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What the Gipsy Means to Future Design

28th February 1958
Page 61
Page 61, 28th February 1958 — What the Gipsy Means to Future Design
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Which of the following most accurately describes the problem?

By P. A. C. Brockington,

A.M.I.Mech.E.

INDEPENDENT front-wheel suspension was originally adopted by •car manufacturers to enable the engine to be moved forward in the chassis; improvements in comfort and road holding were important but secondary considerations. To provide additional body space for a given vehicle weight and size was the main objective.

Optimum use of the chassis overall dimensions in terms of load-carrying capacity is equally important to operators of commercial vehicles, and in assessing a new design in its wider implications this aspect of its possible advantages must be given priority. When judging the merits of a new form of suspension system the space it occupies may, therefore, be a determining factor in its general acceptance by vehicle manufacturers.

Although Flexitor trailing-arm spring units giving independent suspension have been produced by the Spencer Moulton company for trailers and so on for many years, 'and are based on long-established design principles, their application to both axles of the new Austin Gipsy represents an original design concept. This may indicate a method of springing that could be advantageously adopted by manufacturers of larger vehicles of the conventional type.

The fact that no lubrication is, required and the claim that the units can operate indefinitely without replacement will commend such a system to the maintenance engineer. Its suspension characteristics may also appeal to the operator as a satisfactory compromise between those of the conventional leaf spring and the self-compensating constant-periodicity type of suspension, particularly if the cost of the vehicle is favourable. Initial acceptance for incorporation in the first prototype of a new vehicle would, however, be decided by the designer of the chassis frame.

When noting details of the Gipsy which could have a wider application, the most outstanding feature is the combination of the front spring-unit housing and tubular cross-member. In the layout of the Gipsy chassis, front-wheel drive and so on, the space inside the main members would have been sufficient for wishbone springing. To the designers with an eye on the future the suspension might offer scope for further compaction of the chassis components to increase the space available for the payload.

Multi-hush Torsion Unit

If mounting the units to give the required wheelbase necessitated location of the rubber torsion member in an "occupied area," it should be possible to devise a modified multi-bush unit of shallow depth and large diameter giving the same characteristics. Cranking the front trailing arms of the Gipsy to increase the track shows the latitude in another plane which the units permit.

For other applications it should be possible to employ units the size and characteristics of which could be modified to match different arm lengths. The ideal type of suspension with regard to installation is one that gives the chassis designer freedom to locate the wheels in transverse planes best suited to the functioning of the vehicle without being hampered by the space requirements of other components.

Until a transmission system is commercially available which obviates driving shafts, bulky gear casings and so on (without introducing equally bulky components presenting difficult mounting problems), a heavy final-drive assembly, located in line with the driving wheels, is unavoidable. The use of trailing-arm springs should, however, allow the designer to arrange the transmission and suspension components in a layout best suited to uniform stress distribution and weight reduction.

Employing independent suspension units for the rear wheels has .never been seriously considered for larger commercial vehicles in quantity production, on account of the increased cost, the necessity to employ four additional universal joints and the likelihood that tyre wear would be increased. The acknowledged advantages of the system include the rigid attachment of the final-drive unit to the chassis frame, a substantial reduction in unsprung weight and the facility provided for employing inboard brakes at the rear.

Mounting the final drive on a chassis member eliminates the bouncing axle and allows the gearbox to be combined with the final drive, which could provide a reduced loading height for a given spring deflection. For commercial vehicles a low unsprung weight is not of particular value in promoting road-holding, but facilitates rebound control. Location of the rear brakes inboard of the drive shafts would probably be of the, most benefit, in that heat dissipation would be greatly improved, whether or not disc brakes were fitted.

Because the movement of the front wheels lies outside the plane of the spring units, the greater space available should allow the brake drums or discs to be mounted on the inner side of the wheels_ The camber of the wheels does not change as a result of spring deflection and vertical movement does not cause tyre scrub; increasing the distance of the knuckle joint from the wheel plane and length of the stub axle would, therefore, be permissible.

Although the Gipsy is not a heavy vehicle, it is designed for cross-country work, which is liable to impose far higher lateral stresses on the wheels and mounting members than normal running on made roads. This dispels any doubt whether a light trailing arm can be economically produced for vehicles of this weight and capable of resisting severe side loading without undue flexing. It gives promise that the arms of a system applicable to much larger vehicles would not be excessively heavy.

Waterproof

Notable details of the Gipsy frame include complete sealing against the ingress of moisture to the interior of the sections and side welding of the members to obviate joints along the upper and lower lines of maximum stress. Sealing the frame members is of particular importance when a light-gauge material is used, because it eliminates internal corrosion.

Justification for employing a, lightweight frame of the Gipsy type for heavier vehicles might in practice depend on the ammint of torsional flexing which the designer decided should be allowed. In the event of a frame being evolved to reduce torsional flexing to an insignificant minimum (possibly in conjunction with air suspension), this form of construction might be the most suitable. A ,semi-rigid frame might be produced which was adequate for some vehicles without the addition of extra stressed members, but could be readily incorporated in a rigid integral body.

Reverting to the trailing-arm suspension, one of its most important features is the support of the arm by a rubber bush which is not subject to wear. If the shaft had been mounted in closely spaced bearings, a small amount of wear would have allowed for a large 'amount of wheel movement laterally.

The Flexitor elements are lightly stressed in shear only and springing is not progressive. No allowance is made for settling, but the arms can be located afresh to increase the ground clearance. At the front end, the change in caster angle with deflection might be serious on a softly sprung vehicle or on one with a high front-axle loading.

Supporters of air suspension would not accept trailing-arm suspension based on rubber elements as a satisfactory substitute. Even if it is assumed, however, that the advantages of a constant-periodicity self-levelling system would ensure its adoption for most goods and passenger vehicles, the simplicity of the Flexitor type of unit may well justify its use in conjunction with an air •bellows. An existing design of air-suspension system is based on a light leaf spring which acts as the drivingand braking-torque member; a long-life low-cost rubber member which also functions as an oil-less bush for a trailing arm has much to commend it.

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Organisations: Torsion Unit
People: Austin Gipsy

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