Radial Vs Bias Ply Tyres: Differences Explained

Bias-ply tyres were the only option people had for many years until the 1970s. They were perfect in their own way but still suffered from some problems.

Bias-ply tyres don't shed heat quickly which was a big drawback as vehicles became faster. You see even though the technology and machinery was evolving to permit higher speeds, the tyres were just not able to keep up. The machinery was capable, but the tyres of the day made it impossible for the technology to be implemented.

Another problem they were infamous for were their stiff sidewalls. This was due to the fact that they were constructed in such a way that all the plies in the tyre ran from sidewall to sidewall.

Fast forward to the 1980s and Radial tyres were brought into the automotive world to correct some of the unwanted characteristics of bias-ply tyres. For starters, their steel belts dissipated heat much better and secondly, since they had fewer layers of body cords on the sidewall, the sidewalls would allow the tyre to flex more. This meant that the contact patch of the tyre would stay put on the ground a bit better, and the sidewall would provide a lot more feedback. This design also allowed the sidewall and tread to work more independently of each other, a significant difference from what a bias-ply permits.

CONSTRUCTION

Radial Tyres:

Radial tyre construction

Radial tyres are composed of steel belts that run across the tyre radially i.e transversely from bead to bead (red coloured wires in the picture). On top of these thin steel layers, there are alternating layers of body cords that cross each other, which are present only on the tread portion of the tyre. This keeps the sidewall area relatively thin which allows it to function independently of the tread.

In radial tyres, the steel belts run perpendicular (90-degree angle) to the direction of travel. Also, since the construction allows the sidewall and the tread of the tyre to function independently and because the steel belts run radially, there is low sidewall flex and the flex does not get transferred to the tread area. This helps the tyres maintain grip at all times even when if the sidewall is flexing.

Bias Ply Tyres:

Bias ply tyre construction

In bias ply tyres, the alternating nylon belts run at a 30 to 45-degree angle with the central tread line. A major difference here is that these numerous nylon belts overlap rather than cross each other and run all along the sidewall and tread. Thus, there is no segregation of layers between the tread and the sidewall which gives it a stiff internal construction. This makes the sidewall much less flexible.

Also, since the tread and the sidewall are interdependent, it causes the sidewall flex to interfere directly with the functioning of the tread area and the footprint. It also causes these tyres to heat up faster as compared to their radial counterparts.

Radial vs. Bias Ply Tyres

Pros of Radial Tyres:

  • Stay cool even at high speeds and high load
  • Steel belts provide much better protection against punctures while making the tyres more rugged
  • Uniform contact with the ground provides uniform tread wear
  • Provide a much wider footprint that keeps the vehicle stable

Cons of Radial Tyres:

  • Steel belt construction reduces ride quality
  • The sidewall is more prone to damage
  • Produce more noise due to harder tread area

Pros of Bias Ply Tyres:

  • Tougher sidewalls due to the crosshatch construction
  • Smoother functioning on rough surfaces as compared to radial tyres
  • Less expensive than radial tyres

Cons of Bias Ply Tyres:

  • Faster treadwear due to higher traction levels
  • High rolling resistance which reduces fuel economy
  • Roll-over effect causes loss of traction while cornering

Did you know that the steel wires in radial tyres become magnetized with use, so as they rotate, an alternating magnetic field is created. It is quite measurable with an EMF meter close to the wheel well when the wheel is rotating.

Also See: What Is A Pneumatic Tyre?

Why do some vehicles still use bias-ply tyres?

Auto enthusiast folks are usually interested in performance, or in other words -- SPEEED! They need tyres that are able to tolerate extremely high levels of stress and at the same time, provide adequate feedback and confidence to the rider/driver.

Touring and off-roading junkies, on the other hand, are typically interested in other features like the load-carrying capacity and longevity of the tyres. They are essentially looking for a dependable companion for long tours that can carry heavy weights and tolerate shocks.

Luckily, there's a method of tyre construction that shines for each! Bias-ply tyres' are extremely good for tourers and off-roaders due to their stiff sidewalls. They perform admirably under heavy loads and their inherent lack of flex also means that the sidewall won't "wash out" in a turn as easily as a radial.

Bias-ply tyres are also the only tube-type tyre available. Many motorcycle riders of today are still fans of the "classic look" of spoked wheels. In off-road situations, spoked wheels are also usually a little more sturdy as compared to cast wheels.

Radials do one up bias tyres when it comes to their ability to have a substantially larger tread to sidewall ratio. Usually, the width of the tread and the height of the sidewall have similar measurements in bias-ply tyres, however, radial construction allows these two dimensions to be substantially different. What does that mean? It means that something like an extra-wide tyre can be made without a crazy-tall-looking sidewall.

Radial construction has also revolutionized modern sportbikes. With a fairly wide tread, but a very short sidewall, radial tyres are capable of giving excellent feedback to the riders/drivers. They run fairly cool and don't flex very much. This lack of sidewall flex, despite the thin sidewall, allows the tyre to be strong and rigid, so that it feels secure and planted under cornering forces.

Image Credits: agricultural.michelinman.com

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