A UNIQUI
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64-TONNER
Takes the Rough with the Smooth
Road Test
No. 215
VIEWED from a mechanical standpoint, the Unipowered Bedford is excellent in design. Considered. from the operator's position, it has many attractions. The comparatively small engine is enabled by the eight speeds available to exert its power to the best advantage, the ratios being skilfully chosen. The layout of the final transmission system is extremely neat and the efficiency high.
All four bogie wheels are driven ; thus the machine is far better equipped for extricating itself from an awkward position than the trailing-axle type of six-wheeler, whilst the provision of a third differential gear between the two axles is a big contributory factor to keeping down running costs on roads, and enhances roadworthiness generally.
The Bedford 3-ton chassis, which forms the basis of this conversion, was road-tested by this paper on its inception, and a report published in our issue dated August 3, 1934: A description of the performance of a similar chassis, coupled to a Carrimore semi-trailer to form an articulated six-wheeler, appeared on October 12 last. Now we have conducted a third test of the same model, this time in the form of a rigid six-wheeler, and, because of the unique design of the bogie, this report should be of especial interest.
It should be noted, that, although Universal Power ' Drives, Ltd., is not tied to Vauxhall Motors, Ltd., the Unipowered Bedford 6-tonner carries the joint guarantee of the two companies.
The alterations made to the original chassis include the imposition upon the frame longitudinals of straight
channels, which are a tight fit on the parallel portions of the original members, are cut-away where the latter are kicked up and are packed with wooden fillets where the main longitudinals taper. For long-wheelbase chassis, the channels are of the dimensions shown in an accompanying drawing, whilst for shorter machines, steel of 41-in. thickness is used. Throughout the conversion only high-tensile steel bolts are employed.
The equivalent of a bogie crossshaft is formed, in effect, by the relay or transfer gearbox, and the brackets which support it and are bolted below the frame channels. These carry, on their outsides, trunnion pins, upon which the spring-chairs work.
The rear-suspension system incorporates two pairs of inverted semi-elliptic springs arranged so that there is one spring above and one below each trunnion, the ends of the springs being hinged to brackets on the axle ends. This layout affords a high degree of bogie articulation, that is to say, relative vertical movement of the wheels. From the Bedford gearbox to the top • of the transfer case, Which, of course, does not move relative to the frame, there is a shaft, incorporating Hardy-Spicer needle universals to allow a misalignment which does not normally exceed 2 degrees, but obviously varies with chassis distortion.
Referring to the accompanying illustration of the relay gearbox, it will be seen that on the .rear of the propeller shaft are two plain gears of different diameters. These are free on the shaft, but either can be engaged by means of a splined sliding dog. The wheels are in constant mesh with those on the layshaft, the smaller of which also meShes with the first crown wheel.
The last-named incorporates a differential, whence the power is transmitted by two Rzeppa (pronounced jeppa) universal joints and two short shafts, one extending forwards and the other rearwards, to the driving axles which incorporate bevel gears and differentials of orthodox design.
The Rzeppa joints are of the greatest interest. They transmit the rotation at constant velocity and permit a deviation of 37
degrees from a straight fine; that is to say, they allow an overall angle of 74 degrees.
In effect, the inner and outer members of the Rzeppa joint are concentric spheres, one being separated from the other by a close-fitting cage. Grooves of semi-circular section, cut around both effective surfaces in directions approximately parallel to the axis about Which the spheres rotate, combine to form channels, spaced 60 degrees apart, whilst a large ball in each channel transmits the torque from the outer to the inner member,: whatever the angularity of one axis to the other.
A body of normal loading height easily clears the top of the transfer box, although this is just above frame level. The drawback in this respect is less than it appears at first .glance, for the top of the worm case on an overhead-worm machine may quite possibly reach as high a level under certain conditions.
The rear extension of the uppermost transfer-box shaft constitutes a power take-off, without any alteration to the mechanism, and in numerous cases it is convenient to have this at the rear of the vehicle.
Outside the Unipower works there is a piece of rough, soft ground. Here some of the capabilities of the machine were demonstrated to us. In spite of the 6-ton load, no difficulty was experienced in climbing a short, steep gradient ; in pulling out of a hole into which both the driving wheels on one side had sunk, or in negotiating irregularities of surface that brought the articulating characteristics of the bogie into full play. Furthermore, there was hardly any tendency of the wheels to spin.
. Were a differential-lock incorporated, there would be few conditions beyond the powers of this vehicle, without one,its performance is surprising.
There is no doubt that the presence of an intermediate
differential retards tyre wear and economizes fuel. We could not, of course, investigate the former, but are informed that a mileage of 20,000730,000 is usual, with truck •
tyres, With regard to the latter, we obtained a return of over Broekley Hill, between Edgware and Elstree, was selected for an observed hillclimb. The early slope was commenced at 15 m.p.h. in low top gear, low
third" was engaged at 11 m.p.h., and this gear carried us on to the beginning of the 1-in-81 section, which demanded low "second." In this ratio we gradually gained speed from 5 m.p.h. -to 7 m.p.h. at the summit. The bare quarter-mile of steep gradient occupied 1 min. 45 sees. Immediately after the climb the water temperature was 200 degrees F.
On Cocas Hill, 1 in 6, a stop and restart was made. In respect of clutch, gears and engine, the machine's behaviour on this test was faultless, but the handbrake was found to be incapable of preventing the vehicle from running back, unaided by the foot brake. In a machine so well equipped in other respects for difficult conditions it seems a pity that the driver should be handicapped by an imperfect hand brake. However, in a supplementary capacity to the foot brake, it was found to be of considerable value, as is shown by the satisfactory figures revealed in an accoinpanying graph.
Acceleration .results are similarly shown, and the variety of ratios available is a big asset towards compensating for the somewhat low power-to-weight ratio. To get the best out of the gearboxes, however, both mental and manipulative agility are required.
The levers are feather light: but quickness of thought is needed in remembering how they must he moved, and considerable knack mast be acquired properly to operate the relay gear change.
There is another point which, in our opinion, is open to improvement. We
concerting :—The driver wishes to turn slowly to the right: around a blind corner, into a main road, from a road that runs slightly down hill. His right hand is outstretched, his left is turning the wheel, his left foot is gently depressing the clutch, and his right, even more gently, the brake. There is thus nothing to take the torque reaction of his body, which is being merely supported by the seat upon which he sits. A powerful brake-pedal return spring would probably afford sufficient resistance to enable him to steady himself with his right foot. This is only a small point which diminishes with familiarity.
Before concluding an account of our impressions, the light but certain steering calls for comment. Great-attention is paid by the Unipower designers to the question of weight distribution, and their achievements, so far as our experience is concerned, do them credit.
Taking into consideration all its features outlined above —its capabilities and its low running costs—and bearing in mind that its unladen weight is under three tons, and then recalling our own impressions on the road, we arrive at tlie conclusion that this Unipowered Bedford is one of the best rigid six-wheeled 6-tonners made in this country.