Bias Ply Versus Radial Tires
For Your Antique Car,
Traditional Rod Or Street Rod
One decision every hotrod owner will eventually have to face is whether to attach period correct bias ply tires or
the modern radial tire to his or her antique car.
Radial tires were first introduced as original equipment in 1973, making bias ply tires a simple answer for the
traditional rod. Many traditional rod owners would not consider riding on radial tires.
A street rod is not so constrained by convention. Many street rod owners do choose to stay period correct
by installing bias ply tires. Others prefer the modern technology of radial tires.
Let’s take a look at the difference between radial tires and bias ply.
Basic Tire Construction & History
The carcass of a tire is made up of layers of rubber-permeated fabric. Within the rubber base are a series of
plies of cord (most commonly a polyester fabric, sometimes steel or other textiles) that acts as reinforcement to
stiffen and strengthen the tire. The method and direction these plies are applied differentiates between a bias ply
and a radial tire.
Bias Ply Tires
The plies on a cross ply or bias ply tire run with the cords at an angle of about +60 and -60 degrees from the
direction of travel. These plies criss-cross, alternating in direction with each layer at angles to each
other. (The degree of the bias actually varied somewhat between manufactures.)
The fabric was built up on a flat steel drum. The plies were turned up around the steel wire beads and the
combined tread/sidewall applied. The uncured tire was loaded over a curing bladder and shaped into the mould.
This shaping process caused the cords in the tire to assume an S shape from bead to bead. The angle under the
tread stretched down to about 36 degrees. In the sidewall region the angle was 45 degrees and in the bead it
remained at 60 degrees. The low angle gave rigidity to support the tread and the high sidewall angle gave comfort
to the ride.
Radial Tires
Michelin first introduced steel-belted radial tires in Europe in 1948. Radial tires are so named because the ply
cords radiate at a 90 degree angle from the bead cord to the direction of travel, across the tire from lip to
lip.
By preventing the plies from rubbing against each other as the tire flexes, the radial tire allows the side
walls of the tire to be more flexible which provides less rolling resistance, better gas mileage, and longer tread
life.
This flexibility also promotes better adherence to the road, so the tire handles better on both wet and dry
surfaces. The slightly bulging appearance of radial tire sidewalls compared to bias ply tires is also a feature of
this flexibility.
To add further strength to low radial tires, the entire tire is surrounded by additional belts, oriented along
the direction of travel. Originally these belts were made of steel (“steel-belted radials”) but today are often
made from a class of strong, heat resistant, synthetic fibers from the Aramid class, such as Twaron and Kevlar.
Low radial tires separate the tire carcass into two separate
systems:
- The radial cords in the sidewall allow it to act like a spring, giving flexibility and
ride comfort.
- The rigid steel belts reinforce the tread region, giving high mileage and
performance.
Radial Tires and Bias Ply Tires Ride Differently
Motorists were not accustomed to the different springiness degree of slip while steering, so Ford Motor
Company began modifying suspension systems in the 1960s. Less tight steering, and added both isolators to the drive
shaft and bushings to the suspension to absorb the thump of riding over asphalt expansion joints in a concrete
roadway.
With the smoother ride, cars could now be made lighter, and the race for lightweight, fuel-efficient vehicles
was on. And off. And on again …
Appearance of Tires and Wheel Wells
There was also a change in the aspect ratio (relationship of the height and width of the tire cavity) between
the older bias ply and modern style radial ply tires. So they look different.
We think of bias ply tires as tall and skinny, while radial tires are more short and wide. This may have
been a drawback to traditionalist rods a few years ago.
Today specialty tire manufactures use the radial design to make everything from the old bias ply Redline Tires
to Wide Whitewall Tires. Weight, size, and contact area have been taken into consideration when developing
these radial tires for vehicles that were originally equipped with bias ply tires.
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