LEEVENSPARK

Four Wheel Drive (Automobile) The two main traction problems associated with a two-wheel drive (4 x 2) vehicle are loss of traction during cross-country operation and loss of adhesion during acceleration. To provide a solution to these problems, Harry Ferguson, the inventor of the light weight tractor, was the first person to understand the importance of "all-wheel drive". In 1954 he patented the "Ferguson Formula" (FF), which was used on the Jensen car in the early 1960s. In a four-wheel drive (4 x 4) vehicle (four by four vehicles), the drive is transmitted to all the four wheels. The intended use of the vehicle governs the type of 4 x 4 drive system that is offered by the manufactures. Some vehicles are designed to work efficiently both on and off the highway. These vehicles use two-wheel drive on 'hard' surfaces and restrict the use of four-wheel drive for cross-country operation where it is likely to encounter muddy surfaces leading to loss of traction

Bearing Lubrication The whole hub assembly is partially packed with grease. A radial-lip grease-seal is pressed inside the hub next to the larger bearing. The lip of the seal faces this bearing and fits over the cylindrically machined surface on the stub-axle. Over packing bearing hubs with grease causes churning and develops very high running temperatures. Due to high temperatures and excessive mechanical working the grease breaks down and becomes soft. The centrifugal force acting on the grease in the hub pushes soft grease outwards, so that it flows towards the lip seal and seeps past the seal lips. Bearing greases are based on mineral oil and to thicken the oil soaps of calcium, sodium, or lithium compounds are added. The consistency of a grease depends on the viscosity of the base oil as well as on the structure and properties of the metallic soap added. The three most important properties of a grease includes : (i) the melting temperature, at which the grease loses its semi-soli

Bearing Loads on the Front Axle Figure 27.6 illustrates the forces and the reactions on steering knuckle when the vehicle is at rest. The thrust load and the knuckle-pin-bearing load can be expressed in terms of the reaction of wheel on wheel spindle. Let, Rw = The reaction of the wheel on the spindle acting vertically through the centre of contact of tyre on ground.  Rt = The load on the thrust bearing Ru = The load on the upper knucklepin bearing  Rl = The load on lower knucklepin 'B' and 'C represent the centres of lower and upper knuckle-pin bearings respectively.  'A' is a point on the spindle axis in the centre plane of the wheel. Fig. 27.6. Forces and reaction on steering knuckle. The other loads acting on knuckle-pin bearing are those due to the rolling resistance and road shocks. These loads are proportional to the static load and hence can be accounted for.

Types of Front Hub Assembly The stub axle construction depends on whether it is a driving or non-driving hub. Non-driving Hub Figure 27.4A illustrates a typical bearing arrangement for a non-driving hub. This consists of a stub-axle, an externally cylindrical sleeve hub, a pair of taper-roller bearings, a grease-seal, a castellated adjustment nut and split-pin, a washer, and a dust-cap (Fig. 27.7). A centrally flanged cylindrical sleeve hub is fitted over small outer and large inner taper-roller bearings, which are supported on the stub axle. The hub is made of malleable iron or steel cast. The bearings are designed to absorb both radial and axial loads when assembled. The slackness between the taper rollers and the inner and outer races are taken up by spinning the hub assembly while at the same time tightening the adjustment nut until all the free lay has been taken up. The bearings are then preloaded by tightening the nut with a torque wrench to some predetermined torque setting. T

Front Axle and Steering System Front axle carries the weight of the front part of the automobile as well as facilitates steering and absorbs shocks due to road surface variations. The front axles are generally dead axles, but are live axles in small cars of compact designs and also in case of four-wheel drive. The steering system converts the rotary motion of the driver's steering wheel into the angular turning of the front wheels as well as to multiply the driver's effort with leverage or mechanical advantage for turning the wheels. The steering system, in addition to directing the vehicle in a particular direction must be arranged geometrically in such a way so that the wheels undergo true rolling motion without slipping or scuffing. Moreover, the steering must be light and stable with a certain degree of self-adjusting ability. Steering systems may also be power assisted. The chapter discusses the front axle construction and its align­ment, and steering geometry and steering

Wheel Alignment Checking and Adjustment The alignment for suspension and steering linkage is usually checked and adjusted if, GO abnormal tyre were exists, (ii) the automobile handling is felt improper, (Hi) the suspension has been repaired, or (iv) a normal preventive maintenance check becomes necessary. In the past the wheel alignment was measured using a trammel gauge, but nowadays either an optical or an electronic gauge is used. Before tracking the wheels, the following preliminaries should be undertaken. (a) The tyre pressures are checked. (b) The correct load on the vehicle is ensured. (c) The wheels are positioned in the straight ahead position. (d) The vehicle is moved forward to settle the steering. (e) The run-out (buckle) of the wheel in checked and the maximum run-out is positioned so that it does not affect the alignment measurement. Prior to aligning the front suspension and steering systems, the factors to be ascertained that the suspension pivots and ball joints are i

Front-wheel Toe-in or Toe-out Toe-in is the amount by which the front-wheel rims are set closer together at the front than at the rear with the wheels in straight ahead position when the vehicle is stationery (Fig. 26.16A). Alternatively, toe-out is the amount by which the front-wheels rims are set farther apart at the front than at the rear (Fig. 27.16B). Therefore in Fig. 27.16, toe-in =Tr-Tf and toe-out = Tf-Tr. Fig. 27.16. Steering toe-in and toe-out. Toe-in or toe-out compensates for movement within steering ball-joints, suspension rubber bush-joints, and any slight deflection of the track-rod arms or suspension arms when the vehicle is in motion. The objective of the non-parallel stationary alignment of the front steered wheels is for the toe-in or toe-out to be taken up when the vehicle is moving, so that both wheels run parallel under normal driving conditions. Toe-in neutralises the cone rolling effect of front wheels caused by camber angle. The amount of toe-in for any vehic

Castor Caster angle is the tilt of the king pin or ball joint centre line from the vertical towards either the front (negative caster) or rear (positive caster) of the vehicle (Fig. 27.15). The weight of automobile having positive castor tends to turn a wheel inward to allow the body to lower. Negative castor causes an outward turning effect. Wheel castor enables the driver to feel the straight ahead position so that he may steer in a straight path.     During cornering, a torque must be exerted on the steering wheel to overcome the self centring or castor action, which tends to keep the wheels pointing straight ahead. The castor angle produces a trailing effect and hence gives the directional stability. Incorrect castor can produce difficulties like hand steering, pulling to one side when brakes are applied and shimmy. To understand the action of this steering feature, the operation of a simple furniture castor (Fig. 27.15A) fitted to a trolley may be considered. Force acting on the t

Front Wheels Alignment 27.2.1. Need for Front Wheel Alignment For free movement of the road-wheels with the least of effort, opposite wheels must be approximately parallel to each other when the vehicle is in motion along a straight path (Fig. 27.8). Each wheel has a tendency to negotiate a path perpendicular to its own axis of rotation. Therefore, if the front wheels are aligned for converging towards the front, then during movement in the forward direction both wheels try to roll close together. On the other hand, if the wheels are aligned for diverging towards the front, the wheels try to roll farther apart. Therefore due to free rolling tendency the average path followed by both wheels have a continuous tendency to either push together or pull apart. Consequently, while rolling forward, each wheel simultaneously tends to slip laterally, so a continuous cross tread scrub action is established, resulting in excessive tread wear, heavy steering and probably leading to poor fuel consum