Automobile Wheels and Tyres
The automobile wheels support the total weight, resist the strains created during turning and transmit the driving torque for propelling and breaking torque for retarding. The wheels must have well balanced construction particularly for running at high speeds. Unbalanced wheel assemblies cause excessive vibration, which accelerates tyre and king-pin wear.
Automobile control, acceleration and braking occur through the tyres and their contact on the road surface. The tyres must be large and strong enough to support the vehicle on the road. The traction, the force required to make the tyre slip on the contact patch, is the same on the wheel in all directions whether it is accelerating, cornering, braking, or any combination of these. The tractive force to control the automobile drops rapidly when a skid starts so that control is lost. The tyre must absorb, by deflecting, part of the shock from road irregularities.
During normal operation, passenger car tyres rotate approximately 500 revolutions for each km travelled. The chief factors affecting tyre life are inflation pressure, vehicle speed and rate of acceleration, temperature, tyre interchanging, and tyre and wheel balance. A tractor tyre normally is expected to last twenty years, a truck tyre 80,500 km running, a passenger car tyre 48,500 km and a racing tyre 800 km.
The chapter discussed various road wheels and tyres, their construction and other aspects, including wheel balance.
23.1.
Automobile Wheels
The automobile wheels fulfill a number of following objectives.
Structure.
Wheels must be rigid enough to retain their shape under all operating conditions. When subjected to abnormal impact, they should preferably buckle and must not collapse. The dimensional tolerances of the wheel should be accurate enough for carrying out wheel alignment and balancing.
Weight. Wheels must be light so that the unsprung weight is reduced. Light wheels and tyres also follow the road surface contour more accurately so that wheel bounce is minimized, resulting in improved road contact and reduced tyre wear.
Tyre Attachment.
Since the wheel-tyre combination is responsible for the transmission of traction to the road or for steering reaction, the tyre must be suitably located and rigidly secured on the wheel. Aditionally, the design of wheel should be such that the tyre can be fitted easily.
Wheel Mounting.
The wheel attachment must be designed properly for locating, securing, and supporting the wheel. Also the wheel should be easily fitted or removed from its axle-hub.
Cost.
Wheels should be made out of cheaper materials that can easily be fabricated, cast, or forged, with the minimum machining. It should also have better finish and appearance and should not easily deteriorate with age and weathering.
23.1.1.
Pressed-steel-disc Wheels
These wheels (Fig. 23.1) use a formed disc pressed into a rolled-section well-base rim and held in position by spot welding. The formed disc contacts the rim in a number of equal-spaced arcs. The disc has a number of equal-spaced shallow elongated slots immediately under the base of the rim well (Figs. 23.1 and 23.2A), which improves brake cooling and decreases the transfer
Fig. 23.2. Pressed-steel-disc wheel rim. A Car double-hump well-base rim. B. Van 5 degrees seat-angle well-base rim.
C. U-type drop centre 15 degrees taper rim.
of heat from the brake-drum to the tyre due to passage of air through the slots. The disc is fixed to the axle-hub by a number of studs and nuts with conical or spherical seating.
The rim for the car wheel is cold rolled from flat steel strip and the rims for heavier commercial vehicle are hot rolled to the section from steel bars. The steel strip is cut to the required length and then rolled into a circle before the ends are butt-welded. The circular steel strip then undergoes a series of rolling operations to obtain a complete rim. The rim is then expanded to size and the valve hole is punched.
The well in the base of the rim is sufficiently deep to hold the beads of the tyre for mounting or demounting. The portion of the rim where the tyre sits have a 5 degrees taper due to which, as the tyre is inflated, the beads are forced up the taper providing a wedge fit, and a good seal is obtained with tubeless tyres.
Car Well-base Rims.
Wheel rims are designed to minimize dislodging of the tyre, when subjected to heavy cornering. Originally the rim was tapered from the rim flange to the edge of the well so that the pressure acting over a short length of the bead would force back the tyre bead into the well. This can happen due to the interaction of the cornering side-force acting on the wheel and the reaction of the tyre and the road.
This is prevented in the flat-ledge rim profile (Fig. 23.1) due to the provision of a parallel flat ledge between the outer taper adj acent to the flange and the edge of the well. In a close-fitting tyre bead this can happen only due to the pressure acting all the way round the bead. Another approach for improving the bead rim seal and joint is to form a hump slightly in front the shorter taper next to the rim flange (Fig. 23.2A). Once the bead sits over the short taper portion adjacent to the rim flange, it is very difficult for it to climb over the hump, back into the rim well.
Van 5 Degrees Seat-angle Well-base Rim.
These rims (Fig. 23.2B) have high wall flanges, which protect the tyre beads and walls from damage due to external interference. Generally both tubed and tubeless tyres are fitted on these rims. However large stiff tyres cannot be mounted over these flanges, and hence these rims are only used for vans and small commercial vehicles.
U-type Drop-centre 15 Degrees Taper Rim.
Tubeless tyres and large section tyres with relatively flexible beads are used for vans, buses, and medium sized trucks. These tyres use a single-piece well-base wheel rim, having relatively shallow flanges at the edge of 15 degrees taper bead-seats (Fig. 23.2C). This profile of the rim provides a good joint and sound seal between the tyre bead and the rim taper.
23.1.2.
Detachable-rim Wheels
Large tyres for commercial vehicles use more plies in the casing and are therefore relatively bulkier in the bead region. The rims for these tyres have one removable side-flange, which allows the wheel tyres to slide into position, and then the flange can be replaced and locked in place.
Semi-drop-centre Two-piece Rim.
This type of rim (Fig. 23.3A) has inner and outer tapered tyre-bead-seating surfaces separated by a shallow central or near central well. The inner flange can be removed for fitting a tyre. In this design, the well depth only permits the tyre beads to pass over the top of the outer bead-seat taper. The outer flange is detachable and is sprung into a continuous groove formed along the outer edge of the rim base so that it is retained in its working position.
This rim forms an intermediate class between the well-base and wide-base rims. It accommodates heavier beads, which are too rigid for fitting on the full well-base type. This rim is used on light trucks.
Wide-base Two-piece Rim.
This rim (Fig. 23.3B) is made up of one fixed flange integral with the rim base and one split detachable flange. The rim uses 5 degrees taper seats for tyre beads, the one on the detachable flange side being integral with the flange. The outer detachable flange is sprung into a continuous groove formed along the outer edge of the rim base to retain the flange in its working position. During inflation of the tyre the bead sits over the detachable-flange taper, holding it is position. This rim is used mainly on medium-size commercial vehicles.
Wide-base Three-piece Rim.
This type of rim (Fig. 23.3C) contains one fixed flange integral with the rim base, one detachable endless flange, and a separate flange-retaining split lock-ring. The rim uses 5 degrees taper seats for tyre-bead location, the one on the detachable-flange side usually being on an extension of the spring lock-ring. During inflation of the tyre the bead sits over the extension of the spring lock-ring, holding it in position. This rim is used on large commercial vehicles.
Divided Flat-base Rim.
This type (Fig. 23.3D) of rim is integral with the wheel itself. For fitting or removal of the tyre the two halves of the wheel are divided by dismantling the outer ring of bolts, which hold the wheel halves together. These rims are used primarily for large military trucks.
Fig. 23.3. Detachable rims. A. Semi-drop-centre two-piece rim. B. Wide-base two-piece rim.
C. Wide-base Three-piece rim. D. Divided flat-base rim.
23.1.3.
Cast and Forged Alloy Wheels
These wheels are manufactured as a single-piece rim and disc. Car wheels are generally cast or extruded, but truck wheels are forged. Magnesium and aluminium alloys are most commonly used for reduction in weight. Magnesium alloy produces a saving of weight of 30% over aluminium alloys and 50% over steel for similar strength. Magnesium alloys exhibit very good fatigue properties and excellent resilience, due to which they are capable of resisting vibrational and shock loading better than both aluminium alloy and steel. However they are highly susceptible to corrosion and therefore must be provided with a protective surface coating. Although aluminium alloys have relatively less fatigue properties but their corrosion characteristic is also less and they can be readily cast or forged. The section thickness for the rim and disc are required to be greater with light alloys than with steel.
Even though it is a disadvantage to machine the wheel rim and the stud hole flange after casting or forging operation, but this produces close-tolerance wheels. Also most light alloys are better conductors of heat than steel, so that they transfer any heat generated by the tyre or brake more quickly to the wheel-disc for dissipation to the air stream. Light-alloy wheels are more expensive to manufacture than pressed-steel wheels. Aluminium-alloy wheels are cheaper than magnesium alloy wheels. Light-alloy wheels are used for better appearance and lighter weight. Usually an aluminium alloy is preferred for passenger cars and trucks, and a magnesium alloy for sports and racing cars. The wheel-rim profile used for both light alloy wheels and steel rims is similar.
23.1.4.
Wheel Mountings
Car Wheels.
Conical-nut Mounting.
Car steel wheels are generally aligned with conical-taper nuts those fit into the wheel-disc countersink stud holes. The wheel is centralized to the hub axis due to the taper, which also provides a wedge action to the nuts when they are tightened. The wheel then properly located, secured, and held by the stud and nut (Fig. 23.4A).
Fig. 23.4 Passenger car wheel fixing. A. Nut with conical seat. B. Spigot-mounting with conical set-bolt.
C. Nut with fitted shoulder and conical seat. D. Nut with fitted sleeve and conical seat.
Spigot Mounting with Conical Set-bolt.
Figure 23.4B represents the spigot-mounting wheel on the axle-hub, which accurately locates and partially supports the wheel. The disc is secured to the hub by the conical-shaped set-bolts, which also transmit both the driving and braking torques. This method is suitable with both steel and light-alloy wheels and is commonly used on small and medium sized cars.
Shouldered-nut Mounting.
The light-alloy car wheels with the softer metal may distort or tear away at the stud holes when the nuts are tightened, so that these holes are slowly enlarged. To maintain a concentricity of the wheel, the conical end of the nut is extended with a parallel shoulder or sleeve portion, which is a close fit with the stud hole. The wheel is then located, secured and supported by the nut (Fig. 23.4C).
Sleeved-integral-nut Mounting.
A further improvement in car wheels is achieved by incorporating a separate sleeve, which has a conical-and-parallel profile. This sleeve rotates freely relative to the hexagon nut and prevents tearing between the alloy wheel and the stud nut during tightening (Fig. 23.4D).
Commercial-vehicle Wheels.
Conical taper-nut Mounting.
Commercial-vehicle steel wheels are attached to the axle-hub flange by using conical-taper nuts fitting into countersink stud holes (Fig. 23.5A). In
Fig. 23.5. Commercial-vehicle wheel fixings. A. British standard wheel fixing. B. Continental (DIN) standard wheel fixing. C. Spigot-mounted wheel fixing.
case of twin wheels, the wheel stud-hole flange next to the drum is located by a loose conical-taper-shaped washer placed over the stud or by an integral spherical-shaped seat on the stud. The wheel is located, secured, and supported by the nut.
Continental (DIN) Standard Wheel Mounting.
The wheel can also be mounted to the hub by plain nuts with split spherical-faced washers. These washers fit into spherical countersinks formed in the wheel stud holes. The spherical seats centralize the stud holes, -and the washers and the nuts mainly take the relative motion (Fig. 23.5B). The wheel load is su^x>rted by the centre-bore of the wheel flange. In case of twin wheels, a spherical washer or seat supports the inner-wheel flange. /
Spigot Mounting.
With spigot-mounting (Fig. 23.5C), the centre-bore has a close-tolerance fit on the hub spigot for locating and supporting the load. The wheel nuts use an integral flat-face washer, which holds the wheel securely to the brake-drum, whereas the stud takes the acceleration and braking torques. The same mounting layout also holds good for twin r«ar wheel arrangements, the support in this case being between the centre-bore pf the wheel stud-hole flange and the hub spigot. This type of wheel mounting is mostly used for large trucks.
