Aim
To study and prepare report on the constructional details, working principles and operation of the following automotive tyres and wheels.
a) Various types of Bias & Radial Plies Tyres
b) Various types of Wheels
Disc wheel
This type of wheel consists of two parts, a steel rim which is base to receive the tyre and a pressed steel disc. The rim and the disc may be , permanently attached or attachable, depending upon design. A typical steel disc wheel is shown in fig (3). in which the pressed steel disc is welded to the rim.
When the bead of the tyre is rests in the rim, it becomes possible to fit in position of the rim. Without the well it would not be possible to mount or remove the tyre from the wheel. The seat of the rim where the tyre rests usually has a 5 degree or 15 deg taper (not shown) so that as the tyre is inflated, the beads are forced up the taper to give a wedge fit. With tubeless tyres, the taper helps to build a good seal.
The steel disc performs the functions of the spokes. The wheel hub is fitted on the axle. Some slots are generally provided in the wheel disc to allow the air to pass to the inner side for better cooling of the brake drum inside. Since these holes tend to weaken the disc, the holes in modern wheels are swaged which means that some portion of the disc around each hole is turned inward smoothly to compensate for the loss of strength due to holes. A separate cover is also provided on the wheel disc. A hole in the rim serves to accommodate tube valve.
A wheel may be inset, zero set or outset, depending upon the position of the rim in relation to attachment face of the disc. In the inset wheel the centre line of the rim is, located inboard of the attachment face of the disc. Inset is distance the distance from the attachment face of the disc to the centre line of the rim. A zero set wheel is the one in which the rim centre line coincides with the attachment face of the disc while in the outset wheel the centre line of the rim is located outboard of the attachment face of the disc. A wheel whose disc can be mounted on either face to provide inset or outset, thus decreasing or increasing the wheel track is called reversible wheel. Wheel constructed in two parts, which when securely fastened together combine to form a rim having two fixed flanges is called a divided wheel rim.
Wire Wheel
Unlike the disc wheel the wire wheel has a separate hub which is attached to the rim through the number of wire spokes. The spokes carry the weight, transmit the driving and braking torques and withstand the side forces while cornering, in tension. Spokes are long, thin wires and as such these can not take any compressive or bending stresses. All types of loads are sustained by the spokes in tension .The spokes are mounted in a complicated criss-cross fashion installed in the three planes. The component of vehicle weight in the direction of spokes above the hub is sustained by these spokes in tension. Similarly, the driving and the braking torques are taken up by the tension in the spokes in the desired direction as shown by fig respectively. The side forces on cornering are taken up by the spokes forming triangular arrangement. Thus it is seen that the spokes have to be mounted on the wheel. The initial tension of the spokes can be adjusted by means of screw nipples which also serve to secure the spokes to the rim. The hub is provided with internal splines to correspond to the splines provided on the axle shaft. A wing nut screws the hub on the axle shaft. The advantages of this type of wheel are light weight and high strength, and above all it provides much better cooling of the brake drum. It is also easy to change the wheel when required, because only one nut has to be opened. However, wire wheels are expensive due to their intricate construction. The rim of a wired wheel is not capable to fit tubeless tyres.
Light alloy cast or forged wheel
The latest trend in case of automobile wheels is the use of wheels made from aluminum or magnesium alloys cast wheels are generally used for cars while forged wheels are preferred for wheels of heavier vehicles. The main advantage of light-alloy wheels is their reduced weight which reduces unsprung weight. A magnesium alloy wheel weighs about 50 per cent of a steel wheel and about 70 per cent of an aluminum alloy wheel for similar strength. Moreover, light alloys have better conductivity of heat which helps the wheels to dissipate heat generated by the tyres or brakes and thereby run cooler. Further, wider rims improve stability on cornering. Through cast or forged wheels have to be machined, yet this helps to maintain close tolerances and also produce better appearance. Magnesium alloys have high impact and fatigue strength so that they can stand vibrations and shock loading better. However, being tune to corrosion, these have to be given some protective coating. Aluminum alloys do not have high resistance to vibration and shock as in case of magnesium alloys, but these are relatively easier to cast or forge and also less tune to corrosion. Generally, aluminum alloys are used for cars sports and racing cars usually have magnesium alloy wheels. Higher cost is the only disadvantage of light alloy wheels.
Wheel styling being a modern trend in automotive industry, wheel division of Dunlop Limited has designed a range of wheels of cast aluminum alloy and designated this range as Formula D This range also includes composite wheels, i.e. the wheels with cast aluminum body alloy and steel rims.
Tyre
A tyre is a cushion provided with an automobile wheel. It consists of mainly the outer cover i.e the tyre and the tube inside. The tyre tube assembly is mounted over the wheel rim. It is the air inside the tube that carriers the entire load and provides the cusion. .
The tyre performs the following functions
1 Supports the vehicle load
2 Provides cushion against shocks
3 Transmits driving and braking force to the road
4 Provides cornering power for smooth steering
Tyre Properties
Non skidding
This is one of the most important tyre properties. The tread pattern on the tyre must be suitably designed to permit least skidding even on wet roads.
Uniform wear
To maintain the non skidding property, it is very essential that the wear of the tyre treat must be uniform. The ribbed tread pattern helps to achieve this.
Load carrying capacity
The tyre is subjected to alternating stresses during each revolution of the wheel. The tyre material and design must be able to ensure that the tyre sustains the stresses.
Cushioning
The tyre should be able to absorb high frequency vibrations set up by the road surface and thus provide cushioning effect
Power consumption
The automotive tyre should absorb some power which is due to friction between the tread rubber and road surface and also due to hysterics loss on account of the tyre being continuously fixed and released. This power comes from the engine and should be least. It is seen that the synthetic tyres consume more power while rolling than the ones made out of natural rubber.
Tyre noise
The tyre noise must be in the form of definite pattern, a sequel or a loud roar. In all these cases, it is desirable that the noise should be minimum.
Balancing
Balancing is very important consideration for tyres. The tyre being a rotating part of the automobile, it must be balanced statically as well as dynamically. The absence of balancing gives rise to peculiar oscillations called wheel tramp and wheel wobble.
Types of tyres
The use of solid tyres in automobiles is almost obsolete and only the pneumatic tyres are used universally. These pneumatic tyres are of two types viz. the conventional tyre with a tube and the tubeless tyre.
Conventional Tube tyre
Fig gives in a simplified form the cross section of such a tyre. It consists of two main parts, viz. the carcass and the tread. The carcass is the basic structure taking mainly the various loads and consists of a number of plies wound in a particular fashion from the cords of rayon or any other suitable material. Each cord in each ply is covered with resilient rubber compounds and all the plies are insulated against each other. The term ply rating which is often used in tyre industry does not indicate exact number of plies in the tyre. It is only a relative index of tyre strength and load carrying capacity. A four ply rating tyre, may have only two plies. In order to prevent the tyre from being thrown off the rim, the plies are attached to two rings of high tension steel wire. These rings are made to fit snugly against wheel rim thereby anchoring the tyre to the rim. These rings are called beads.
The tread is the part of the tyre which contacts the road surface when the wheel rolls.It is generally made of synthetic rubber and the design of the tyre tread depend on various tyre properties viz the grip, the noise and the wear. The tread is moulded into a series of grooves and ribs. The ribs prove the traction edges required for gripping the road surface while the grooves provide passage for quick escape of any foreign matter such as water etc. Traction edges and sipes are provided on the ribs. Sipes are the small groves moulded into the ribs of the tyre tread, these increase the traction ability of the tyre by increasing number of traction edges. As the tyre flexes on the road surface, the sipes open to provide extra gripping action. The sipes appear to be shallow but in fact these travel the entire depth of the tread. The design of the tyre tread has a direct effect on the tyre life, its handling characteristics, quality of ride comfort, noise, and traction. Between the head and the tread the outer rubber covering of the carcass is called side wall. The sidewalls are designed to flex and bend without cracking when subjected to continuous deflection while running. In other words, the sidewall material must have high fatigue strength. At the inner edges, beads are formed by reinforcing with steel wires. This provides the tyre with strong shoulders for bearing against the wheel rim. All plies are tied to the beads which prevent any change of shape.
Inside the tyre, there is a tube which contains the air under pressure. The tube being very thin and flexible, takes up the shape of the tyre cover when inflated. A valve stem is attached to the tube for inflating or deflating the same.
Tubeless tyre
This type of tyre does not need a separate tube; instead the air under pressure is filled in the tyre for which purpose a non-return valve is fitted to the rim (fig 6) The inner construction of the tyre is almost same as that of tube tyre, except that it is lined on inside with a special air-retaining liner.
The tubeless tyres possess following advantages over the conventional tubed tyres.
1. Lesser upsprung weight – being lighter, unsprung weight is reduced, and ultimately reduces wheel bouncing.
2. Better cooling – In case of tube tyres, heat in the compressed air has to pass through the tube material, i.e. rubber, which is not a good conductor of heat. Since there is no tube in the tubeless tyres, hence heat passes to the atmosphere directly resulting better cooling thereby increasing the tyre life.
3. Slower leakage of air – since the inner liner in the tubeless tyres is not stretched like the tube, it retains the air better resulting in its slower leakage.
4. Simpler assembly – Only the tyre has to be fitted over the rim. There is no danger of the tube being punctured during assembly.
5. Improved safety – In case of any tiny hole being caused in the tyre, the same can be repaired simply by plugging, whereas in case of the conventional tyres it takes quite some time to remove the tube for repair. Apart from this, a tubeless tyre retains the air pressure for longer period even when punctured by nail provided the same is held in place.
Carcass Types
Carcass or skeleton of the tyre is of 3 types
1 Cross ply or bias ply
2 Radial Ply
3 Belted bias type
The tyre is named after the particular type of carcass it contains as this is the main structure taking the stresses while in operation.
Cross Ply type
In this type, the ply cords are woven at an angle (30deg – 40deg) to the tyre axis. There are two layers which run in opposite directions. However, the cords are not woven like wrap and weft of ordinary cloth because that would lead to rubbing of the two layers and thus produce heat which would damage the tyre material (fig 1).
Radial ply type
In this ply cords run in the radial direction i.e. in the direction of the tyre axis. Over this basic structure, run a number of breaker strips in the circumferential direction. The material for the breaker strips must be flexible but inextensible, so that no change of circumference takes place with change of the amount of inflation. Without the breaker strips, radial plies would give very soft ride, but there will not be any lateral stability. The extensible breaker strip behaves like a girder in its own plane and provides the directional stability (fig 2).
Belted bias type
This is a combination of cross ply & cross ply tyres. The basic construction is the bias ply over which run a number of breaker belts. The belts improve the characteristics of the bias-ply tyre to a large extent. (1) the stresses in the carcass are restricted and the tread area is stabilized by belts. This results in reduction of tread-scrubbling and thus appreciable increase in tyre life (2) the breaker belts hold the tread flatter against the road surface, thus causing increase of traction and safety, (3) the belts increase the resistance of the tyre against puncture.
Reference Books:
1. Automotive Mechanics – Crouse/Anglin
2. Automobile Engineering by Dr Kirpal Singh
3. Automobile Engineering by Pitman
Viva Questions
1 Describe the requirements of an automobile wheel
1 What is a light alloy? Discuss the merits and demerits of light alloy in automobile wheels
2 What are the advantages of a disc wheel
3 What are inset, zero set and outset wheels
4 How is the vehicle weight supported in case of a wire wheel
5 What are the advantages of a wire wheel
6 Why wire wheels can not be used to mount a tubeless tyre
7 Name the different materials for automobile wheels
8 State the advantages of (i) Magnesium alloys (ii) Aluminum alloy wheels
9 What are the functions of an automobile tyre
10 State various desirable properties of an automobile type
11 State various factors affecting tyre life
12 How does a tyre wear in case of (i) over-inflation (ii) under-inflation
13 How tyre is designated
14 What is type carcass
15 What is a function of a breaker belt
16 State the factors which road grip of a tyre depends
17 What is static unbalance and dynamic unbalance
18 What is the reasons for higher braking efficiency on wet roads in case of radial ply tyres
19 What is ply rating of a tyre
20 Draw cross section of an automobile tyre and show on it various constructional features
21 How is tyre manufactured? Explain briefly.
22 Compare the radial and bias ply type carcass tyres
23 How does heat affect the tyre life
24 State reasons for the following tyre defects:
(i) Ply separations
(ii) Uneven wear
Tread cracking