Drivers Fact Sheets
Tyre Safety – Know Your Tyres
Tyres perform several important functions and are the only part of your vehicle that comes in contact with the road. On average, the part of a tyre in contact with the road is the size of a postcard.
The entire weight of your vehicle and its contents is are supported by the tyre and air inside the tyre.
It’s against the law to have:
- Car tyres with tread worn to below 1.6mm;
- A mix of radial and cross ply tyres;
- Over or under-inflated tyres;
- Tyres with cuts, lumps, bulges or tears;
- The wrong sort of tyre fitted to a vehicle or trailer.
- All new tyres must meet European standards for load/speed performance, shown by an ‘E’ or ‘e’ mark on the sidewall.
- All retread tyres supplied in the UK must comply with the British Standard AU144e, and be marked with this number. This proves they have been thoroughly tested and meet the same performance standards as new tyres.
- Only buy second-hand tyres if they are marked ‘part-worn’ next to the E or BS mark, to show they have been properly checked for faults.
Checking Tread Depth
Most tyres have tread wear indicators, usually six or more small ribs across the bottom of the main tread grooves. When the tread surface is level with these ribs, the tyre needs to be replaced.
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.
The accepted normal tread depth when new is 9mm. Many tyre manufacturers state that they design tyres to function as well at 1.6mm as they do at 9mm. This is despite industry testing having 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.
Europe-wide, the recommended point for change is 3mm. So at what depth do ministerial cars in the UK have their tyres changed ? 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. Until any legislation is in place you can make up your own mind, scrape by on 1.6mm, or be safe on 3mm. The choice is yours.
Lorry and Truck tyres currently have a 1mm legal minimum tread depth in the UK which many are trying to drive up to 1.6mm – for exactly the same reasons as they want to see 3mm for car tyres.
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 for car, van and SUV treads is 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. You need good tyres to drive safely. They affect the steering, braking and acceleration of your vehicle.
Tyre Wear Visual Check
Check for obvious signs of wear
Exposed tread bars (replace)
Irregular shoulder wear (have inspected)
Centre wear (have inspected)
Checking Tyre Pressures
- Look in your handbook or consult your garage or tyre dealer for the recommended pressures for your vehicle.
- Tyres can lose 1 psi (pounds per square inch) per month under normal conditions Additionally tyres can lose one psi for every 5% C temperature drop.
- You should check the pressure at least every two weeks, and only when the tyres are cold. Even a short trip to the local garage will warm up the tyre and raise the pressure.
- Driving on under inflated tyres is almost certain to cause serious damage, always inflate tyres to the suggested pressure.
- Maintaining the correct pressure is the easiest, yet most important thing you can do to get the best performance, economy and safety from your tyres.
- Lower pressure increases heat; excessive heat results in tyre damage.
- Under inflation of tyres can cause them to experience uneven or rapid tread wear, as well as lead to an increase in fuel consumption. In addition, under inflation reduces your vehicles braking and handling capabilities, and can ultimately lead to serious tyre failure.
- Under inflation can create an overload on tyres resulting in tyre damage.
- Different driving conditions require different pressures. For example, a higher pressure is usually recommended for high speed driving or when carrying or towing a heavier than normal load. Seek advice on what is best for you and your car.
- Where possible, carry your own tyre pressure gauge.
- Sealing valve caps must always be used, the best being the metal type.
- Avoid using ‘sealants’ or other liquid preparations to prevent deflation. These may cause the valve to stick open slightly, causing pressure loss and indirectly causing corrosion of steel belts.
- Don’t forget to inspect the spare tyre
- Every week inspect tread and sidewalls for cuts and abrasions, bulges, unusual wear and road damage.
- If the tyre receives a severe impact, ask your tyre retailer to check for internal damage. Do not repair cuts in sidewalls of radial ply tyres.
- Take action immediately to rectify any unusual sounds or vibrations. Tyre balance and vehicle wheel alignment should be checked regularly, (ie every 10,000km), especially if your tyres are subjected to rough roads or aggressive driving.
Regular rotation of tyres is a proven method for promoting even wear and therefore extending tread life. If uneven wear has occurred, this may be due to worn suspension components, vehicle misalignment or incorrect tyre balance. These concerns cannot be corrected by simply rotating the tyres. Your local tyre retailer should correct these problems.
Front to back same side rotation is acceptable when tyres are wearing unevenly. If the spare is included in the rotation it should be exchanged with the tyre allocated to the right hand rear position
For maximum mileage rotate your tyres every 5000 to 10000km and follow the correct rotation patterns.
Rear and four wheeled drive vehicles Front wheeled drive vehicles
Interpreting The Numbers On Your Tyre
This section helps you to understand and recognise the meaning of the size markings on the side wall of a tyre.
|Width of the tyre (mm) “Section Width”||Aspect Ratio (%)||Tyre Construction(Radial)||Rim Diameter(inch)||LoadIndex||SpeedSymbol|
Width of the tyre measured in millimetres from sidewall to sidewall. The tyre in the diagram above is 205 millimetres wide
Aspect ratio is the ratio of a tyre’s width to its height. A 65 series tyre, therefore, is a tyre whose height is equal to 65% of its width. Manufacturers make tyres with different aspect ratios to perform different functions. The most common are as follows.
75 and 70 Series
Popular on many small and medium cars where they provide good ride, comfort and durability.
Tyres with 65 series aspect ratios are popular fitment on many medium and large family cars. Improvements in handling while retaining good ride are key performance strengths of this type of tyre.
Further improves handling, particularly steering response as side wall height is reduced. They are a popular fit on sports touring vehicles.
55, 50, 45 Series And below
Very low aspect ratio tyres developed for the new range of higher performance vehicles where ultimate grip during braking, steering and traction are the priorities.
This tells you how the tyre was put together. The “R” stands for radial, which means that the the ply or ply cords are alligned at 90 degrees to the circumference. These cords are layers of fabric that make up the body of the tyre, and run radially across the tyre from bead to bead. A “B” indicates the tyre is of bias construction, meaning that the body ply cords run diagonally across the tyre from bead to bead, with the ply layers alternating in direction to reinforce one another.
While radials improve performance the fact is that they are not very good for bad roads and overloading. The suspension of a vehicle has, also, to be modified to accept radials. Radials offer much better grip of the road especially on surfaces which offer poor wheel traction like wet or slippery roads.
The “15” above indicates the rim diameter in inches. It is the diameter of the tyre bead seat ledge in the rim. Most tyres are built to inch standards for rim diameters. However, some tyres are built to millimetric rim dimensions. Always match the tyre’s rim diameter to the wheel rim diameter. This is important for safety.
NOTE: A millimetric rim has a different shape than an inch rim; they are not interchangeable.
Tyre ‘Load’ And ‘Speed’ Markings
With few exceptions, tyres new or retreaded are required by law to carry indications of the tyre’s load carrying and speed capabilities. These are moulded on the sidewall as a service description comprising a Load Index (e.g.‘94’ in table 1) for load carrying capacity and a Speed Symbol (e.g. ‘H’ in table 2)for speed capability. It is strongly recommended to always fit tyres that have a speed capability and load index at least equal to or higher than those originally specified by the vehicle manufacturer.
|Table 1 Load Indices and Related Maximum Loads|
|Load Index||Load KG||Load Index||Load KG||Load Index||Load KG||Load Index||Load KG||Load Index||Load KG|
|Table 2 Tyre Speed Symbol Marking|
|Speed Symbol||Maximum vehicle speed for which tyre is suitable|
Tyres carry a three digit age code on the sidewall indicating the month and year of manufacture. For example 129 means the tyre was manufactured in December 1999.
Tread Patterns And Profiles
Most people don’t realise it but tyres are fashion accessories. 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.
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.
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 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.
Directional And Asymmetric Patterned Tyres
Some tyres have patterns where their direction of rotation is important to achieve their full performance. These are known as ‘Directional’ pattern tyres and the direction of rotation is marked on the tyre’s sidewall. Additionally some tyres have patterns which are different on the inner half of the tread than compared to the outer half. These tyres, known as ‘Asymmetric’, have their sidewalls marked ‘Outside’ and/or ‘Inside’ or similar wording.It is important with both these tyre types to observe the fitting markings on the tyre sidewall.
Runflat Tyres (RFT)
RFT’s represent the latest in a long line of safety initiatives being made available today. Safety has become something of a catch cry for a society which has become less accepting of the trauma associated with all manner of accidents. The value of developing and adopting new technologies to keep us safe is well recognised. Furthermore, consumers have demonstrated a willingness to pay a premium for safety features they understand. Just as anti-lock brake systems, traction control, stability control and air bags have trickled down from upper models to base models, the very same is expected of RFT.
RFT’s also provides more comfort and convenience for the motorist. Within the specified speed and distance limits (typically 80 km/h for up to 80 km), the driver can either complete their journey or travel to the next qualified service station without the worry of changing a flat tyre. This means vehicle occupants can continue travelling to important meetings or events even when tyres are totally deflated.
I confirm I have read the fact sheet ‘Tyre Safety – Know Your Tyres’.
Name of Driver………………………………………………………….Vehicle Reg………………………………….