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THE IDEAL CHASSIS PC SPECIAL CONDITIONS
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TO most minds the phrase " special conditions," when used in a transport connection, suggests rough country. Any conditions, however, that are not normal presumably may be regarded as " special." The meaning, therefore, should have a much broader application than merely to matters of surface.
Atmospheric temperature and pressure, availability of supplies of fuel, water, oil, spares, tyres and so forth should really be included, together, perhaps, with considerations of the class of goods or passenger transported and the length of the journey. Ques tions of terrain, nevertheless, still seem to remain uppermost.
Pre-eminent in equipping for special conditions is the practice of employing more than the usual number of wheels for traction, Where ground is soft, and incapable of exerting the necessary rtiaction to both the weight and the driving forces applied by the wheels, obviously local loading can be lessened by increasing their number.
Before the arrival of the big-section low-pressure tyre the load per square inch of contact between tread and ground was so limited, even when twin tyres were used, that additional wheels were the only solution. Accordingly, six-wheelers were employed almost uni versally for cross-country purposes, and naturally the extra pair of wheels was driven. It was easier to obtain the additional traction thus than by driving the front wheels, With the arrival of big-section lowpressure tyres, however, the duplication of the back wheels for weight carrying could be avoided, and, (..,f course, traction was much increased, deeply cross-grooved treads being an important contributory factor. It is probable that such a four-wheeler is equal, if not superior, to a highpressure-tyred six-wheeler with double drive.
Additional driving wheels, however, had been proved such an asset that further development logically took the form of employing the front wheels for driving as well as the back, and a step forward was made of such magnitude as to bring the modern wheeled vehicle almost level with the track-layer in respect of its capacity for negotiating rough country.
The all-wheel-drive type must be subdivided into two further groups— machines with rigid axles and machines . with independent suspension. Under extremely exacting conditions, the former type has shown itself highly satisfactory. In addition it is easier to produce. An excellent degree of articulation can be permitted by the use of a high-built chassis, and suitable transmission layout.
In connection with the latter type there are certain constructional problems which handicap the designer. If carclan shafts be used to transmit the drive to each wheel, these probably have to be shorter than where two rigid axles are driven from a roughly central gearbox. Therefore, the angularity of the universal joints is considerably greater. This is but one of Many features that tend towards complexity of design.
For the ideal, desirable features inchide simplicity, ample up-and-down movement, tolerably constant velocity, enclosure of working parts, and provision against loss of drive through a spinning wheel.
On the first score rigid axles appear to be at an advantage, and possibly, but not necessarily, on the second, third and fourth. There is, however, a design of independently sprung all wheel-drive machine in which no universal joints are incorporated and all working parts are enclosed and run in To Check Wheelspin.
Provision against loss of drive through one, or more than one, wheel failing to make adequate adhesion with the ground can be obtained by locking the appropriate differential. There is scope here for designers, for a satisfactory automatic lock, which comes into action so soon as the speed difference exceeds a predetermined magnitude, has not yet appeared.
Alternatively, a means for equalizing the weight supported by each wheel would diminish the tendency for any one to slip, and the incorporation of a device of this nature should not be so difficult a problem. A point to watch, however, is that the action of the apparatus to equalize pressure between near and off-side wheels, which works in exactly the opposite way to a stabilizer, does not detract from the roadholding qualities of the vehicle in ordinary circumstances.
If a hydraulic device were to be employed, possibly a hand-controlled valve could be incorporated which would render it a stabilizer in one position and a weight equalizer in the other.
Turning now to other considerations, we find in the producer-gas vehicle a recent advance towards the ideal chassis c64 for localities where liquid fuel is scarce. Numerous countries overseas are well supplied with timber, and, at present, charcoal is eminently suitable for gas producers. Where charcoal, anthracite or coke is available and liquid-fuel scarce, the producer-gas machine has big claims to be considered as approaching the ideal.
Reference was made earlier to the need for enclosure of working parts. This introduces the extremely important matter of providing for the exclusion of impurities, notably sand. Under special conditions, air cleaners, filters and so forth have harder tasks than in operation on concrete and tarmacadam highways at home. Our ideal chassis must certainly be adequately equipped in this respect.
Furthermore, protective coverings of leather, or other suitable material, should be fitted to vulnerable parts such as brake piston rods, steering joints, and so forth, where bare metal cannot he otherwise concealed. Not only are cleaners needed on engine main air intakes, but also on the intake orifices of vacuum brake cylinders, air compressors, etc.
As for extremes of temperature, there is nothing the designer can do to prevent water from freezing. We insist, however, that large-bore accessible
drain cocks be proeided at every point in the cooling system where water could possibly lodge, and that an easily renewable breaking piece be incorporated in the
pump drive. • To guard against boiling is easy; a radiator of adequate cooling and water capacity and a powerful fan are eSsentials.
Prevention of tyre overheating is more difficult, but julst as Important, if not more so, . because a hot tyre may cause a fire. ' Even if the brake drums tie generously ribbed, " the heat has still to be convected away from the dangerously adjacent tyres. Ventilating wheels are probably the soundest scheme, as they create an air current over the drums as they revolve.
Big Engine Desirable.
Low atmospheric pressure is yet another difficulty to contend with. Its main effect is simply to reduce engine output. Accordingly our ideal chassis must have a highpower-to-weight ratio. At the sacrifice of speed one could use a lower overall gear, or, better still, specify a relatively low direct ratio and an overdrive for use under easier conditions. Yet another scheme is to supercharge, and it is satisfactory to realize that the supercharger has almost reached a state of development that would permit its inclusion in an ideal chassis.
Besides the presence of foreign bodies in the air through which vehicles operated under special conditions have to travel and which their engines have to induce, there are also corrosive influences. Precautions, therefore must be taken to prevent damage by this cause to susceptible materials. Rustproofing, for example, must be thorough, or metals employed which are resistant to such attacks, and attention must be paid to the insula tion of electrical equip ment.
British manufacturers are fully aware of the requirements of operators whose vehicles work under special conditions. In most cases they have obtained information first hand. The idea, once prevalent, that they lagged behind their foreign competitors has long since been dispelled. They must not relax, however, for developments are continuously occurring. Operators, also, should respond by selecting machines well equipped for severe operating conditions.