OZONE ATTACKS RUBBER- AN EXPENSIVE LESSON

I had an expensive lesson on the destructive effects of ozone on rubber recently.
Ozone occurs naturally in the air, commonly after electrical storms, of which we’ve had a few lately.

But in this case, the destruction originated from an electric motor, immediately under the rubber seal that was supposed to keep the water in my pool’s heating system. What’s that got to do with tyres? Well, when tyres are stored, like on your caravan or on a seasonal piece of farming equipment, care should be taken to protect them from sunlight, and electric motors.

Otherwise, ozone from electric motors will attack the rubber, BUT ONLY WHERE IT IS STRETCHED!
Wide lacy cracks appear, and grow very quickly, as shown in the photo. The victim is shown in the front of the first photo, a new seal in the background. This appearance is typical.

So the temperature sensor was ejected, a geyser of salty water covered everything for 3 metres around, the pumps eventually ran dry, and an expensive repair bill resulted. All this for a 50 cent rubber grommet in the wrong place.

When removed from the pipe, the side of the seal inside the pipe was in A1 condition. This is shown in the second photo- it’s been turned over. There’s no ozone in the water, then- only chlorine.

Another place where this type of cracking occurs is in the bent rubber hoses in your washing machine. That can do a lot of damage too, replacing tiles, carpet, timber, coreboard cupboards, and maybe carpets too.

Punctures and “Run-Flats”

I hope that it’s been raining where you are. We needed it.

However, one side effect that can get us involved, is the greater risk of a puncture.

Why? Because wet rubber cuts easier, and is pierced easier, than dry rubber. When the tread rubber is almost gone, then the incidence of punctures increases markedly. A fair statement is that the rate of punctures doubles when there is only the last ten percent of the tread thickness left.

So it’s only flat at the bottom, you are told by unsympathetic observers, while you struggle with over- tightened wheel nuts, and having to unload the boot to find that long neglected spare wheel. And of course, it’s still raining.

So what do you do about it? An ounce of prevention, and all that.

First- is there a spare wheel in the wheel well. Is there a wheel well? Good question. Many modern cars/SUV’s don’t have a spare. They supply a can of gunk to seal the tyre, and get you home. Supposedly.

Not much use though when confronted by the tyre ‘failure’ in the accompanying photographs.

This is what is known as a “run flat failure”. The tyre invariably is on a rear wheel, and has been run in a straight line, deflated, at speed for a considerable distance. The sidewall “knuckles under” midwall where the rotating tyre hits the ground, and the heat and distortion actually melts the reinforcing cords in the tyre casing. So after around 10 km, it fails, disastrously. As the driver of this utility explained, “it happened on the freeway, mate”.

You’d need a pretty big can of gunk to fix that, mate!

Then they started the unenviable task of finding a replacement tyre. Being a utility, it was shod with commercial (or L.T.) tyres, which even though it was a Holden, aren’t always carried by retail tyre stores. Took the whole afternoon.

So make sure that your vehicle has a spare, that it’s got air in it, and can be accessed. It doesn’t matter if your new car has a steel wheel, with a different sized tyre on it, when all the rest are spiffy low profiles on mag wheels- it has been matched and approved for rolling diameter ,and load carrying capacity, and it’s legal.

If it’s a used car, make sure that it’s got a spare. Period!

Our readership survey shows that the greatest interest in articles on our website relates to articles on tyre pressure. Since basically, this is the one parameter that the user has under their direct control, this is probably not surprising. So I thought that I would expand a little further on the subject.

The modern steel belted tyre has made a myth of many strongly held beliefs that go back in history to the days of bias tyres, sometimes called CROSS PLIES. Some of these were that under-inflated tyres wore on the shoulders, while over-inflated tyres wore out in the centre of the tread first. This was before those pesky Frenchmen invented radial tyres and front wheel drive (traction avant) motor cars. Today, front wheel drive tyres wear more rapidly on the front, with rapid wear both in the crown and the shoulders of the tyre, because it is steering and driving both at once; while on a rear wheel drive car, the rear tyres wear in the crown, and the front tyres wear on the shoulders, because that is where they are doing their main work- developing grip.

The steel belts of a radial tyre are laid criss-cross under the tread running at a low angle to the direction of rotation. They brace the tread. The tread rubber, and the tread pattern particularly, are then held stable, and the pattern’s major function of providing drainage channels for water trapped under the tyre to escape, is assured.

So what is the role of tyre pressure?

First, and most important, is that it is the air that carries the load. The air pressure inside the tyre and rim tensions the tyre cords in the casing, which allows the tyre to support the weight of the car. The quickest way of proving this is the extreme method of letting the air out, and seeing how far you get- like nowhere.

So assuming that there is air in the tyre, how much is enough?

According to the Tyre and Rim Association load tables, 4 tyres under a Holden Commodore inflated to 250 kPa (36 p.s.i.) can carry a load of 650 X 4 kilograms= 2.6 tonne.

Now you would admit that that’s a pretty overloaded Commodore.

Since a V6 VT/VX series Commodore with two passengers in it, with luggage for two in the boot weighs:-

Car tare weight 1665kg (V6), 1682kg (V8), when fully fuelled

2 passengers weigh 73 kg over the front axle, 63 kg over the rear,

Luggage for two………….27.2 kg

Total weight (mass) is therefore around 1845kg, give or take a few, according to the Tyre and Rim Association Standards. Optional extras also have to be added in as well.

This represents only 71% of the load carrying capacity of the tyres when they are inflated to 36 p.s.i., which is their maximum. And according to the Tyre Standards, only 23 p.s.i. is required to carry the load of the car when operated at normal highway speeds. With that load on it, and with that pressure in it, the tyre is the shape that the engineers designed for it to operate satisfactorily.

Yet the car engineers do not as a matter of course, specify pressures that are barely adequate to carry the load of the car and its occupants. So there has to be more involved than just load carrying. A placard for the VX/VT series Commodore appears below. Note particularly the pressures specified for the 225/60R15 tyres on this model- 29 p.s.i., and the 225/55R16 tyres at 26 p.s.i. under load conditions similar to those quoted above.

Note also that further pressure increases are specified for (a) extra load (all seats occupied, and their luggage too; and (b) extremely high speed (above 160 km/k) in this case.

Motor car engineers, and the driving public, expect that their cars will “handle”. This is not necessarily for

use at high speeds in today’s highly regulated road systems ( read radar traps) but is something else called steering response. A popular catch phrase for this a decade ago was “radial tuned suspension”. Nowadays, phrases like “turn in” get bandied around.

From the load tables, around 24 p.s.i. is enough to satisfy the tyre engineers that their tyre is inflated to its design shape that will be durable, and hold together under the loads the tyre is carrying. With such a pressure, the modern car would handle like mush, and drivers would not be happy. Car engineers specify pressures in the region of 28 to 34 p.s.i, with appropriate corrections for high speed. A tyre placard which specifies the pressures for a Holden Calais appears below. You will note that these are up in the 35 p.s.i. range for the same sizes as were fitted to the VT/VX series above.

Note also that NO pressures increases are specified for a full load, only for speeds above 160 km/h. In other words, the standard inflation pressure can cope wit high load conditions.

The Calais used to be thought of as half way to a limousine ride, but with 35 p.s.i. in the tyres, the limousine buyer will be disappointed. The steering response (ability to change direction quickly, as in negotiating a course of witches hats) is extremely good. Drop the pressures to 29 p.s.i., and it’s a different motor car. Softer ride, not nearly as twitchy for the type of buyers who buy a Calais. After all, these are the type of buyers who want a bit of luxury, and all the mod cons added in.

So what does extra pressure do? One- it shortens the footprint, which lessens the time available for the tyre to drain water out from under it. In compensation, it increases the pressure under the contact patch. How much, and whether it entirely compensates for less drainage time, is a matter entirely for the design of the tread pattern, the speed, depth of water, and how much tread pattern depth remains on the tyre. Naturally, the less pattern depth, the more drainage is impaired- worn tyres don’t perform as well as new tyres in the wet.

The second thing that air pressure does, once the load and speed requirements have been met, is stiffen the sidewalls. Experienced tyre men caught without a pressure gauge, compress their thumb into the sidewall in the shoulder area, which has no bead stiffeners, and no steel belts in that part of the tyre. They get very good at estimating pressure in the tyre. Incidentally, a tyre has to be below about 17 p.s.i. for it to “look flat”.

So a turning input from the steering, down through the steel rim, and the flexible rubber sidewall, will get to the road surface where it has to do its work, with more sensitivity- things happen quicker, and more predictably if the tyre is pumped up “hard”. The tyre distorts less in a sideways direction under extreme cornering forces too. Sporting drivers say that the car “handles”.

High performance and premium quality tyres have complex bead and lower sidewall structures and low profiles, so that the message can get to the road quicker WITHOUT the necessity to run high pressures, which can give a bone shaking ride, and transmit a lot of thumps and bumps from concrete road joins and the like to the cabin. There are more components in the tyre to accomplish this, which is why they cost more.

So what caused the engineers who designed the VE Series Holden Calais to specify such high pressures. The answer will be found in the delays that accompanied the release of the new VE model Holden. Loaded with extras, the VE Calais did not meet the intended design parameters for fuel consumption. It ended up with the engineers chasing a figure 0.1 litres per 100 km lower. The solution was to increase the tyre pressures. This lowers the rolling resistance of the tyres, and improves fuel consumption. There is a lesson here for all of us, if your fillings in your teeth can stand the ride. Increase the tyre pressures, and your fuel consumption will reduce. Even maintaining the pressure at the intended design pressure will pay off in lower fuel consumption.

So have a look for your tyre pressure placard somewhere on the car, or in your car handbook. Mostly, it’s on the driver’s door or door pillar. It specifies the pressures that the car engineers are happy with to make the car ‘handle’ the way they want it to. If the fillings in your teeth can’t cope with it, accept that there is a penalty involved, paid for at the fuel bowser.

Or else just call it a “Sports Calais”!

DISCLAIMER.

The writer drives a VX SS Commodore, and would just love to get his hands on a VE 6 litre “SS”, because “it handles”.

Tubeless Tyre Valves

The little rubber and brass valve that holds the air in your tyre, and admits new air, is one of the world’s most successful inventions. William Schraeder designed its fundamentals nearly 120 years ago.

The little “springy thingy”, called the “valve core”, that screws into the brass valve really hasn’t altered all that much in that time, and all you need to remove it and let the air out, is a slotted valve cap. Or you can just depress the little button in the centre and you get the same effect, only slower.

Yet when you buy a new tyre, the fitter always replaces the valve. Why does he bother?

The modern tubeless snap-in valve is compressed into a hole in the rim to provide a seal. A brass stem is adhered to a rubber skin, with a domed shape on the inside of the wheel to prevent it being blown through the hole by the air pressure.

 Over time, the degree of compression is lowered (it doesn’t fit as tightly). It may even crack around the groove in the rubber which lodges in the rim hole due to flexing.

The valve actually flexes as the wheel revolves, particularly if it is a long one designed to protrude past the wheel trim. Ultra-high-speed photographs have shown the valve actually touching the rim at right angles at very high speeds. Also the heat during service causes the bond of the brass to the rubber to deteriorate, and if this bond ruptures, the stem blows out, and the tyre goes down quickly.

So reliability is what it’s all about. It’s much better in the long run to replace it after one tyre life.

You can contribute by using dust caps or valve caps, and giving a blast of air around the valve before you clamp on the air chuck, which you should do monthly. If you suspect a leaking valve, a “dob of spittle” on the end of your finger into the brass stem is the tried and true method. If it bubbles, first check the valve core is tight. If it is, loosen it, let some air our, then retighten to dislodge any dirt that might be there.

If it still leaks, replace the core. Unscrewing the core right out will let all the air out, and coincidentally clean the seat that the “springy thingy” seals on. To do this, you need a slotted metal valve cap, or a valve tool, and a kindly service station operator to assist if needed.

The metal clamp-in valves are different. These are used in some alloy wheels, where the thickness of the metal around the hole is too great for a snap-in type. But they are even more desirable when high speeds are the norm. Unlike a snap-in, they do not flex, and they sandwich two air seal washers under compression to get the air seal. So even though they cost more, they last longer, because it is not as necessary to replace them after every tyre life.

Want to know more? See our “All about tyres” section or our “Inflating Tyres Safely” post.

Here’s the plug! Stuckey Tyre Service

Stuckey Tyre Service is one of Australia’s premier suppliers of car tyres, whether for vintage or motorsport application, or everyday road use. We supply all the major premium tyres. Our sales office and warehouse are located at 828 Sydney Road Brunswick, Australia.

Servicing the demands of Australia’s leading motor racing teams has provided us with unrivalled knowledge of the best performance tyre and wheel combination for every application, road or track. From the most exotic European sports car to the average family sedan, we at Stuckey Tyre Service have a carefully selected range of tyres and alloy wheels to enhance the road performance, safety and appearance of your car.

At Stuckey Tyre Service you can take advantage of the ultimate precision fitting and balancing service where the utmost care is taken with your valuable tyre and wheel purchase. In particular we take great pride in being able to balance a wide variety of specialty wheels including wire wheels for historic applications. The most advanced fitting and balancing equipment is used by skilled technicians whose work is trusted by Australia’s top race drivers at speeds over 300Kpm.

We at CarbonBlack love sending customers to the Stuckey team.

Tyre Tread Compounds

There’s a great deal of confusion amongst car enthusiasts, particularly the “rubber burners”, on tyre tread compounds and their make-up. One enthusiast on a car blog announced that tyres weren’t made from rubber at all, but from oil. You know what- he was mostly right!

Oils ain’t oils, and rubber ain’t rubber any more.

Tyres contain 3 or 4 different rubbery materials. One is natural rubber; which is the juice of a tree, which is coagulated using acetic acid, smoked and dried. The others are all made from oil, and are called “polymers”- another term is “long chain macromolecules” but don’t worry too much about that. It is now possible to make “natural rubber’ from oil too, but it’s cheaper to let the tree do it.

These various rubbers can be mixed together in different ratios in giant blenders to make a compound. At the same time, other important ingredients are added to make the resultant product tougher and stronger. A typical tyre compound may contain 10- 14 ingredients, all added for a specific reason. The most important of these is carbon black, of which there are many types.

A tyre typically has 7 to 11 compounds, each doing a specific job, be it encasing the bead wires, keeping the air in a tubeless tyre, flexing the sidewall, sticking the layers of nylon or polyester together, and so on. But there is only one compound that hits the road where it all happens, and that’s the tread compound.

This is basically the only criterion on which the motorist can judge the performance of the tyre, so it receives the most comment from car enthusiasts. The tyre engineer and chemist can vary the compound formula to maximise/minimise any tyre characteristic that he requires.

A typical passenger tyre tread compound contains as the base polymer styrene-butadiene copolymer, around 35% carbon black to reinforce it, and maybe some silica. These increase the abrasion resistance, tear strength, and cut resistance. Without them, the tyre would go gooey, and wear out very rapidly. Remember those old crepe rubber soled shoes?

Vulcanisation chemicals such as sulphur, zinc oxide, stearic acid, and accelerators make up 3-5%; antioxidants and antiozidants to stop it perishing or cracking, processing aids such as oil, resins, tackifiers to aid in the lay-up of the assembly are all incorporated.

Some tyres have two tread compounds- either side by side (very rare), or a cooler running undertread compound under a harder, hotter running cap stock. Most, however, only have one compound in the tread area.

The rubber used in tyres is normally a copolymer ( mixed and then polymerised together) of 23% styrene, and 77% butadiene. However, this ratio is not set in concrete, and specialty rubbers of different proportions of these two refinery products can be made. For example, “cling rubber”, which was widely touted for its improved wet grip, is 40% styrene, 60% butadiene. The resulting rubber ran hotter, and wore out quickly under Australian conditions. A 90% styrene, 10% butadiene rubber is used to make floor tiles, not car tyres.

Another rubber developed for use in tyres is polybutadiene. Butadiene is the most common feedstock from a refinery. However the resultant polymer suffers one big disadvantage- its wet grip is poor. Its big advantages are however, that it stands up to extreme abrasion much better than other rubbers, and runs cooler. Back in the days when the Armstrong 500 Miler was run at Phillip Island on standard tyres and rims, and the track was not a smooth hot mix like it is today, Harry Firth won the race by changing only one tyre, whilst everyone else changed at least twelve on the very abrasive track. But he lost 3 seconds a lap, because down on the ocean side, the tyres wouldn’t grip to racing levels. The tyres were made from a high proportion of polybutadiene in the tread, specially airfreighted out for the race.

We’re not as skilful as Harry Firth was then, and the Australian motorist puts “grip in the wet” as the top desirable characteristic from his tyres, so its use is now mainly confined to truck tyres in blends with natural rubber, where heat is the main enemy of tyre performance.

So ultimately, it’s the “grip” of the tread compound that drives, steers, and brakes the car, through the contact patch, around the area of a size 12 shoe. It does this by slipping! Sliding generates friction, and this causes things to happen. All tyres slip, particularly driven and steering tyres, which is why they wear out. No friction- no progress. Try driving and steering on black ice sometime to see what I mean.

“Rubber burners” overlay this with “sticky friction” by heating the tread surface till it starts to revert- goes gooey. Lots of smoke! On top of this, tyres generate heat internally from the stresses generated by flexing (the hysteresis loop). As the rubber warms up, the rubber changes its grip characteristics, provided that the compound hasn’t degraded to the gooey stage (“goes off”). This occurs generally in the thickest part of the tyre under the greatest load, like the outside shoulder of a tyre being driven on a banked circuit. That’s why you see tyre technicians who are evaluating tyres, drive a thermocouple needle into the shoulder of the tyre tread- the thickest part. The electric blankets on the wheels ready for a change onto a race car are there for the same reason- so that the car will handle similarly to the old, warmed up tyres.

The position of the white stripes in the tread grooves of the Formula 1 cars indicates the type of compound used in the tread. The tread grooves are there to slow the cars down, even when the road is dry. Race team managers under F1 Rules have to use at least two types of tyre during the course of the race. This adds another source of tactical variation for managers to consider, as though they haven’t got enough on their plate. But the race result may have been decided in a tyre development laboratory in Kobe or Luxemburg or wherever, since so much data has been accumulated on the vagaries of each circuit, and the tyre compound that performs best on that circuit.

It’s almost time for the Melbourne Grand Prix- so enjoy your viewing

Should Tyres have a use-By Date?

Channel 7’s “Today Tonight” program on Friday 5th December, picked up on earlier publicity originating from an American T.V. program “Twenty-twenty”. An aggrieved customer in South Australia complained that he had been sold Light Truck tyres that were already 14 years old when fitted. One tyre had separated its steel belts from the tread ring, causing damage to his mudguard, and raised the risk of an accident.

The British Rubber Manufacturers have recommended that tyres more than six years old should not be sold, but there is no law requiring this anywhere in the world at present. The American Rubber Manufacturers Association states that there is no scientific evidence to support a six-year limitation on the life of a tyre.

The Channel 7 program cut pieces from the sidewall of the tyre, and did a “tensile test”, pulling on the test piece till it broke. Pieces cut from the (used) 14 year old tyre broke at a lower tensile than from a new tyre. Why they tested the sidewalls, which are a different rubber compound to the tread/steel belt area, it is not known, but it is not surprising that testing two tyres made 14 years apart would give different test results. The reason? The tyres were different!

Tyres are warranted for their life by the manufacturer. Occasionally tyres, like many products, are subjected to a recall program. To enable identification of these, a code is branded into the sidewall, which is used world wide, and is a requirement of the American Department of Transportation. It is called the DOT code. Practically all tyre manufacturers worldwide use this code.

The code details the actual factory in which the tyre was made, the design, and among others items, the last appearing group lists the week and year the tyre was made. 3 digits for the ninetees, four digits for the noughties. Examples then are 489 for the 48th week of 1989, 2604 the 26th week of 2004.

Tyres are generally 6 months to 2 years old by the time they are fitted to your car as replacements. The original equipment tyres are generally one week to six months old, dependent on whether the car was made here, or imported.

The Australian tyre market is so fragmented, with many makes and models of vehicles sold, that the supply chain for replacement tyres is very long, and large stocks are held at distribution points to meet market requirements. For example, the 11 hectare distribution centre at Somerton, Victoria, can hold up to 11 million tyres. Naturally, efforts are made through inventory control to ensure quick turnaround of stock going into the store, to reduce holding costs.

Eventually, tyres are shipped out to your local tyre store. Here they should be stored in racks, in a “cool, dry place”. Many tyre storage areas paint their tyre storage area windows with blue paint to screen out U.V. This is because tyres get harder with age. The vulcanisation process continues at a very slow rate, and protective agents such as antioxidants and antiozidants incorporated into the mix diminish in effectiveness with prolonged storage. Walk into a darkened tyre store, and you can smell the rubber. A somewhat doubtful farming practice used to be that tractor tyres were stored by the farmer to “harden them up”, and possibly improve tread wear. Really, all it did was increase the risk of buttress cracking.

Unless stored correctly (read “All About tyres/Storing a tyre” on our http://www.carbonblack.com.au site), the tyres will eventually craze or crack most severely where the tyre is resting on the pipe rack. This is because stretched rubber is attacked by ozone in the air. Ozone is generated by electric motors and lightning, so maybe the shop compressor is the culprit. However, tests done in the past have never been able to show that tyres stored this way will not give a satisfactory life. The deformations caused by the pipe rack run out as soon as the tyre gets run in on the vehicle- say 10 kilometres, depending on the temperature.

The real sleeper in all this is your spare wheel. Stored in the boot, or under the tray of a light truck, it is subjected to high summer temperatures, and may lay there undisturbed for six years or more if you don’t have to use it. Our discussion on what to do about that is contained in “All about tyres/original equipment”. Basically, it has missed out on six years of design improvements whilst sleeping in the car boot, or lying in the dealer’s racks waiting for a sale, or in the South Australian’s case, 14 years.

So should tyres have a “Use by Date?” It would appear that provided they have been stored correctly, there is not a problem with tyres encountered in the usual course of trade. Besides, somewhere out of Broken Hill or Wilcannia or somewhere like that, you will be pleased to find that the tyre service has your badly needed tyre, even if it is a bit dusty.

All that applies to tyres also applies to automotive car batteries of course, for all the same reasons, except that a lead acid battery does in fact have a finite life, and has to be stored correctly with its charge maintained until it is sold. The warranty period then kicks in once it is sold.

Ageing Tyres and Road Safety

A recent survey carried out for the RAC Foundation in the UK has revealed that although nine out of ten drivers understand the link between tyre age and road safety, up to an estimated three million in the UK- do not.

According to the research, owners of low-mileage cars, vintage and classic models, caravans, motor homes and trailers need to be aware of the risk of tyre fatigue. The spare tyre is also at risk of age-related deterioration, as it is often unused even though other tyres may have been replaced over the years.

Simplifying the checking process could be one answer to the problem, as seven out of ten motorists said they would like to see the current age coding system replaced by an easy-to-read ‘year of manufacture’ date on the tyre.

The Foundation was also concerned that 45% of drivers questioned did not know the minimum legal tread depth for car tyres and among women drivers, this figure rose to 60 %.

Although it is quite easy to check a tyres’ age from the sidewall code it only tells you when the tyre is made, not its physical condition.The danger signs of ageing can include:

– cracks or crazing on the sidewall

– tread separation, cracks or bulges

– deformation of the tyre carcass

– discolouration and fading of the rubber

In response to these findings the UK RAC and the UK National Tyre Distributors Association have launched an eight-week tyre safety campaign urging drivers of low-mileage vehicles to get their tyres inspected.

Obama’s “Inflate Your Tyres”

Irrespective of your politics it is good to see that one US presidential candidate supports proper inflation of tyres as a way of promoting fuel efficiency and thus helping the environment. He must have been reading Davids’ blog on tyre pressure!

Exploding oxygen cylinders on Qantas: what’s it got to do with tyres?

Qantas have found out the hard way that when oxygen cylinders operate at very high pressures and ‘fail”, they can do an extreme amount of damage.

From the evidence released so far, it seems that the valve screwed into the top of the bottle ‘let go explosively”, coming through the floor and hitting a door handle, luckily not a passenger. The bottle then took off like a rocket would under the same circumstances, straight through the side of the plane. This failure can be due to the thread being over-tightened and stretched as the valve is inserted. The same thing can happen to wheel nuts over-tightened on their studs, the over-stretched thread fatigues, and the wheel comes off because the wheel nuts were too tight!

So what’s this got to do with tyres? Well, I once heard a ‘supersingle”, a 15R22.5 Tubeless tyre with 105 p.s.i. of air in it, on the FRONT wheel of a truck, blowout (fail catastrophically was the term used) from 5 kilometres away on the Murray Valley Highway. That gives you an idea of the force involved when a tyre blows out.

Any pressure vessel is designed and tested to a much higher presuure than it’s operating pressure – the “factor of safety”. . For example, the air compressor operating in the corner of the workshop has a safe operating pressure stamped on it, and the “Test Pressure” as well, which will be several times higher than operating pressure.

So what do you reckon the “Factor of Safety ‘of a tyre is? It doesn’t just sit there like the oxygen cylinder. It operates in a dynamic environment, having to cope with all kinds of abuses such as heat, potholes, broken road edges, and the like.

The answer is experience. Each tyre is designed to do a particular job. For example, an off-road tyre would have a different factor of safety to a speedway or race car tyre, which hopefully will never hit a pothole at speed. An off road truck tyre should have a stronger casing than a highway design, if service experience dictates it. This is achieved by either using more cords in the casing, or stronger cords. In other words, the design is different.

An interesting twist on all this is that it is the rim that fails before the tyre casing bursts when due entirely to over-inflation. There is so much side force on the rim flanges that they buckle and fail. To check their computer calculations of tyre casing strength, tyre engineers regularly blow up their tyres on very very strong rims in a safety cage. But they do it with water! They might get wet, but they don’t get hurt.