How many cars today come equipped with bias tires?
The answer is 'None'.
Then why are the car manufactures still publishing the recommended tire rotation for bias tires in their owner handbooks.
Also, why hasn't the tire industry or the service industry attempted to correct them?
They don't know?
They don't care?
They're afraid to?
Whatever the reason, we on the front line of the service industry are stuck trying to service the customer and trying to answer simple questions we have no answers for. Thus, making us appear ignorant about a simple service like a tire rotation.
You would think the tire manufacturers would be the ones with the final say on this matter but look at the different rotational patterns published as being acceptable throughout the industry.
Are they saying 'any' rotation pattern is acceptable?
Looking at the patterns above, it is obvious that answer is 'Yes'.
There's something seriously wrong in Denmark.
Here's my solution:
If the vehicle has Bias Ply tires, you can select any pattern you need to to counteract any unusual tire wear because of the biased ply construction. Since the cord is working agaist itsef, it is resistant against deflection, making it a good choice for slow moving, off road vehicles where the concern is focused on sidewall strength and resistance to punctures. This construction also maintains its original shape throughout the life of the tread, so changing direction in a rotation is not an issue.
Radial tires, on the other hand, are designed to conform to variations of the road surface because of the flexibility of the sidewall. This also allows the tread to stay flat on the road during cornering, give way to bumps and potholes and reduces rolling resistance which also reduces internal temperature from friction and extends tire life.
This radial ply construction, although nearly perfect as a road tire, does have an interesting character; it changes shape during wear-in. Most would question this comment but I have proven it.
I had noticed that after rotating tires in a pattern that changed the rolling direction of the tires, some models returned with tire noise and/or instability on the road and the cure was to return the tire back to its original direction of travel.
I suspected it was due to the radial construction but I needed some proof.
Mounting 4 new 15 inch Michelins on a customer's car, I marked the rims, tire beads and tread shoulders with spots of paint perpendicular to the center line and would monitor these tires under normal driving conditions over the next 8 months.
It only took 8 weeks to see the change I was half expecting. The spot on the tread shoulders move ahead of the center line just under 1/8 inch. This meant the tread face had rotated in relation to the bead making these tires now directional, in nature.
This was the reason why rotating the tires that changed direction of travel created problems.
Here's what the rotation pattern for most radial tires should be;
In addition, if you have a used, loose tire that you are not sure where it should be mounted, you can feel the wear pattern on the tread. Move your hand forward and back along the tread surface. You will feel the sharper edges in one direction. Since the leading edge of the block of tread comes in contact with the road surface first, it wears the edge a little rounded. The trailing edge of the tread block will have a sharper edge leaving the clue as to what direction the tire was used to turning.
Now, when it comes to heavily constructed truck tires, low profile and specialty tires with reinforced sidewalls, I doubt that this 'directional break-in effect' would be as drastic as an average tire, but I will stay with this rotation pattern whenever possible because I don't want to be the cause of a problem.
Here's something else I've noticed that is unique to radial tires; During the 'Break-in' it is critical that the wheel alignment, tire pressures and balance be accurate because these factures will determine the end pattern of the 'directional break-in effect'. If the pattern ends up poor, there is no reversing it and the life expectancy of the tire will be shortened.
Pass this information on to every tech and tire specialist you know and especially your customers who really count on you to know precisely how to take care of their vehicle.