WHEELS AND HUB OF VEHICLES

 ROAD WHEELS AND HUBS

The hub is the centre portion of the wheel and is what attaches the wheel to the suspension. The spokes radiate out from the hub and attach to the rim. The rim is the outer part of the wheel that holds the tire. While many people refer to wheels as "rims," this is technically incorrect. Basically, the rim is the portion of the wheel which used to retain tire on the wheels or locks tire with the wheel.Three main types of road wheel are produced for cars and commercial vehicles, and these must be designed and constructed to meet certain essential requirements according to their specific applications. The design features necessary for all wheels may be summarized as in the following sections.

Lightweight construction: Each wheel adds significantly to the unsprung mass of its suspension mechanism. Since it is easier for a shock damper to control the bouncing movements of a small rather than a large unsprung mass, it follows that for good road holding the wheel should be made as light as possible. For cars, this requirement has been helped by modern body styling trends demanding smaller diameter wheels than were used in the past.

Strength and rigidity: These are not one and the same requirement. A wheel must possess the sufficient strength to withstand all normal service loads imposed upon it, as well as being resistant to accidental damage. It must further be rigid or stiff enough as a structure to minimize bending deflection under steering and cornering loads. Again, the smaller diameter wheels used in modern cars offer an advantage in this respect.

Tyre retention The design of the wheel rim must permit the mounting and removal of the tyre and retain it securely in position when inflated. As regards the former requirements, heavy-duty tyres used on commercial vehicles generally require a detachable flange rim as compared with the simpler one-piece rim adopted for cars. For tyre retention taper seatings are provided adjacent to the rim flanges so that the beads of the inflated tyre are forced to climb these tapers, thus wedging themselves against the insides of the rim flanges. The rim must also accommodate the tyre valve assembly. Following the development of the tubeless tyre in the late 1950s, American safety regulations created a requirement for a retaining hump on the outer bead seat and adjacent to the well of the wheel (Figure 22.18), which later became accepted practice elsewhere. Its purpose is to ensure greater safety by preventing the tyre bead from sliding off its seat in the event of sudden deflation or hard cornering with underinflated tyres.

Wheel retention:  It is, of course, an essential requirement for both cars and commercial vehicles that their wheels should be simple to remove and refit (even though the jacking procedure may sometimes leave a lot to be desired!). Established practice is for each wheel to be attached to a flanged hub by an inner ring of either studs and nuts, or screws (Figure 22.19). In a non-spigot type mounting, these serve the dual purpose of retaining the wheel endwise and locating it radially. They are therefore provided with either conical or spherical seatings (Figures 22.19a,c,d and 22.21b), which exert a wedging action to prevent their loosening against the wheel. Under no circumstances must these two different forms of nut or screw seating be interchanged. Less commonly, the nuts or screws may be left-hand threaded on the left (near side in the UK) of the vehicle and right-hand threaded on the right (offside in the UK). For modern heavy-duty commercial vehicles, and increasingly also for greater accuracy of the wheel mounting on some cars, the function of radial location may be performed by the wheel being made close fitting over the end of its supporting hub, it then being known as spigot mounted. In the case of commercial vehicles, flat-faced nuts with captive washers are used to retain the wheel (Figure 22.19e), while for car wheels either conical or flat-faced screw serves to retain pressed-steel and cast alloy wheels respectively (Figures 22.19b and 22.21a).

It is imperative that the wheel nuts or set bolts are tightened progressively in a diagonally opposite sequence and are

neither under- nor over-tightened. Under-tightening is especially hazardous in the case of heavy vehicles since there is then a distinct possibility of a wheel becoming detached on the road. The manufacturers of heavy vehicles specify the correct torque and re-torque tightening values to be used, which are typically based on lightly oiled threads, so it is most important to consult their detailed instructions and also the British Standards Code of Practice BS AU 50: Part 2: Section 7a: 1995. These are intended to ensure that sufficient tension is being generated in the wheel studs and by the same token clamping force imposed on the wheel, which will prevent any fidgeting movements at its mounting interface and subsequent loosening of the wheel nuts. It, therefore, follows that the mating surfaces of the wheel mounting must be clean and preferably free of paint.

Economic and accurate manufacture Obviously, motor vehicle wheels must be suitably designed for mass production methods to keep manufacturing costs down to a reasonable level, but they must also be made to exacting tolerances to minimize both radial and lateral runout, as measured at their rims. These features contribute to smooth running and satisfactory tyre life.

Types of road wheel 

The types of road wheel used on modern cars and commercial vehicles fall mainly into the following categories: pressed steel, aluminium alloy and detachable rim.

Pressed steel: This type is fabricated from a dished and flanged disc pressed into a rolled-section rim and retained by spot welding, a process that has superseded riveting to provide more reliable air sealing for tubeless tyres (Figure 22.20). The joint between the disc and its rim is not continuous but interrupted by a series of equally spaced ventilation slots derived from the original polygon-shaped disc blank. These may be supplemented by swaged ventilation holes in the dishes portion of the disc so that in combination they not only induce a flow of cooling air over the brake assembly but also minimize heat transfer from the latter to the tyre. The wheel fixing holes are formed in the central portion of the disc. A one-piece construction is used for the well-base rim, so called because a well is formed around the middle of its base to facilitate removal and refitting of the tyre. The tyres are provided with taper settings, and in the case of pressed steel wheels for cars, a 5° angle of taper is used. This type of wheel generally meets all the requirements listed earlier and, compared with its one-time rival the wire-spoked wheel, it possesses the additional advantage of being easy to clean.

Aluminium alloy: The demand for attractively styled wheels manufactured from a high-strength aluminium alloy has continued to grow

in recent years. These wheels are generally produced from one-piece castings and since they must be finish machined, their manufacture is to closer tolerances than steel wheels (Figure 22.21). For high-performance cars, a two- or three-piece construction in forged aluminium alloy may be used. This typically comprises a centre section with either a single or divided rim held together by a ring of small diameter, bi-hexagon headed, high-tensile screwed fastenings. An air sealing gasket is required between a divided rim. Aluminium alloy wheels possess certain technical advantages. The lighter material permits the use of thicker metal sections for greater rigidity. Similarly, the rims can be increased in width so that correspondingly wider tyres may be used for improved car handling. In this context, the lighter wheels also contribute to better road holding, by virtue of reducing unsprung weight of the suspension system. Apart from allowing a saving in the overall weight of the vehicle and therefore the fuel it consumes, the lower rotational mass or ‘flywheel effect’ of the wheels absorb less energy in speeding them up or slowing them down during acceleration and braking. Furthermore, the better heat conductivity of this material permits cooler running for the tyres and brakes, also the pattern of the wheels can be designed to induce more air to flow over the brakes. The possible disadvantages of aluminium alloy wheels are their reduced resistance to both accidental damage and corrosion, although a wide variety of protective finishes are now applied to them. Aluminium alloy wheels of the ventilated disc type are also used to a limited extent on heavy vehicles, where they offer similar technical advantages, especially in terms of heat dissipation from the brakes. However, an important commercial consideration is that they confer valuable weight savings, which can be translated into the greater payload, which is the actual useful weight carried by the vehicle.

Detachable rims: This type of construction is confined to commercial vehicle applications and it arises from the need for ease of mounting and removal of their heavy-duty tyres. Apart from rim design, it is basically similar to the welded pressed steel wheel used on cars, although the disc portion may be of either constant or variable-section thickness. This is so that the necessary strength requirements can be met without producing an excessively heavy wheel. It is usually deeply dished not only to accommodate the bulk of the brake drum but also to provide the sufficient offset for twin rear wheels to be mounted back-to-back. Ventilation apertures are generally incorporated in the disc. With detachable rim wheels an integral flange is formed around only one edge of the rim, the other flange being detachable and taking the form of either a split spring ring, which locates directly in a groove around the edge of the rim, or the combination of an endless rigid ring and a split locking ring likewise sprung into the grooved edge of the rim. These two forms of detachable rim construction, known as the two-piece and three-piece respectively are used according to whether the wheel loading is medium or heavy (Figure 22.22). In both cases, taper bead seatings are provided for the tyres. Another type of wheel that may sometimes be encountered on heavy vehicles and particularly those used by the-military is the divided wheel. It is so called because it is constructed in two main parts, which are clamped together by an outer ring of bolts and nuts (Figure 22.23), The complete wheel then being secured to its hub by another inner ring of nuts. Wheels of this type are typically used in conjunction with massive tyres that are designed to operate at very low pressures. The tyres are so hefty, in fact, that they could not be pulled off an ordinary rim owing to insufficient give. The divided wheel allows the wheel to be taken out of the tyre when the inner and outer halves have been separated, instead of attempting to take the tyre off the wheel. A safety note is given later with regard to handling precautions for divided wheels.

REFERENCES: Light and heavy vehicle technology by M.J. Nunney