About when did it become permissible to criss cross radial tires when rotating? I remember the early BF Goodrich Radial TA's could only be rotated front to back, on the same side.
The origin of
NOT cross rotating tires goes back to the days of the Firestone 500 - the late 1970's, when steel belted radial tires were first introduced. There was a rubber chemistry problem. The chemical that was used to promote adhesion (
HMT - hexamethylenetetramine) of the rubber to the copper in the brass coated steel wires, caused an acid in the presence of water, which corroded the steel. Water vapor can penetrate through rubber, so figuring this out was difficult, and Firestone was the last to do so - and they were stubborn about admitting they had a problem.
One of the things that was discovered during investigations was that the first detachments of the steel wire from the rubber occurred on one side of the wire - and to slow down the spread of these separations into a more generalized separation, it was recommended that steel belted tires
NOT be cross rotated. I was not a fan of this approach.
Once all the tire manufacturers figured out the chemistry and changed it, the problem disappeared and there wasn't any need to recommend against cross rotating tires.
HOWEVER, these kind of things linger on - and even though it's been over 50 years, the recommendation is still remembered. If you'll notice, the USTMA (US Tire Manufacturers Association) says NOTHING about not cross rotating tires.
