Tire sidewall height vs rolling resistance?

[ksa8907]

Originally Posted by lasitter

My truck currently has Kumho Solus KR21 235/75R15 tires, 108T XL 2183 lbs weight rating, 6.5" tread width and weighing 28-29 pounds. These are not LRR tires.

The General Tire Grabber HTS is the only (non-winter) LRR tire available in that wheel size, so I am considering buying that tire, and also tires for 16-18" wheel sizes, where there are many more options.

One question I'm trying to answer is this: Given that I'm already getting very good highway fuel economy (up to 32mpg @ 55mph under ideal conditions), what is the biggest bump in fuel economy that I might get as a result of tire selection?

Then: How much would I have to give up in terms of driving and performance characteristics, in order to achieve those benefits?

General Tire Grabber HTS LRR 235/75 R15 109T XL 2271 lbs./51psi 12.0/32" 32lbs 7.2"
Michelin Latitude Tour HP LRR 255/65 R16 109H SL 2271 lbs./51psi 10.5/32" 33lbs 8.5" (owners hated it)
General Tire Grabber UHP --- 255/65 R16 109H SL 2271 lbs./51psi 12.0/32" 34lbs 7.7"
Pirelli Scorpion Verde+ AS LRR P235/70 R16 106T SL 2094 lbs./44psi 11.0/32" 30lbs 8.0"
Pirelli Scorpion Verde AS LRR P235/70 R16 106H SL 2094 lbs./44psi 11.0/32" 31lbs 7.2" (2011 Production)
General Tire Grabber HTS LRR 235/70 R16 106T SL 2094 lbs./44psi 12.0/32" 33lbs 6.5"
General Tire Grabber HTS LRR 235/70 R16 106T SL 2094 lbs./44psi 12.0/32" 33lbs 7.0"
General Tire Grabber UHP --- 255/55ZR18 109W XL 2271 lbs./51psi 11.0/32" 32lbs 8.5"
Pirelli Scorpion Verde AS LRR 255/55 R18 109V XL 2271 lbs./50psi 11.0/12" 36lbs 9.0"
Continental CrossContact LX20 LRR 255/55 R18 109H XL 2271 lbs./51psi 12.0/32" 35lbs 8.6"

I'm very suspicious about tire manufacturer claims regarding LRR. No one is releasing CRR data for their products. That said, I'm generally aware of the claims made for LRR tires.

The tires above cover lots of different options and strengths. I'm going to have dedicated winter tires so that one tire does not have to accomplish everything.

Just looking at the tires above, and considering the weight, PSI inflation, rubber patch on the road, and approximate sidewall height, and forgetting price, which ones would likely show the greatest improvement in MPG?

The greatest difference in weight above is the Verde+ at the 16" wheel size with 30 lbs. and 8" of rubber on the road, and the 18" wheel with Verde with 36lbs and 9.0" of rubber on the road. That's a pickup of 2-8 pounds per corner for me. What does that cost me just in weight, or in terms of the six pound difference between the Pirelli's?

Compared to my current 6.5", it looks like I can have a lot more tire on the road if I want it, and that might represent a big improvement in stopping and handling. That's partially why you'll see the UHP summer tires in the list, in case MPG improvement isn't enough to matter.

It also looks like I can shave 2-3 pounds off the wheels. Still working on that.

If cooper makes the discoverer htp in that size you may want to consider it. We put those on my wife's car a couple years ago, 30k miles later they have about 50% or so tread left. I have been very happy with them.

[gumby79]

A closed sholder will yield better FE ,however at a cost of traction. Not a problem when running different winter tires and a possi.
Also imo they are quieter

[Focus-Ak]

In post #13 I provided a chart of tire profile vs. rolling force, with tire width and rim diameter as parameters (and note that I have just now corrected the chart in that post).

Below is a similar chart for tire width vs. rolling force, with tire profile and rim diameter as the parameters. This one looks a lot more uniform. Once again (as in the chart in post #13), it seems like the rolling force data for the two blue 195 data points were reversed in the source data I used. I that were changed, the blue 70 and 75 lines would not cross.

[Focus-Ak]

In post #13 I provided a chart of tire profile vs. rolling force, with tire width and rim diameter as parameters (and note that I have just now corrected the chart in that post).

Below is a similar chart for tire width vs. rolling force, with tire profile and rim diameter as the parameters. This one looks a lot more uniform. Once again (as in the chart in post #13), it seems like the rolling force data for the two blue 195 data points were reversed in the source data I used. I that were changed, the blue 70 and 75 lines would not cross.

Both charts are good guides; they just show relationships among the three factors in different ways, together providing a "sensitivity analysis" for which factors have the greatest effect on efficiency.

[lasitter]

Focus-Ak: Thanks for updating the charts.

I have a couple of additional questions / observations.

This is showing rolling force for non-LRR tires or LRR tires?

And: As you would expect, most of the data points are for passenger cars like the Prius. My truck has factory options for 215/75R15 and 235/75R15, and I'm running the latter (non-LRR). The smaller tire can do much less in the way of load carrying.

The chart has one line for 16" wheels, and it seems to suggest lower rolling resistance. I have insufficient load options in 16" so I've just ordered a set of Bridgestone Dueler H/L 422 Ecopia XL in 235/65R-17.

I'm hoping by reducing combined wheel/tire weights and switching to an LRR tire my rolling resistance and economy will be the same or better, at least after some of the tread wears away.

[Focus-Ak]

Actually, it is clear from the chart that resistance goes up with diameter. The difficulty with the chart is that there are no "60" profile tires (or any with lower profile than that) to compare directly to the two 16" tires that were in the study. The questions, for which the study has no data to answer, are: (1) what would the data points be for 16" tires with profiles of 65, 70, 75, 80; and (2) where would the data points be located for 13", 14" and 15" tires that have a profile of 60? I think we can imagine how in very rough terms how those relationship might play out - that is an expectation or rough prediction.

As others have noted in this thread, there are other factors at play regarding total efficiency: (3) engine RPM at highway cruising speed (in which diameter functions opposite to the charts); and (4) the effect of tire weight both rotationally and linearly when the car accelerates (which reinforces the diameter relationships shown).

[CapriRacer]

Focus-AK and lasitter (and anyone else who views this thread).

Be very careful!

When you go to compare tires for your vehicle, they have to be tested at the same conditions. The data in the charts above is NOT. Different sizes are tested at different loads. If you want to compare different sized tires, you should be looking at RRC values (RRF divided by the test load) - and if you do, you will find that wider tires are ever so slightly better for RR. See my web page for a more detailed discussion:

Barry's Tire Tech

PLUS, the data in the above charts is for a single line of identical tires (except for size). The tires happen to be non-LRR, but that doesn't matter. What you should be getting from the data is the relative values for different sized tires.

[MeteorGray]

Going back to this thread's title question: "Tire sidewall height vs rolling resistance?"

With a given wheel diameter for a car and all other things being equal, for less rolling resistance is it better to have more rubber in the tire's sidewall with a smaller metal hub/rim, or is it better to have less rubber in the tire's sidewall with a larger metal hub/rim?

Stated another way, for lower rolling resistance is it better to achieve a 26-inch-diameter wheel with a 16-inch metal hub/rim or an 18-inch metal hub/rim?

[gumby79]

Baced on this chart of rrc. From bearys tire teck.
Using the formula he provides for wheelsize conversion as p+10 mm/-5R+1%
Eg:P175/80 r13 = P185/75 r14 out side diameter. Should be equal

P175/80R13= ~10.5RRC
P185/75R14= ~11.1. RRC
P195/70R15. NA I used P+20/-10R+1. To adjust next closest in this tire availability. As I have dune in my past life as a tire repairman@Winston Tires for 16"aswell
aswell.
P205/65R15= ~10.8RRC
P215/60R16= ~10.2RRC
THE DATA SAID:
All things the same except wheel size, the shortest. And the tallest are. Ranked1&2 for this Goodyear tire for cars not LT.

Question
On your hafe tone truck are you running LT or P (passenger car) this will make a large difference in FE?

[Focus-Ak]

Originally Posted by CapriRacer

Focus-AK and lasitter (and anyone else who views this thread). Be very careful! When you go to compare tires for your vehicle, they have to be tested at the same conditions. The data in the charts above is NOT. Different sizes are tested at different loads. If you want to compare different sized tires, you should be looking at RRC values (RRF divided by the test load) - and if you do, you will find that wider tires are ever so slightly better for RR. ...

In looking into it, right you are concerning the need to use RRC. I thought Lambilotte had said he did not find correlation within the RRC tests, but he instead said his RRF and RRC tests were not correlated to each other. Adding to my confusion, his scatter plots in his presentation all were based on RRF. Below are charts I made of his RRC data, done in the same way I did above for RRF. The RRC data is all over the map, and although the above charts are not, that is because the increasing test load with increasing tire size is the dominant factor. (Whereas the data we are interested is for same load [i.e. weight of the same car] across a variety of tires that have an equal or higher load rating.)





Lambilotte has remarks that may be of interest in testimony in 2007, from page 64, here http://www.energy.ca.gov/transportat...TRANSCRIPT.PDF. He refers to charts in the PPT that is linked on Bruce's website (URL in earlier post).

Also of interest are some conclusions in Ch. 12 (by T.J. LaClaire) in NHTSA's "The Pneumatic Tire (2006) http://www.nhtsa.gov/staticfiles/saf...HS-810-561.pdf," ff. p.507. Worth quoting here, I think, for discussion:

Effects of tire design parameters on rolling resistance

[T]he roles of several design parameters are quite clear and their impact on rolling resistance is generally consistent. These can be confidently applied when reductions in rolling resistance are sought. The effects of the tire mass, crown shape, the use of a cap ply, and dimensional influences are discussed below. … Beyond materials, perhaps the single most important factor in tire design impacting rolling resistance is the mass. Since the deformation of rubber throughout the tire is the primary source for all energy dissipation responsible for rolling resistance, removal of rubber material will reduce the rolling resistance. Tread depth and width are two key design parameters that affect the tire mass, and both parameters have an important effect on rolling resistance. As noted in the previous section, rolling resistance decreases nearly linearly with tread wear. The total reduction in rolling resistance between new and fully worn tires may exceed 20%. A reduction of tread depth in the initial design will reduce the tire rolling resistance from the start in a manner similar to that observed in the stabilized tread loss period of the tire life. It has been observed that reductions in the shoulder tend to have a larger impact on the rolling resistance than in other regions of the tread … .

Similar to tread depth, reductions in the tread width can be used to remove mass, thus reducing the rolling resistance. Care should be taken, however, to ensure that the tread width does not decrease to a point at which the stresses in different regions of the tread become very non-uniform or excessive. Excessive loading of the shoulder portion of the crown can occur if the tread width is excessively reduced, which will have adverse effects on rolling resistance in addition to other performance aspects of the tire. Of course, the wear life of the tire will also decrease when tread rubber is eliminated from the tire. Reduction of mass from other parts of the tire also improves rolling resistance, but tends to be less effective than crown modifications. Nonetheless, it is worthwhile for rolling resistance … to remove as much mass as possible from a tire if the additional rubber is not necessary for a specific function.

The curvature of the crown has an important effect on the stresses generated in the con* tact patch during rolling, and also on stresses in the belts. Although a flatter crown (decreased curvature) is not always better, this is generally the case. It results in reduced lateral bending since achieving good contact with the road requires less flattening of the summit. Lateral stresses in the contact patch are also reduced, so that shear stresses in the tread are generally lower. However, the tire shoulders can be excessively loaded if the crown radius becomes too large. This causes the stress levels there, and thus the energy dissipation, to increase. Additionally, if the carcass shape in the meridional plane remains the same, a flatter crown will result in an increased thickness of tread in the shoulder regions of the tire. In this case the mass effect may counteract any reductions in energy dissipation obtained through flattening, and the rolling resistance may increase as a result.

The addition of a nylon cap ply tends to increase the rolling resistance by several percent, and this difference can be even larger if the tire that was not initially designed for use with a cap ply. Although a nylon cap helps reduce rolling resistance at high speeds, … this is not the case at more typical operating speeds. Below about 120 kph the rolling resistance is larger when a nylon cap is present due to several effects. First, adding the cap ply simply adds mass to the tire. Second, the added rubber and the cords dissipate energy. Although energy loss in the cords is not as significant as that in a similar volume of rubber, it may contribute up to 2% to the rolling resistance due to hysteretic energy dissipation as the cords are cyclically stretched and relaxed. Additionally, the cap ply stiffens the crown region circumferentially, which influences the stresses both in the contact patch and the belts, and it restricts growth in the shoulders when the tire is inflated thus making the crown profile more round than in a tire without a cap ply. These effects require that the tire design be adjusted when a cap ply is used. Otherwise, the rolling resistance may be impacted negatively.

Various studies have been conducted on the effects of tire size … The results have been quite mixed, probably because of interactions among the many variables of tire design. One parameter that appears to have a clear effect is the tire outer diameter: a larger diameter tends to reduce the coefficient of rolling resistance. Pillai and Fielding-Russell [27] found that the coefficient of rolling resistance is approximately proportional to the outer diameter raised to the -1/3 power, [Equ. 2.16], for tires of similar construction. This relationship holds for a wide range of tire sizes within the same tire line, using the same construction and materials. Pillai and Fielding-Russell used this relation to predict the effect of aspect ratio using different combinations of other dimensional variables. They considered section height, section width and seat diameter, which were interrelated with the tire outer diameter to predict their effects using Eq. 2.16.