What is the difference between the two?
The main difference 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 today is polyester. The method or the “direction” these plies are applied, with relations to each other and to the center line of the tread, differentiates between a “bias” and a “radial” tire. The plies on a bias tire run approximately 45 degrees to the center line 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 1970’s. The plies on a radial tire run 90 degrees to the center line of the tire and basically overlap instead of crisscrossing. This new design, actually developed during the Second World War , allows the side walls of the tire 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 1974.
Physical comparison of the older style “bias ply” tire and the modern style “radial ply” tire also reflects a change in “aspect ratio”, seen in the relationship of the height and width of the tire cavity. The cavity of the earliest tires was basically round, 100 aspect ratio (inflate an inner tube outside of 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; the height of the cavity was 82% of the width, wider than the earliest tires but still some what tall and skinny. Profiles changed in the mid 1960’s to 78 and even70 aspect ratios providing lower profile tires with more tread face on the road and shorter side walls, a little “firmer” ride but more “responsive” handling. When the Radial tires came on the scene they were built with the lower aspect ratios, therefore we generally acquaint bias ply tires as tall and skinny, while the radial tires are considered short and wide.
As the aspect ratios decreased (tires became shorter and wider), tire engineers determined that “belts” could be layered under the tread to provide better tread face integrity. These belts were originally constructed of rubber permeated fiberglass mesh; we referred to them as “fiberglass belts”, then later steel mesh; which we refer to today as “steel belts”. Bias ply tires were “belted” during the 1960’s, particularly on the lower aspect ratio “performance tires”, but for the most part, in our minds, we only correlate belted tires, more specifically “steel belted” tire with modern radials.
So what does all this mean to you as an antique or collector car owner?
It actually means a lot. The bias ply tire offers originality, a concept that is foundational to our hobby whether you have a completely original car or a customized street rod. The bias ply tire has, and continues to provide utility service and esthetic appeal for folks who desire a period look and originality. They are as safe and reliable, even more so today with modern materials, as they ever were, and let’s face it; we drove on bias ply tires for over 70 years on much worse roads they we have today.
Is the radial tire better?
Of course it is. It’s the “new improved” tire. In fact, it’s a better tire today than it was when it was first introduced as Original Equipment in 1973. Its design is better for road and hwy use. It is safer; it provides higher gas mileage, longer tread life, and better handling. How much safer, how much higher, how much longer, and how much better? It depends on the specific vehicle, how much it is driven and most importantly, how it is driven. Should you install radial tires instead of the original bias ply? This question ultimately has to be answered by the owner of the vehicle. It comes down to form over function; is it worth what you gain by installing radial tires compared to what you give up by not installing original style tires.
Bias Ply to Radial Conversion Chart
Coker Bias/Radial Conversion
Ref: Coker Tire
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