All About Tyres

     

When to Replace Your Tyre

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When is it the Right Time to Replace Your Tyres?

Don’t think of that beer just yet! Firstly, it is important to understand the dynamics of tyres and when those dynamics are being threatened by wear and tear, use and abuse That time could be anytime. Read more

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Tyre Tread Wear

Unlike physical structures like skin and bone, tyre rubber cannot re-generate itself. Once it’s gone, it’s gone. New tyres are then needed. However, protection of the structure, but reducing aggressive driving – starting, stopping and cornering in particular – can lengthen the life of the rubber on your tyres. Read more

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When is it time to replace your tyres?

Identification of telltale signs of wear on your tyres will assist in keeping your driving experience and your safety to the highest required levels. Forming a habit of checking your tyres can help in this identification process. Read more

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When is it the Right 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’.

We wish!

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Tyre Tread Wear

TYRES WEAR OUT. ‘Isn’t that great!’ say the tyre dealers. ‘Need some new tyres already, and gee, do they cost a lot’, say the motorists. Of course the tyre companies do make a tyre that never wears out. It’s called the ‘spare wheel’. (Joke)

Tyres wear out because they grip the road by developing friction. Friction is a study in itself, but without it, you would get nowhere – no acceleration, no braking, and no steering. This all occurs where the tyre meets the road. The less movement of the rubber under the footprint, the less the tyre wears. The less heavy braking, or fast cornering you do, the less the tyre wears. The reason – in order to develop friction, the tyre must slip to develop grip. In order to corner, the tyre surface must distort at the contact patch by slipping. This is happening all the time. When your tyres chirp as you accelerate, or squeal as you brake violently, you are only hearing what goes on all the time.

The A.B.S. braking systems on modern cars pick the point of maximum grip, which is just before the tyre starts to skid, to optimise their brake performance. A.B.S. systems are calibrated to pick this point of maximum grip. They ‘let go’ just before the tyre starts to slide, pulsing far more rapidly, on and off, than even a skilled motorist could do, to maintain maximum braking power. So realise that it is not the brakes that stop the car, it’s the tyre grip.  If there is no grip available, like on ice, then you have trouble going anywhere at all!

Due to excessive movement of the tread rubber, the bias ply tyre, which was the normal tyre 30 years ago, reached its engineering limit. It couldn’t go much faster, got too hot, wore fast, and distorted at high speed. Michelin invented and marketed the steel belt radial in 1948, patented its method of construction, and the world’s tyre engineers had to wait till the patents expired. An interim radial tyre, called the Pirelli Cinturato, was developed in this period, which didn’t infringe the patents. At one stage, 44 tyre companies around the world were making tyres of that design.

You were doing well to get 25,000 kilometres out of a set of bias ply tyres, and with steel belt radials, this has doubled. Putting steel belts on a radial casing allowed the tyre to develop its cornering power at a lower slip angle, so it didn’t wear as fast. Incidentally did you know that hooning around roundabouts can wear your tyres up to eight times faster than straight ahead running?

However, the first radials were down on wet road holding, being skinny by today’s standards. In other words good tread wear and good grip proved difficult to combine in the one tyre. It still is.

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When is it time to replace your tyres?

Tyres have grooves in the tread pattern. In fact, grooves are the tread pattern. But the depth of the grooves varies, either for cosmetic, drainage, pattern stability, but mostly, because the law requires them to. If you look on the sidewall of the tyre, up near the tread, at around six places around the buttress you will see tiny letters ‘T.W.I.’, for ‘Tread Wear Indicator’ This marks the spot where the tread groove is 1.5 mm shallower than the rest of the groove around the tyre. All the grooves across the crown of the tyre are shallower at that point.

When the tyre wears down to a depth of 1.5 mm of pattern remaining, a bar of solid rubber about 12 mm wide appears right across the tyre tread. The remainder of the groove circumference is still 1.5 mm, and your tyre legally is due for replacement. Before these bars were used, the usual test was to place a match head in the groove. If you could see the match head proud of the tyre surface, it was worn out. Nowadays, who carries matches? The disposable lighter has made them obsolete. Who smokes? So these T.W.I. indicators are a very valuable tool in enforcing a legal requirement for a roadworthy vehicle. The garage mechanic, or the policemen, are trained to look for them.

They can also be misleading. You might be led to think that your tyre is absolutely perfect until the bars appear. Be aware that the ability of the tyre pattern to shift water from under the tyre reduces progressively as the tyre wears down. That is the only purpose of the pattern after all. The deeper the pattern grooves, the better the wet road hold, but there is a practical limit on how deep the pattern can be. 20% deeper pattern would not necessarily give 20% more tread life from the tyre.

So, every 60,000 kilometres or so, get onto our web site, and enter the challenge to once again equip your vehicle with these black doughnuts. ‘60000! I’ve never had that much out of a set of tyres’ you say. That’s because you are probably driving a front wheel drive. The front wheels are doing all the work (slip), accelerating, steering and braking, while the rear wheels are trundling along for the ride, holding up one corner each. Easy!

     
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