One of the more annoying problems with owning a car is when the air escapes gradually from your tire*, leaving you with a flat.

A more lethal problem is a sudden blowout at high-speed. According to Michelin statistics, there are around 23 000 accidents each year in the US due to punctures. Around 500 people lose their lives each year (in the US) as a result of such accidents.

*The Brits spell this word tyre, and that's what I'm going to do.

But how often do you get a flat?

It depends, to a certain extent, on how rich you are. There is no doubt that the first owner of a BMW or Lexus gets far fewer flat tyres than the fourth owner of a Volkswagen Rabbit or Golf.

Most people who own a one-year-old Mercedes have the money to keep it properly maintained. Owners of 6-year-old Volkswagens are less likely to find the money to keep their vehicles properly maintained. According to statistics, anyway.

But the industry average shows that of the four wheels on your car, each of them is likely to see a puncture every 150 000 km or so. Which sort of means that an average car is likely to see a puncture every 40 000 km or so. Once in two to three years sounds about right.

Punctures, however are rarely catastrophic. They can be classified by how fast the tyre loses pressure. If the pressure all drains away in less than a second, the puncture can be called catastrophic. The sudden loss in pressure will almost certainly lead to loss of control, especially when driving at highway speeds.

Even if the pressure loss happens in 10 seconds, it is fairly dangerous. But if the pressure drains away in an hour or longer, there is plenty of time to bring the vehicle to a controlled halt, and probably get it to a garage or service centre.

Over 90 percent of punctures allow more than an hour between the incident and the tyre all effective pressure. This data from Pirelli.

That's the good news. The bad news is that most cars nowadays are so comfortable that drivers rarely notice when the pressure has dropped by half. Most drivers will continue driving with only a quarter of the air left in their tyres. Many will continue to drive with a flat, oblivious to the dramatic increase in rolling resistance and poor handling that a flat brings.

So, legislators have tried to compel car makers to fit pressure warning systems. These systems alert drivers to gradual pressure loss in their tyres, and allow them to top up the air in good time to keep them mobile. In the United States, the T.R.E.A.D. act means all new cars have to come with pressure warning systems that alert the driver when there is 25 percent pressure loss.

In Europe, the industry has resisted this, so few cars have warning systems. Those that do tend to be either luxury cars or super sports, or city cars, but most do not. Car chassis engineers do not like the idea; very few car buyers seek such a warning light and some of the cheaper systems on the market give so many false warnings that many drivers switch the warning off, or cover the indicator light with a sticker.

There, however, are many different solutions to the problem of pressure loss over a 24-hour period (or longer). Describing them all is a separate node, but I covered some of them in the spare tire node.

However, the fact that there are now so many alternative solutions to this problem, means that, in future, more and more cars will come without the fifth wheel. Eliminating the spare saves weight: think of the tyre, the wheel and the toolkit required to replace the wheel. It saves money. It allows more cargo space, or more passenger space and it improves design flexibility.

Maintaining correct pressure in the tyre saves fuel and improves tyre wear as well.

What to do in a high speed blowout

When travelling in a straight line, it is relatively easy to come to a safe, controlled halt, even after a high speed blowout. Unfortunately, a high speed blowout on a curve is all but impossible to control, even for professional drivers.

The most important thing to do in a high speed blowout, is to grip the steering wheel firmly, and keep the car travelling in the desired direction, but don't do anything else. If travelling in a straight line, there is enough time for an alert driver to get a good grip on the wheel and maintain control.

On a curve, however, things go wrong so fast, and the forces are so great, that few have any chance of keeping control, or recovering from the resulting skid.

In a corner, the lateral forces on the flat tyre will tend to pull the rubber off the metal rim which does a couple of things. First, it means the wheel comes into direct contact with the road. This has a number of effects, the first of which is that there is no significant grip. Unless the rim gouges a rut into the road surface. If that happens, the car will probably roll over. Second, it means the car only has three points of contact with the road. If it is unbalanced, then the rim is going to dig into the road surface, with similar, catastrophic results.

Even with run-flat tyres, which are designed to support the weight of a car under zero inflation pressure, a professional will not be able to prevent a serious spin, before bringing the car back under control. I've seen it under test conditions and it's quite a sight.

Even in a straight line, a flat tyre dramatically increases the drag on that wheel, which will, in turn, pull the vehicle round. It is like applying a brake to just one wheel. That will make the steering wheel want to turn. With or without power steering, it is vital to keep the steering wheel under control. Keeping the front tyres pointing in the correct direction is the most important part of retaining control.

If the blowout is in the front wheel, then things are a little easier. If the blowout is in a rear wheel, then the rear end will tend to fishtail around, requiring great skilll on the part of the driver to keep the vehicle under control.

In addition, the three remaining tyres will be near their limit of their grip, so applying the brakes is likely to be dangerous, especially in the wet, or on ice or snow. Instead of braking, take your foot off the gas, switch off cruise control and let the vehicle slow down naturally.

If on the freeway, indicate to move across to the slower lanes and onto a safe part of the road. As soon as you feel confident enough to take a hand off the wheel, switch your hazard warning lights on. Use a hand signal, if necessary: get a passenger to stick their arm out of the passenger side to show a determination to move across. You may have to be firm and pro-active about this, as other drivers will not be aware you are involved in an emergency. They might not give way readily.

Once at the side of the road, get the vehicle as far away from other traffic as possible. Your tyre is already damaged beyond repair, do not be concerned about driving along at walking pace, to somewhere less dangerous.

A How-to list of things to do when changing a tyre is included in the spare tire node.

Log in or register to write something here or to contact authors.