Sand, rock, gravel, grass and mud are just a few examples of how varied off-road terrain can be – not to mention that many of these surfaces vary in depth and consistency.
Given this nearly infinite range of terrain possibilities, it’s no wonder that tyre manufacturers don’t quote off-road traction figures. In fact, the only recognised tyre-traction test is one that’s conducted on asphalt (tar), and even then, the asphalt has to comply with a globally recognised (industry) standard.
Considering the above, the subject of traction is the most controversial of all tyre-design topics, particularly off-road where many terrain types are loose in terms of traction, as opposed to firm, like asphalt. In this way, off-road tyres need to possess design features that address the added problem of loose, rutted, and abrasive terrain.
So, what are these design features? Well, the answer lies in what we call TCC: Tread, Carcass and Compound.
Without wanting to over-simplify the subject: the looser the terrain, the more aggressive the tread pattern needs to be. But what does aggressive mean? In general terms, an aggressive tread pattern is one that features a large percentage of open space (voids) between the tread blocks, as well as deeper tread grooves.
Although deeper tread is often associated with on-road mileage and value, (which is true), in the off-road sense, a deeper tread pattern will also increase a tyre’s traction, puncture-resistance and durability.
Siping also plays a fundamental role in how well a tyre performs off-road. A common misconception is that the role of siping is restricted to wet-tar performance, when in fact it also plays a key role in the flexibility of the tread over rocky terrain.
However, the drawback of siping is that each sipe-groove acts as a potential breaking point and tread-block weakness. Put another way: Cut and Chip damage typically occurs where the sipe groove exits the tread block.
Fortunately, sipe technology is evolving. In some cases, rather than exiting the side of the tread block, modern sipes are often cut internally, within the tread block. This radically improves Cut and Chip resistance off-road.
Another major breakthrough in sipe technology is the introduction of micro-gauge siping, which is really just a fancy way of saying ‘thinner sipe grooves’. However, the benefit of a thinner-gauge sipe is that the groove can now be cut much deeper into the tread block, and thus provide better grip throughout the tyre’s life – not just for the first few thousand kilometers.
The Cooper AT3 LT is a great example of a tyre with deeper treads, as well as micro-gauge sipes that are both internal, and exiting of the tread block. All of these features vastly improve the tyre’s off-road traction, longevity and cut and chip resistance.
The additional benefit of a micro-gauge sipe is that the tyre is far less likely to pick up stones that then become trapped within the groove, leading to potential stone drilling and punctures.
Another major advancement in off-road tread design can be found on the sidewalls of certain tyres, where a purposely-engineered flex groove is moulded into the tyre’s shoulder. This groove helps to dampen impacts on uneven terrain, but more importantly, it dramatically improves sidewall flexibility at low pressures, which means more grip and better off-road safety.
However, the subject of traction isn’t solely dependent on tread block shape and size, which brings us to…
Much like the foundation of your house, the carcass of a tyre accounts for up to 70% of the tyre’s strength. It’s also where much of the force and friction (acting against the tyre) is absorbed.
Load-carrying capacity, pressure performance, heat tolerance and puncture resistance are all features directly linked to the construction of a tyre’s carcass.
But the list doesn’t stop there; the rigidity of your tyre’s carcass will also play a role in your 4x4’s steering response, particularly when it comes to large-size vehicles and SUVs that need to perform an emergency manoeuvre or evasive steering action. A strong carcass will reduce fishtailing and body roll, while allowing the vehicle to quickly return to centre steering.
To sum up, buying an off-road tyre with a weak carcass is like building your house on sand.
Rubber is rubber, right? Well… no.
Most of us accept that plastic composites come in a variety of densities, strength and flexibility, and the same applies to rubber. It all comes down to what mix (or ‘recipe’) is used.
Various additives play various roles in how well a tyre’s compound reacts to the demands of off-road terrain. The problem with many of these additives is that they’re either costly to buy, or the process of getting them to mix evenly into the rubber is costly. Ultimately, it’s a game of economics and market positioning: some tyre brands aren’t out to create the world’s toughest off-road tyre, and therefore position themselves as a budget alternative, made from budget ingredients.
A great example of a costly additive is the addition of Coupled Silica. Although silica has largely revolutionised the performance of off-road tyres, very few manufacturers choose to use the additive because of its high cost, and the difficulty of getting it to chemically infuse so that it’s evenly distributed within the rubber compound.
So, what’s so great about silica? Well, most low-quality rubber compounds have high rebound characteristics. If you think of a solid rubber ball being dropped from head height, the ball will bounce back almost as high – let’s say to 80% of the original height. If you infuse the same ball with silica, the rebound rate could drop to more than half that.
In terms of performance, this means: more contact with the road, less bounce, less friction, more grip, and more mileage.
Large tread blocks, internals sipes and a durable compound – three features that contribute to the S/T MAXX's reputation for off-road longevity.
Next month, we’ll take an in-depth look at carcass construction, as well as the subject of load index and why it’s more important for 4x4 owners than they may realise.
Until then, feel free to send your tyre-related queries to firstname.lastname@example.org or visit www.tyrelife.co.za for more expert tyre advice.
The TyreLife Team
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