Sophisticated machines, advances in production methods, and free air all have an influence on the dynamics of tyre manufacturing, marketing, performance, cost and longevity. This section attempts to explain how.
The Engineering (and Benefits) of Tyre Uniformity
Keeping the ride a smooth one is a function of sophisticated machines which can shave off a millimetre of difference. Read more to find out how
More Than Just Hot Air
One of the few things in life that is still free is Air. The right amount is what makes a substantial difference to the ride. One of the laws of physics is generally used to guide the use of air in tyres. Find out which one
Confucius said: ‘Stepping in the shoes of someone else may not be ideal’. But for economical reasons, fitting a worn tyre with a new tread compound helps to keep not only the costs of tyres down for truckers, but also keeps the retreading industry in business. Find out why
Riding a tube was part of tyre structure of yesteryear. Riding a tube today only applies to surfers! Tyres today are tubeless, and this innovation helped reduce the downside in a number of tyre related practices. Find out more
The Engineering (and Benefits) of Tyre Uniformity
A tyre is a lay-up of fabric layers, rubber strips, and steel wire. These individual components are assembled on a collapsible drum, inflated to the toroidal shape we are familiar with, and vulcanised in a mould. As each layer is applied, dependent on the component and its purpose, some overlap may be required to maintain the integrity of the tyre casing under pressure. You would call this a ‘join’.
Sometimes these ‘joins’ can be seen as a slight bulge in the wall of the tyre late in the day, when the sunlight is at an acute angle to the wall. They are essential to the make-up of the tyre, and are not a fault in the construction. However, they can have an effect on the ride quality of the tyre if they are too large, or are too close together.
Think of a tyre as a series of springs, like you see on the Rovers currently on Mars. As the tyre revolves, each spring is compressed in turn. However, if one spring is ‘stronger’ than the rest, when that spring hits the ground, it will cause the axle to lift, which, if it is at a speed which harmonises with the spring rate of the suspension, can cause a vibration. The tyre may be perfectly round, the rim may be perfectly round, the assembly may be perfectly balanced, and yet there is still a vibration due to ‘non-uniformity’.
This causes car designers quite a bit of angst, particularly if the vibration occurs at a speed that the car is commonly driven.
The tyre manufacturers have sophisticated machines which can measure this ‘non-uniformity’ and, by grinding away a miniscule amount of rubber from the tread at the ‘stiff spot’, can correct for it. Strangely enough, they can take a round tyre and grind it slightly out of round, so that it behaves as though it was round. The degree of uniformity required is specified by the car designers and their N.V.H. (Noise, Vibration and Harshness) engineers.
Rather than do this after the tyre is built, the tyre and production quality control engineers spend much effort in sequencing the tyre building process so that the cause of the non-uniformity is reduced in the first place – by accurate control of the component dimensions and staggered placement of the joins of controlled overlap to minimise their effect. This is what you pay for when you buy a good quality tyre. ‘No Vibrations!’
More Than Just Hot Air
The atmosphere is air, which is a mixture of gases. Mainly it is an inert gas called nitrogen, mixed with around 20% oxygen, which is what we need, and other gases such as carbon dioxide, which is being blamed for everything, it seems. It also contains water vapour in varying amounts, dependent on the temperature and the humidity.
Oxygen is separated from air, to be used for a huge variety of very useful applications, such as medicinal, welding, metal cutting, and as part of the process, nitrogen is separated in prodigious quantities. It is a by-product of oxygen production. And here is the rub – the nitrogen produced has no moisture in it, and is inert – it will not support combustion – which requires oxygen.
One application touted for nitrogen is in tyre applications where either the moisture content of the air, or the risk of fire, makes a dry, inert gas very acceptable.
In our humid tropical summers, the typical tyre service air compressor should be fitted with an after cooler, which cools and therefore de-humidifies the air, provided the air reservoir is drained regularly. Unfortunately, not every compressor has them, and it is not uncommon for airlines to contain water.
A law in physics states that the total pressure is the sum of the partial pressures. If moist air under pressure gets hot, it will rise further in pressure than dry air.
Some pressure increase is normal under operating conditions, as the tyre gets hot due to flexing. 2 to 3 p.s.i (15 to 20 kPa) increase is normal for passenger radials. If it is much higher, the tyre is either under-inflated or overloaded. This is why all inflation pressures are specified as ‘cold inflation pressure’, and a check first thing before setting out is the only meaningful measurement.
Nitrogen is used to inflate heavy aircraft tyres, which are already operating at exceedingly high pressures, in the region of 230 p.s.i. It removes one source of risk. It also stops unauthorised service people adjusting tyre pressures, since they are unlikely to have a source of bottled nitrogen.
Some heavy truck fleets are using nitrogen filling, but the claimed advantages of more casing life appears to be stretching it a bit. What is does do is dissuade the truck driver from deflating the steer tyres to get a more comfortable ride, since steer tyres are usually inflated to 110 to 120 p.s.i. Many of the fuel tankers you see on the highway also use nitrogen inflated tyres, which appears worthwhile to the operators because of the lower fire risk.
The economics of the heavy trucking industry rely on the retreading industry to keep their tyre costs down. Truck tyres are commonly retreaded twice. However, trucking firms monitor their tyre performance closely. Tyres are rotated through steer, drive, and trailer positions, to maximise their life and the number of retreads they can attain.
The casual observer may think that this is not desirable, because of the strips of rubber that can be seen alongside the main trucking highways. But it is not the retread that has separated, it’s the casing components that have separated, which can be seen from the strands of steel wire attached to the rubber tread strips.
Retreading bonds a new tread compound to a freshly prepared surface of the old tread. The process, called ‘buffing’, presents a scored creped clean surface to the new rubber. New retread rubber can then be applied using unvulcanised rubber spirally wound around the tread, procured with a bonding layer sandwiched between the old and new, or unvulcanised strip rubber with a bonding layer on its underside. All systems have their advocates. The bond of new rubber to the old is quite strong, and service reliability of the retread is not usually a problem. Trying to get the last bit of life out of a tired casing is.
Aircraft tyres are retreaded using the spiral wrap hot application process, and six or seven times is quite normal. The casing inspection standards are quite rigid though, as you can imagine.
Nowadays, passenger retreads do not enjoy the same acceptance. Several factors have influenced this:-
- Casing mileage has been doubled with the introduction of steel belted radials compared to bias ply.
- Motorists’ expectations have risen regarding performance. Motorists realise the difference that a set of good tyres can make to their car.
- Casing reliability standards deteriorated, considering that the retreaded radial was expected to do four times the life of a new bias ply tyre.
- The relative costs and efficiency of manufacture of new tyres lowered the cost difference between the two processes. Similar things happened to the engine reconditioning industry.
However, when times get tough and the pocketbook can’t meet the cost of new tyres, retreads are always there, and are infinitely better than a bald tyre once it starts to rain.
Can you believe that there was a time when every tyre had a tube in it! Punctures were frequent, and to ‘pinch the tube’ when fitting the tyre was common. It still happens with bicycle tyres. They need airtight rims to be tubeless. Nowadays practically the only time a motor tube is fitted is when an off-roader has a puncture, hasn’t the gear to inflate a tubeless tyre, and goes to the toolkit to fit the spare tube which an experienced off-roader always carries.
The most graphic way to illustrate the benefits of a tubeless tyre, is to inflate a party balloon. Stick a strip of masking tape to the balloon, and pierce it with a pin. The balloon goes down, but slowly. Now prick it away from the strip, and it goes ‘bang’. The balloon surface is not under tension to the same extent under the strip of tape.
In other words, a puncture in a tubeless tyre gives a ‘slowout’ not a ‘blowout’, gaining in safety and reliability. Indeed, depending on where the tyre is punctured, you may only notice an annoying slow leak before you wake up to the cause. Often, you can hear the puncturing object hitting the road, before you realise that is causing you problems.
To discard the tube was a revolutionary modification to the way tyres were serviced. Previously anyone with a small compressor could fit a tyre and tube assembly. However, a tubeless tyre requires a high volume blast of air injected into the tyre interior, to force the beads out of the wheel well onto the bead ledges, and seal. The tyre-fitting machine incorporates this into its design, so that fitting tubeless requires a reservoir of air and a fitting machine, taking it out of reach of the handyman. Once the beads are seated, it is a different matter.
Early tubeless tyres struck problems. Air that leaked into the casing could gather at a particular spot, and cause a bubble to develop. Rubber itself is permeable to air, so the design incorporates a relatively thick layer of rubber (the tubeless liner) on the interior surface of the tyre. The inside of the bead area has to be sealed too, and exposure of the casing cords due to tearing of the bead toe during fitting can leak air into the casing. That’s why the tyre levers used have no burrs, and a lubricant is used on the beads.
Big improvements have been made in the compounds bonding the layers of rubber and casing cord reinforcement over the years as well. The adhesives used to bond the rubber to the plies are quite sophisticated, because they have to be flexible and long lasting, over the life of the heat prone tyre.
The type of valve used with a tubeless tyre snaps into the hole in the rim, under compression, to maintain the seal. Tubes could retract their valve stem into the interior of the tyre when the tube deflated, and ruin the casing. With tubeless, provided you change the tyre before the sidewall is damaged from being run flat, there is a good chance that the tyre is capable of being returned to service. It should always be examined INSIDE after a puncture. If the serviceman recommends that you fit a tube to your punctured tyre, you should realise that you are robbing yourself of the many benefits that a tubeless tyre bestows.
However, when a tyre is punctured in an area that cannot be repaired with a tubeless repair satisfactorily, then it is an option to fit a tube, albeit an expensive one. There are limitations to which part of the tyre can be repaired. The shoulder-buttress area where extensive flexing takes place is such an area.