Tyre Safety And The Law

Tyre Care Tips

Tyres are expensive, and of all the consumables in motoring tyres come second only to fuel in terms of cost. As with fuel consumption, the way you drive can impact on your tyre consumption.

If a tyre does develop a problem, it is useful to be able to identify what that problem is, so that you can prevent it recurring. The main problems are wheel alignment and incorrect tyre pressure. Don't just assume that when you have had your alignment checked that it will be correct; keep an eye on your tyre wear. Similarly, air pressure needs to be checked, even on recently fitted tyres.

However, your driving habits may impact on your tyre wear. It has been said that high speed driving can overheat tyres and result in the degradation of the rubber, resulting in the breakdown of the tyre. This is true, but generally your car should be fitted with tyres with speed ratings above the maximum speed of the car itself. Also, that heat build-up usually only occurs when the tyre is under-inflated. So high speed driving alone is not usually the sole reason for tyre failure - or else our motorways would be littered with cars needing tyres replaced.

Tyre wear is accelerated under hard driving, so the heavy footed driver, both on the accelerator and brake is likely to see higher tyre wear. If he adds high speed cornering he can expect to increase tyre wear even more.

Harsh braking wears down tread quicker, and it can lead to flat spots, especially on older cars with oval brake drums or uneven discs.

Inflation is key to keeping tyres in good safe condition. The air in the tyres warms up very quickly, even on a cold day, and when the tyre starts rolling it generates some heat, which heats up the air in the tyre, which expands thus creating a higher air pressure. So, when you drive a mile to the garage to set your tyre pressures, and they are at 32lbs instead of 30lbs, and you drop them by 2lbs you have actually reduced the pressure from the correct 30lbs.

Tyre pressures should always be checked at cold before you move the car - which is why tyre pressure monitoring systems that function as soon as you put the key in the ignition are better than ABS fed systems. (Most cars sold in Europe with TPMS have electronic sensors in the wheels that check tyre pressure automatically).

Incorrect tyre pressures have various outcomes. Firstly too low a pressure can result in uneven wear on the shoulder of the tyre. It can lead to overheating and tyre breakdown. It makes the suspension feel softer, and can make the car's handling sluggish and slow to react to steering input.

Too high a pressure results in uneven wear around the centre of the tread. With less contact on the road it makes wheelspin and skidding more likely, accelerating wear in the high pressure contact area. The car will have very light steering and in some situations it will be over-responsive to steering input and have a propensity to skid.

There are very few circumstances where reducing tyre pressure on the road can be recommended. Not even in snow and ice. However, if towing, or running a heavily laden vehicle it may be a requirement that the tyres are inflated to a higher pressure - see your vehicle owners' handbook for details.

Balancing your wheels is important for several reasons. Firstly, an imbalance in the wheels, especially front, will be felt as a vibration through the steering at some or all speeds. It can increase noise levels and make the car uncomfortable to drive. On the short to long term it can increase mechanical component wear in the track rod ends, ball joints, steering rack and even the wheel bearings can fail. If you have your wheels balanced after a new tyre fit or a puncture repair, be wary of any detrimental changes to your car's "feel", it may need the wheels rebalancing.

Another area that causes increased tyre wear is that of road surfaces. There isn't a great deal that the driver can do, other than be aware that on some road surfaces - even those that appear smooth, there may be a multiplying factor on tyre wear. A worst case example would be that tyres that might do 30,000 miles in the UK, might, under similar driving conditions in some areas of Norway or Sweden only do 20,000 miles or less. This is because the Scandinavian road surfaces are built to offer better surface drainage and grip due to their adverse weather conditions.

Spotting the Faults

Tyre pressure is best checked with the tyres cold, in the morning. Tyres should be inflated to the vehicle manufacturer's recommendations. However, when you buy replacement tyres you should check that they need to be inflated to the same level - some tyres operate batter at slightly different pressures.

Balancing. Any feel of vibration should be investigated, and the first stop is wheel balancing. If you are not happy with the response from one tyre fitter, use another.

Alignment is one of the bugbears. Every tyre depot claims to do wheel alignment but many just don't get it right, and that includes some very big names. So watch for uneven tyre wear and at the first sign of poor alignment or you can feel your steering pulling to one side under normal driving on a flat road get your car in and have it checked. You will be told that you may have kerbed the car, hit a pothole or similar, but have it checked by someone you can trust and who lets you see quite clearly what they are doing. If your tyre fitter doesn't use laser alignment then take it somewhere that does, and ask to see the figures.

Accounting for general wear requires tyre rotation. Most owner's manuals show the proper cycle for rotation.

Punctures can sometimes be avoided by removing objects just stuck in the tyre. Beware however, or removing penetrations from inflated tyres. You may be able to drive to a tyre depot with a nail in the tyre. This is because radial tyres have a soft butyl liner that seals around small penetrations to prevent sudden air loss. This minimises the risk of high speed sudden loss of pressure, and it allows a get to safety option. If you remove a screw or nail, the tyre will deflate quickly and you will need a roadside tyre change - not always the best place to do the job.

Tread Depth Law and The Effect of Tread Depth on Tyre Performance

Current tread depth legislation requires that car tyres must have a minimum of 1.6mm of tread in a continuous band throughout the central ¾ of the tread width and over the whole circumference of the tyre.

However, despite the law, it is generally recognised in the tyre industry that the legal limit is an extreme. Many tyre manufacturers state that they design tyres to function as well at 1.6mm as they do at 9mm (the accepted normal tread depth when new). That is a surprising statement for any tyre company to make, but some have said just that.

So, if a tyre performs as well at 1.6mm as it does at 9mm, what happens at 1.5mm? Is there a sudden drop in performance? Actually there isn't, because industry testing has shown that when a tyre reaches around 3.5mm in tread depth, the level of performance in the wet, in particular, starts to deteriorate, as does its dry handling characteristics.

The recommended point for change is accepted Europe-wide as being 3mm. So much so that ministerial cars in the UK have their tyres changed at, you guessed it, 3mm.

Why then the current legal limit of 1.6mm? There are several arguments against the change, some of which you may question. One is that the sudden change from 1.6mm to 3mm would have a serious impact on the pockets of hundreds of thousands of motorists who are already struggling to keep their cars on the road. Another is that it would require changing all the tyre moulds in use to increase the tyre wear indicator depth to 3mm. And of course Europe plays a part, as there would not be universal implementation of 3mm tread depth, requiring double standards in production and possibly in policing.

The reality is that since tyres are now a global commodity it would almost require a global adoption of 3mm as a minimum. It doesn't take an Einstein to counter the arguments against 3mm, but until the legislation is in place you can make up your own mind, scrape by on 1.6mm, or be safe on 3mm. Your choice.

But before you make that choice, it might be worth your while having a look at the video indicated at the bottom of this article. It might make you change your mind.

Truck tyres currently have a 1mm legal minimum tread depth - which many are trying to drive up to 1.6mm - for exactly the same reasons as they want to see 3mm for car tyres.

Incidentally - you may wonder why the normal new tread depth is generally around 9mm. This is to do with the slip, ie distortion in a tyre block, and its level of hysteresis. If you take an eraser and holding it vertically, draw it across a desk, you will see that it distorts before it loses grip - that is what we call "slip". If you increase or decrease the length of rubber you are flexing, the slip increases or decreases. It becomes obvious that too much slip would make a vehicle unstable. Remember that the compound is a compromise too and the ratio of slip to tread block and the compound used is all finely tuned. The industry norm is for car, van and SUV treads, 9mm.

The Impact of Tread Depth on Tyre Safety

The braking and grip performance of tyres in wet weather deteriorates considerably once the tread depth reduces below 3mm. This is because the main function of the tread pattern of a tyre is to evacuate water. As the tread depth decreases it gradually loses the ability to evacuate all water from the road surface under the tyre and the car will eventually aquaplane.

Many tyre tests have shown that the wet braking distances of a new tyre compared with a tyre with only 1.6mm of tread left on it are huge and can be the difference between life and death.

Tyre Rotation

Tyre wear rates differ depending on the axle on which the tyres are fitted and whether the vehicle is front or rear wheel drive. To extend the life of your tyres it is advisable to change tyre positions on a regular basis. Different tyre manufacturers may recommend different rotation periods, ranging from 2-6,000 miles. It is advisable to switch tyres from left to right periodically as well as from the back to the front of the vehicle.Certain high performance tyres are position specific and should not be rotated. If in doubt, contact a specialist tyre dealer.

There is some dispute over the fitting of a pair of new tyres to a car. Simple logic suggests that the car needs most traction at the drive axle. In some cases that would be true, for instance if you were constantly driving in muddy conditions and needed the grip to keep you moving. However, irrespective of the vehicle, front or rear wheel drive, or even 4x4, the typical vehicle is designed to understeer in skid conditions. That is, it ploughs forwards.Generally, backing off the power and the brakes will recover the vehicle from the skid, though not always.

By putting the grippier tyres on a front wheel drive car, it increases grip at the front, but equally increases the difference in traction/grip between the front and the rear tyres, so the rear tyres lose grip early and as a consequence the car is at risk of going into oversteer. For the average motorist oversteer usually means ending up rear end first into the field (if they are lucky). That argument can also apply to rear wheel drive vehicles, it is not about getting the power down on the tarmac, it is about keeping the rear end of the car in contact with the road.

With four wheel drive vehicles, rotation of tyres is much more important and all four tyres, ideally, should be replaced at the same time. So, location should not be an issue. If it is, the same rule applies, newer tyres to the rear.