Tires, tires, tires! We all own 'em, drive on 'em, and try to smoke 'em, but how many of us know how to read them and understand what that sidewall is trying to tell us? If many of you just put your hand down, you're not alone!
In the old days, it was pretty easy to pick out a set of skins for your truck, but that was back in the heyday of bias-ply tires, whereas today there is a mind-boggling amount of size choices in modern radial tires, especially with the popular rise of larger-diameter wheels in the last 10 years. But what's the difference between bias-ply and radial tires, you ask?
For those too young to know what bias-ply tires are and to refresh the rest of us, we took this question and many more to the antique and classic tire gurus at Coker Tire. They told us that the main difference between bias-ply and radial tires lies in the tire's construction. The basic carcass of all tires is made up of layers of rubber-permeated fabric. These layers are referred to as plies, and the most common fabric used in tires today is polyester. The method, or the "direction" these plies are applied, with relation to each other and to the centerline of the tread, differentiates between a "bias" and a "radial" tire. The plies on a bias tire run approximately 45 degrees to the centerline of the tread, alternating in direction with each layer; thus, they crisscross or run in 90-degree angles to each other. This design or style of construction was common on all tires provided as original equipment on U.S.- built cars until the early '70s. The plies on a radial tire run 90 degrees to the centerline of the tire and basically overlap instead of crisscrossing. This new radial design, actually developed during World War II, allows the tire's sidewalls to be more flexible, which provides less rolling resistance, providing better gas mileage and longer tread life. This flex also promotes better adherence to the road, thus better handling on both wet and dry surfaces. The radial tire found early acceptance in Europe, and finally became standard equipment on most U.S. passenger cars by '74.
A physical comparison of the older-style bias-ply tire and the modern-style radial-ply tire reflects a change in aspect ratio that is seen in the relationship of the tire cavity's height and width. The cavity of the earliest tires was basically round, which gave them an aspect ratio of 100 (inflate an inner tube outside the tire and its cavity is basically round; the height and the width of the cavity are the same). Through the years, most bias-ply tires had an 82 aspect ratio, meaning the height of the cavity was 82 percent of the width, wider than the earliest tires but still somewhat tall and skinny compared to radials. Profiles changed in the mid-'60s to 78 and even 70 aspect ratios, providing lower-profile tires with more tread face on the road and shorter sidewalls, which gave them a little firmer ride but more responsive handling. When radial tires came on the scene, they were built with the lower aspect ratios, therefore we generally think of bias-ply tires as tall and skinny, while radial tires are considered short and wide.