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The Tyres on Your Car
Underline to the heading, The tyres on your car.


Tyres are something that many drivers don’t really pay very much attention to, and yet their importance should never be under estimated. Complacency regarding tyres can kill, and that statement is not made with the intention of being dramatic, but to merely state a simple fact.

When we talk of safety concerning tyres, we are not just talking about tread depth, as that would be far too obvious. There are other issues with our rubber that can be far more sinister, because the dangers are not nearly so well understood.

This article has been written with the intention to advise you on what checks you need to make before you take to the road, and how your tyres can tell you if you are staring at a potential disaster.


Why Are Tyres Made The Way They Are?

Before we get into all that it would be useful to gain a better understanding of tyres and learn more about the job they have to do. For that we need to look at what considerations have to go into the manufacture of a vehicle tyre.

A tyre mnufacturer has to,
  • Design a tyre that will comply with the road vehicle legislation applicable to the use of the vehicle to which it is fitted and within the country that it is being used.

  • Design a tyre that is the most appropriate dimensions relative to the wheel rim upon which it is to be fitted.

  • Design a tyre that can be easily inflated and be able to remain so inflated.

  • Design a tyre that is will be appropriate relative to the maximum achievable speed of the vehicle to which it is fitted to and be able to cope with being used at that road speed.

  • Design a tyre that will be constructed in such a manner as to cope with the type of work the vehicle to which it is fitted will be expected to perform.

  • Design a tyre that has the ability to disperse water from the tyre contact area with the road surface upon which it is being driven, and therefore reduce the risk of aquaplaning.

  • Design a tyre that is able to withstand a certain degree of abuse without any adverse effect.

  • Provide details of a recommended pressure that is appropriate for the use of that tyre when fitted upon a vehicle.

  • Design a tyre that provides a comfortable ride for the vehicle occupants without any drop in performance.

  • Have the maximum degree of mileage life as possible, balanced with the required level of grip performance.

  • Design a tyre that has the appropriate external dimensions to work in harmony with the dimensions of the vehicle to which it is to be fitted.

Where Tyre Grip Comes From

When we talk of tyres we will instantly make an association between that word and the word grip, but what is grip and where does it come from?

A tyre gains its ability to grip the road surface through friction between rubber and tarmac, and that friction can only be present if there is something pressing it onto the road. Those with an interest in motor racing will probabaly be familiar with the term, Downforce, but whereas the racing car may gain downforce though its aerodynamics, downforce for the standard road car is provided by good old fashioned gravity. In otherwords, it is the weight of the car that gives our tyres grip on the road.

It is true to say the heavier the vehicle the greater the friction between the road and tyre, and you could be forgiven for thinking that if you increased the weight of your vehicle that will mean it will hold the road better, or stop in a shorter distance.

However, whilst an increase in vehicle weight will provide additional tyre grip, because the the additional weight provides greater downforce, that weight also produces greater Kinetic Energy, which places greater demand upon the performance of the tyre. Therefore, any additional grip advantage produced by the additional weight is lost.


Tyre Tread Depth

The tread pattern of a tyre is only there to disperse the surface water on the road, and this is because a vehicle being driven on bald tyres in wet weather would be exremely difficult to control, as a cushion of water would form between the contact surface of the tyre and the surface of the road, causing loss of grip.

This condition is known as Aquaplaning, and if there is anyone reading this who has experienced aquaplaning first hand they will know that is it extremely frightening. It is therefore important to understand what the legal requirement is concerning the minimum permissible tyre tread depth.

Actually, if you drive on bald tyres on a dry road, you will experience better grip than if you were driving on a brand new set. This is why competition cars, that are driven on a race track, are fitted with Slicks, which is the name in racing terms given to tyres without any tread to be used in dry conditions. However, to do this on the public road would be illegal.

The large bocks of tread on a dual purpose vehicle tyre that is ideal for on road and off road use.The minimum tyre tread depth requirement for cars, and goods carrying vehicles with a gross plated weight of 3,500kgs, is 1.6mm. This tread area has to form a continuous band around the whole circumference of the tyre and for at least 75% of the overall width.

That band also has to be in the centre of the normal tread area. However, whilst that is the legal minimum, our advice is that in practical terms the legal depth is grossly inadequate, as even at 1.6mm your car will be very susceptible to aquaplaning and really any tyre should be replaced well before it wears down to that level.

A good rule of thumb to use when making judgement if it is worth keeping a tyre on your car is that if it looks a bit dodgy then it is dodgy and should be replaced.

However, don’t just look at the tyre tread pattern, as there are other tyre safety issues that may need to be addressed. As well as tread pattern, look out for tears, lumps, bumps, splits and cuts. If you have any doubt as to whether the tyre is still servicable, scrap it and get another, as the result of not doing so could cost far more than the price of a new tyre.


Tyre Inflation Pressure

What else is there to know? For a start there is pressure, but not the kind you get from your spouse or partner because you haven’t done something that you were supposed to do. Ask yourself how often you give your tyre pressures a thought, and for those who do give it a thought, how many actually make the physical effort check to see if the tyres of your vehicle are inflated according to the manufactures specification?

Tyre pressures are vital in the Ride Drive on the road survival guide, and the importance of this subject should never be underestimated.

Air pressure within the tyre is what keeps it the right shape whilst you are driving and allows it to do the job it was designed to perform. If you have a difference of pressure between two tyres fitted to the rear of your car, for example, under the wrong circumstances this will cause severe vehicle instability, even if that pressure difference is just a couple of pound per square inch. This is because different pressures within tyres will produce a different rate of what is called Slip Angle.

A large tangled mass of old vehicle tyres showing various stages of wear and tear.Without getting into technical jargon, the Slip Angle is basically the rate of creep of a tyre across the road surface when the vehicle is cornering, and when there is a difference across an axle, the vehicle can be very difficult to control.

You may have heard that you should never mix tyres of differing structures on the same axle, and this will have been in reference to the Cross–Ply tyres and radial tyres. However, Cross–Ply tyres are practically obsolete now and so the chance of you coming across a Cross–Ply
tyre is pretty remote.


Every Day Driving Conditions May Now Reveal a Problem

If you are driving your car with tyres inflated at different pressures, whilst you are pottering around the town in traffic and driving at fairly low speeds, you would probably never notice there was anything wrong – unless that difference in pressure was a huge one.

There is a high chance that nothing would go wrong for you whilst driving in that environment, and you may not even notice anything untoward on slightly more free–flowing suburban route either. But then get out on the open road, or the motorway, and begin to build you speed above 40 –50mph, and the next corner you come to could be your last.

A car can actually be quite stable, even on the wrong tyre pressures, providing those pressures are the same across a particular axle, and do not differ by any great amount to what is recommended. However, the pressure within one tyre, as compared to its opposite on the same axle, only needs to differ by a small amount to cause instability and potential loss of control by the driver.

The same applies of you were to have too much of a pressure difference from front to rear. Larger differences in pressure between the front and rear pairs can cause a devastating effect to the behaviour of the vehicle.


Sudden Tyre Deflation – The Blowout

You will often hear of people discussing tyre blowouts, mainly because this is something that many will fear will happen to them. A tyre blow out, to give it a definition, is the sudden and total deflation of a tyre due to the collapse or failure of its structure.

There is a tremendous amount of rubbish talked about blow outs and something that is severely misunderstood among the motoring population. Blowouts do not just happen, they are caused, and that cause, 99.9% of the time, is through neglect. That means not completing simple maintenance and regular visual checks, coupled with a lack of awareness, allows for a tyre blow out situation to occur.

A 5 stud alloy vehicle road wheel fitted with a rugged off road type of vehicle tyre.As mentioned earlier, the air pressure within a tyre is necessary to maintain the integrity of the tyre profile. If you steadily reduce the air pressure within a tyre you will reach a point, long before it becomes flat, where the tyre walls begin to bulge outward where it is in contact with the road.

If that car is driven on the partially deflated tyre the bulging tyre wall, now less rigid due to lost air pressure, will begin to distort and even ripple as the wheel rotates.

The rippling effect of the tyre wall causes friction within the rubber, causing heat. All the time a car is being driven along on a tyre in this condition, it is generating heat within itself and the temperature will continue to rise for every metre of distance over which it is driven.

Eventually, there will come a point where the tyre will become so hot that it can no longer cope, and that is when it will suddenly let go with a bang. That is a blowout.

If you were to realise what was going on before you got that failure point, and if you were to try to put your hand on the tyre just after you have come to a stop, you could burn yourself quite badly. There is a terrific amount of heat generated under these conditions, so much so that under inflated tyres on a driven vehicle have even been known to ignite.


Always Fully Investigate Faults

Let us not get carried away here, but instead ask ourselves why the tyre has lost pressure in the first place? After all, loss of pressure is the cause of the problem, and is the beginning of this chain of events leading up to a tyre blowout.

The most common reason for loss of tyre pressure is that you have picked up a nail, or similar, that has penetrated the body of the tyre and has remained there embedded in the rubber. With a nail or screw acting as a plug the tyre doesn’t deflate quickly, and a foriegn body such as this will allow the air to seep out slowly, and you have a slow puncture.

However, it might even be that the pressure loss is not caused through a slow puncture, but will be due to the tyre pressures not being checked for so long the tyre has gradually deflated, little by little, through age and neglect, as rubber is a natural product and actually has micro pores in it. It could be that you have been running around for weeks like this, but because you have been the only occupant of the car you have got away with it. Now, when you have got three other people on board, and a boot full of luggage, things are very different.


Tyre Deterioration and Degradation

A stack of old worn out car tyres in different tyre sizes and profiles.Talking of age, have a look at the walls of your tyres, not just a glance, but a really good close look. Do you see any signs of a small crazed pattern in the rubber – like a mini crazy paving design? If so, the wall of the tyre is perished and therefore weakened.

Sunshine and salt spray are the biggest culprits when it comes to causing rubber to degenerate and is most common on those vehicles that are used infrequently, and which stand in the open air. Touring caravans are the most affected in this category, as they are often stood in some outside storage space for eleven months of the year before spending a couple of weeks on the road during the family holiday.

Any sign of deterioration in a tyre then get rid of it, as keeping substandard tyres is foolish and may cost you or someone else a life.

Another aspect of tyre deterioration you may wish to consider concerns your spare wheel. Where is it carried on your vehicle? Most 4x4’s have them mounted upon the rear loading door, which is fine providing it is covered over. If it isn’t covered then it will deteriorate and perish very quickly in the light of the sun.

Those spare tyres mounted beneath the vehicle will deteriorate too, as they receive all the salt spray and muck that is flung around under the car; coming up from the road wheels as it is driven along. Always check them on a regular basis too.


The Effects of Tyre Damage

When did you last hit a kerb or pothole? Have you bumped up a kerb whilst trying to park, or run over a piece of debris on the motorway? If you have done any of these things, what damage has it done to the tyre?

Sure enough, when you have maybe looked at your tyres afterwards, to see what sort of condition they are in, they may have appeared to be fine, but that will only be a visual check made from the outside. What damage has the tyre really suffered and how could it affect your safety?

If you have just banged your car up a kerb you may well have caused havoc to the inside of the tyre, and you won’t see what’s there unless you take the tyre off the rim.

Broken ply cords and layer separation of the rubber banding within the tyre body is a common symptom of bumping up and down kerbs, but from the outside you will never see it. However, get out on the motorway, and start making progress along the road, you could find out the hard way that you are in trouble. The golden rule here is to never ever assume anything, or you could pay dearly.


Learning to Recognise What Your Tyres Can Tell You

When you are cleaning your car use this as an occasion to check your tyres for stones or other objects that may have become wedged in the tread grooves. Harmless as this may seem at first, these can wriggle and worm their way through the rubber and end up causing a puncture.

If you don’t clean the car often, check for foreign bodies in the tread anyway. If you find you have one tyre that has gone soft on you, don’t just put air into it to blow it up again, investigate the reason as to why, as probably there will be a nail or screw stuck in it.

Another thing you can do, and this concerns mainly the front wheels, put the flat of your hand across the tread area and with light pressure only, and run your hand around the circumference of the tyre and do this A colletion of partly worn vehicle tyres showing various signs of tyre wear.backwards and forwards. What you are feeling for is whether the tread has become stepped or feathered.

To explain this further, if you imagine stroking a cat or a dog, and you try to do so by running your hand from its tail towards its head, you will notice it feels totally different to when stroking the animal in the more normal direction.

If there is a difference to the feel of your tyre tread, according to the direction in which you are running your hand around it, then you need to investigate this further.

Tyres tell us a lot about the state of health of a car, and a feathered tread can be a symptom of defective or worn shock absorbers, wheel bearings, steering joints, wheel misalignment or incorrect wheel balancing. The condition of the tyre tread can even give you a clue as to whether your brakes are binding (binding means failing to release properly). Check across the tread area to see if there is a difference in tyre tread depth from one edge to the other, as this will be another symptom of a wheel alignment, or tracking fault.


Toeing In and Toeing Out – That’s Tracking For You

Demonstrating the angle of wheel alignment as it will appear on a rear wheel drive car.Front wheel drive cars have the front wheel alignment set at toe–out, which means the leading edge of the front wheels are set up to turn slightly away from each other.

This is because the engine is driving the front wheels, and as you drive, the torque through the driven wheels pulls them into near enough perfect alignment. Rear wheel drive cars have the front tyres toeing in, so they splay outwards as they drive forwards, as you can see in the diagram. You can’t see any of this with the naked eye, as the change in angle is only a small number of degrees.

The front wheels, and even the rear wheels in certain cases, will never be set completely parallel with each other, and this is set to compensate for the forces that are acting upon the wheels whilst the vehicle is being driven.

A tyre that is scrubbing away the tread around the outside edge will tell you the wheels are toeing too far inward, as in one or both will be steering slightly toward the other as you drive.

Scrubbing of tread on the inside edge indicates one or both of the wheels are toeing too far outward and so effectively trying to steer away from one another. Excessive wear around the centre of the tread area is a symptom of that tyre being driven upon whilst over inflated, whereas excessive wear to the area of both shoulders, as compared to the centre, indicates use of
the tyre in an under inflated condition.


Learning About Tyres Helps You Make The Right Choices

Lastly, when you take your vehicle to a tyre centre the staff should have been trained to a high standard, and therefore should be able to advise you as to the most appropriate tyre to choose for your car. However, it doesn’t hurt for you to know a thing or two about tyres before you attend, as being better informed will give you more confidence in making your choice.

Every tyre has a ply and speed rating, and information concerning this will be printed upon the wall of the tyre. Whilst the maximum speed limit for UK roads is 70mph, you should be fitting tyres to a 140mph car that are capable of coping with that sort of speed, so as to be compatible with the vehicle type.

When dealing with top of the range cars, particularly sports models, many have tyres that have what is known as Directional Tread. That is to say the tyre has to be fitted to a wheel so that it has the same rotational direction. In other words, you will have left and right–handed tyres, and to put a right hand tyre on the left side of a car will not be a good idea. Your tyre centre staff should know all of this, but it is always best to keep an eye on what is going on.


Julian Smith
Ride Drive Limited

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The Tyres on Your Car

     
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