TYRE CONSTRUCTION – LOAD INDEX, PUNCTURE RESISTANCE & STEERING CONTROL
TYRE CONSTRUCTION: THE NUMBER ONE TYRE FEATURE NO-ONE TALKS ABOUT
Puncture resistance, payload capacity, heat tolerance, body roll, steering response and pressure performance... are just some of the features directly linked to the construction of a tyre’s carcass.
Yet, despite the obvious relevance of these features off-road, you’ll seldom hear any mention of carcass construction in a campfire debate or off-road tyre test.
"You’ll seldom hear any mention of carcass construction
in a campfire debate or off-road tyre test"
So, what gives? Why are so many people (including tyre dealers and 4x4 magazines) ignoring the one thing that accounts for up to 70% of a tyre’s strength and performance?
Answer: Because it’s a difficult thing to gauge.
The carcass of a tyre is the foundation on which the rest of the tyre’s design is built; and much like when buying a house, there’s not much you can do visually to determine if the foundation is solid.
Similarly, a tyre’s carcass is a difficult thing to gauge without cutting the tyre in half. But before you cast the subject aside as an “unfortunate unknown”, the truth is that there is an effective way of gauging a tyre’s carcass strength. So effective, in fact, that the numbers literally speak for themselves.
A quick glimpse at the sidewall of any all-terrain and mud-terrain will reveal a load-rating figure between 1 100kg and 1 700kg.
Typically, most OE all-terrains and factory-fitted SUV / Pick-Up tyres will hover around the 1 100kg mark. In contrast, most aftermarket (Light Truck) brands, such as Cooper, will vary between 1 300kg and 1 700kg per tyre. So, if a high Load Rating is so important, why aren’t vehicle manufacturers fitting stronger tyres to their 4x4 models?
You can determine your tyre’s load capacity by looking at the small print on the sidewall. This particualr tyre (a Cooper STT PRO 285/75/16) has a load capacity of 1 700kg!
It’s a good question, and the answer is quite simple: economics. Most vehicle manufacturers are doing their utmost to minimise costs while remaining competitive in price, fuel-economy and comfort. That said, vehicle manufacturers generally source a tyre for the following qualities:
- They must be low cost
- They must be lightweight for better fuel economy, and
- They must be relatively pliant for better on-road comfort.
However, this raises another question: If four OE tyres can handle 4 400 kg in total, isn’t that more than enough? The Gross Vehicle Mass of a typical double-cab is about 3 000 kg.
That may be true in a numbers game, but do you really want to push your tyres to the limit in a remote, off-road environment that has a high concentration of hazards and obstacles? Tyre-failure is far more likely to happen on a steep, rocky track when the weight of your vehicle is predominately on the front tyres, and you’re forced to make static steering inputs over sharp rocks.
Many of us also add heavy accessories to our vehicles with auxiliary fuel tanks, water tanks, aftermarket bumpers, a winch, an extra spare wheel – and of course, we may also attach an off-road trailer or caravan.
Generally speaking, a Light Truck tyre boasts a far stronger construction than a Passenger or SUV alternative. Look for the letters ‘LT’ when purchasing a tyre for heavy off-road use.
The load-capacity of a tyre is directly linked to its air-pressure rating. A tyre with a Load Rating of 1 100kg will most likely have a maximum air pressure rating of 3 bar. By way of contrast, a Cooper S/T MAXX with a Load Rating of 1 400 kg will safely hold 5 bar. That’s roughly 30% more strength than an OE tyre.
But the benefits of a high Load Rating don’t end there…
DEFLATION, HEAT AND PUNCTURE RESISTANCE
Although many tyre manufacturers object to the practice of tyre deflation, there’s no contesting the benefits of deflating one’s tyres off-road.
A deflated tyre will:
- Vastly improve off-road traction
- Dramatically increase off-road comfort
- Improve puncture-resistance
- Reduce stresses transferred to your shock absorbers, and
- Greatly reduce cut and chip damage off-road
But how much should one deflate off-road? Sadly, there’s no one-size-fits-all answer to that question, but the short answer is: As much as you can without compromising the structural integrity of the tyre.
So, the ability to deflate a tyre off-road is in direct proportion to the risk of deflating in the first place, i.e. heat damage.
The more you deflate a tyre, the greater your chance of generating dangerous heat levels within the sidewall, and thus the greater the risk of causing total tyre failure. This is particularly true in very hot climates where road temperatures can reach 60ºC plus. Of course, the amount of weight your vehicle is carrying, and the speed at which you drive, will also determine how much you can deflate before heat becomes a problem.
Cutaway view: Budget tyres typically use budget steel cords, rather than high-tensile steel. This feature also determines the tyre’s carcass strength and load-bearing capacity.
Needless to say, a tyre with a strong carcass and high Load Rating will radically outperform a weaker tyre in terms of staying cool, and allowing more deflation without generating as much heat. That means more traction, more comfort off-road, more shock-protection, fewer punctures, and less cut- and chip damage from a tyre that’s better constructed to endure the stresses of off-road deflation.
Next month, we’ll look more closely at cut and chip damage – what causes it, how to prevent it, and why some tyres are more prone to it than others.
Until then, you can send your tyre-related queries to firstname.lastname@example.org or for more expert tyre advice visit
The TyreLife Team