By Jake Venter
Tyre design is constantly evolving, with the result that new ideas and designs are appearing on a regular basis. This issue we’re looking at two topics that get quite a lot of people hot under the collar, and we’ll also look at the date code that can be found on many tyres.
The latest designs are self-supporting, meaning that they’re equipped with reinforced sidewalls that are able to support the vehicle’s mass even if the tyre is completely flat. They will allow the car to travel on a flat tyre for at least 100 km at 80 km/h, with only a slight reduction directional stability.
Many people don’t like run-flat tyres. The stiff sidewalls are responsible for a considerable reduction in ride comfort, and they’re also significantly more expensive than normal tyres. They seldom last for more than 25 000 km. However, they do make even more sense on an off-road vehicle when you’re far from civilisation than when you’re cruising on the open road. Another option, which has been around for many years, is to fit a circumferential band inside the wheel. This prevents the tyre beads from collapsing into the well as soon as the pressure drops, and allows the vehicle to travel on, but at a speed that’s well below 80 km/h. In both cases the tyre will be damaged beyond repair.
Nitrogen in tyres
The tyres on racing cars and aircraft are filled with nitrogen instead of air because the absence of oxygen is a safety factor in case of a fire. Filling tyres with nitrogen is also beneficial because it is usually supplied dry, while most air compressors can only deliver moist air. The presence of water vapour will have a long-term corrosive effect on valve seals as well as the wheels.
Some vendors selling nitrogen maintain that nitrogen-filled tyres keep their pressure longer, because of the absence of oxygen. The claim is that the oxygen molecule is smaller than the nitrogen molecule, so that the former will leak out quicker over time, but I find this difficult to believe. There is in fact almost no difference in molecule size, and the leak rate should be about the same. It’s worth remembering that an air-filled tyre already contains about 80 per cent nitrogen.
Using old tyres
One often sees a car running on very old tyres. This is understandable, because tyres are expensive. However, one must be aware that an old tyre can suddenly disintegrate. Tyre companies regard a 5-year-old tyre as too old to trust. In fact, some car companies are advising motorists to replace any tyre that is more six years old, no matter how good it may appear to the untrained eye.
Most modern tyres carry a TIN (Tyre Identification Number) code in small letters somewhere near the bead. This code follows after the factory identification code and shows the date of manufacture. Three numbers indicate a tyre made before the year 2000, and here the digits stand for the week and year of manufacture. For example, on a tyre with the code NAA375 the NAA is factory identification and the numbers imply that the tyre was made in the 37th week of either 1985 or 1995. If there is a triangle indentation after the last number the date will be 1995, and if the indentation is not there it will be 1985. Four numbers indicate a tyre made after 1999. For example, the code DFK1204 means that the tyre was made in the 12th week of 2004. DFK is again a factory reference code.
Small cracks anywhere on a tyre are a sign that it is getting past its prime. These cracks usually start at the bottom of grooves where there is an abrupt change of section, but later become visible all over the tyre. Larger cracks tend to break chunks off the tread. Sometimes the sidewall layers separate over time and this often forms a visible bulge.
Three factors are at work to break tyres down:
1. Natural rubber is weak in tensile strength, but if it is exposed to sulphur while under high pressure and temperature the sulphur will link the rubber molecules together so that it becomes strong but elastic. This process is called vulcanisation, and continues for the rest of a tyre’s life, but at a slower pace, due to the effect of sunlight, heat and movement. However, if it carries on for too long, the tyre becomes hard and brittle again.
2. Oxygen and ozone in the air causes the rubber to harden and go brittle.
3. Water in the atmosphere and inside the tyre, as well as on the road, breaks down the bond between the steel plies and the rubber, so that the tyre weakens.