NEW 4-15 OUTFIT HANDLES WELL
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THE main attraction on the stand of J. Brockhouse and Co. Ltd. at the Commercial Motor Show was undoubtedly its new 4-15 twin-oscillating axle semitrailer. With the introduction of this model, Brockhouse re-entered the market for semi-trailers of this layout which it had virtually left in 1939.
The general design of the 4-15 was described in our issue of September 14 last. Pressed-steel frame side-members, cross-members and out-riggers give a clean appearance to the semi-trailer. The side-members are 15 in. deep and reduce to 7.375 in. above the coupling area. Each of the twin-oscillating axles is carried on a trailing link pivoted at the front end on bearings mounted on the underside of .a full-width cross-beam welded beneath the side-members. The trailing arms are of solid-drawn square tubing and pivot on oil-impregnated plastic-asbestos bushes with phosphor-bronze thrust washers. The axles are mounted in trunnion brackets underneath the trailing arms and pivot in P.T.F.E.-impregnated bushes.
Aeon pillow-type hollow rubber springs are located on plates on the undersides of the frame side-members above the centre line of each of the axles, bearing on the top face of the trailing arms. These provide the sole suspension media, no rebound springs being fitted, but at the end of each of the trailing arms check cables are connected to the side-members to restrict rebound. Stops limit the oscillation of the axles to 100 on either side of the vertical. Girling 15.25 in. by 4.25 in. two-leading-shoe brakes are employed. The tyres are 10.00-20 (l4-ply).
The unit exhibited at the Commercial Motor Show was in fact the prototype model on which extensive tests at the M.I.R.A. proving ground had been carried out by Brockhouse, and after the Show it was made available to The Commercial Motor for testing purposes. Full tests on semitrailers are, of course, not practicable because of the dependence of the unit's performance on the tractive unit to which it is coupled. Most of the work carried out was therefore concerned with the assessment of the handling qualities of the unit and the performance of the brakes and suspension.
For the tests, the 4-15 was coupled to a Seddon Mk. 15/10 unit. The semi-trailer platform was loaded with concrete blocks weighing 15 ton 1 cwt. which, with tractive unit and semi-trailer kerb weights of 3 ton 19.5 cwt. and 3 ton 5.5 cwt. respectively and two people in the cab, made the gross weight of the outfit 22 ton 10.5 cwt. Individual axle loadings at this weight were: front axle 3 ton 9.75 cwt.; rear axle 8 ton 6.25 cwt.; and semi-trailer axles 10 ton 14.5 cwt. It will be seen, therefore, that although the 4-15 • is sold as a 15-tonner, a 16-ton payload could be carried (Provided it is correctly distributed) without exceeding the maximum legal axle loadings on the tractive unit rear axle and semi-trailer axles of 9 and 11 tons respectively. This, of course, applies in the case of the outfit as tested, that is with the Seddon tractive unit—the actual load possible depends entirely on the unladen weight of the prime mover used.
Braking tests produced very good figures for the outfit. Because brake tests with an articulated outfit give no real -indication of the amount of work being done by the semitrailer braking system, the brake-test procedure used on normal road tests would have shown very little. The procedure adopted was, therefore, to take Tapley meter readings for stops from 20 m.p.h. applying the semi-trailer brakes only. This was done by operating the hand-control valve mounted on the steering column of the tractive unit and figures of between 30 and 32 per cent were obtained.
Full-pressure braking stops for the complete outfit gave a figure of 58 per cent on the Tapley meter, which comes within the range of figures obtained by Brockhouse themselves on their proving tests.
On all the stops the outfit behaved perfectly; there was no tendency to jack-knife and the tractive-unit rear wheels marked the road heavily whilst the semi-trailer wheels marked only faintly. The stops when operating the trailer brakes only were particularly good and even on these the trailer wheels did not lock, there being only light marks on the road.
A figure for the rolling resistance of the complete outfit was also taken. This turned out to be 45 lb. per ton, which figure again includes the effect of the tractive unit. It is difficult to obtain a figure for the rolling resistance of the semi-trailer alone, but the figure obtained for the complete outfit is commendably low, being about the average for a loaded four-wheeler.
There was no noticeable delay in the braking system. The elaborate equipment required to measure this was not available, but delay is usually easy to "feel ", particularly if excessive. Credit for this must be the mounting of the Clayton Dewandre Air-Pak for the air-hydraulic braking system on the headboard, so enabling the shortest possible air lines to the tractive unit control valve to be used. Another advantage of mounting the servo system on the headboard is that good accessibility is afforded for main tenance purposes. •
General handling of the outfit was very good; the low rolling .resistance being particularly evident on bends, as is to be expected with a twin-oscillating axle layout. On the brake tests there was no tendency for judder to occur and the rubber suspension gave a good ride. It was not possible to remove the load to make empty runs but by the nature of the Aeon rubber springs used there should be a relatively soft ride in this condition; much softer in any case than with a leaf-spring layout.
The time taken to uncouple and couple the tractive unit and semi-trailer was checked finally. These operations were done by the Brockhouse driver who took 2 minutes seconds to uncouple and 1 minute 57 seconds to recouple the outfit.