Learner Zone
Motor Vehicle HubWheels and Tyres
A tyre is a ring-shaped component that surrounds a wheel’s rim to transfer a vehicle’s load from the axle through the wheel to the ground. It provides traction on the surface over which the wheel travels.
alignment
Wheel alignment (or tracking) is an important part of car maintenance. It refers to the angle and direction at which tyres are set. Hitting kerbs or driving at speed through a pothole can result in wheel misalignment. Poorly aligned tyres can affect road holding and decrease the life of tyres.
- Uneven or abnormal tyre wear
- Drag or pulling to one side
- Steering wheel vibration
- Scrubbing or squealing noises at low speeds
Are all signs of wheel misalignment. A tyre that is ‘toed in‘ will show wear on the outer shoulder of the tyre, while a tyre that is ‘toed out‘ will show wear on the inner shoulder. Different types of vehicles require different toe settings to allow for the way that wheels either pull towards each other or apart.
Tyre wear is not the only area of the car that can suffer if wheels are incorrectly aligned. Braking distance, driving quality and fuel consumption can all be adversely affected.
Additionally, wheel balancing ensures that the weight of the wheel and tyre is even around the axle, and is achieved by placing tiny offsetting weights at specific points around the wheel.
If there is uneven tyre wear or tyre ‘wobble’ at 40-50mph, it is likely that the wheels need re-balancing.
Watch this short video to learn more about wheel alignment.
markings
The markings on the tyre sidewall are shorthand for a wealth of information, from the tyre model to the load index, speed rating, tyre size, construction, and much more.
Using the following sequence of tyre markings as an example: 225/45 R 18 95 H
The first number to appear in the sequence is 225. This number is the nominal width of the tyre (in millimeters) from one sidewall to the other.
The next number in the sequence is 45. This number is the tyre aspect ratio – the height of the tyre’s profile outward from the rim represented as a percentage of the tyre width. This number is calculated by dividing the tyre section height by the tyre section width. So, if a tyre has an aspect ratio of 45, the tyre height is 45% of its width.
Next is a letter instead of a number. This letter indicates the type of construction used within the casing of the tyre, which in our example is R for ‘radial construction’. Other examples are B for ‘bias-ply’ or D for ‘diagonal construction’. Radial tyres are the most common on the road.
The next number 18 represents the diameter of the wheel rim in inches.
The number after the rim diameter represents the load index. 95 is a code for the maximum load a tyre can support when fully inflated.
Last in the sequence is the speed rating. Letters ranging from A-Z represent the speed rating. Each letter denotes the maximum speed a tyre can sustain under its recommended load capacity. H is equivalent to a maximum speed of 130 mph (209 kph).
tread
The tread is the part of the tyre that makes contact with the surface of the road. There is a great deal of variety in tread patterns. They are unique designs that enhance a vehicle with grip and handling for specific driving conditions. One can identify a make of tyre by the tracks it leaves on the road.
Every tyre tread has four constituent parts:
- Ribs are the raised section of the tread pattern, made up of tread blocks;
- Grooves are deep channels that run circumferentially and laterally around the tyre;
- Tread blocks are the raised rubber segments that make contact with the road surface;
- Sipes are small, thin slots moulded into the tread blocks.
The ribs, grooves, tread blocks, and sipes can be arranged in a unique pattern to modulate the tyre’s performance in critical areas, which in turn provides tyre manufacturers with the ability to develop tread patterns to address specific driving needs like wet braking, dry handling, aquaplaning resistance, and traction on ice and snow. There are three broad categories of tyre tread pattern.
Symmetrical – the most common type which is suitable for passenger car tyres, but not for high-performance use. Tyres with this design have continuous ribs or independent tread blocks across the entire face of the tread, and both halves of the tyre feature the same pattern.
Directional – tyres with directional tread patterns are designed to roll forward in one direction only. It has lateral grooves that meet in the middle of the tyre tread, resembling the shape of an arrowhead. The V-shaped grooves are more capable of resisting aquaplaning at high speeds by displacing water efficiently through the tread pattern. It also provides excellent handling on snow or mud. A good all-season or winter tyre is likely to have a directional tread pattern. The extra traction is also useful for performance tyres on high-performance vehicles.
Asymmetric – a tyre with an asymmetric pattern features two separate tread designs, one on the inner half and another on the outer half of the tyre. The inner tyre tread is responsible for water displacement and protection against aquaplaning. The outer tyre tread has rigid tread blocks for higher lateral stiffness, which provides high grip when cornering and driving on dry surfaces, and quieter interior noise.