WHY SOME ALL-TERRAINS CUT AND CHIP OFF-ROAD
Nothing ruins a holiday quite like a tyre puncture, which is why most off-road travellers favour puncture resistance above all other tyre features. However, with the exception of sidewall damage and excessive tread wear (due to under inflation), a common cause of off-road tyre damage relates to cut-and-chip performance.
In most cases, this happens at high speed and on long gravel roads, where impact forces against loose or partly embedded rock cause cuts and chips to the tyre’s tread belt.
Unfortunately, cut- and chip damage cannot be repaired, and over time, it will greatly affect the traction performance of the tyre as well as its life-expectancy, which can be reduced by as much as 70%.
So, what makes one tyre more susceptible to cut and chip than another? Well, let’s look at that question from a different angle and ask: If we engineered a tyre specifically for extreme cut and chip resistance, what would it look like?
First off, the tread blocks would be as large as possible, for the simple fact that a large tread block will shoulder impact-forces far better than a smaller tread block will. Secondly, the tread-block design would follow an elemental shape, as opposed to a geometric one. And lastly, the tread blocks would have no siping.
An example of an industrial use off-road tyre with no sipe grooves and large, elemental shaped tread blocks.
As you may know, sipes are thin slits that are cut (or moulded) into a tyre’s rubber surface. The process was first invented in the early 1920s by John Sipe, who patented the idea after cutting slits into the soles of his shoes. However, it was only in the 1950s that the tyre industry adopted the process on a large scale.
Today, several tyre manufacturers are experimenting with various shapes, forms and types of siping; but no matter how the technology evolves, tyre sipes are widely accepted as a major contributor to the traction performance of a tyre on wet tar, snow and ice.
The problem, however, is that tyre sipes also produce weakened edges within the tread block, by creating potential cleave-off zones that are too small and unsupported to resist the impact forces and abrasiveness of a rocky trail or high-speed gravel road.
An example of cut & chip damage where bits of the tread block have broken off at their weakest point: the sipe groove.
So, if your sole intention were to engineer an extremely durable tyre – built for extreme cut-and-chip resistance – it’s quite likely that the tyre would have no sipe grooves at all.
Of course, such tyres do exist in the industrial and agricultural sectors; but, for passenger vehicles, 4x4s, pick-ups and SUVs, a variety of traction needs must be met. This is why you’ll still find siping in Cooper’s most extreme off-road tyre products, such as the Discoverer STT PRO and S/T MAXX. But, there’s a twist…
A feature that’s extensively used in Cooper Tire products is the inclusion of internal sipes. This means that the sipes are cut within the tread block, and do not cross the tread-block edge. This feature vastly improves the tyre’s resistance to cuts and chips by ensuring that the sipe slits do not weaken the tread-block edges, while still maintaining the benefits of traction performance in wet conditions.
The Cooper S/T MAXX features limited siping that is both internal and partly exiting of the tread block. Note how each tread block is generously sized to endure the impact forces of off-road terrain.
Some all-terrain tyres, however, are specifically designed for severe icy roads and wet-weather conditions. These tyres generally have a tread pattern, structure, and material-compound geared for that purpose, and in most cases, they showcase the 3 Peak Mountain & Snow (3PMS) symbol on their sidewall.
GOOD NEWS & BAD NEWS
Many of us are familiar with the M+S All-Season symbol found on various all-terrain tyres; but in the case of 3PMS tyres (such as the Cooper AT3 4S and various other tyre brands), these products are specifically designed for unusually cold, wet and icy conditions. In fact, in certain parts of North America and Europe, 3PMS tyres are often a legal requirement between the months of October and March.
Tyres that achieve an extreme weather rating will showcase the universally recognised 3 Peak Mountain & Snowflake symbol on their sidewall. In most cases, these tyres need more sipe grooves in order to achieve their 3PMS rating. The downside, of course, is that this feature exposes the tyre to cut and chip damage off-road.
Unfortunately, the qualities that make a tyre 3PMS rated are often the qualities that make a tyre susceptible to cut & chip when used extensively on gravel roads. Some of these features may include:
• high levels of tread-block siping, which produce vulnerable edges that are more likely to be cleaved off through contact with sharp gravel surfaces
• smaller tread blocks that are geometrically less stiff than larger tread blocks
• higher voids or openness in the tread pattern, which increases the tread block contact pressures with sharp gravel surfaces
• tread cap stocks that are biased towards low-temperature compliancy, rather than ultimate rupture strength
The new Cooper Discoverer AT3 4S is a great example. Thanks to the AT3 4S’s generous siping design, purpose-engineered tread compound, and sidewall-marked 3PMS rating, this tyre is specifically designed for all-season conditions and extreme wet-weather roads. However, should this tyre be used extensively off-road (where the Cooper A/T3 LT and XLT would be a better option), this misapplication will soon cause the tyre to break down in the form of cut & chip damage.
The new Cooper AT3 4S is a severe weather-rated tyre with a 3PMS rating. Note the tyre's generous siping when compared against the more off-road specific A/T3 LT and A/T3 XLT products.
FUTURE TREAD DESIGNS
Of course, tyre technology is constantly evolving: some day, we may very well get to enjoy the benefits of a tyre that is as capable in severe winter conditions as it is durable on gravel roads. But, until then, the qualities of a 3PMS tyre are generally not compatible with long-term use on rocky terrain or abrasive gravel tracks.
Next month, we’ll take a closer look at the Cooper S/T MAXX, and reveal why this tyre has become the preferred choice of safari rental companies, the mining sector, and 4x4 tour guides. Until then, be sure to check the sidewall of your tyre for the M+S or 3PMS symbols, and ask yourself: Is this the right tyre for the job?
The TyreLife Team