Jump to the Comparison Section - for those in a hurry.

On a radial tire, the plies are run from bead to bead directly across the tire (radially). In other words, the cords are perpendicular to the bead. Successive plies are just layered over the existing ones with all cords being parallel. A bias ply tire runs the plies at an angle to the bead (biased). Different layers have opposing angles, criss-crossing across the tire.

On an unbelted tire, like most bias ply designs, the tread rubber is molded right on top of the plies. For a belted tire, like a radial, a flat layer of material, usually a steel mesh, is placed between the body and tread. The belt is just a weave of metal fibers that overlap in a pattern very similar to the criss-crossing bias ply material. Each belt adds an additional layer in the tread area but leaves the sidewall area untouched.

Inside the tire, the air under pressure presses equally in all directions, allowing the tire to act as a spring. As the weight of the vehicle compress the tire, the air resists. The sidewall must deform allowing the tire to flatten. The contact area between the road and tire multiplied by the air pressure in the tire should equal the load on the tire. However, this is not exactly the case and is the key to many of the differences between the radial and bias ply tires. As mentioned before, the cords that make up the plies can be thought of as a thread. They are easy to bend but resist being stretched (tension).

Bias ply tires are rounded in the tread area because there is nothing beyond the tread rubber to flatten them out. As the tire rolls, the center section must deflect more than the edges, leading to higher contact pressure in the middle. The tread blocks also tip inward. All this results in a contact area smaller than expected.

Also note that since the body carries some of the load, the tires can heat up very quickly. The lateral and longitudinal flattening of the tread area when rolling compounds the problem. Excessive heat or load can cause the plies to separate. Strict attention to air pressure is crucial to safety and tire life.

If a radial did not have a belt, the tire would deform too much. Under any kind of cornering load it would just roll sideways on the rim. Furthermore, concentrated loads in the tread area would cause the cords to separate because the plies have very little resistance in the direction perpendicular to the cords. The bias construction of the belt resists the twisting and separating forces.

A split in the sidewall of a radial is known as a radial crack. It appears as a slit perpendicular to the bead or tread area. High lateral loads, twisting, and bending can cause this during off-roading or operating with too little air pressure. This can also occur under high torque conditions like when a spinning tire suddenly gets traction or accelerating a high powered vehicle on the street, more common with lower tire pressure. Bias ply tires can experience ply separation usually from excessive heat or overloading.

While the bias ply's sidewall is very strong, a sharp object can easily pierce either design's body or sidewall.

Nylon is quite stiff when cold, giving bias plies that distinctive square tire ride when cold. Polyester is less affected by cold.

Since the bias ply sidewall carries some of the load, it is very stiff. Since the radial's belt distributes the load across the tread area, the sidewall (and body) can be made less stiff, allowing the air to do the work. Hence, the radial's sidewalls aid in absorbing shock.

The flexible body and the addition of a belt gives radial tires a nice, flat tread. The load is more evenly distributed across the width and the tread blocks don't fold in. The rounded shape of the bias ply causes the center portion of the tread area to be further compressed than the edges, concentrating the load and tilting the tread blocks. Therefore, radials have better directional stability. The belt in a belted bias ply does help to flatten the tread and increases the directional stability of the bias ply.

The shock absorbing flexible sidewall of the radial allows the tire to conform to irregularities, important for off-roading. Combined with the flat tread area, radials have excellent traction. They also respond to airing down very well. The stiffer construction of bias ply and belted bias ply tires inhibit the tire from conforming and makes the contact patch smaller. However, the flatter tread and distributed load in the belted version does improve its traction.

As noted above, the belt material is usually in a bias ply pattern. When placed on a radial, there is flex point where the belt meets the radial sidewall. On a bias ply, the construction is homogeneous, or on a belted bias ply, the sidewall and belt are in the same pattern. Hence, they don't have this flex area. This allows bias ply tires to use tread blocks up to or extending down the sidewall to aid in protection and traction.

A belt not only flattens the tread area, it maintains the flatness. Under cornering loads, the tread does not bend as much preventing the tire from rolling on the rim. Radials and belted bias plies handle better.

Tires with a steel belt have better protection in the tread area. A sharp object that slices down might not cut through the metal mesh. The belt does not add any extra protection against punctures though.

For severe off-road driving, the bias ply is a stronger tire. The nylon sidewalls resist abrasions more than the polyester. More importantly, the bias construction tolerates twisting and bending from rocks and roots. Since the sidewall is as strong as the rest of the body, it can take lateral loads from rocks and roots without splitting. They can survive abuse that would destroy a radial. The radial's sidewall plies don't reinforce each other, making them very vulnerable to splitting from twisting, bending, and side loads. The weak sidewalls can also bulge out quite far, exposing them to danger. The bias ply can use extremely aggressive treads for excellent traction. Sidewall tread blocks can help protect the sidewall further and to add traction to climb out of ruts and up rocks.

For more moderate off-roading, the choice is not clear. A case can be made for either design. While the radial has the aforementioned disadvantages, the radial's flexibility gives it a smoother ride. It also conforms to irregularities, and with the belt, distributes the load evenly for excellent traction. They also air down well for a larger footprint for floatation.

Similarly, we already pointed out the benefits of the bias ply off-road. However, they suffer from a stiff sidewall resulting in a rough ride. They do not conform to irregularities or distribute the load as well. The smaller contact area reduces traction. Airing down has less effect on the contact area. A belted design helps in the traction but not much else.

The belted bias ply, as noted above, gives the bias ply the flatter tread area, increasing directional stability, handling, and traction on and off-road. However, the body is still extremely stiff.

There are also some belted bias ply tires that are using polyester instead of nylon. The polyester allows a little more flex to absorb shock, allow the tire to conform to obstacles, and to allow better aired down performance. Furthermore, they do not have the rough ride when cold. By increasing the denier of the cords, the polyester tires can have equal load ability as the nylon counterparts. Some abrasion resistance is lost however. I am very excited about this design.

Some radials have more layers in the sidewall to increase strength. However, they are still laid in a radial pattern and cannot resist twisting and scuffing like the criss-cross bias ply pattern. The extra plies can also decrease flexibility, hence ride and traction.

If using a bias ply on the street, strict attention must be paid to air pressure. Since they are stiff, they look inflated even with very little air pressure. Driving with this condition can lead to ply separation because they heat up excessively. Radials on the street need to have the correct air pressure too to prevent radial cracking or tread separation (those rings of tread material on the side of the highway). If using radials off-road, try to keep the air pressure up to decrease the sidewall bulge and to prevent splitting.

Many people swear by one or the other. As with most things, there is not a clear cut superiority of one over another. Interco, for example, makes all three designs to satisfy different needs. Remember that bias ply tires served the industry for many years before the invention of the radial, and the technology has improved them dramatically. On the flip side, radials were invented for a reason.

Related Links