Tyre Performance

When a Tyre Stops Working

A tyre is designed to keep your vehicle in contact with the road. When it stops doing that it is no longer working and your vehicle develops a skid. The dynamics of skidding are for another website; here we briefly look at why your tyre stops working.

The micro detail of a tyre would show its micro irregularities in its tread surface interacting with similar micro irregularities in the road surface. Rather like a lizard climbing a sheet of glass, its microscopic hairs on its feet clinging to microscopic roughness on the glass.

When the car is sitting still, the tyre is sitting still and there are no dynamic changes in that interface between two surfaces. When the tyre begins to roll though those micro interfaces create a resistance that results in grip. Close examination of the tread block will show that as force is applied by accelerating (or cornering) the tread block distorts allowing the tyre casing to shift slightly in relation to the tread interface with the road. This distortion of the tread block is called "slip". The amount of slip in the tread block is determined by its size and its compound. A small block will have more slip than a large block, a deeper block more than a shallow block. And the amount of "slip" is also determined by how much the compound will allow itself to be distorted.

Whatever the compound and the tread depth, once the slip has reached its maximum level, the tyre casing then distorts, this happens both in acceleration and deceleration and also in cornering.

When the tyre casing reaches the limits of its distortion, the tyre has nothing left to give and, suspension settings accounted for, the tyre loses its grip at the point of micro interface with the road and the tyre skids across the tarmac.

It is highly unlikely that any normal driver would ever notice the tyre actually deforming. However most drivers will experience from time to time what happens when the tyre stops working: The skid.

There is one area where this is of critical importance in tyre choice. Most car drivers select tyres that suit their vehicle and its everyday use. There is one exception. The SUV/4x4 driver. There is a propensity amongst some 4x4 drivers to opt for the biggest, chunkiest tyres they can find. If they actually spend a great time of their driving off the road and in conditions that require those chunky tyres then there is no problem with that selection. However, when the vehicle is on the road for the majority of its life there are safety issues to be considered.

Those big chunky mud-plugging tyres may work well in the mud, and may even give comfort in snow, but, they offer a false sense of security on asphalt in the wet, and can add metres to the stopping distance, even in the dry. Not really important unless that extra metre or so happens to be occupied by another vehicle, or a pedestrian.

Those issues on stopping distances also apply to car and van tyres. Though the visual differences are not always so obvious. In tyre tests there can be varied results, tyres can be good in one area of measurement and not in another. So, when making your tyre choice do listen to the experts, read any tyre reports you can find. Most reputable tyre fitters have copies of test reports lying around. And remember, the most important metre in your braking distance is the very last one.

Tread Patterns and Profiles

Most people don't realise it but tyres are fashion accessories. No, you won't get away with giving your wife or girlfriend a pair of 255/35R16 as a set of earrings, but, when a car is designed, an integral element in the designing of a car is to include the tyre width, profile, diameter and pattern as a core factor in the design.

Whilst the tyre will always be required to do its job, of providing grip, traction, adhesion and being an element of the car's suspension, it nowadays also has to look good. It has to look good on the car and in the showroom. That makes the car tyre a fashion item.

It is impossible to tell from looking at one tyre tread whether it works better than another tyre of a similar style. It used to be pretty simple, a question of bars and lugs, and blocks. Now we have a different approach to tyres and how they work, and that means that things can get very confusing for the tyre buyer: Though they need not be.

Back to Basics

Very early on it was realised that tyres worked better if they had a tread pattern. Designs were arbitrary and Dunlop actually produced a tread pattern that left an imprint of the brand name Dunlop as it ran over soft surfaces.

There is a history of tread design that can be followed through the years. But essentially, there was the bar or lug type tread with solid lugs across the tyre tread traction. Then there was the circumferential channel pattern that gave lateral grip. A combination of the two basic patterns led to the block type tread pattern that survives in many tyre patterns today.

Interestingly, all three early type patterns are still available for specific uses today. However, modern motoring's focus has changed slightly, and whilst grip and traction are still core values, today we are more focussed on wet and dry handling and braking ability.

The best tyre on dry, level road surfaces is a slick. However, road surfaces are rarely dry, and the roads that we drive on every day - even the smoothest of them, are a far cry from racetrack surfaces, and as a consequence the road tyre needs to be harder wearing, so it has a firmer compound (generally), it has to have grip and traction, and it also needs to resist aquaplaning.

The tread on a modern car tyre is considered to be a water pump, designed to express water from between the contact patch and the road surface. How it manages to do that is the subject of many millions of pounds of investment for each of the leading tyre manufacturers.

For the tyre buyer, generally speaking, the greater the number of channels in your tyre's tread the greater its ability to pump water away. However, the design of those channels may make the tyre more or less efficient. A wide tyre may require a different approach to water dispersal than a narrow tyre. So that trendy looking tread you find on a Porsche tyre, may not be suitable on a narrow tyre fitted to the family saloon - and vice versa.

A recent trend has been the development of high performance tyres with circumferential grooves and tread bands that offer different characteristics across the tread of a tyre - these tend to be asymmetric and directional. Another trend is for the "single tread" where the tread pattern is such that the "land" area of the tyre never breaks contact with the road and the "sea" area (the troughs) channel water away from under the tyre. Both these tyre patterns are claimed to be quieter than block type tread patterns.

Noise Travels

Tyre noise is a big issue nowadays. Cars have become so quiet that often the loudest bypass noise comes from the induction system and the tyres. Tyre pattern can make a great deal of impact on the noise a tyre makes. The noise actually comes from the leading edge of the tyre block making contact with the road surface, and the trailing edge snapping back as it breaks contact with the road. So, the more blocky a tyre is, the noisier it will be: Perhaps something to consider if road noise is intrusive in your car. Winter tyres are always going to be louder than their all season counterparts because they are blockier and have many more sipes (thin slices in the tread block) to give better grip and pump more water away.

Noise is a comfort issue and comfort is a big issue for vehicle manufacturers and tyre makers. The tyre is an integral part of your vehicle's suspension. It absorbs the first and all minor impacts with variations in the road surface. It softens the ride and suspension settings at the point of vehicle design and manufacture take into account the Original Equipment (OE) tyre design. So, when replacing your car's tyres it is always a good idea to buy, at least with the first replacement, the same tyre as is fitted OE. By the time your car needs its second or third change of tyres the original specification of the suspension will have been diminished by wear and tear and it becomes feasible to fit non-OE tyres without any real impact on the car's feel since it will rarely feel like new by that stage anyway.

Comfort Profile

Comfort is also impacted by profile. Now this is where we are all becoming fashion victims. Cars are coming with ever larger wheels and lower profile tyres. The technical reason is that the larger wheels allow the manufacturers to fit larger brakes, and therefore make the braking of the car more efficient. In utilising larger wheels the car requires thinner, lower profile tyres to stay within the style and design characteristics of the vehicle, and also within the ability of the tyre industry to produce suitable tyres, and the aftermarket to deal with them. A vehicle that leaves the factory with 18 inch rims and 20 profile tyres will have had its suspension designed to deal with the harsher ride created by the low profile tyres. However, if a car comes with 70 series on a 15 inch rim and the owner changes to 18 inch and 20 profile (as an example), he will definitely have a harsher riding car as the lower profile tyre has a stiffer, less supple sidewall and will absorb far less of the surface undulations than the car fitted with the 70 profile.

So, unless you drive on excellent road surfaces for most of the time the low profile tyre is potentially harsher. If you fit low profiles to a car not designed for them, you will gain in looks (perhaps) but suffer in the ride. Your handling on smooth roads will improve, but on poor roads and potholed surfaces your handling will only be of use to you as you dodge the rim smashing voids in the asphalt.

If you want to know more about tread design then you will be best served reading the product news and the tread descriptions.

Low Profile Tyres - Performance Characteristics

Compared to conventional passenger car tyres, low profile car tyres have greater width ratio to cope with the demands of higher performance vehicles.

The key performance advantages are:

• Improved handling and grip
• More traction and braking power

Disadvantages are:

• Increased road noise
• A harsher ride
• Less resistance to aquaplaning due to the amount of rubber on the road

Run Flat Tyres

All the latest technology brings the modern motorist the run flat tyre - by whatever name you wish to call it - a tyre that you can drive on when the pressure has dropped.

Actually, it isn't new technology at all, just more modern interpretations of old ideas using modern materials. The first patent for a pneumatic tyre was taken out by Robert W. Thomson in 1846. Thomson called it the Aerial Wheel, and one of his patents included a tyre with multiple inner tubes - another of his suggestions was to fill the tyre cavity with horsehair or sponge to support the tyre and prevent it collapsing onto the rim. In essence both these methods are in practice today in one form of run flat tyre or another.

If that isn't enough to convince that run flat technology isn't new, then let's go back to a small conflict in Europe in the middle of the last century. The War Office supported research into self supporting tyres. In one version the air in the tyre was replaced by a rubber ring with voids in it - This was the PT Support tyre and it was expensive and heavy, but it worked. In one report a raid on enemy lines was subjected to three hours of air attack. Three of the vehicles were set alight, and one put out of action by gunfire. The remaining three all drove 70 miles back to safety with punctures in every tyre.

There was also a run flat that used thick sidewall construction and a rim filler to retain the beads in place. This was revisited in the 1960's with the Tyron band - still available and selling well around the world.

Today's modern run flat tyres are essentially designed around those original principles - though perhaps the tyre manufacturers might disagree.

There is the Goodyear Run-on Flat - this is a tyre which is designed with stiffer than normal sidewalls, which use a temperature resistant compound to fend of destruction by heat generated by running flat.

There is the Michelin Pax system, which uses a well filler and support system to keep the tyre on the wheel and prevent the tyre from being destroyed by the rim in the event of a flat tyre.

Continental has the integrated wheel system, IWS, a stiffer sidewalled tyre combined with a rim and bead system that retains the tyre on the wheel.

And in truck tyres there is the Greatec system which has a tube inside the tyre that when the tubeless tyre is punctured, the tube fills the void and supports the tyre in a "get-to-safety" role.

The key aim in all of these systems is to keep the tyre on the rim for as long as possible and to delay the destruction of the tyre, allowing the driver to make his way to a place of safety.

The one thing all these tyres have in common is the need for a tyre pressure monitoring system (TPMS).

TPMS is essentially a method of monitoring the level of pressure in a tyre and alerting the driver if there is a loss of pressure. Allowing him to reinflate the tyre or have the tyre swapped (usually a professional job as TPMS and run flat tyre equipped cars rarely have a spare tyre.

There are two basic types of TPMS, one is fed data by the vehicle ABS and traction control system which can identify a differential in wheel rotation - a flat tyre turns more slowly than an inflated tyre, so that differential can be used to flag up a pressure difference. The disadvantage of this system is that the vehicle needs to be moving for it to operate.

The other system uses chips in the wheel well of the wheel and tyre assembly to monitor pressure and report to the on board computer. This gives an instant pressure reading and can even be used to limit the speed of the vehicle. The minus point here is that these sensors mean that extra care has to be taken when fitting tyres, possibly even special equipment. They are also expensive to replace if damaged.

Most European vehicles with TPMS are manufactured with wheel mounted version.

Tyre Choice by Season

Summer Tyres The main characteristic of the summer tyre is excellent driving and braking performance on both dry and wet roads, in temperatures above freezing throughout the spring, summer, autumn and winter. A simple block-shape tread pattern ensures the tyre's contact area remains as rigid as possible, ensuring maximium grip at all times by optimising friction with the road surface. If the tyre has any special indication on it, it means this tyre is for summer usage.

Winter Tyres This type of tyre is suitable for snow and ice covered roads. The tread pattern features a heavily siped, fine-block design with grooves that are deep and wide, providing excellent driving traction and braking performance on winter road surfaces. The tread rubber used in winter tyres provides good friction characteristics and suppleness even in low temperatures. At present, winter tyre design is focused toward braking performance and driving control on icy roads. Recent developments in tyre pattern design and rubber technology have greatly improved winter tyre performance.

All-season Tyres These tyres are designed to be used all year round to save having to change the tyres to suit the season. All-season tyre patterns are more complicated than those of the summer tyre because of the longer block-edge for winter usage. More sipes appear in the all-season tyre than summer tyre to improve the braking and driving performance by maximising the effect of edge. It is important to remember that in regions where winter temperatures are very low and heavy snow falls are frequent, braking performance will rarely be 100 percent so it is safer to use winter tyres. All-season tread designs will provide good all-around traction for varing road conditions, but still provide good treadwear and tyre noise characteristics.

Information courtesy of Kumho.

Tyre Ageing

All tyres undergo an "ageing process" due to exposure to the environment even if they have never been used or have only been used occasionally.

This phenomenon is caused by a number of factors including:

• the process of aftercuring, whereby the tyre continues to culcanise very slowly. This is caused by factors such as light, heat and movement and makes the tyre become stiffer and more brittle.
• oxidation involving oxygen and ozone from the atomosphere causing hardening of the rubber and reduced elasticity
• plasticisers drifting from the tread area to other parts of the tyre, resulting in increased hardness.

Tyre ageing can be minimised by proper storage in a cool, dry environment, away from direct sunlight and away from electric motors (such as air conditioning units) or other sources of ozone.

Interestingly, tyres, which are infrequently used or not used at all, age and crack quicker than tyres, which are in regular use. For this reason tyre manufactureres tend to recommend including the spare tyre in any tyre rotation process. Spare tyres, which are old should be used with caution. If in doubt, get the tyre checked by a tyre expert.

Caravan tyres or tyres on other vehicles that are parked for long periods are particularly subject to ageing and owners of these vehicles are generally advised to jack the weight off the tyres and cover them so that they are not exposed to sunlight.

How long before tyres become too old?

This is rather like asking "how long can a piece of food be kept before it goes off?" and therefore it is something on which the tyre manufacturers have some difficulty in agreeing. No expert can definitively say how long a tyre will last as a tyre's life depends on many factors such as the type of rubber compounds used, temperature, usage, humidity etc.

However, Bridgestone's website points out that many tyre companies, including Bridgestone, offer manufacturing warranties on tyres for 5 years from the date of manufacture, which has resulted in many vehicle manufacturers advising against the use of tyres that are more than six years old.

The British tyre industry has not been able to agree on a definitive statement relating to tyre ageing. As a result of this the National Tyre Distributors Association is understood to be preparing a members' advice document for release in early 2008, which is said to be based on a similar leaflet produced by the German tyre dealers association (BRV)

The BRV's advice on the subject is, in fact, based on a joint statement dating back to 2001 and prepared in conjunction with Bridgestone, Continental, Dunlop, Goodyear, Michelin and Pirelli, which advises dealers to make sure tyres are not sold, which have a production date that lies more than 5 years in the past.

In terms of tyres already fitted to vehicles the BRV advises a maximum age of 10 years for car tyres and 6 years for tyres fitted to caravans, trailers of other vehicles likely to be standing around for long periods.

How do I check the age of my tyres?

All tyres are marked with a serial tyre identification number. The last three digits (for tyres made before 2000) or the last four digits (for tyres manufactured after 2000) denote the week and year of manufacture. For example, a tyre carrying the number 3506 denotes a tyre manufactured in the 35th week of 2006.