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Air Suspensio I Disc Brakes

23rd November 1962
Page 52
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Page 52, 23rd November 1962 — Air Suspensio I Disc Brakes
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IT can now be revealed that the use of a patented arrangement of master levelling valves, located at the centre of vehicle roll, is an all-important feature of the air suspension system applied to Guy Wulfrunian doubledeckers and Victory coaches, and the success of the 53 Wulfrunians in the fleet of the West Riding Automobile Co. Ltd., Wakefield, has demonstrated that the system offers multiple advantages to the fleet operator. Technically, the master valves are of special interest because they reduce air consumption to a minimum, and in general the system can be commended on the grounds that it affords a greatly improved ride under all conditions of loading, reduces suspension maintenance costs and eliminates damage to the bodywork by vibration.

It will be recalled that a double wishbone type of independent suspension is employed at the front, controlled by two Guy-Firestone rolling-lobe diaphragm springs, and that four .similar springs are used in the suspension system of the drop-type double-reduction rear axle. Air is fed from an engine-driven compressor having a capacity of 10.5 cu. ft. per min. to a frame-mounted reservoir which is charged to a pressure of 140-160 p.s.i. and supplies the air-hydraulic braking system of the disc brakes as well as the master levelling valves. These are used in conjunction with two wheel valves at the rear and two at the front, whilst each diaphragm spring is equipped with a primary surge tank. The rear primary tanks are each supplemented by an auxiliary surge tank, but at the front the use of auxiliary tanks has been discontinued on the West Riding Wulfrunians to reduce the static displacement caused by a high platform load at the front entrance.

Valves at Roll Centre

The rear master valve is attached to a body member immediately behind the axle in such a position that the link connection of the actuating lever is coincident with the body roll centre and the transverse centre line of the wheels, so that body roll has no effect on valve opening. The linkage employed between the actuating arm of the front master valve and the lower wishbones of the suspension affords a similar reaction with regard to roll at the front end, the valve itself being bolted to the chassis in this case.

In operation, the response of the master levelling valve to an increase or reduction in body loading is instantaneous in that a small load increase immediately opens the port supplying the wheel valves (through the master valve) to atmosphere. The wheel valves, however, provide a measure of independent control, the supply of air to the diaphragm springs and the release of air to the master valve depending upon displacement of the actuating lever from the neutral position in each case. Moreover, angular movement of the lever in either direction of fewer than 5 degrees permits a limited flow of air only, so that the air springs are charged and air is released at a reduced rate if wheel movements are small. Air flow to or from the wheel valves is impossible unless the master valve lever is in the appropriate position.

Reduced Air Consumption

Because no air is lost when the vehicle rolls or the body is displaced from the vertical on account of road camber, overall air consumption is greatly reduced compared with a conventional system operating on the time-lag principle. During experiments with a prototype vehicle fitted with a conventional levelling system and a large air storage cylinder, air loss was so great that the vehicle had to be stopped every 25 miles to allow the compressor to recharge the cylinders. Employing roll-centre levelling valves reduced air consumption by up to 75 per cent and eliminated the need for air storage. On average, the compressor now operates about 36 per cent of the total running time.

Unladen, the weight on the front axle is 4 tons 14 cwt. 2 qt. and the rear axle is loaded to 3 tons 15 cwt. 2 qt., laden figures being 6 tons 3 cwt. 2 qt. and 7 tons 2 cwt. 2 qt. respectively..

According to Mr. R. Brooke, chief engineer of the West Riding Automobile Co. Ltd., the suspension system promotes consistent braking when the vehicle is unladen, or partially laden, as well as improving the ride. With the original rolling-lobe diaphragms there were one or two cases of failure resulting from internal abrasion on full bump, but the use of a modified type of diaphragm promises to extend the life of the units indefinitely. The diaphragms can be replaced as easily as a fractured leaf spring and can be renewed at half the cost, which represents a valuable

reduction in overall maintenance costs. A number of vehicles have covered over 60,000 miles without renewal of a diaphragm.

Mr. Brooke considers that the full potential of an air suspension system can only be realized if it is used in conjunction with independent springing or, at the rear axle, if two air springs are employed on each side.

The virtual elimination of vibration will, it is considered, enable the body to be retained in service for the full life of the chassis without maintenance, apart from repairs necessitated by accidents. More immediately, the suspension provides a valuable saving in day-to-day maintenance by obviating the need for routine tightening up and so on. It is pointed out by the development staff of the Firestone Tyre and Rubber Co. Ltd., makers of the Airide springs, that the company enables a system to be employed giving low spring rates about the normal ride position without sacrificing roll stability on bump control, which is made possible by progressive load-deflection characteristics. The valve/bump-rubber device incorporated in the spring isolates the surge tank from the air spring at a predetermined point in the bump stroke, and this augments the rapidity with which the air pressure and spring rate are effective for both slow (isothermal) and fast (dynamic) movements, and the accelerative deflections, which would otherwise be experienced with the low-frequency suspension, are thereby eliminated.

It is emphasized by the company that mounting the front air springs direct onto the stub axles and the two rear springs of each wheel on an axle beam obviates the undesirable type of static and dynamic load distribution associated with lever arms. After the prototype vehicles had been tested over a substantial mileage, modifications were made to the front air springs to cater for the increased frequency of occurrence of high-amplitude rebound strokes resulting from the low-frequency suspension characteristics.

Disc Brake Benefits Complete freedom from fade, consistent performance under all operating conditions, ease of pad replacement and absence of heat-soak into the bearings, are features of the Girling split-circuit disc-brake system of the Guy Wulfrunian that are particularly commended by Mr. Brooke. Of these, the favourable heat-dissipation characteristics of the rear axle-brake assembly are of special interest, because the elimination of the heat-soak problem virtually disproves the general assumption that disc brakes are not suitable for heavily loaded rear axles. It is notable that a normal type of hub grease is employed.

It is emphasized by Mr. Brooke and the technical staff of the Girling company that an important factor in the installation of the rear discs is the layout of the assembly, which was designed to incorporate brakes of this type. The discs are of the ventilated type and are integral with a heavy sleeve member, flanged at the outer end for attachment to the hub, thus providing an air-insulation space between the sleeve and the drive shaft, a long heat path from the disc to the hub and an additional heat-dissipation area. The elimination of heat-soak is the more remarkable on account of the low floor height of the vehicle and other "enveloping " features that tend to restrict air flow.

Pads Easily Changed A semi-skilled fitter with the aid of an apprentice and labourer can change all the pads of a vehicle in one hour. The work can, therefore, be performed during the night, whereas it is necessary to renew brake-drum linings during the day shift 'because the job can only be undertaken by highly skilled fitters. The use of disc brakes directly increases vehicle serviceability in this way and the increase is augmented by the virtual elimination of complaints by drivers regarding brake efficiency. An attendant advantage of disc brakes is that the pad wear can be easily checked without dismantling the assembly. The Girling company state that there has been no known case of disc fatigue failure.

Because of the hilly terrain in the operating area, the freedom from fade provided by the disc brakes is an important safety factor. When descending steep hills, the provision of adequate braking power is no longer depen c4 dent upon the engagement of a low gear, and, although thi: may encourage more prolonged use of the brakes (an( thus increase the wear rate), it promotes driver confidenci and reduces driving strain. Applying the brakes to the limit of efficiency has been discouraged by artificiall! increasing the pedal pressure required for maximun retardation.

Absence of squeal tendencies is also a valuable featurs of the disc brakes, particularly in the case of hospital route: on which it is essential to prevent squeal. With drun brakes, it is frequently necessary to employ arelativel3 low friction material to ensure silent operation. ,Freedon from glazing of the friction material is also a meritoriotr feature of disc brakes. Drum brake facings of the hardei type that offer a long wearing life tend to glaze witl repeated light application, with a consequent reduction it efficiency. Full efficiency can only be restored by remova of the drums and roughening the facing surface.

Consistent Operation A special advantage of hydraulically operated disc brake! cited by Mr. Brooke, compared with the cam-operated type is that leverage variation is eliminated with changes of peda travel and with wear of the friction material. Consistenc of the brakes is partly attributed to the light contact maintained between the pad and the disc in the off positior (combined with automatic adjustment) which obviates los' motion. Scoring of the disc is less severe than drurr scoring in similar operating conditions.

A Ferodo DS5S pad material is employed, the co-efficieni of friction of which is about 0.3. This material has a helpful characteristic in that friction increases slightly witlan increase in temperature, which is a valuable feature with regard to fade resistance.

In Mr. Brooke's opinion, disc brakes would be entirely satisfactory if the cost of the pad material were substantially reduced, which is mainly dependent on an increase in demand. The cost of renewing the friction material increased by 50 per cent compared with drum brakes, and on average the life of the pads is about 60 per cent of that given by orthodox linings. It is possible, however, that the more useful stopping power of the disc brakes accounts in part for the reduced life of the pads.

Some difficulty has been experienced in obtaining equal braking effort from all the discs, probably as a result of the hydraulic system being a two-part type with a high boost ratio, which is susceptible to changes in seal friction in the low pressure system. This has been partly overcome by employing air-diaphragm-type actuator units operating two hydraulic master cylinders in place of an air-servo hydraulic system. Although obtaining equal wear of all the pads still appears to offer a number of problems, it is not regarded as an impossible target.

It seems evident that both the air suspension and disc brake systems, although initially expensive, bring substantial benefits to passengers, drivers and the operators. As with almost any design which departs considerably from orthodox practice, continued development work has proved highly beneficial. Although the Wulfrunian as originally introduced was of outstanding interest because of its advanced specification, the various modifications intro duced over the past three years have enabled more of the potential advantages of the original design to be obtained. It is interesting to note that the basic features of the original version continue to be standard.

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Locations: Wakefield

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