ONE MORE AXLE Three More Tons
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As announced in "The Commercial Motor" dated March 25, Vauxhall Motors, Ltd., have approved two six-wheeler conversions for certain Bedford normaland forward-control 7-ton chassis. Each type has been fully road-tested by John Moon. This is his report on the performance of a vehicle modified by the Primrose Third Axle Co., Ltd., Clitheroe, Lancs.
8 RIEF details published on March 25 indicated the lively, economical performance given on test by the Bedford-Primrose six-wheeler with a payload of 10i. tons. My trial proved, furthermore, that, although not intended for cross-country running, the Primrose bogie has good off-the-road traction characteristics.
Unlike many modifications of this type, the two Vauxhall-approved assemblies have been designed by Vauxhall Motors in conjunction with the conversion concerns. Not only has this reduced the number of nonstandard components used, but the development work has produced a particularly well-matched braking system. The efficiency of this was shown by the Primrose conversion's ability to stop from 30 m.p.h. in 55„ ft.
The Primrose conversion looks complex because of its trailing quarter-elliptic springs and coil springs, and its special hydraulic distribution system. Brief inspection of the layout, however, shows this impression to be false. The simplicity of the system belies its effectiveness.
The vehicle offered for test was based on a Bedford SL1 13-ft-wheelbase forward-control 7-ton chassis, which, in basic form, has a gross weight rating of 10 tons 5-f cwt. The price of the chassis-cab with Bedford oil engine is £1,143.
The Bedford two-speed axle is mandatory for either of the conversions. This is because the single-speed axle has a maximum load rating of 7 tons 111 cwt., whereas the two-speed axle's load rating is just over 8 tons. The price of this axlo puts £95 on to the basic chassis price, added to which there is another £10 15s. for the Bedford chassis flitch plates to which the Primrose extension members are attached. The Primrose-conversion price is £537, making £1,785 15s. in all.
These chassis extensions are in. thick and 10+ in. deep with 3-in.-wide flanges. The extensions are buttwelded to the flitch plates and riveted to the side members at appropriati points. No flitch platevre offered with certain of The chassis suitable for conversion (the J6S, J6L and SE models), in which case Primrose use long channel extension members extending to the back* ofj the cab, but with their loiyer• flanges removed forward o the bogie so that the effective section at this point corresponds to the normal L Bedford flitch plates, The staidard rear-axle main and helper springs are retained for the conversion. Cast-steel brackets are attached to the side members behind the rear hanger brackets. These provide the pivots for the trailing leaf springs, which locate the trailing axle and absorb braking torque. Loading on the third axle is carried by two main coil springs with a rating of 1,500 lb. per in. Lateral location of the axle is given by curved slides, against which the outer edges of the trailing leaves' ends bear. Telescopic dampers with a working stroke of 9 in. are added.
The hydraulic compensating device consists of two hydraulic cylinders mounted vertically betWeen the chassis frame and the driving axle, and piped to tWo 'other cylinders at the top of the " funnels " which house 'the upper ends of the coil springs. The pistons of these rear cylinders act upon the lops of the coil springs through a simple arrangement of buttons and locating collars. The operation of the system is fairly straightforward. As the load onthe driving axle is increased, the forward cylinders are compressed and fluid is forced into the rear cylinders, so deflecting the coil springs below their normal unladen length. Thus as the loading on the rear bogie increases, the relative weight distribution between the driving and trailing axles is automatically adjusted from a ratio of 2.7 to 1 unladen to 1.8 to 1 laden.
This ensures that traction is maintained by the driving axle when the vehicle is empty, as well as reducing unnecessary tyre wear and scrub at the trailing wheels. When laden the 1.8 to 1 distribution maintains a high proportion of the load on the driving wheels for traction, and gives a satisfactory share of loading among the eight tyres.
A failing of many early third-axle conversions was the inadequacy of the braking system. Today it is -widely accepted that the standard vehicle servo cannot provide enough power for an additional pair of brakes, and an extra servo is frequently fitted. The relationship of this to the standard servo is important, however, and much of the development work carried out by Vauxhall engineers on this conversion has been associated with the braking system.
Effective Braking System
The resulting combination is effective but simple. The normal Bedford tandem hydraulic master cylinder is retained, and this is boosted by the standard Clayton Dewandre upright-vacuum servo. The tandem cylinder operates the frontand trailing-axle brakes through independent hydraulic circuits. A separate hydraulic circuit is added for the driving-axle brakes and it is boosted by a Clayton Dewandre Hydrovac suspended-vacuum servo controlled by the line pressure in the main braking system.
Thus the axle which is carrying the most weight of the three receives the most braking and this holds true whether the vehicle is laden or empty because of the hydraulic compensating system. The hand-brake linkage is also simple but was effective enough to produce a retardation meter reading of 40 per cent. when applied from 20 m.p.h. The layout incorporates a conventional compensator on the trailing-axle beam controlled through a cable fed from an additional compensator at the base of the hand-brake lever. The other side of this compensator actuates the driving-axle brakes through the normal linkage.
The Primrose trailing axle consists of Bedford drivingaxle tubes joined together by an additional central tube (which %replaces the differential case of the normal axle). Standard hubs, bearings, brake drums and brake assemblies are retained.
The vehicles available with the Primrose conversion are 1'3 I 3 •
the normal-control J6S and J6L 7-ton chassis and the forward-control SE, SL and SS 7-tonners. When the Bedford 300-cu.-in, oil or petrol engine is fitted in any of these vehicles the maximum gross weight rating is 15 tons, but when the Leyland 0.350 105 b.h.p. oil engine is installed (in S-type conversions only), the gross weight rating is raised to 15 tons 8 cwt. With the Leyland engine the kerb weight is increased by approximately 3 cwt., compared with the chassis with the Bedford oil engine. In all cases 8.2520-in. (12-ply) tyre equipment is standard.
With Colin Prebble of Vauxhall's and myself aboard, the six-wheeler was tested at a gross vehicle weight of 15 tons 01 cwt., and individual axle-weight checks showed that the driving axle was carrying 3 tons 81 cwt. more than the trailing axle. The kerb weight of the vehicle, which had a 21-ft. drop-sided body. was 4 tons In cwt., and in the
1 4 kerb condition the driving-axle loading was 2 tons 01 cwt., whilst that of the trailing wheels was 15 cwt. The test load consisted of sand evenly distributed over the body floor.
Braking tests were carried out first and these revealed good retardation characteristics. Indeed, during the course of one stop from 30 m.p.h. a Tapley-meter reading of 87 per cent. was obtained, although the usual average maximum retardation figure is about 69 per cent.
When braking from 30 mph., the front and trailing wheels locked, resulting in a bad pull to the right which could, however, be overcome by steering-wheel correction. Hand-brake effectiveness was shown by the Tapley-meter readings of 40 per cent, obtained from 20 m.p.h., and the power was such as to lock the trailing-axle wheels.
Acceleration figures were entirely satisfactory for this
class of vehicle,., and for, both setsof tests the two-speed axle was left in low ratio. The direct drive acceleration performance was particularly good in view of' the comparatively small engine, and the engine and transmission were perfectly smooth in this set of ratios at road speeds down to 8 m.p.h.
Two sets of fuel-consumption figures were taken over the usual Bedford six-mile undulating test course. between Barton and Clophill. I drove for the first test and had fairly clear traffic conditions. Without exceeding 33 m.p.h., I was able to Maintain an average speed of 26.9 m.p.h., the
resulting consurription rate being 12.6 m.p.g. .
Prebble drove for the second test and encountered rather more traffic, which dropped the average speed to 25.1 m.p.h. but improved the consumption rate to 13.2 m.p.g. Both Prebble and I had to engage third-low when ascending the northern side of the hill at Clophill.
For the hilt,climbing tests, the vehicle was taken to
Bison Hill, a incline with an average gradient of 1 in 10+ and sections of up to 1-in-61severity.
The climb was made in an ambient temperature of 44' F. and lasted 5 minutes 54 .seconds. This is -a good time for a vehicle of this power-weight ratio and is Only 11 minute longer than the time taken by .a standard Bedford 7-tonner with slightly higher axle ratios running at a gross weight of 10 tons 124 cwt.
The ascent raised the engine-coolant temperature from its normal 155' .F. to 162.° F., which indicates that the engine is by nomeans under-cooled. and that the vehicle would be quite suitable for continuous hill working in much higher -ambient temperatures. The lowest ratio used during the climb waS bottom-high. This was engaged for 21 minutes, and the minimum road speed on the steepest section was 4 m.p.h.
Fade Resistance Check To check for fade resistance, I coasted the six-wheeler down the hill in neutral, using the foot brake to restrict the maximum speed to 20 m.p.h. On the lower slopes of the hill, where the gradient is not so severe, I engaged top gear and applied full throttle to give the brakes something to work against. The total time of the descent was
2 minutes 24 seconds.
The Tapley meter gave a reading of 45 per Cent. after a 20 m.p.h. " crash" stop at the bottom of. the hill. This showed comparatively good anti-fade characteristics in view of the severity of the test. Exactly the same retardation rate was achieved with the 7-tonner tested in August, 1958, on the same hill. The pedal travel had increased by 3?,in., and recovery from fade was fairly rapid.
Returning to the 1-in-61 section, I stopped the vehicle •
and the hand brake held it on this slope with ease. I was surprised at being able to Make a restart in bottom-high, as I had quite expected to have to use bottom-low. The vehicle crawled 'away from this slope reasonably well, which indicated that gradients of 1 in 5 should not-present starting difficulties in bottom-low.
Traction tests were carried out on the approach to some waste ground at the back of the Vauxhall works. -The gradient on the road leading up to this is such that-at least one other six-wheeler conversion investigated by Vauxhall engineers, before the decision was made to adopt the Primrose and the other, failed to reach the test slopes.
The site of .the test was on a short 1-in-5 loose-surfaced bank, and although the off side trailing axle Wheel .came • clear of the ground for an instant, the vehicle successfully. surmounted this obstacle when travelling forward.
Slight Wheelspin The real test was in reverse, however. When backing towards the bank, slight wheelspin occurred, and once on the slope the off-side driving Wheel came clear of the ground. But traction was restored when the hydraulic compensation linkage forced it down, and the bank was surmounted—an excellent performance.
As a final test, a cheek was made on the rolling resistance' of the vehicle to see whether the trailing axle was causing any undue resistance. The meter gave a reading Of 37 lb. 'per ton when' the resistance was checked at 20 m.p.h. in top gear, and this result is little more than normally obtained with the standard Bedford 7-tonner. It can be assumed .that the Primrose conversion has little adverse effect in this respect.
So far as handling is concerned, I was impressed with the lightness Of the steering. Six-wheelers are prone to heavy steering, bid, In the case of this' conversion the frontaxleloading is ton less than that.of the normal Bedford 7-tonner, thongh castor action was not reduced and there was no tendency towards steering wander.
The vehicle rode extremely steadily and there was little roll on corners.. At one stage I sat comfortably on the test weights immediately over the rear bogie. Little bounce occurred, mainly because of the provision of telescopic dampers at each of the three axles.
The brake-pedal action was firm without being hard, and little travel was necessaryto attain full braking efficiency when the drums were cold. On this particular chassis the changing characteristics of the Bedford twospeed axle were a little sluggish, but this is unusual for this axle, which normally gives extremely fast, clean changes. A maximum speed of 45 m.p.h. was recorded on the open road.