Wheels and Tyres
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Wheels and Tyres.
Tyres on modern vehicles fall into two main categories know as Radial and Cross-ply.
Radial tyres are fitted to most light vehicles as they are smaller and lighter, giving reduced under wing space and light suspension un-sprung weight. They are also very good at running at high speeds safely.
The tyre shown via the link below has a carcass of either a steel or man made fibre supported structure known as the ply belt, which runs from rim to rim RADIALLY. Over this is laid a tread support structure and finally the rubberised compound is laid over the top to form the tread.
Radial diagram -
The tyre depends on the structure to be safe so any damage to the underlying structure is both dangerous and illegal. The tyres must also be suitable for the vehicle weight and speed.
Cross ply tyres have the carcass plies running at different directions to each other as shown by the picture (see link below). These plies run over the whole width of the tyre including the tread area and this makes the sidewall much stiffer and also makes the height of the tyre slightly different. This has the effect of making the tyre much better over rough ground as the sidewalls are less likely to collapse and experience damage while driving. However this makes the tyre gain more heat so on motorway driving the friction increases fuel consumption and the stiff tread does not perform so well and causes the tyre to have less grip than a radial.
Cross ply diagram -
Due to the fact that the grip the tyre can produce differs, it is illegal to mix cross ply tyres with radial. The only acceptable configurations are to have cross ply on the front axle, or cross ply all around the vehicle. Everything else is illegal and dangerous.
The tyres are identified by the markings on them.
The tread pattern vary depending on the use the tyre is being put to, i.e. off road, motorway, drive axle steer axle etc but generally on light vehicles the tread pattern is designed primarily to disperse water.
The tyre is identified by the markings shown on it. Radial tyres are always marked in mm across the width.
The markings 195/65 R 15 91 H
Mean
195mm from outside to outside measured across the width.
65% of the width is the distance from bead to tread in profile.
R means Radial
15 inches from bead to bead which is the wheel size.
91 is the weight rating referring to the standards table.
H is the speed rating referring to the standards table.
The tyre fitted to the vehicle must be suitable in terms of size, speed, weight and safety. If it is not the fitter is the person responsible if any accidents occur as a result.
Safety checking.
Light vehicle tyres must have at least 1.6mm of tread over at least 75% of the tread area and visible tread over the rest. The tyre must not have cuts in the sidewall greater that 10mm or 10% of the wall height whichever is the less. Any cut that exposes the tyre carcass is illegal. There should be no bulges or damage to the tyre or sidewall. Tyres must be suitable for the vehicle and the same size on each axle. Some tyres have a tread pattern which must be fitted in a certain direction, so called Directional Tyres and the tread is designed for moving water from the surface of the road. These will have direction markings on the wall. Some tyres have to have one side fitted to the outside to help with cornering or because of the carcass design. These are called Asymmetric Tyres and will have markings to indicate which side is outside.
Tyre Pressures
The tyre pressure must be correct for the use the vehicle is being put to, so a chart is normally referred to find the correct pressure. The pressure may be quoted in Bar,(i.e. 2 Bar) or pounds per square inch, (i.e. 30 psi).
Tyres wear unevenly for a number of reasons. These may be imbalance, tracking out of alignment, worn dampers, under or over inflation, steering system wear or misalignment or many other reasons.
Wheels differ in their structure and design and these must be suitable for the vehicle. Wheel nut fixings differ depending on whether the wheel is steel or alloy, and of the mounting design. The pictures show stud mounting and hub mounting types.
Alloy wheels are often based aroung a hub fixing to help support the wheel.
The wheel also has a drop centre called a well to aid tyre filtting and also has an offset, that is on different wheels you may observe the wheel fixing centreline to be in different positions between different wheels.
Wheels need to be balanced to avoid wear and vibration and to avoid short tyre life.
Tyre inspection
Equipment
Depth gauge, pressure gauge
Results
Check item / NSF / OSF / NSR / OSR / Spare / NotesPressure
Wear
Note 3 breadth points
Damage
Type
Size
Load index
Speed index
Valve condition
Tyre/rim seating
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