[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.