How Do Little Grooves And Ribs Reduce Irregular Wear?
Why does irregular wear so often start on rib edges?
Irregular wear is caused by irregular abrasion, and irregular abrasion is caused by uneven pressure against the road surface in different areas of the tread surface.
So the pressure on rib edges is uneven?
It can be. And once you understand why uneven pressure can cause irregular wear, you'll see that many different kinds of irregular wear result from this single cause.
What causes uneven road surface pressure?
Let's start with a very simple example. Imagine a dual assembly, but that one tyre has a bigger diameter than the other. Clearly, the smaller tyre doesn't press against the road surface as hard.
Does that mean it wears slower?
Quite the contrary. The smaller tyre, the one not pressing against the road as hard, actually wears faster often much faster - than the larger tyre.
Why is that?
Because the smaller tyre has a smaller circumference, it should take more revolutions to cover the same distance as the larger tyre. But because both tyres are bolted together, they have to make an identical number of revolutions to cover a given stretch of road.
The only way the smaller tyre can do that is to slip and skid as it is dragged along by the larger tyre. The big tyre wears out the smaller tyre, while the smaller tyre acts as a brake on the larger tyre. This is one reason dual matching is so important.
Two tyres with different diameters cannot cover the same distance in the same number of revolutions, unless they're bolted together.In that case, the smaller tyre is dragged along by the larger one, greatly accelerating the wear on the smaller tyre.
What does this have to do with a single steer tyre?
A rib tyre is like several tyres bolted together. If one or more ribs are smaller than the others, they may wear much more rapidly than the larger ones, for the same reason that the smaller tyre in the dual assembly did. The result can be a wear pattern called rib punch wear, sometimes seen on multi-rib tyres.
A multi-rib tyre can behave much like several tyres bolted together. Smaller ribs can wear very rapidly, resulting in a condition called "rib punch wear."
How does this relate to rib and shoulder edge wear?
What can happen to a multi-tyre assembly or multi-rib tyre can happen to a single rib too. Just as a tyre or rib may not press as hard against the road surface as another, different parts of ribs may not press against the road surface as hard as other parts.
The closer you are to the edge of a rib, or to the shoulder of a tyre, the less pressure there tends to be. That can result in more rapid wear at those edges. The pressure differences may be tiny, but over tens of thousands of kilometres, tiny differences in abrasion can cause shoulder edge wear or river or wavy wear.
Is this where those little ribs and grooves come in?
Yes. One of the first irregular wear-reducing designs is what is called a Defense GrooveTM feature. (As seen on the Firestone HP 3000)
This groove creates a narrow rib along the shoulder of the tyre. The pressure is less here, and the rib wears more rapidly than the shoulder rib beside it. This increases the road surface pressure at the edge of the shoulder rib, helping to equalize pressure across the entire rib - just what we need to promote even wear.
And, as the tyre wears, the defense rib continues to wear more rapidly than the shoulder rib, keeping the pressure up on the shoulder rib edge for continued irregular wear protection. In a sense, it sacrifices itself to protect the adjacent rib.
The Equalizer Rib TM design on the Bridgestone R227 looks similar. Does it do the same job?
The DefenseGrooveTM feature on some Bridgestone and Firestone steer radials creates a small rib that wears more rapidly than the shoulder rib beside it, helping to equalize pressure on the shoulder rib and thereby prevent shoulder edge wear.
The Equalizer Rib feature of the Bridgestone R227works much like Defense Groove designs, but helps protect the main ribs rather than the shoulder ribs
What about the Side Groove concept?
This is a new concept, developed by Bridgestone Japan. The shoulder ribs of this R227feature Bridgestone's new Side GrooveTM design. This groove is in the side of the shoulder, instead of in the tread itself. It helps equalize pressure over the surface of the shoulder rib, but in a different way. Because it can open and close in response to changing shoulder edge forces, you might say it acts almost like a shock absorber or load leveller for the shoulder edge.
The Side GrooveTM design of the Bridgestone R227equalizes shoulder rib road surface pressure under a wide variety of condition5, to help prevent shoulder edge wear. Its position on the shoulder also prevents it from picking up and retaining stone.
Is this better?
For some applications. Because the Side Groove structure is positioned away from the road surface, it doesnt pick up and hold small stones - which can cause irregular wear to start. And, it is far less likely to be torn or damaged during turns.
Because of its ability to open and close, the Side Groove design does an excellent job of equalizing shoulder rib road surface pressure, even under some fairly severe side forces.
Will all new tyres incorporate this design?
No. As we've said before, no single design is best for every application. Very long distances line haul operators will find that either a Defence Groove or Equalizer Rib design will probably provide excellent irregular wear resistance.
Haulers who make some shorter runs may get better results with th Equalizer Rib desing, because it copes so well with turning and side forces. And, operators who make mostly short hauls may find that neither feature is particularly helpful.