Originally Posted By: xtell
I've been only rotating my tires front to back because in the late 1970's I did cross rotations and a couple of times the steel belts came apart. Are the tires that are made now a days (aside from the tires that have a one way tread pattern) able to be mounted to roll in the opposite direction without the issues of tread separation like we had in the past? This is not a joking post, I'm serious. I have not cross rotated tires on my personal vehicles or work trucks since the 70's.
Back in the 1970's - the early days of steel belted radial tires, there was a chemical - hold on, I'll look it up - HMT (hexamethylenetetramine) - that was commonly used as an bonding agent in tire rubber. Unfortunately, it produced amines which caused corrosion in the presence of water.
Also, it was discovered that the separations caused by this corrosion first appeared on one side of the wire and later grew to the other side of the wire. This lead folks to recommend not changing the direction of rotation of the tire - and rotating the tires on the same side of the car accomplishes that.
I am of the opinion that that recommendation was a way to shift the blame off the tire manufacturer - that the separation, once started, was going to grow around the wire anyway and that the torque on the tire was why the separation appeared on one side first - AND - it's only that the torque due to forward motion is much more frequent than the torque due to braking.
Oh and HMT was replaced by HMMM (hexamethoxymethylmelamine) and the corrosion problem went away - and so did the reason for the same side rotation recommendation.
Please note: I am not a chemist and what I wrote above about the chemicals is based on a conversation I had with a chemist who was involved in the 1970's in analyzing what was going on. I hope I got it right.
- BUT -
The separation mechanism for current tires (and tires from the Ford/Firestone situation 17 years ago) is different. Those separations start just off the wire, in the rubber matrix, on the underside edge of the top belt. It grows around the edge, but more importantly, between the 2 belts, until it reaches the point where the centrifugal forces exceed the adhesion - and the top belt detaches from the bottom belt. A cap ply not only reduces the amount of stress, but also acts as a restraint against the centrifugal force. A tire with a cap ply will still have a separation, but it detaches much, much, much further down the road.
This is a long way of saying that ALL tires currently made - and that includes directional and asymmetrical tires - can be rotated in any pattern without fear of compromising the structural integrity of the tire. In fact, no tire manufacturer I know of currently recommends same side rotation (as opposed to cross rotation).