Hi,

Here you go. Unfortunately I could not find enough technical information about the Continental tires because key elements like tread width or revolutions per mile were missing. So I went to TireRack and picked light truck tires that seemed somewhat like the Continenal.

My recommendation is to go to TireRack and pick tires that can provide: psi, tread width and revolutions per mile. Plug these into the row and add the other data needed to make sure it will fit on your wheels. Once you've narrowed your field, pick your poison.

Note that the maximum tire pressure mileage improvement only happens if you use it. If you run a lower value, your mileage will fall off.

GOOD LUCK!
Bob Wilson

 

It gets complex very fast. Changing the tire specification not only involves potentially changing the tread width and rolling radius (which is not necessarily a pure function of circumference and inflation and which in turn changes the distance travelled) relative to a given inflation pressure. It also changes the distortion constants used in the above formulas for the tires sidewall and tread compounds.

 

Until California solves the problem by posting the measured rolling resistance, the right approach, all I have is a hack model versus the typical tire advertisement ("Looks fine, runs a long time.") Compared to no model at all, it is like having at the least, a life-ring from a sinking ship.

This model isn't great but it gives a useful indication of which way to go to try and improve mileage. There have been enough 'lay' reports of disasters, usually associated with wider treads and service departments letting air out of tires, to believe the model is 'good enough.'

Bob Wilson

 

What about overall weight of the tire?
For instance, the OEM Highlander Hybrid Michelens weigh 28 lbs
The replacement goodyears I chose (44lb max sidewall vs 32) weigh 32 lbs.

There must be some penalty for unsprung weight and accellerating the mass.

MM

 

bwilson4web — Your diagram (in post #7) illustrates the problem nicely. The length of the tread which is forced into contact with the road starts out as the curved portion of the circle "below" the road in your diagram, and is deformed into the straight portion (the chord of the circle) whose length is shorter. The steel belts under the tread are very "stiff" to longitudinal expansion/contraction, and so there must be some horrible "squirming" contortion of the belts going on as the tire gets deformed. The resilience of the tread rubber presumably allows this continual squirming to take place, but this results in energy being dissipated in the form of heat (i.e., "rolling resistance") in doing so. Does the belt deform into a longitudinally undulating shape, do you think? I haven't found any discussion of this mechanism in any of the sources I've consulted so far. Interestingly, since the higher the rolling resistance the greater the rate of heat generation in the tire (all else being equal), one could potentially use the steady-state tire (tread or, say, air) temperature rise above ambient as an indirect measure of its rolling resistance.

phoebeisis — Here are some actual numbers for the P215/60VR16 94V (Michelin Energy MXV4 S8) tires on the TCH, as near as I can glean them:
Unloaded radius r0 = 0.333 m = 13.1 inches
Loaded radius r1 = 0.3118 m = 12.27 inches
That's a radial difference of 0.021 m = 0.83 inches, which is quite significant. I don't have the tread width handy, but knowing it and the car's weight (and front/rear weight ratio), one could compute the required length of the tread contact patch for each tire in order to support the car's weight. From this one could then compute the actual longitudinal tread length change that must take place.

Stan

 

Hi Stan,

I found one set of Bridgestone charts that discusses hysteresis but has no detail. I've got a couple of SAE tire papers too but other than comparisons of belted versus radial tires, there is not much there. I suspect we're getting into the region of "proprietary knowledge." The only other open source has been some discussion of silicon used in tire 'rubber' to reduce rolling resistance. However, the exact mechanisms remain somewhat imprecise.

BTW, I have been thinking about how to make small model tires that may be useful for evaluating the effects of diameter, tread width and inflation. I'm not ready to publish anything, yet.

It occurs to me that uniform, flexible rubber foam mat material is pretty easy to obtain. I could use a "hole saw" to cut tire 'slices' from the material. Then using a threaded bolt and washer, make tires of variable tread width. Using different size hole saws, I can make tires of different diameters. The 'pressure' would be fixed by the properties of the material. Then using a treadmill setup, I could measure the drag as a function of diameter and/or tread width.

As for inflation pressure, my Sumitomo tires are rated at 51 psi, which is what I run. I can perform a series of runs using my Graham scanner and directly read out the rolling drag at a constant speed of 24 miles per hour at pressures of: 51, 45, 40, 35 and 30. I'll need to carry a pump to reinflate them after the runs. I'll also need to measure the radius to the road as a function of tire pressure, which I suspect will be modest.

This would give us a way to determine:

Sum_RR = a*F(width) + b*F(diameter) + c*F(pressure)

Bob Wilson

 

bwilson4web — That sounds like a very interesting idea. You certainly are a "do it" type of guy! Let us know what you find.

Stan

 

As a result of a previous, rather lengthy thread in the Camry Forum with Stan, I have been doing something very much like this. I haven't published anything yet becasue the weather in Washington has been so wet that I have not had consistent pavement conditions, and even had to throw out data with the potential for inaccurate resistances distored by partial leading edge hydroplaning. I was using 31 MPH (50 km/h) because that is the number that Michelin told me they use for published rolling radii (standard ISA day at sea level). I also used different inflation pressures from 30 to 45 at 5 pound increments. (Not quite so bold as Bob on 50 PSI.)

The prelimiary results are indeed modest. I have calculated variances of up to .092 (in) on a static loaded radius of 12.25 inches at reference speed. Speed is by far the more important variable, with observed results that yield a variance of up to .348 inches under (nearly) identical pavement conditions over a 10-60 MPH range.

 

I can appreciate the problems of dealing with rain in your climate. I remember visiting the Cascades and hearing nothing but the constant sound of water drops.

Bob Wilson