Merityre Article Archive

Low Cunning

Fashion has followed performance, which is why ultra low-profile tyres have become more common. Every car tyre has an aspect ratio, which is a comparison of its height in relation to its width. A ‘full profile’ tyre has a section height that is 80 to 85 percent of its width; such tyres are rarely seen these days, unless a classic vehicle is riding on them.

Aspect ratios have been falling over the years. 70 percent profiles came along, followed by 65 percent, 60 percent and so on. Nowadays, 45 percent profile tyres can be seen on high-performance cars and the current low figure is a mere 35 percent. Beware though, ultra-low profile tyres may look thrilling and they certainly offer tremendous performance. There is, however, a twofold cost. The lower you go, the more ride comfort suffers. What’s more, wheel damage from touching a kerb, or hitting a pothole or debris in the road, is far more likely.

Formula One Facts

A Formula One car can accelerate from rest to 99.42 miles per hour and decelerate back to zero in four seconds. Even though such a car weighs only a little over half a tonne, the breathtaking forces involved clearly give the tyres a hard life. Here are some captivating facts from F1′s sole tyre supplier, Bridgestone.

The company produces 40,000 of its Potenza F1 tyres each year and takes 1,800 tyres to each grand prix.

During the 2008 grand prix season, 19,435 laps were completed on Potenza F1 tyres.

A Bridgestone tyre fitting team can fit 56 car tyres per hour, that’s 14 sets of tyres. Sixty of the company’s personnel attend each grand prix, taking with them 12 tonnes of specialised equipment.

At just over 186 miles per hour, Bridgestone wet weather tyres displace 61 litres of water per second.

Potenza F1 tyres are built to last – for around 93 to 124 miles under racing conditions.

Back to the Real World

Now for some less esoteric facts and figures, from the everyday world of motoring.

About 75 percent of British motorists are driving on under-inflated car tyres.

What this is doing to their carbon footprints isn’t known but it is costing them over 2 billion pounds every year.

Approximately 12 per cent of the cars in the UK have at least one defective tyre.

Another 12 percent have a tyre with less than two millimetres of tread, which is legal but decidedly unsafe.

Ninety percent of Britain’s motorists don’t know the pressure to which their tyres should be inflated.

Continental Tyres carried out a study, which discovered that 40 per cent of British motorists claim never to have checked the pressure in their car tyres.

More than 40 per cent of drivers admitted that they didn’t know how to check tyre tread depth. Seventy per cent had no idea of the legal minimum tread depth, which is 1.6 millimetres over 75 per cent of the tread’s width, in a continuous band.

About 20 percent of drivers confessed to driving on tyres that would probably not meet legal tread limits.

These sobering figures, apart from highlighting the need for a certain amount of education for UK drivers, suggest something else. It’s comforting that the average UK driver isn’t in charge of Formula One car tyres!

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In many respects, modern cars are computers riding on tyres. Much of what happens below decks may seem to be under the driver’s control but in fact, computerised assistance is ever-present and sometimes, the computers take over. Here, we can take a look at some computer-controlled elements that specifically relate to the work of car tyres. We’ll find ourselves knee deep in acronyms but don’t worry, each will be explained.

Lets start with ABS. Which is? The Anti-lock Braking System. ABS first appeared in 1929, when Gabriel Voisin, a French aircraft and car engineer, developed a working system for aeroplanes. In the production car world, Chrysler later teamed up with Bendix to produce ‘Sure Brake’. This, a fully computerised all-wheel ABS system was available on the 1971 Imperial. Not to be outdone, General Motors stopped car tyres skidding with ‘Trackmaster’, a rear-wheel only system that first appeared on their 1971 models.

So how does ABS help car tyres? A typical system has a central electronic control unit (ECU), a speed sensor at each wheel, and hydraulic valves within the braking system. Under hard braking, a car wheel that is rotating significantly slower than its counterparts is on the point of locking. The ECU detects this and commands the appropriate hydraulic valve to reduce the braking force at that wheel. As the wheel speeds up again, the ECU reimposes full braking force – this can happen up to 20 times per second.

What does this mean in practice? When the ABS keeps the car tyres at the point of locking but doesn’t let them lock, the car is being braked to the limit of the tyres’ adhesion. This represents maximum braking efficiency. Moreover, skidding car tyres have actually lost their grip. The driver may be applying maximum braking force and locking the wheels but braking and steering control are lost. Under the influence of ABS, it becomes possible to still steer the car around an obstacle.

Now for another acronym, which describes a system that has evolved from ABS. Electronic Stability Control, or ESC, adds two further sensors to the ABS armoury. The first, a gyroscopic sensor, can detect when the car’s direction of travel doesn’t accord with the position of the second, the steering wheel angle sensor. The ESC’s software can then apply braking force to up to three of the wheels individually. Consequently, the car tyres’ grip is used to assist in making the car travel in the direction the driver has chosen. CBC, or Cornering Brake Control, is a refinement in ESC. This system can control the ABS, telling it that the wheels on the inside of the curve being taken should be braked harder than the those on the outside. It also dictates how much additional braking force is applied.

Paradoxically, an ABS-related system can help car tyres to retain grip under acceleration. A Traction Control System, a.k.a. A TCS, detects a driven wheel that’s spinning and briefly applies braking force to allow the spinning car tyre, or tyres, to regain traction. More sophisticated TCS systems can control the throttle level simultaneously, cutting power to the spinning wheels.

As you can see, these electronic aids to safety are complicated. Different manufacturer’s calling them by alternative names further complicates the issue. For example, some call their TCS an ASR, an Anti Slip Regulator. The above is a very simplistic explanation of such systems and there are other, similar systems in use. The bottom line is that they all help a car’s tyres make maximum use of the grip they offer.

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