Understanding the difference between radial ply and cross ply tyres, and their relative strengths and weaknesses, is important when it comes to deciding between the two. Being able to identify the advantages and disadvantages of each will not only inform us on our next tyre purchase should that be our end goal, but also build on our general understanding of automobiles.
Construction
The significant tyre construction difference between the two tyre ply types is the way that the ply cords are arranged. In cross ply tyre construction, the cords are arranged at a 45-degree angle to the centre line of the wheel and crisscrossed. In a radial ply tyre, the cords are arranged at a 90-degree angle to the centre line of the wheel and overlap.
Cross ply tyres have stronger and less flexible sidewalls as a result of this construction difference, which is advantageous in protecting the inner tube that is essential in cross ply construction. As radial tyre construction does not require an inner tube, the flexibility of the sidewall is advantageous in terms of absorbing vibrations and generating less heat.
Advantages and disadvantages
The advantages of radial tyres are significant. They offer improved ride comfort and handling, greater fuel efficiency, are significantly safer than cross ply tyres, and by the 1970s, had become standard on all road cars in the UK and US.
The disadvantages of radial tyres are the vulnerability of the flexible tyre sidewall to damage when overloaded or underinflated, or to impact damage, as well as the increased cost (though radial tyres will last longer before needing to be replaced). Cross ply tyres are now most commonly used for off-road vehicles, construction/agricultural vehicles and classic cars.
Fuel efficiency
A report from the late 1970s by the U.S. Environmental Protection Agency found that on average “radial tyres improved vehicle fuel economy by 5%”. Though it also noted that “for a typical vehicle owner travelling about 10,000 miles or more per year on tyres underinflated by 3 to 4 PSI, the annual fuel savings that would result if this underinflation was corrected is over 45 litres per year”. Thus, when driving on radial tyres, we can be sure we are using the more fuel-efficient tyre type of the two, however, if we don’t keep our tyres properly inflated, we will lose a lot of that advantage.
Ride comfort and handling
The flexibility of the tyre sidewall in radial tyres means greater absorption of shocks, bumps and impact than cross ply tyres. Radial ply tyres are more stable and their design ensures the maximum surface contact with the road.
The internal structure of the first radial tyres was the so-called “fly trap”. The impact of radial tyres on driver comfort is so pronounced that the majority of tractors now come fitted with them, despite the increased tyre strength and rigidity of a cross ply design. For farmers driving long distances in tractors, the reduced vibrations and increased traction are huge differentials.
Safety
It can be incredibly dangerous if we run tyres which have lost a significant amount of their tread. The rigid nature of cross ply tyres means that heat dissipates less quickly and the tyre will degrade more faster than a radial tyre, increasing the likelihood of an accident due to loss of traction. Cross ply tyres also have a lower speed rating than radial tyres. As a result, when driving on cross ply tyres at high speeds, we are more likely to have a blow out than if we were driving on radial tyres.
There are additional risks associated with mixing the tyre types on the same vehicle. For instance, it is illegal to mix tyres of a different construction, such as cross ply and radial ply tyres, on the same axle. It’s uncommon for cross ply or bias belted tyres to be used in the UK, and radial ply tyres are the standard fitting for vehicles sold. The mixing of ply types on the same vehicle is to be avoided at all costs from a safety perspective.
In a situation where using a mix of tyres is unavoidable, it is essential that radial ply tyres be fitted to the rear axle, and cross ply tyres fitted to the front axle. This is due to the respective effects that the two tyre mixes will have on the steering of the vehicle. Fitting radial ply to the rear axle and cross ply to the front axle will produce a tendency to understeer, i.e. the vehicle will turn less tightly into a corner than it is steered. When we fit the tyre plies the other way around, an oversteer effect is created, i.e. the car turns at a tighter angle than it is steered. Understeer is generally considered to be preferable to oversteer due to the ease with which it can be controlled.