Merityre Article Archive

All car tyres may seem to be the same but they aren’t. Road tyres have different characteristics designed to maximise their performance for specific types of use. However, these differences pale into insignificance when you consider the different tyres offered for competition use.

Take rally car tyres. These must cope with a huge array of surfaces in innumerable different climatic conditions. In addition, rally car tyres usually have to be road legal and must last longer than racing car tyres.

Examining two types of rally car tyre in more detail gives an idea of how the various factors in the tyres interact. Gravel tyres have to clear away the top coating of soft dirt or loose gravel, just as car tyres must clear away rainwater. To do this, they have big, chunky blocks that are made of relatively soft rubber. This lets the tread’s blocks grip while protecting the reinforcing plies that lie beneath them. The tyre tread and carcass’s being rigid enough to resist tearing while being sufficiently elastic to return to their initial shape guarantee toughness and a long life.

Snow/ice tyres can boast an effective grip-enhancing addition. Some can have up to 380 carbide-tipped studs in their tread. This is tough on the surface beneath the tyre but the surface can be tough on the studs themselves. At 75mph, each stud hits the snow or ice 17 times…per second! The impressive part – other than that (most of) the studs are still present when the car stops – is that the car is running on a total of about 50 studs at any one time. The total area the studs offer is about the size of a postage stamp.

In some instances, the tyres have no tread at all. Such tyres are called slicks and are used in drag racing. ‘Top Fuel’ dragsters are the fastest and can reach 330mph in less than 4.45 seconds – the time it takes them to cover the quarter mile. Accelerating faster than the space shuttle launch vehicle or a catapult-launched jet fighter requires as much grip as can be found.

It’s common for all kinds of dragsters to do something memorable. The driver places the car so its rear tyres are sitting in two puddles of water laid down near the start of the drag strip. The driver then applies power, spinning the rear wheels in the water. The immediate result is huge clouds of rubber smoke and steam around the tail end of a car that’s standing virtually still at practically full throttle. Yet this isn’t for show. After a ‘burn out’ like this, the tyres’ tread surfaces are extremely hot and sticky – and therefore grippy.

Often, drag racing car tyres look distinctly soft as the car approaches the start line. This is unsurprising, as the tyre pressure used is around the 12 to 15 psi mark. Why so soft? For two reasons. Soft tyres give better grip but they have another benefit. As the wheels rotate faster, the tyres enlarge. This is like putting bigger wheels on the same car; drivers get a higher top speed from the tyres’ radial expansion.

As you can see, even from just these two forms of motor sport, specialist car tyres are essential. They do what’s asked of them and in doing so, they increase the performance of the car involved in whatever sport is concerned. Think about this the next time you find yourself believing car tyres are all the same!

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All car tyres are round, black things with sidewalls and treads, and they need to be inflated from time to time. So, they’re all the same, aren’t they? In fact they aren’t all the same and to say they are is just like saying all houses are the same.

This is about the difference between common aspects and specifics. True, every house has a roof, doors and windows but a two-up, two-down terraced property and a mansion, though they’re both houses, are clearly very different. The same goes for tyres, albeit less obviously. Here, we can consider five distinct types of tyre, and these will illustrate the differences.

Standard or all-season tyres are the most commonly fitted type of car tyre. As their name implies, they’re the jack-of-all-trades of the tyre world. They work well in both dry and wet conditions. They aren’t too noisy on most road surfaces and their fairly hard rubber compound gives them a long life. These factors represent compromises, of a kind that work for most requirements – and for most drivers.

Performance tyres, on the other hand, balance certain factors in order to enhance certain characteristics. Car tyres of this kind don’t last as long as standard tyres but their softer compound yields better grip. Similarly, performance tyres don’t appreciate colder conditions, which is why they’re often called ‘summer tyres’. They also don’t readily suffer significant wear. Performance cars – and demanding drivers – deserve performance tyres.

Winter tyres are car tyres for use when snow and ice are liable to be encountered. These tyres have a more marked tread pattern, which is great for giving the best grip and preventing snow and ice from packing in the treads’ grooves. They are also made using compounds that remain flexible even when thermometers are reading low. Some even have integral metal studs to maximise grip. The costs for these specific characteristics are twofold. First, winter tyres create a lot of noise, even on smooth roads. Second, in normal conditions, they wear very quickly and can damage the road surface. The kind of conditions where you’d need winter tyres are obvious, as is the fact that they can’t be used year-round.

All-terrain tyres are the top of the range tyres for unfriendly conditions. Their big, open, chunky treads can grip on loose surfaces such as gravel and sand. Knowing that such surfaces are liable to be in rough country, manufacturers make all terrain tyres with tougher, stiffer sidewalls so they’re less susceptible to damage from potholes and debris. These tyres are sturdy and generally long-lived, at the cost of a considerable noise level on smooth tarmac. There is a still more specialized tyre for adverse conditions. Mud tyres have an extremely large tread block pattern and are suitable for use only in muddy conditions.

Car tyres of this kind are often used on the kind of four-wheel drive vehicles that are actually taken on rough roads; this includes 4x4s that specifically go off-road regularly.

Finally, run-flat tyres are car tyres that are capable of still being used even though they’re no longer inflated because of a puncture. Now appearing on more and more modern cars, they allow you to retain control of the vehicle despite being at zero pressure. However, they can run flat only for a given distance and at reduced speed – your car’s handbook will have the necessary details.

Which type of car tyres you choose to buy is clearly a ‘horses for courses’ decision, largely depending on what you want from the tyres. However, should you be in any doubt, the staff at any good tyre bay will be happy to give you their professional opinion on what type will be best for you.

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Here’s something about car tyres that’s both interesting and thought provoking. Take a look at the sole of your shoe. Why? Well, you may not know it like the back of your hand but here, you’re looking at your shoe’s sole for a reason. The area of the sole of your shoe is approximately the same as that of a car tyre’s contact patch, the amount of tread it has on the road at any one time. Multiply this area by four and it represents all that’s stopping your car, and anyone on board, from sliding off into the scenery when travelling.

Now, say your car weighs one tonne. The car tyres’ four contact patches therefore have to keep one tonne on the straight and narrow. Or do they? In fact, they must often cope with greater loads. When you drive round a corner, you feel centripetal force (not centrifugal force) pushing you towards the outside of the car. Your car tyres create this force and while it’s unlikely that your car can create a cornering force of 1g, it might well do so under very heavy braking. So, during accelerationg, cornering and braking, that four shoe soles’ area is coping with keeping you on the road, rain or shine, summer or winter.

The law says that the minimum depth of the tread on your car tyres must be 1.6 millimetres, across the central ¾ of the tread around the complete circumference of the tyre. If you haven’t this amount of tread on your tyres, you might get 3 penalty points on your driving licence and a £2,500 fine. This is per tyre, so multiply it by four for the worst-case scenario.

There is, however, an even worse case scenario that affect car tyres. RoSpa, the Royal Society for the Prevention of Accidents carried out some tests. On a hot rolled asphalt surface, a car with barely legal tyres travelled 36.8 per cent further before stopping. On a smooth concrete surface, the same car went 44.6 per cent further. Unsurprisingly, RoSpa recommends tyres be changed when the tread depth falls to 3 millimetres.

There’s another risk that worn car tyres create in the rain. You may have winter tyres, summer tyres, asymmetric tyres or run flat tyres. All have treads and when the treads are badly worn, their contact patches lose their ability to quickly dispose of surface water. When a tyre aquaplanes, it’s riding on a thin film of rainwater it can’t get rid of. Then, you have no appreciable grip and therefore very little control. There’s a memorable black and white picture, taken when Dunlop Tyre’s technical team identified aquaplaning in 1962. In it, a Mark Two Jaguar is shown at speed on a wet test track. White segments painted on its tyres’ walls show that the front tyres aren’t rotating at all – scary. Car tyres may have improved since then but like its cause, aquaplaning remains.

So, running your car tyres down the hilt tread-wise may be economical but it could cost you far more than you thought possible. Happily, you can keep a check on your tyre treads’ health. All tyres have tread wear indicator bars moulded into the tread pattern. When – or preferably a little before – these are no longer lower than the tread surface, it’s new tyre time. You can also buy a tread depth gauge from most motor parts outlets. Failing all else, you can check the tread depth using an old 10 pence piece. The ring of dots around the rim indicates 1.6 millimetres when the edge of the coin is pushed into the remaining tread. However, Matthew Dent didn’t retain this element when designing the ‘Royal Shield’ coins introduced in summer 2008.

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In the past, people used to change car tyres by hand. Some still might but this arduous procedure has been semi-automated for a long time. Here’s how professional tyre fitters change a car tyre, using a tyre machine.

 As the tyre to be replaced will probably have been in service for a long time, it’ll be reluctant to part from its wheel. The beads, where they abut the wheel, will be firmly stuck and this is where the tyre machine’s first weapon comes into play. After unscrewing and removing the valve to let the air out of the tyre, the fitter will position the wheel in the jaws of the bead breaker. One press of a pedal and the bead is ‘broken’ – separated from its location.

 Next, the fitter will lay the wheel flat on the tyre machine, clamping it into place. Since wheels – especially alloy ones – are both delicate and expensive, the equipment is designed to not touch the wheel rims directly. Having levered the upper bead upwards over the wheel rim, the fitter will engage the hooked head of the tyre machine under the bead. Another pedal press sets the machine rotating, to skin the tyre off the upper rim. Repeating this process for the lower bead separates the old tyre from the wheel.

 A little preparation of the wheel comes next. The old valve body is sawn off inside the wheel and if necessary, the inner faces of the wheel rims are cleaned. The old wheel balancing weights are removed. Having pulled a new valve into place with a special tool, the fitter will coat the wheel rims and the new tyre’s beads with a special compound. This is a lubricant that eases the fitting process though sometimes, a material with sealing properties may be needed.

 The penultimate stage of tyre fitting is essentially the reverse of the removal of the old tyre. The fitter uses the tyre machine to force the new tyre on to the wheel, bead by bead. Finally, a compressed air line is used to inflate the tyre, until its beads pop outwards to seal against the inner faces of the wheel rim. Throughout the process, the tyre fitter has done the controlling, while electricity and compressed air have done the heavy work.

 This leaves just the wheel balancing. The fitter will have already positioned the tyre correctly in relation to the wheel, using a coloured dot marking on the tyre. However, balance weights are virtually always required and there’s a machine to help with this task. The tyre fitter will clamp the wheel onto the machine. When the guard is closed and the machine starts, it will take the wheel up to a fixed speed, usually about 200 revolutions per minute.

 Using its onboard sensors and electronics, the balancing machine points out where the wheel weights should be fitted and how much they should weigh. Having fitted the weights – stick-on or clamp-on as appropriate – the fitter will run the machine once more, fine-tuning the balance with other weights if necessary.

 As we can see, what used to be an hour or more’s hard work is done in minutes. What’s more, if you watch the proceedings from the waiting area, you’ll know exactly what’s happening in the tyre bay!

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Car tyres can be punctured and go flat. When it’ll happen is unpredictable, although it’s never at a convenient time, but you can change a wheel quickly and safely. Details follow, assuming you have the necessary equipment; you may wish to acquire some of the items mentioned.

There is a cardinal rule about wheel changing – never attempt it on the hard shoulder of a motorway or busy road. If reasonably close to an exit or turnoff, you can ‘nurse’ your car to a safe spot but if you can’t, call your breakdown service. Trying to change a wheel inches from a busy traffic lane is far too dangerous to contemplate.

Your safe spot needs to have a firm, smooth surface. Park your car, turn on its hazard lights and set up your warning triangle. Then find the equipment you need. Your car will have a jack and wheel nut wrench in its toolkit and the spare wheel will be either in or under the car. The handbook will tell you where the spare lives and how to extract it. Before starting, have any passengers get out of the car and stand well away from the car and passing traffic.

And so to work. Ensure your car’s handbrake is firmly applied and that the car is in gear or, if automatic, is in ‘park’. Then, chock the wheel diagonally opposite to the one you must change. Now to position the jack. Place it exactly where the handbook says – nowhere else will do – and raise the car just enough to check the jack is firmly seated. Remove the wheel trim if one is fitted, cutting through any cable ties retaining it. Alternatively, if there are any, remove the wheel nuts’ caps.

Loosening the wheel fastenings can involve applying considerable force. Ensure the wrench engages completely before starting to turn each fastening – they’re usually loosened anticlockwise. The car’s weight stops the wheel turning and all you do now is slacken the nuts or bolts. Always apply force downwards. You may stand up and press on the wrench with your foot but support yourself against the car. If one fastening is an anti-theft locking nut/bolt; there’ll be a fitting you can use with the wheel wrench to loosen it.

Next, jack the car up until the wheel with the flat tyre is off the ground – and never place any part of you between any part of the car and the ground. Now, remove all but the topmost wheel nut/bolt. Then, you can support the wheel, remove the last nut or bolt and take the wheel off. Note that a wheel and tyre combination can be heavy so make sure you lift it with a straight back and a firm footing.

Now to fit the spare wheel. Lift it into place on the hub, align the nut/bolt holes and fit the topmost one. Turn it finger tight then replace the remaining fastenings, also finger tight. Lower the jack and remove it, then fully tighten the fastenings in a diagonal pattern. You can now put the equipment you’ve used – and the offending wheel and tyre – back where it belongs.

All finished now? Not quite. Nowadays, wheel fastenings must be torque tightened to a specific degree. You also need to check the pressure in the spare tyre, adjusting it if necessary; and of course, have the punctured tyre repaired or replaced. The first two tasks should be carried out as soon as possible, for safety’s sake.

Two further points remain. The first concerns making wheel changing as easy and safe as possible. Refinements exist and you may want to take advantage of them, buying a pair of heavy gloves and a reflective waistcoat, and acquiring anything else you might need. You may also want to practise a wheel change at home, when it’s warm and dry.

The second and final point concerns ‘space saver’ spare wheels. Your car may have one and restrictions of speed (usually 50mph) and distance apply to them. In some cars, after a space saver spare is fitted, some dashboard warning lights may illuminate. Some ABS, traction control and automatic gearbox sensors can be confused by the presence of a space saver spare. Once again, your car’s handbook will tell you about this.

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