Part six of an extract from All-Terrain Tactics, by Glyn Demmer.
Today we take them for granted, but in 1844, when Charles Goodyear is said to have developed vulcanised rubber that later became the core ingredient of automobile tyres, it was big news.
Until then, horse carriages and bicycles rode on uncomfortable wooden wheels – so the pneumatic, air-filled tyres that John Dunlop initially developed for bicycles in 1895 were a blessing. No more sore back-sides, and no more dentures falling out on a cobbled road!
So here’s the lowdown on those vital pieces of rubber that do service on our vehicles.
In a radial-ply tyre the body cords run in a radial direction across the tyre’s circumference, from bead to bead. Cords are made of nylon, polyester or rayon for steel-belted tyres. Breaker belts are made of steel cords and serve to brace and stabilise the tread in the road contact area. Radial cords, running as they do from bead to bead, reduce inner deformation in the shoulder and side-walls.
A typical radial-ply tyre’s construction
The tyre’s side-wall markings Nominal section width (mm)
Following the rim diameter would be the load index and speed symbol.
The load index
Simply put, this is an internationally recognised numerical code denoting the maximum load in kilograms that a tyre can carry at a specified speed in km/h indicated by the speed rating symbol.
Tyres are produced to various speed ratings denoted by a speed symbol.
Tyre speed symbol markings – maximum speed
Speed symbol L M N P Q R S T U H V
Speed (km/h) 120 130 140 150 160 170 180 190 200 210 240
WARNING! Many 4×4 accidents occur on the open road, on tar roads, where a combination of incorrect inflation, high speed and overloading causes heat build-up in the tyre. The heat build-up is caused by excessive deformation of the tyre and leads to destruction of the tyre components and a failure. It is wise to bear these factors in mind. If you are in any doubt, consult your local tyre dealer.
Aspect ratio: this is the ratio between the tyre’s height from bead to crown and its width from side-wall to side-wall, expressed as a percentage.
Bead: The bead is the part of the tyre that comes into contact with the wheel rim. Tyre beads are made of high-tensile steel and anchor the tyre to the rim.
Footprint: This is the tread section that makes contact with the road surface.
Shoulder: This is the area where the tyre’s side-wall and tread meet.
Side-wall: The side part of the tyre extending from the bead to the shoulder.
Tread: The patterned surface section composed of cleats or lugs formulating the pattern or design. It is also the part of the tyre that comes in contact with the road surface. Various types of tread pattern exist to
optimise the tyre’s performance to suit various road or performance conditions, such as AT (all terrain), MT (mud terrain), and so on.
Flotation: A tyre’s ability to support a vehicle while under moving conditions across yielding terrain.
Deflation is sometimes controversial, but it remains a vital technique that should be practised and implemented. Here are a few guidelines:
* Deflation improves traction, but also reduces ground clearance
* Wheelspin with deflated tyres may cause the tyre to “de-bead”, or to come off the rim. This is more prone to occur in rocky or sandy conditions
* Tyres should be deflated when cold, not hot (hot air expands and gives higher tyre pressure readings). You could unseat a tyre from its rim as it cools down
* In rough terrain, deflation also allows tyres to absorb more shock
* In rocky terrain, deflated tyres can mould themselves to rocks as they go over them
Tip: If you absolutely have to deflate hot tyres, add at least 0,5bar as a precaution against “de-beading”.
Sand: Lower pressure to increase traction. It is worthwhile to deflate the front tyres only, as this will compress the sand for the back tyres to ride over. As hot sand has little moisture, it is looser and more difficult to drive on.
Water: Do not lower pressure in water as obstacles under water may not be visible. It also lowers the vehicle’s ground clearance.
Rocks: While deflation allows better grip, be careful not to spin or pinch a softer tyre as this could unseat the tyre or damage the side-wall.
Mud: Slightly lower pressure will increase the surface area and improve flotation, but beware of hidden obstacles.
Tip: Modern 4×4 tyres are designed to be used at the recommended pressure under most conditions. Use deflation as a last resort to overcome severe conditions. Re-inflate tyres as soon as possible. Select your tyres according to the road conditions where most vehicle usage takes place. It is important to consider the following factors: size, width (this has an influence on flotation), and load/speed rating.
* Don’t mix radial-ply with bias or cross-ply tyres. Always carry a pump, puncture repair kit and a good tyre pressure gauge in your 4×4
* Fit metal valve caps with rubber seals to prevent foreign particles entering a valve
* Understand your tyres’s limits with regard to load as well as speed rating
* Check wheel balance and alignment after extended rough off-road travel
* Carry two spades if possible
Today we tend to drive on radial-ply tyres, as they offer good traction, extended life and a passenger car-like ride on tar. The typical all-terrain multipurpose tyres brake well under most conditions and perform well off-road. Side-wall bulge was a problem with older radial-ply tyres, but nowadays light truck technology has resulted in a stronger and stiffer side-wall.