so yes the two types should never be mixed in any way on an axle or on a car so if the spare is an old cross play and teh car has radial tyres the spare should be discarded. the same is true if it is an old car where it is fitted with cross plies and a radial is the spare the spare should be thrown away.
I had a friend who called me with handling problems on his Buick Wildcat. Not much of a car guy, he had a radial and a bias ply mix on each end of the car....... at a mere 20 MPH the steering wheel was rocking back and forth so violently you couldn't control the car's direction. I put the radials on the front, and moved the bias ply tires to the rear, and it drive fine......But that first configuration was absolutely crazy!
Been there, done that!
It's the sidewalls of the tyre. Radials flex a lot more than crossply
Different handling characteristics and grip levels.
A radial has soft, squishy sidewalls (which is why they can sometimes look like they're flat, even when they're not) and a very stable tread, which stays flat to the road, a crossply has very stiff sidewalls which can 'roll' the tyre tread across the road and give less grip.
It's more of a theoretical problem than a real one nowadays, you'd be hard pushed to find a car made in the last 30 years to which crossplys can be fitted. They're pretty much the preserve of the old classic car enthusiast who wants to keep their car original.
Sidewall flex on a radial tyre is not transmitted to the tread. The footprint only lengthens. All sidewall flex is transmitted to the tread on a bias ply tyre. The footprint deforms and there is an increase in tyre slip. These two technologies should never be placed on the same axle.
I've never understood why mixing cross ply and radial tyres on the same axle can cause a car to crash or affect the safe handling of a car.
Is it because the treads are different or some other reason?
