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.

Good Tyres, Bad Tyres, What’s the Difference anyway?

Because so much of the detail of a tyre is hidden from view, and it doesn’t mean much to the average tyre buyer anyway, the customer feels quite entitled to ask “Why does this tyre cost more than that tyre, and what does the difference mean to me anyway?”

Because more often than not, the tyre is presented in a vertical stack alongside other tyres, the salesperson is quite likely to launch into a comparison of tread and buttress design and width, tread pattern design, accompanied by claims of superior mileage, roadholding, reliability, and “it’s on special this week only” sales presentation. The reason is that either the customer can see these things for themselves, or can conceptualise, or are prepared to accept because the salesman obviously knows more than they do.

The question remains though – “why does this tyre cost more than that tyre?” It’s a valid question from the customer’s point of view. Why do some tyres cost more than others, and is it worth it to buy the more expensive tyre?

So start with “how recent is the design?” Most new tyre designs (sizes, patterns, constructions) are brought into production to meet the requirements of the design engineers of new cars. If they didn’t ask for particular improved tyre attributes, then the design process would stagnate. They drive the improvements, to meet design parameters that they want to incorporate in their new car design. This process goes on worldwide, all the time.

The tyre company, needing their business, designs, qualifies, tests extensively, government certifies their new tyre design, and submit prototypes to the car company for evaluation on their new design car. To this stage, this has cost a great deal of money in technical resources, tooling costs, mould manufacture, and qualifying testing. Then they wait while the car company engineers evaluate their tyres against others from competing tyre companies. So there is no certainty that these prototype tyres will ever see enough of a production run to amortise their development costs.

Remember, each new car has at least 4 new tyres, possibly 5.

So hurrah, at last the car company accepts the tyre for production, and contracts for supply at a particular rate at 12 hour’s notice is arranged, at a price that is barely adequate.

Then, after two to three years, replacement tyres are required by the car buyer from a retail tyre store, in competition with tyres from all over the world in the same size. This is possible because of currency alignments, and because tyres are all made to conform to the same standards regarding size dimensions, speed and load carrying capacity.

But there emerge major differences in appearance, because the car engineers may have specified a quiet riding tyre for a saloon, whereas more eye-appealing tyres from say Europe in the same size may have been designed for a more sporty vehicle; or advertising campaigns, consumer reviews may influence both retailer and buyer; the reputation of the brand definitely carries weight; word of mouth approval; bulk package deals from wholesaler to the retailer; or simply the skill of the salesman in influencing the customer’s choice, based on questioning the customer as to the application of the tyre. Always in the background, is the appeal of low price.

Another 3 years on, another 60000 kilometres, time’s moved on, probably the car’s changed hands, the pattern is no longer available (the moulds do wear out), fashion has changed, tooling costs have been recovered, so the price of the product has been lowered to meet competition and retain market share. Besides, 18 inch wheels have superseded 15 inch- that wasn’t so long ago, was it! Your once newly developed tyre has now become the price leader into the tyre shop so that hopefully you will buy something better, more modern, better performing, more costly.

Tyres are all fat and black, look the same from the outside, they’re almost all truly round these days, and the detail of the construction differences are inside the casing. However, small differences inside add up to small improvements in braking, handling, cornering, steering response (lane changing ability), quietness, and harshness over concrete road joins, durability under high speed/high load conditions, and other measurable improvements. All carry a cost, improvements are small, but when it comes to the crunch, may make a difference to your comfort or wellbeing. Just the design of the tread pattern, the scrambling of the tread elements to break up the noise generated, can add considerably to the cost of the mould. Then you have to have the I.T. expertise to be able to produce the noises the pattern makes on a computer first.

If you buy a bad tyre, it will be with you for a long time.

Tread life isn’t the be all and end all. A survey of Australian motorists some years ago showed that the quality most desired in a tyre was the ability to stop, and handle, in the wet. Perhaps the average motorist is more discerning than they are given credit for!

If you would like to know more, have a look at the blog on www.carbonblack.com.au, and the “All About Tyres” section too.

Wire Failure – from Ipods to Tyres

FAILURE – not a pretty word is it? Yet I had two failures this week, both due to FATIGUE.

My steam iron cord failed just where it comes out of the rubber tube at the end of the handle, and my I-pod just where the ear bud cord comes out of the plastic.

Why there? The iron had a long rubber tube, and a spring shaped thingy as well wound around it. Yet it still “failed” there. The wires inside were charred, but very fine. The I-pod wires were just fine, and broken, if you get my drift.

Why do they make them out of such fine wire, you were going to ask. Well if they made these flexible leads out of a thicker wire, they would not last long at all. Witness when you want to bend a coat-hanger till it breaks. It doesn’t take long, and it gets quite hot to hold where you’re bending it.

But like the iron and the I-pod, it will always break first where it is being flexed the most. This needs a bit of explanation.

So it is with tyres. If nothing else destroys it, such as road damage, the tyre will fail where it flexes the most. Wrong! It fails first where the greatest differences between flexibility (the tyre sidewall) and rigidity (the bead/lower sidewall); or upper sidewall to tread and belt area exists. It fails due to fatigue because by then, it will have rotated and flexed at these parts of the tyre, on average 30 to 45 million times for a passenger tyre, and 100 to 130 million times for a truck tyre.  Truck tyres go further because they’re inflated harder, so don’t flex as much, though they may have worn out three tread lives by then- tyre speak for been retreaded twice.

After that, the carcass of the tyre is not worth retreading because it is approaching the unreliable stage due to fatigue. Reliability is highly prized- and highly priced you might say.

Passenger tyres go at least twice as far as they used to 30 years ago, so a large chunk of the fatigue life built into the tyre is consumed in the first tread life. This is why retreading of passenger tyres has declined to such an extent. It is also why the motorist should look after his tyre pressures. The flatter or more overloaded the tyre, the more the tyre deflects as it rotates, and eats into its reserves against the ultimate failure- fatigue.

Incidentally, the wires in the steel belts of tyres are cables of wire made up if strands of fine wire, just like in the steam iron. The wires in the bead, which locks the tyre on the rim, don’t flex, so they are more like a coathanger wire. The iron had lasted quite well really- at least the fatigue beat the corrosion inside the steam chamber!

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.

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 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.

Online pursuit of tyres is no longer tiresome

Sydney, 29 November 2007 – CarbonBlack TyreXchange (CarbonBlack) today announced the launch of its independent CarbonBlack Tyre Scorecard. The new quarterly report details consumer tyre purchasing habits featuring data gathered at the point-of-sale on www.carbonblack.com.au. The CarbonBlack Tyre Scorecard explores pre-purchase consumer triggers for tyre brand choice, brand substitutes, brand preferences and performance feedback. The longitudinal study of consumer behaviours will track the impact of tyre branding activity over time and examine the performance of many existing tyre industry marketing strategies. The October 2007 edition is the first report in a series of high level summaries focused on the automotive industry.

According to CarbonBlack’s founder and managing director, Ms. Jodi Stanton, “Our research indicates that the marketing strategies to tyre consumers undertaken by tyre manufacturers, and distributors vary widely,” Ms Stanton said.

“Obviously some of those strategies are working while others are not – which means marketing expenditure is not being properly targeted. Our research will help direct marketing activities to minimise the risk of missing consumer purchasing triggers, previously difficult to do with more traditional marketing channels such as TV and radio”.

Amongst a wealth of information, the independent report identified:

• certain brands experienced far greater loyalty than others;
• consumers increasingly relied on other consumers to influence their purchasing decision;
• many consumers had reasonably high probabilities of switching brands given the right influence;
• no single brand produced greater than 60% support when considering purchasing that same brand of tyres again (refer graph 1);
• marketing campaigns can produce swings of up to 14% in brand awareness with consumers (refer graph 2);
• the most in-demand tyre requested by purchasers does not even factor on the list of most recommended by tyre dealers (refer graph 3 and 4).

For Stanton the unique aspects of the report will enable the tyre industry to understand what is involved in the purchasing cycle. “Our data is sourced during key stages in the purchasing decision making process when consumer intentions are aligned with their behaviour rather than intention,” she said.

“The report provides ongoing information about the value of brands of tyres in the Australian market that has been collated from information sourced at the time of transaction, from consumer profiling and surveys from consumers actively in the market to purchase tyres.

The results are a fascinating and an extremely valuable representation of the pattern of buying and the actual triggers used by consumers – it is not the theory that a general buying survey might generate,” Ms Stanton explained.

“This will allow tyre industry marketeers to assess the performance of their current messages and also allow them to compare performance against competitors. “

Consumers are growing more sophisticated each year. However, marketing messages tend to be more around umbrella brands and price based promotional campaigns versus specific product differentiation, leaving consumers with little choice but to focus on price.

The first in a series, the CarbonBlack Tyre Scorecard will be followed by reports covering industry issues such as pricing and promotional strategies and channels to market.

The next edition of the Scorecard (January 2008) will be released in February 2008.

Determining the “lead” of your front tyres on a F.W.A. tractor.

Let’s deal with the first question- why do you need to know what the lead is anyway?
Before you start, read the article on ‘Tractor tyres for Front Wheel Assist Tractors‘. This should be ringing warning bells that a little serious study should be undertaken before you need to replace the tyres on your Front Wheel Assist tractor- most frequently the front tyres first.
When the time comes, you might find that these tyres are radials, made in Romania or somewhere (anywhere!), and that you can’t get them, except from the tractor dealer at an exorbitant price. Suddenly your tractor, now 3-5 years old, isn’t such a good acquisition after all.

So some homework is required.

If you fit tyres that don’t match the design engineers’ dimensions, then you run the risk of the tyres failing due to a torque buckle in the sidewall, the transfer case between front and rear drives failing (expensively), or in any case, the tractor chewing up fuel, and getting expensive to run.

There are two ways of checking the lead of the tractor with replacement tyres:

• The book method. If you have the rolling circumference of the front and rear tyres available from printed catalogue information, and are prepared to consult either the tractor handbook for that particular model, or even better, check the specification plate or get under the tractor to check the stamping of the drive ratio on the transfer case, you can do a “book calculation”. Aren’t pocket calculators marvellous! There have been many cases though where the manual says one thing, the transfer case another, so beware. Other forms of the same information are “rolling radius”, or “tyre diameter”, “static loaded radius”, or “revolutions per kilometre”. Note however, that neither “tyre diameter” or “static loaded radius” are fully satisfactory, because they do not allow for the flexing and slip of the tyre as it rolls. They are, as specified, static (stationary) measurements. However, the book method is a way of getting started on the choice of alternative tyres to suit your tractor, and will save you a lot of time in the field.

• The field test method. Our laboratory had a very expensive testing machine installed. On the front of the machine there was a very small plate. You had to lean well forward to read it. It stated “One test is worth a thousand opinions”. The advantage of a field test is that it tests the tractor in the configuration that it is actually going to be used in. The test is carried out on a hard surface. Disengage the front wheel assist and the differential lock to ensure that all wheels are independently free to roll. This may involve keeping it at idle, and check that F.W.A. is not automatically engaged if the engine is turned off. (check the handbook if unsure).

• This method requires a straight, fairly level hard surface up to 100 yards long. A tractor tyre has between 20 and 26 lugs on it, which extend down the sidewall of the tyre. This means that you can divide the circumference of the tyre into around 20-26 segments, so put a splodge of a bright colored paint on the buttress in contact with the ground at the time. (six o’clock position). Hammer a stake into the ground opposite the paint splodges, front and rear, or just lay it on the ground, if you’re sure you won’t trip over them.

• Then tow the tractor forward, with two people alongside, one counting the front revolutions, the other the rear, so you need three people. After ten revolutions of the rear tyres, stop, and lay a stake opposite the splodge of paint, which should be at 6 o’clock. The front tyre splodge won’t be at six o’clock, but probably somewhere up in the air. Count the number of lugs that it has completed in its last partial revolution, keeping in mind the direction of rotation.

Here you have two options. Either place a stake opposite the six o’clock position of the front tyre, and estimate the number of lugs traversed in the last partial revolution (for example 7 lugs out of 24), or better, roll the tractor forward till the splodge is at six o’clock, and lay the stake opposite.
The rolling circumference of the front tyres is given by the distance traversed by the number of revolutions (either partial or complete, the distances are different), and for the rear tyres, the distance traversed divided by ten, so you need at least a 10 metre tape and a couple of markers.

An example, using the whole number of revolutions for the front tyres:

Front tyre

Tyre Size: 13.6- 28
No of revs: 13
Distance b’twn stakes: 50.18 m
Rolling Circ: 3.86 m

Rear tyre

Tyre Size: 23.1-30µ
No of revs: 10
Distance b’twn stakes: 48.57 m
Rolling Circ: 4.857m

You’ve now arrived at the ratio between front and rear tyre rolling circumferences, in this case
4.857 divided by 3.86, equals 1.258
This ratio has to be lower than the transfer case ratio, which can be found on the tractor specification plate, stamped on the casing, or in the handbook. (The latter is the least reliable).

The objective is that the front tyres when driven, have to be rolling FASTER than they want to be when free rolling, and trying to lay down more track than the rear tyres. This generates a “lead”, which makes the tractor easier to drive and steer, it pulls better and is more efficient, particularly so in loose soil. On hard ground (roads), disconnect the front drive to avoid axle windup.

The front tyres are DRIVEN, on average 2% faster than the rears considering the amount of track they lay down. The extra 2% or so is taken up in soil slip, which is what makes it more efficient. In the example above, a transfer case ratio of 1.28 would give a lead of 1.75% (1.28 divided by 1.258, minus 1, multiplied by 100). By the way, the replacement tyres don’t HAVE to be radials, as long as they match the dimensions and the rim widths, they’ll be O.K. Be aware though, that bias ply tyres vary in diameter with pressure, while radials don’t. With bias ply, pressure adjustment can be used to give a final “fine tune” of lead, provided that the usual operating parameters are observed on minimum and maximum pressure, and that you can tolerate the harder ride that might result!

So vent out all that water ballast in your rear tyres. That’s “old technology”, and not applicable to F.W.A. tractors. Reap the benefits at the diesel bowser.

Stretch Limos

Makes you feel envious as they glide by, does it? It shouldn’t- just console yourself that they wear out their front tyres much faster then the long wheel base Fairlane or Mercedes that they were derived from.

You see, a car turns in a circle round an imaginary point some distance out from the side of the car. The rear wheels track following the fronts, the outer wheel on the circle travelling further than the inside wheel. The differential takes care of the difference. The front wheels behave differently. The inside front wheel turns in a tighter circle than the outside, because it is closer to the imaginary point, the “centre’ of the turn. The angle of the outer wheel is not as acute, because it travels in a wider circle. It’s 2 metres or so further away because of the width of the vehicle.

Now when the vehicle modifiers stitch in another two metres or so of bodywork, and a longer tailshaft and underpinnings, the relationship of the front wheels to this imaginary point is affected. The front wheel angles no longer match the designer’s intentions, and so the tyres “scrub” in ways that were never intended. They wear out fast.

This is why your front end mechanic doing an alignment checks the “toe out on turns” at full lock, to check that nothing has been bent. The steer angles of inside and outside are different at full lock.

If you wish to see the differing angles, look at the front wheels of a forward control vehicle like a Toyota Hi-Ace, or a vehicle with the ability to do really tight turns. And if you customarily do very tight turns, such as garbage trucks manouvering, or hooning around the inside lane at roundabouts, then you have joined the stretch limo brigade, wearing out your tyres faster, because the imaginary point is now much closer than the designer engineer accommodated in his design.

Tyre Review Interpretation by Allan Henry

Allan Henry is the “Major Accounts And Business Development Manager” of Assist Australia. Today, he writes about the interpretation of “tyre reviews”.

Carbonblack offers tyre reviews from experts, tyre dealers and tyre buyers to make it easy for you to buy tyres. But the most important thing is to interpret correctly these tyre reviews… Read this article and make the right tyre choices.

Tyre Performance Parameters:

• Wet grip

• Dry grip

• Wet braking

• Steering response

• Noise level

• Tread wear.

Ranking of importance based on:

1. Safety – the first priority
2. Ability to control the vehicle – also a safety issue
3. Comfort & fatigue factors – on long trips, also a safety issue
4. Cost factors

Note that cost factors may be very important to some drivers, but less so to others who may be prepared to pay a higher price to gain greater safety.

Criteria	Why is it important?	Ranking of importance (more stars = more importance)
Wet grip	Affects steering & braking on wet roads	***** Safety issue
Dry grip	Affects steering & braking on dry roads	***** Safety issue
Wet braking	Ability to stop on wet roads	***** Safety issue
Steering response	Affects driver’s ability to steer the car	**** Driver control issue
Noise	Comfort and Fatigue factor	*** Fatigue issue
Treadwear	Cost issue	Individual – important to many drivers

1. Wet Grip

Refers to a tyre’s level of grip on a wet road, when cornering, braking or accelerating.

Obviously, the water depth on the road surface and the speed of the vehicle are very significant factors in the level of wet grip. However, the tyre tread design, tread rubber compound, and tyre construction all have a significant affect on a tyre’s wet grip, as does the remaining tread depth, or amount of tread wear. Operational factors – tyre load and pressure, and environmental factors such as ambient temperature also play a part. The type of road surface – bitumen, or concrete, and macro and micro surface roughness, and amount and type of surface contaminants (such as oil) also have significant effects. Generally, coarser road surfaces improve a tyre’s wet grip. However, a factor within the control of all drivers – the speed at which the vehicle is driven – is one of the most crucial factors.

Most people have heard of the term “aquaplaning”, and some understand that it results in a loss of control of the vehicle. This is because the tyre tread is unable to clear away the water on the road surface, and the tyre in effect, “rides on a cushion of water”, much like the planing hull of a speedboat. It is necessary for the tyre to make contact with the road surface so the driver can control the vehicle’s direction and speed.

When there is water lying on a road surface, the tyre tread must squeeze the water out of the contact area (or “footprint”) as the tyre rolls along the road to make contact with the road surface. This takes a small but finite amount of time. As the speed of the tyre increases, the amount of time the tyre has to remove the water becomes less and less. Within the “footprint” of the tyre, the proportion of the area in contact with the road gradually becomes less as the speed increases, and when aquaplaning occurs, there is no contact at all. Note though, that at typical highway speeds in heavy rain, the “footprint” area in contact with the road surface can be less than 50% of the normal contact area. The tyre’s grip is also commensurably reduced.

It is sobering to consider the amount of time the tyre has to remove road surface water from under the tread at normal vehicle speeds – see the table below, calculated for a typical tyre footprint length of 160mm:

Speed		Time to travel footprint length Seconds
km/hr	m/sec
60	16.67	0.0096
80	22.22	0.0072
100	27.78	0.0058
120	33.33	0.0048

The time the tyre takes to travel one footprint length, is the maximum time it has available to squeeze out the water on the road surface, before all contact with the road surface is lost!

2. Dry grip

Refers to a tyre’s level of grip on a dry road, when cornering, braking or accelerating.
The level of dry grip basically determines how hard the tyre can brake, accelerate or turn the vehicle through a corner or curve. The level of grip available is affected by many factors, including:

• Tread rubber compound and tread pattern design,

• Tyre shape, aspect ratio, and construction

• Interaction with a vehicle’s suspension

• Operational factors – load on the tyre, inflation pressure, and

• Environmental factors – the road surface, contaminants, and weather conditions

3. Wet Braking

Refers to a tyre’s level of grip on a wet road when braking.

Refer to “Wet Grip” for more information.

4. Steering Response

Usually refers to how quickly a tyre responds to a sharp steering input from the steering wheel, as in (the first part of) a lane change manoeuvre.

This is a tyre performance parameter that helps the driver control the vehicle, particularly when the unexpected happens and an evasive manoeuvre is necessary. Up to a point, the faster the steering response, the better, but there is an ideal level. If the response is too quick, it can make the tyre/vehicle combination “twitchy” or over-sensitive to driver inputs. Car and tyre manufacturers go to enormous trouble to ensure the steering characteristics designed into a car and tyre combination, suit the intended purpose of the vehicle. A sporty car and tyre package will have different steering response designed in, than would be the case for a family sedan.

Generally speaking, the steering response of low “profile” or low aspect ratio tyres is quicker than higher profile or high aspect ratio tyres, which is why low aspect ratio tyres are specified by car manufacturers for their sports oriented models.

5. Noise Level

Refers to the volume of noise generated by the tyre when a vehicle is moving. A tyre generates noise as it rolls along the road surface, mostly from the tread pattern of the tyre.

The aim of tyre designers is to design the tread of the tyre so that it generates a noise as close to “white” noise, and with as low a noise level, as possible. That is, the noise will have no significant tonal characteristics, and be very quiet. The original tyres fitted to basic family sedans tend to be the quietest tread designs. There are a number of reasons for this. One is that these vehicles have less noise reduction material fitted (and car manufacturers therefore specify a quieter tyre for the vehicle), another is that the mix of tyre performance parameters will be different from that required for a sporty or prestige vehicle. High levels of wet grip for example, generally requires a more “open” tread design, which will usually result in a noisier tyre. As a general rule, the more aggressive or “sporty” the tread design is, the more likely it is that the tyre noise will be louder and more annoying. Some SUV or 4WD tyre designs have very aggressive tread designs to provide good grip in off-road, muddy conditions. These designs can be very annoying on bitumen roads, and tiring for the passengers during a long trip on sealed highway roads.

Tyre noise also varies significantly on different road surfaces. On smooth, polished road surfaces, noise generated by the tyre tread predominates, whereas on coarse, new road surfaces, tyre construction and materials also have a significant influence.

If tyre noise, or the lack of it, is important to you, it may be best to replace your worn tyres with some of a similar design and quality to those originally fitted by the car manufacturer.

6. Tread Wear

Usually refers to the distance the tyre has covered before the tread is legally worn out, when the Tread Wear Indicators (TWI) are level with the worn tread surface.

Many, many factors affect the tread wear of a tyre. Tyre tread design, tyre construction and materials, the load on the tyre and the inflation pressure, the speed of travel, wheel alignment, driver inputs (gentle or aggressive steering, braking and acceleration) and many environmental factors all have a greater or lesser influence on the tread-wear of a tyre. Even the age of a tyre (since it was manufactured), and the remaining tread depth have an influence on the rate of tread-wear.

Motorists generally cannot control the environment in which they drive – the type of road surface and layout (smooth or rough, straight and level or curved and hilly), and the weather conditions (wet or dry, hot or cold), but can maximise the tread-wear from their tyres by:

• Maintaining the correct tyre pressure (check at least monthly). If the size of tyre fitted to the car is listed on the tyre placard, use the recommended pressure as the minimum pressure to use.

• Regularly “rotating” the tyres’ position on the vehicle (every 10,000 km is usually a reasonable frequency)

• Maintaining the vehicle wheel alignment to the car manufacturer’s specifications.

• Braking, accelerating and cornering smoothly and gently

Low tyre pressures increase the rate of wear on a tyre, as well as reducing or eliminating the tyre’s reserves of safety. Poor wheel alignment can also cause very rapid wear to all or part of the tyre tread surface, and render the tyre unroadworthy. The worse the tyre misalignment is, the more rapid the rate of wear. Similarly, rough or aggressive driving will also significantly increase the rate of wear. For example, a small increase in cornering speeds will result in a much greater rate of wear.

When is it Time to Replace your Tyres

When is it time to replace your tyres? Should I wait till they’re bald? When are they bald?

There are two schools of thought. One is based on commonsense, the other the law. The law has to define things clearly, so that it is unambiguous.

The law states that tyres are illegal when they are worn down far enough to show the tread wear indicator bars in a band across the tread. How far across the tread (all the way, or merely two ribs of the design) is defined in the State Roadworthiness or Vehicle Inspection regulations. Not every State has an annual vehicle inspection prior to re-registration.

The tread wear indicator bars are a raised portion of the tread groove, approx 12 –15 mm wide, which show as a bar around the tyre in about six places when the tread is worn down to 1.5 mm of tread pattern remaining. There is an assumption here that the tread pattern will wear evenly across the tread, which is far from the case, common causes being misalignment, or heavily cambered wear on front or rear independent suspensions which can wear the very edge of a tread pattern prematurely.

However, the bars serve a useful purpose, and are much more realistic than the old ‘stick a match head in the tread groove, and if you can see the head, it’s bald’ trick.

That leaves commonsense.

Tread patterns are there only to remove water from under the tyre, so that it can grip the road, not a film or sheet of water. So if you live in Innisfail or Tully in North Queensland, your requirements for wet road adhesion are somewhat higher than for Uluru. It isn’t generally realised that the ability of a tyre to clear water away from under it, declines as the tread pattern wears away, simply because the grooves aren’t as deep. This is progressive. It doesn’t suddenly fall away with only 1.5 mm left; it has been falling away all the time since new. So if you have noticed that the tyre performance isn’t what you expect, you slip and slide in the wet, it’s time to have a look at them.

If you know that the big wet is due, that you are traveling into a high rainfall area, that it’s getting close to winter in the southern States, it’s time to check your tyres. Who knows, the drought might even break with a ‘big Wet’.