if we are talking about steel belted radials which just about all cars and trucks have now.

But let's assume it does that. The friction which causes rolling resistance, as I understand it, mostly comes from the hysteresis or energy loss from flexing rubber, and most of that loss is in the tread region of the tire followed by the sidewalls. If we're flying along at 1000 mph such that the centrifugal force on the tires is making them pointy, they probably won't be flexing very much at the point of road contact so yes I suppose there could be less r.r.. But that situation doesn't exist on the street, that I know of. LSR cars don't even have rubber tires that I know of. Dragster tires do lots of funny things with speed and centrifugal force so that might be kind of along the lines of what you are thinking.

If you're in a Tempo which generates so much aero lift that it's about to go airborne (LOOK OUT HERMIE LOOK OUT!!! :eek: ) I suppose since the loading on the tire has been reduced to near nothing, that the tread and sidewall flexing has been reduced too, thus lowering rolling resistance.

By definition, rolling resistance (force) is dependent upon the friction coefficient between wheel and surface.

Fr = µ * m * g

(friction coefficient x mass x gravity constant)

Er, no, rolling resistance is coefficient of rolling resistance * normal force. Friction force is coefficient of friction * normal force. Unless you're doing a burnout/drift/launch, this doesn't matter as far as fuel consumption goes because the tire is not slipping. Rolling resistance is the one that matters.

Point of order--

The tires are often slipping a little bit. That's what happens when they turn, different parts of the contact patch slip. This is especially true when you are cornering quickly, but it is also true when you corner at more normal speeds.

...But the dominant force when going in a straight line would be rolling resistance, yes.

(I'm now trying to resist getting sidetracked into a discussion about the "coefficients" and how they are not constant....)

-sod

lets say a car weight 2000 pounds, and weight is evenly distributed between all 4 tires.

so, 500 pounds a tire. lets say your tires are inflated to 50 psi.

that means there are 10 square inches on the ground.

now lets say we spin the tires really frigging fast (like say at bonneville speedweek at 200 plus mph).

the centrifical force actually causes the tires to "stretch" and become round. Yes, you will actually have less surface on the ground, and you easily reach a point where the tire can be come uninflated and you don't crash until you slow down (not recommended).

all neat stuff, but all kind of insignificant for purposes of drag. you STILL have 500 pounds of force pushing down on the tire, and like serial said, it is the coefficient of drag times the normal force, and neither of those has changed significantly.

Not so. I've measured it and the correlation isn't direct.

Perhaps this will help:

Barry's Tire Tech

It would be most interesting to see the results of the (not completed) "air or tire" test on the Ford van!

P.S. We don't even have to measure... think of the extremes: a tire with 500 psi and a 500 lb load isn't going to have a one square inch contact patch and a tire with 1 psi isn't going to have 500 square inches on the ground. However, there isn't even a small "zone" where that air/weight/area notion works.