Figure 1 shows the following tire terminology:
- Section Width. Width of the tire at its widest point (outer sidewall to inner sidewall) including “growth” due to inflation.
- Minimum Dual Spacing. Minimum distance from the center of one tire assembly to another on a dual tire axle. Minimum spacing precludes tire rubbing, excessive heat generation, and stone damage (those caught between tires). This spacing has implications in pavement analyses since the center-to-center tire spacing must be modeled. Figure 2 shows typical center-to-center tire spacing.
- Nominal Rim Diameter. Used, along with Section Width, to describe tire sizes.
Tire Size Example
If a bias ply truck tire has a designated tire size of 12.00 – 24, then the section width is approximately 12 in. and the nominal rim diameter is 24 in. For truck tires, the design rim width is about 2.5 to 5.0 in. less than the section width (difference depends on tire size).
Bias and Radial Ply Tires
Tires are classified based on their internal construction as well as their external features. The two major classifications, bias and radial, refer to the orientation of their internal plies (or chords, Figure 3).
Bias tire plies (cords) are laid diagonally during manufacture, while radial tire plies are laid radially during manufacture which results in a more flexible tire wall. These different construction methods result in different tire performance, which leads to the following advantages and disadvantages:
Advantages of Radial Tires Compared to Bias Tires:
- Reduced fuel consumption. About 20 percent of truck fuel consumption is due to rolling resistance. Thus, a 5 percent reduction in rolling resistance will produce a 1 percent fuel saving (Fitch, 1984). It is reasonable to expect a reduction in rolling resistance of about 30 to 40 percent, thus a 6 to 8 percent fuel savings.
- Softer ride. Because of their ply layout, radial tire sidewalls flex more than bias-ply tires, which generally results in a larger, more stable ground contact area and softer ride.
- Less vibration. Due to radial tire construction, tires have less tendency to have tire lug-induced vibrations.
- Extended tire life. As bias ply tires rotate and deflect, the bias (or crossed) plies interact and generate heat (Michelin, 1986) which accelerates tire aging. Therefore, radial-ply tires may last as much as 30 percent longer than bias-ply tires doing comparable work.
Disadvantages of Radial Tires Compared to Bias Tires:
- Poor transport handling characteristics. Tire sway increases as speed increases. This is probably due to low lateral stiffness that produces poor handling.
- Less forgiving of abuse. Overload or under-inflation will cause sidewall bulge that is more susceptible to sidewall damage and puncture.
- Radials track more consistently in a wheeltrack. This is particularly true if the wheeltrack has an existing depression or rut. These tracking differences are attributable to the ply layout (the bias (cross) plies have greater tendency to “crawl” out of a wheeltrack depression than a radial ply constructed tire). The net effect is that radial tires, due to ply layout, likely contribute to some of the wheeltrack rutting on Interstate highways.