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For many years, people relied solely on tread depth to determine when to replace a tyre. If the tread measured above the minimum 1.6mm required by South African law, they assume the tyre still has life, regardless of how old it is, which can be a fatal mistake.
Unfortunately, old tyres can be extremely dangerous, regardless of tread depth, which is why many carmakers recommend replacement at six years from the date of manufacture.
According to Utites.com if not used, tyres last for 6-10 years, depending on the storage and environmental conditions. Overall, the time limits for stored tyres are much the same as for tyres that are being used. The National Highway Traffic Safety Administration (NHTSA) and official manufacturers suggest a tyre is only 100% safe to use until it turns 5-6 years old. However, some admit that a tyre can be operable up to 10 years if you check it for issues annually after the 5th year.
The time that stored tyres last is influenced by two groups of factors:
This group includes oxygen, ultraviolet (UV) light, ozone, and heat damage.
Oxygen is the main element responsible for rubber deterioration, as it breaks rubber down from both outside and inside. Most tyres are inflated with compressed air, which is 21% oxygen. So, unlike UV and ozone damage, the process of oxidation is twice as fast.
Special antioxidant compounds are used in rubber, but they only slow the aging process down. If the polymer structure gets altered by oxygen in any way, it results in rubber deterioration.
When tyres are exposed to sunlight, the rubber immediately starts absorbing UV radiation. The process is the same with both natural and synthetic polymers, which make up rubber compounds. The process of deterioration, due to the impact of UV, is called photo degradation.
Tyre manufacturers try to fight this with the help of carbon black. It’s a finish that is put on a tyre and absorbs sunlight, transforming it into heat. However, any shield loses its ability to stabilize UV radiation after some time. So, the rubber is eventually exposed to the destructive rays, and the tyres last for less time.
In brief, ozone is a type of oxygen with an extra atom in it, which can be found in the stratosphere and troposphere. The type of ozone that is destructive to rubber is usually a result of manmade pollution. The impact of the gas on tyres was first noticed in the 1950s, with rubber deteriorating faster in large cities than in small towns.
Cracks caused by ozone and weather impact. Image source: waterloohonda.com
Tyre manufacturers try to fight ozone damage by putting special compounds in the products they produce. These waxes and oils can help if the tyre is being used, meaning that the compounds are brought closer to the surface. However, as there’s no circulation when tyres are simply stored, the compounds become useless and tyres don’t last as long.
It’s important to remember that cracks from ozone impact are irreparable.
Heat, combined with oxygen, accelerates rubber aging with the process of thermo-oxidative degradation. According to NHTSA research data, tyres last less time in hotter climates, and consequentially, they fail quicker, no matter whether they are ridden or stored.
This group includes temperature and light, ozone exposure, humidity, and deformation.
Temperature and Light
It’s recommended to store tyres in a cool place, preferably not warmer than 77 F and not colder than 32 F. The storage should also hide the tyre from direct sunlight or strong artificial light, as both usually emit UV.
Tyres have to be stored in a place without ozone-producing equipment, which includes electric motors, fluorescent lamps, generators, etc. Anything that can make electric discharges has to be taken away from the room. Excess exposure to ozone can cause cracking when any pressure is applied.
There should be no water bodies or any other source of humidity near or within the storage room. If exposed to excessive humidity, tyres last for less time – they become covered with condensation, which is highly undesirable. Liquids are also potential sources of ozone, which is destructible for rubber.
Flat spot on a tyre caused by storing being mounted. Image source: ford-trucks.com
Tyres should not be stored mounted or inflated, but vertically on a slightly elevated surface. Such a position will put the least amount of pressure on the tyre. If it’s impossible to store them vertically as there is too many and not enough room, make sure the pile isn’t higher than 6 feet. In such a case, there’s no extreme pressure put on the sidewalls of the bottom tyre. Deformed tyres will not regain their form when pressurized – on the contrary, they may break.
There are certain things you can do to improve storing conditions and make your tyres last longer:
After they are clean and dry, put each tyre in an airtight plastic bag – this will keep the oils from evaporating. You have to make sure you take as much air out as possible and close the bag tightly.
A tyres service time expires because of the rubber aging, as the material is always exposed to oxygen that makes the particles become harder and less flexible. As a result, the rubber starts to crack outside and inside, which may cause tread or steel cord separation and complete tyre failure.
Furthermore, stored tyres last for a limited amount of time as they aren’t lubricated. When you ride a tyre, the heated oils within it circulate and grease the rubber, preventing premature drying. When it’s in storage, the oils and emollients dry out, causing known consequences. Since even long-lasting rubber eventually ages, it’s recommended to not use a 10+ year old tyre
It’s very important to determine the age of the tyre you store by reading the date code on its sidewall (Please refer to last weeks article). Remember, the age is counted from the year of manufacture, not the date you purchased it. If you buy a tyre that was stored in a shop for 5 years, you may be wasting your money.
Getting rid of an unused spare or a tyre with plenty of tread may be the hardest thing for a thrifty owner to do. But if it's old, that's exactly what the owner should do.
As hard as it is, of all your vehicle's components, tyres have the greatest effect on the way it handles and brakes. So if the tyre store recommends new tyres at your five-year checkup, spend the money and don't put it off. Your life could depend on it.
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