What is the Right Tire Choice for Your Classic Car or Vehicle, is Bias Ply or Radial?
Tire Construction Methods
The terms “Radial” and “Bias Ply” are used to describe the internal construction of a tire. The difference between them may seem small, but it has a significant impact on the way the tire performs.
Tires are not just big rubber donuts. They are made by stacking layers of rubber and reinforcing fabric. Cotton was once used to reinforce tires, but now nylon, polyester, rayon, fiberglass, and even Kevlar are used. Each layer is called a “Ply”. The difference between radial and bias ply tires is the way the plies are laid in relation to each other.
Bias Ply Construction
- The reinforcing cords are laid diagonally across the tire (usually 30° – 40° from the direction of travel), from bead to bead.
- Each additional ply has the reinforcing cords laid at an opposing angle to the ply underneath it, creating a crisscross pattern.
- Most bias ply tires are 4 ply, meaning they have 4 layers of reinforcing fabric cords.
- More plies = Stronger tires
- 6 ply, 8 ply, and even 10 ply tires are available for heavier vehicles.
- More rubber is then applied over the plies to create the tread.
* **One way to spot a bias ply tire is by recognizing the “Pie Crust” edge on the tread**
In a bias ply tire, the sidewall and the tread are both supported by the same reinforcing plies. This gives the tire certain characteristics.
Radial Construction
- The reinforcing cords are laid at a 90° angle from the direction of travel, from bead to bead.
- Each additional ply is laid in the same direction as the one underneath it.
- All the reinforcing cords run parallel to each other.
- Reinforcing belts are added between the radial plies and the tread.
- Belts are made from woven strands of steel, nylon, Kevlar, etc.
- More rubber is then applied over the belts to create the tread.
In a radial tire, the sidewall, and the tread function as two independent parts of the tire. Radial tire construction has progressed since the early 1970’s and has many benefits over bias ply construction.
It is a decision that every collector vehicle enthusiast faces at some point in his or her life. Do you want the period correct look and feel of a bias ply tire, or do you want the comfort and reliability of a modern radial? Both types of tire construction have pros and cons in the collector vehicle world, but it is important to be educated on the matter before spending money on a set of tires.
Modern black wall radials to fit 14- and 15-inch wheels are still readily available at most local tire stores, but they just do not have the correct look. Although modern radials from your local tire shop are safe and provide an adequate ride quality, they certainly do not add to a collector vehicle’s authenticity like a set of specialty tires. You have the decision of bias ply or radial, and of course the all-important decision of whitewall, redline, white letter or black wall, so choose carefully and hit the road with confidence.
Bias Ply Tires
The specialty tire market has typically been divided into two segments—bias ply and radial. The term “bias ply” refers to the internal construction of the tire. In a bias ply tire, the cords (plies) run at a 45-degree angle from bead to bead. This diagonal pattern is crisscrossed with each layer of ply cord and creates a very rigid structure. In the vintage car market, bias ply tires feature a narrow tread profile, with a sharp shoulder that usually has a piecrust look. The narrow tread pattern, and the fact that most bias ply tires mount to narrow wheels makes for a sidewall bulge that is easily identifiable and adds to the look of vintage cars.
Firestone Bias Ply Tire
Bias ply tires are strong, but the rigidity offers some interesting handling characteristics if you are accustomed to driving on radial tires. The rigid construction means that bias ply tires do not conform to the road surface as efficiently as a radial tire. This creates a “wandering” sensation, as the tires tend to follow the ruts and breaks in the pavement. This wandering is not hazardous, but it requires the driver to pay more attention, especially at highway speeds.
The major advantage to a bias ply tire that completely overrides the finicky handling is the authenticity. Bias ply tires were used on American automobiles until the ‘70s, so if you are building a car from that era, a bias ply tire is an appropriate choice.
Radial Tires
Classic Redline Radial Tire
Radial tires are a popular upgrade for collector enthusiasts who plan to get out and drive their old car. The more modern construction offers great improvements, including ride quality, wet weather traction and tread life, so it is a more practical option for most folks. The advantages are created by the tire’s construction, which consists of ply cords that run 90 degrees in relation to the bead. This radial orientation of the cords allows for a more flexible design, which conforms to the road surface, and allows the tires to skim across the ruts and breaks in the surface. This makes for a pleasant ride quality, even on rough roads, which would cause the wandering sensation with bias ply tires.
In most cases, a radial tire will have a wider footprint, even if it is the equivalent size to a bias ply tire. For instance, a 6.70-15 tire (common for late ‘40s to mid-50s passenger cars) has the P-metric radial sizing equivalent to a 205/75R15 tire. However, the radial equivalent features a wider tread surface by more than one inch. This increased contact patch offers more traction and more stability, while the rounded shoulder provides additional traction in hard cornering situations. Radial tires also feature tread siping, which helps evacuate water from the tread surface, and thus, increase wet weather traction. The drawback to the wider footprint is the modern appearance with the rounded shoulders and wide tread profile
The Choice?
A radial tire allows the sidewall and the tread to function as two independent features of the tire. A bias tire consists of multiple rubber plies overlapping each other. The crown and sidewalls are interdependent. The overlapped plies form a thick layer that is less flexible and more sensitive to overheating.
In today’s world it often seems as though radial tires are the clear-cut choice for any classic car owner. While it’s true that a radial is generally a better choice for a classic car that spends most of its time on the pavement, there are still some strong advantages to a bias ply tire when it comes to an old car or old truck that spends quite a bit of time off the pavement. In many ways, bias ply tires are arguably a better choice for pure off-road use, especially when you are after the big, burly aggressive look. While there is no right or wrong answer to the radial vs. bias ply question, hopefully now there is a better understanding of where each type of tire performs better so that consumers can make more educated buying decisions.
There are now a lot of choices in the vintage car tire market for tires; so, the choice is yours as to the direction of the tire choice.
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