Radial type tyre manufacture

1 The manufacture of radial tyres begins with a number of raw materials: pigments, chemicals, some 30 types of rubber, carcass cords, bead wire and so on.

The process begins by mixing the basic rubber with manufacturing oils, carbon black, pigments, antioxidants, accelerators and other additives, each of which adds various properties to the final mix.

These ingredients are combined in huge mixers known as Banbury machines, which work at extremely high temperatures and pressures. After spending a long time in these mixers, the result is a hot, black, rubbery mixture.

2 The cooled mix may have a number of shapes. Most often, it is formed into carefully identified slabs which are then shipped to the processing factory. These factories process the masterbatch by repeatedly passing it through huge rollers, adding ingredients and mixing these in order to obtain the final composites, which are then transported to the roll mill, where they will be cut into strips which will become sidewalls, treads or other parts of the tyre.

A different type of rubber is used to cover the fabric which forms the carcass of the tyre. These fabrics, supplied on huge rollers, are also specialised and are as vital to the integrity of the tyre as its rubber components. A number of fabric types may be used: polyester, rayon or nylon. Today, tyres for most private vehicles have polyester carcasses.

3 Another hoop-shaped component is known as the bead. Made from a core of high-tensile steel wire, it ensures the tyre fits tightly on the rim. The bead core consists of ribboned layers of steel filaments covered in rubber. The rubber is then rolled around itself several times and secured by clips until final assembly with the other parts of the tyre.

Radial tyres are made using one or two machines. First, a double layer of synthetic rubber acts as a hermetic inner lining, replacing the inner tube.

4 Next come two layers of cord, forming the carcass plies. Two reinforcement strips, known as "apexes", are added above the bead, while two other chafer strips are added to the bead to protect from scratches during fitting on the rim.

The machine on which the tyre is manufactured virtually gives it its final shape and size. The tyre is inspected to ensure all the components are correctly placed prior to moulding.

5 At this stage, the steel belts which increase the resistance of the tyre to punctures and provide a firm grip on the road are added. The tread is the last component to be installed. All the components are then pressed together by rollers. The radial tyres, known at this stage as "green tyres", are now ready for inspection and vulcanization.

6 The tyres get their final shape in the vulcanization mould, akin to a huge waffle iron. Tyre identification and tread markings are engraved in the mould and transferred to the tyre during hot vulcanization. Vulcanization lasts for between 12 and 25 minutes depending on the size of the tyre. When the press opens, the tyre is removed from its mould and transported by a long conveyor to the final quality control and inspection station.

7 If there is the slightest defect - or even the suspicion of a defect, however slight, even in terms of appearance only - the tyre is rejected. Some defects are spotted by the expert eye and hand of inspectors, while specialised machines find others.

Inspection involves far more than a superficial examination. Some tyres are removed from the production line and x-rayed to detect any hidden weaknesses or internal defects. In addition, quality control engineers regularly cut open randomly selected tyres and study every detail of construction which may affect performance or safety.

8 All the components are combined: the tread and sidewalls, supported by the carcass, are fixed to the wheel by the bead, covered in rubber. Except for certain details, all tyres use the same basic elements: steel, fabric, rubber, and lots of hard work, attention, styling and mechanical engineering.

Tyres

Tyres play a vital role in the behaviour, roadholding and comfort of vehicles.

Tyre references

Tyre sidewalls bear a standard reference marking.

Example: 195/65 R 15 91 H

195 refers to the width of the tyre in millimetres

65 is the height of the tyre sidewall as a proportion of its width (here, the sidewall height is approximately 65% of 195 or about 126 mm)

15 denotes the diameter of the rim for this tyre, in inches.

R specifies the tyre structure

R or ZR: Radial (ZR indicates high-performance tyres)

91 refers to the load capacity

rating

max. kg per tyre

and H to the speed rating:

speed rating

Q = maximum speed (km/h)

performance ratings:

The standard UTQG (Uniform Tire Quality Grading System) (mandatory for US DOT standard tyres) indicates tyre performance, as scored with respect to three aspects: wear, grip and resistance to heat.

Wear (treadwear): graded from 60 - 620, reference 100; the higher the index, the better the treadwear

Grip (traction): A A (best), A ,B or C. The grade is awarded following braking tests on a wet test surface.

Heat resistance (temperature): A (best), B or C. This refers to the tyre's heat-dissipation abilities, with C being the minimum acceptable grade.

Other references:

XL, EL or RF: reinforced tyres with increased load ratings

NHR: not approved for road use

MB, OWL or ORWL: white markings on tyre sidewalls

A: asymmetric profile: asymmetric tyre design.

RFT: Bridgestone anti-puncture tyre

Run Flat: Pirelli anti-puncture tyre

Pax System: Michelin anti-puncture tyre

M+S: Mud and snow treads

TT: tyre with inner tube

MFS, FR, ML or G1: includes rim protection ridge

BSW: Black Side Wall

Vehicle Manufacturer Code:

A* or *: approved for BMW

B or AU: approved for Audi

J: approved for Jaguar

MO, MB or A-MO: approved for Mercedes

PE: approved for Peugeot

N0,N1,N3 or N4: approved for Porsche

K1: approved for Ferrari

Tyre wear

There is a ridge in the largest groove of tyre treads. This is the tread wear indicator. If this is at the same height as the rest of the tyre, in other words if it is in contact with the road, the tyre requires replacing. The legal minimum tread depth is 1.6 mm. Worn tyres make the tread less effective and can lead to aquaplaning (a total loss of roadholding) on wet roads.

Abnormal wear

Tyres which are worn at the tread edges:

The tyre is under-inflated, leading to poor fuel economy, increased running temperature and premature edge wear.

Tyres which are worn at the centre of the tread:

The tyre is over-inflated, leading to premature wear.

Patchy wear:

Can be detected by feeling the tread in both directions. This type of wear is generally due to worn shock absorbers or poor wheel balancing.

Asymmetric wear:

If the tyre wear is uneven, the wheels may be poorly aligned.

Fitting the tyres with the least wear to the rear

Placing new tyres to the rear is often recommended to ensure that vehicles understeer on wet road surfaces. This is because it is difficult to recover from oversteer (when the rear wheels lose grip), which requires skilled driving abilities, whereas understeer (when the front wheels lose grip) can be corrected by slowing down, which is the natural reflex of any driver in this event.

Tyre pressures

Tyre pressures should be checked at least once a month. Tyres should be checked cold, when they have not been driven for several hours, or only after a short distance driven at a low speed. If tyres are checked when hot, 0.3 bar should be added to the recommended pressure.

Using nitrogen to inflate tyres offers more stable pressure than traditional air inflation. Tyres inflated using nitrogen generally have green valve caps. Nevertheless, even with this type of inflation the pressure should be checked monthly.

Highway Code prohibitions:

Article 3:

"The installation of the following on vehicles and trailers covered by section II of the Highway Code is prohibited:

3.1 Tyres with different structures, except for spare tyres for temporary use.

3.3 Different types of tyre on the same axle, whether single or double tyres.

3.4 Tyres specifying a load rating or speed category symbol lower than the maximum capacities specified by the vehicle manufacturer."

Article 9:

The provisions of this order apply to the following:

- tyres manufactured after January 1, 1995;

- new vehicles registered after January 1, 1995.

When the Highway Code prohibits tyres of different structures being fitted on the same axle (article 3 points 3.2 and 3.3)

EC Journal No. 1  129/105 of 14/05/1992 specifies that the tyres must be:

- of the same brand

- of the same dimensions

- for the same category of use (e.g. road, snow, all-terrain)

- have the same structure; radial or cross-ply

- have the same speed rating

- have the same load capacity rating

winter tyres