You could mix cross-ply (called bias-ply in the US) and radial tyres, but the steering may feel odd and the car would become unsafe at high speeds, especially around corners. This is because the tyres are designed to handle road stresses in different ways.
All tyres are reinforced with cord, made of nylon or another textile. A cross-ply tyre has its layers of cord laid at an angle, or bias, to the centre line of the tyre, creating a herring-bone pattern beneath the tread. When the car goes around a corner, the whole tyre, including the tread surface, leans outwards. This lessens the contact pressure with the road on the inside edge of the tyre, reducing traction.
A radial tyre's cord layers lie at right angles to the tyre centre line. Look at the wheel from the side and the cords run radially from its centre, which is how the tyre gets its name. The tyre also has belts, usually of steel, beneath the tread. In a corner, the radial's tread tends to stay flat on the road, while the body of the tyre flexes towards the outside of the bend. There is less lifting of the tread from the road, giving better traction.
When a car with all radials or all cross-ply tyres loses traction in a corner, all four tyres theoretically break loose at the same time, allowing you to control the slide with steering to prevent spinning. This is what racing-car drivers are doing when you see them sliding around a corner.
With radials on the front and cross-ply on the back, the back wheels will lose traction first, causing the rear of the car to spin and resulting in loss of control. With the tyre mounting reversed, the situation is worse: the front will break loose first, causing immediate loss of steering control. The car would veer off to the outside of the corner.
Rick Dieckmann, Fort Jones, California, US
