WHEELS AND TYRES

WHEELS AND TYRES

The importance of wheels and tyres in the automobile is obvious. Without the engine the car may be towed, but even that is not possible without the wheels. The wheel, along with the tyre has to take the vehicle load, provide a cushioning effect and cope with the steering control. The various requirements of an automobile wheel are:

1. It must be strong enough to perform the above functions.

2. It should be balanced both statically as well as dynamically.

3. It should be lightest possible so that the unsprung weight is least.

4. It should be possible to remove or mount the wheel easily

5. It material should not deteriorate with weathering and age. In case the material is susceptible to corrosion, it must be given suitable protective treatment.

3.4.1 Types of wheels

There are three types of wheels, viz., the pressed steel disc wheel, the wire wheel and the light alloy cast wheel. Out of these, the first type is used almost universally, because of its simplicity, robust construction, lower cost of manufacture and ease in cleaning. The wire wheel is the earliest type of wheel but presently its use limited to certain sports cars, because it is lighter, heat dissipation is better and it can be fitted and removed very easily. However, tubeless tyres cannot be fitted over wire wheels which are also difficult to clean. the light alloy cast or forged wheel is the most recent type, whose use is ever increasing in both road and sports cars. The use of light alloy (Aluminum and magnesium alloys) makes it possible to use wider rims, which allow low aspect ratio (i.e. wider) tyres to be fitted, thus improving road adhesion, especially on corners. Further, light alloys being good heat conductors, dissipate heat produced by tyres and brakes more efficiently than steel. However, these are relatively costly and more prone to corrosion. Composite cast wheels with steel rims and light alloy centres have also been used.

1. Disc wheel

This type of wheel consists of two parts, a steel rim, which is generally well-based to receive the tyre and a pressed steel disc. The rim and the disc may be integral, permanently attached or attachable, depending upon design. A typical steel disc wheel is shown in Fig. 3.33 in which the pressed steel disc is welded to the rim.

When the bead of the tyre is resting in the well, it is possible to pass the tyre over the opposite edge 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 degree 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 make a good seal.

The steel disc performs the functions of the spokes. The wheel is fitted on the eale by the bolting to a flange attached to the latter. 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 inwards smoothly. This more than compensates 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, zeroset 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 in board of the attachment face of the disc. ‘Inset’ is the distance from the attachment face of the disc to the centre line of the rim. A zeroset 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. a wheel constructed in two parts, which when securely fastened together combine to form a rim having two fixed flanges, is called a divided wheel.

For heavier vehicles, e.g., buses and trucks, the large tyres are bulkier in the bead region. For such tyres, therefore, the well rims as described above are not used. Rather flat based rims are used which may be of 2,3,4, or 5 piece construction. In these types of wheels the lock ring and the loose flange have to be removed first to remove the tyre. Further these rims can be used only with the tubed tyres because they cannot be made air tight which is the basic necessity if tubeless tyres are used.

2.Wire Wheel

Unlike the disc wheel the wire wheel has a separate hub, which is attached to the rim through a 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 cannot take any compressive or bending stresses. All types of loads are sustained by the spokes in tension (Fig 3.34), The spokes remounted in a complicated criss-cros fashion in all the three planes. The component of vehicle weight in the direction of spokes above the hub [Fig. 3.34 (c)] 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 Figs. (3.34 (a) and (b) respectively. The side forces on cornering are taken up by the spokes forming triangular arrangement as shown in Fig. 3.34 (d). Thus it is seen that the spokes have to be mounted on the wheel in a complicated manner. 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 very 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. Further, because the rim of a wired wheel has holes, it is not possible to fit tubeless tyres on wire wheels.

Pneumatic tyres are of two types.

1. Tube-Type-Tyre: A tube-type-tyre is shown in fig. 3.35. In this type, the tube and the tyre are mounted on the wheel rim. The inner tube, which is an endless tube of rubber, in inflated with air and thus provides a cushing medium. The air is forced through a valve provided on the tube and is retained under pressure in th tube. A cap is screwed  down tightly over the end of the valve stem to safeguard against air leakage and to keep dirt from entering the valve.

Rayon cord impregnated with rubber is used for making a tyre. Over the cord, the outermost portion i.e. tread is built up which is of synthetic rubber. A special device is used to form the layers of cord (i.e. plies). The tread material and the side wall are applied and vulcanized into its position by the process of “Vulcanizing”. The vulcanizing is a process of heating the rubber under pressure. The rubber molded by this process will conform to the desired form and impart the necessary characteristics.

The number of plies to be provided on the tyre will depend upon  the use to which the tyre will be put. Usually plies-tyres are used for passenger-car. Tyres used for buses and heavy duty trucks may be made up of the plies.

Tyre sizes

The size of the tyre is marked on the side of the casing. The size of the tyre is selected according to the size of the rim which vary from vehicle to vehicle. For example a tyre is marked as 8.25X20. This means that the tyre fits on a 20 inch rim that it is 8.25 inches larger in radius than the rim without load when properly inflated. Thus the diameter of the tyre when inflated (without load) is, (8.25 = 20 + 8.25 ) 36.5 inches. Similarly tyres are available in different sizes as 7.50 X 16,000 , 15,900 X 20, 9.25 x 20 etc.

Tyre pressure

The air-pressure maintained in the tyre depends upon the type of tyre and the purpose for which it is used. For example, cars and scooters etc. are provided with low-pressure tyres and the pressure is kept 22 to 28 pounds per square inches p.s.i. While the heavy duty trucks and buses are employed with the tyres inflated upto 100 pounds per square inches.

Classification of tyres

1.         Construction

            (i)         Grow-ply tyre

            (ii)        Radial tyre

            (iii)       Belted bias tyre

2.         Application

            (i)         Passenger car tyres

            (ii)        Light truck tyres

            (iii)       Truck & bus tyres

3.         Road Surface

            (i)         Snow tyre

            (ii)        Spiked tyre

            (iii)       All-weather tyre         

            (iv)       Sand tyre

2. Radial tyre. With the arrival of the “highway age” and it high speed driving, the importance of tyres began to draw special attention and this led to the interest on the radial tyres.

In the common tyre, the carcass is made up of layers of cords arranged in bias from  (cross weave). In the radial tyre, the layers of cords are arranged in circumferential direction and parallel to each other. The breakers over the plies are also of cords arranged in nearly circumferential direction. The radial tyre has a flatter cross section in comparison to the rounder cross section of the common tyre.

The radial tyre is constructionally stronger against cornering smaller rolling resistance, and produces less heat, making it chiefly suited for high speed riving. These tyres first found use in sport cars but now they are being used to a large extent in ordinary passenger cars, even through this type of tyre has disadvantages of picking up vibrations and noises more readily.

The shock absorbing properties are better in radial types as they can be readily on rough roads.

The rolling resistance in radial tyres are less as to reduced internal friction will increase the fuel efficiency. The friction also develop the heat in tyres which in bais ply have life upto 60% less compared with radial tyres.

The braking efficiency is better than bias ply as it have greater sidewall flexibility.

The other disadvantages of radial tyres are reported in slow speeds as it increases steering efforts and more friction with the roads. 

3.         Tubeless tyre

The tubeless tyres do not use an inner tube. The tyre is mounted on the rim in such a way that air is retained between the rim and the tyre. The rim used with this type must be sealed. On the tubeless tyre, air is inflated in the tyre through a valve which is mounted on the wheel rim. The tyre bead is made in such a way that it will rest firmly against the flange of the tyre rim. This will help to maintain the air pressure in the tyre after it has been inflated.

If by chance a nail or any sharp object is driven into the tube type tyre, then it will puncture the inner tube and air will be released immediately through the valve hole. But in a tubeless tyre practically very little or no air will be lost from the tyre due to nail puncture so long as the nail remains in the tyre. So any defeat or injury on the tubeless tyre will cause a slow leak but a sudden blow out can be there on a tub-type tyre. So it is advisable to get the tubeless tyre inspected periodically so that all foreign objects cold be removed.

Repairs of the tubeless tyre become rather difficult when some objects are allowed to remain in the tyre as they try to enlarge the puncture. Repairs which are of ordinary nature can be done without removing the tyre from the wheel. This is carried out by the use of a special tool. Sometimes the hole can be repaired from inside by the use of special rubber plugs. These plugs  are designed to be inserted from inside without the necessity of removing the tyre from rim. Retreading of the tubeless tyre is done in the same manner as the tube type tyre.

3.4.3 Causes of Tyre Wear

The treads of tyre wear varies in relation to the type of road surface, the number of curves, the amount of traffic, the temperature and the driving habits of the driver. Abnormal wear can be accounted for by such conditions as incorrect-pressure, faulty wheel alignment, faulty brake adjustment and over loading. The probable causes of tyre wear and their remidies are discussed below:

Probable causes of tyre wear

1. Over inflation: The wear in the centre of the tyre tread is increased by excessive internal pressure. The air pressure also causes the increase cord stresses which reduce the tyre life. Apart from this the riding comfort is affected and more ear noises e.f. rattles or squaks become accentuated.

2. Under-inflation : Under inflation causes the side-walls of the tyre to bend sharply as the wheel revolves, so that the line the cord breaks and utimately the tyre blows out.

3. Front-wheel-Misalignment : This is a frequent cause of rapid tyre wear. Despite increased steering drag, the irregular tread wear will occur, which will slow feathered edges with a ‘filed’ appearance caused by the constant side grinding effect.

4. Rear-wheel Misalignment : The rear wheels will not be parallel to the centre line of the chassis if chassis frame is distorted. Serious tyre wear will take place due to sideway drag.

5. Defective wheels : wheels can be distorted due to accidents or kerb impact and therefore do not rotate truly on their hubs. As a result excessive tyre wear may be caused. Wheel rims more than 1.5 m.m out of true should be discarded.

6. Excessive Speeds : The tread wear increases fairly with the car speed. Other factors affecting tyre wear are condition of -road surface, curves, temperatures, unbalanced wheels and tyres, neglect of small cuts and oil or grease on the corners.

7.  Excessive Wheel Camber : It results in excessive side wear on the treads. This may be due to incorrect adjustment or some severe accident: which leads to excessive wear in the king-pins and suspension bearings.

8.   Flats on Tyres: Flats are caused by violent braking, resulting in wheels locking, so that the tyres dragged over the road without turning.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course