Long-distance Economy on Overdrive By
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John F. Moon
A.M.I.R.T.E.
Air-pressure Brakes on Conuner-Scammell 10-11ton Unit Stand Up Well to Severe Tests
THE first melium-weight primemover to be supplied in standard form with a Scammell coupling gear and air-pressure brakes is the Coinmer 10-11-ton unit, which 1 have tested as an 11-ton out
fit. Until recently this automatic coupling gear had not been fitted to tractors with air brakes, but the layout of the new system is largely similar to the well-known vacuumbraked version. The essential difference is that the vacuum-servo cylinder which actuates the semitrailer brakes through the king-pin linkage is replaced by an air-pressure un it.
Running at gross train weight of 16 tons 81 cwt. (a 31-cwt. overload) the outfit showed extreme economy on a 97-mile trunk run. This economy was helped in no small way by overdrive gear . which is offered as optional equipment.
On this run, which was completed at an average speed of 28.2 m.p.h., a consumption rate of 14.1 m.p.g. was returned. This gives a time-load-mileage factor of 6,500—making the highest ever recorded by The Commercial Illotor.
The 10-11-ton prime mover is derived from the 7 ton load carrying chassis and is powered by the Rootes threecylindered oil engine, no petrol engine option being available. The engine is mounted below the seats of the three-man forward-control cab. This allows the wheelbase to be extremely short, giving increased el 0
rnanceuvrability and greater semiti ailer length without exceeding the maximum legal length a 35 ft.
Nevertheless, the tractor rides well, and the two main disadvantages of most forward-control designs — reduced driving comfort and poor engine accessibility--are avoided by reason of the position of the power unit. Driving visibility is reasonably good, although the range of forward vision would be substantially improved were a deeper windscreen to he fitted.
The braking system fitted to the prime mover tested consisted of a Girling hydraulic circuit actuated through a Clayton Dewandre airpressure unit, this being offered as alternative equipment to the standard vacuum-servo unit. Power is supplied by a single-cylinder compressor, which replaces the exhauster assembly on the engine, and the brakes are controlled by a three-stage pedal valve, the actuating pressures of which vary from 10 lb. to 75 lb. The semi-trailer brake servo is controlled by a hydraulic reaction valve in the tractor circuit.
Uprating the Searnmell semitrailer from 10 to 11 tons involves basically little more than fitting 9.0020-in. (14-ply) tyres instead of the 12ply tyres employed on the lighter version. No modifications have been occasioned by the air-pressure braking on the tractor—the standard semi-trailer can be used with tractors with either vacuum or air systems.
A test load totalling 10 tons 1411 cwt. had been provided, almost 44tons of this being carried by the driving axle. The discrepancy between the loadings on the driving axle and the semi-trailer axle is intentional and is caused by the king-pin position and the body rear overhang. A higher loading on the tractor would, in any case, overload the 7.50-20-in. tyres, which are the largest that can be used with Scammell gear.
The outfit had covered over 7,000 miles before my teSt, so it was mechanically free. The main fuel tank was carefully topped up to overflowing point, in preparation for a longdistance fuel-consumption run. On leaving the Commer works the outfit was taken along the A6 road as far as Bedford, the first 10 miles being covered in 21 minutes. The 3.6 miles through Bedford took 10 minutes to negotiate —traffic congestion not being so acute as usual— and, after joining the Al road at Wyboston, a check showed that the first 30 miles from Luton had been completed at an average of 29.8 m.p.h.
On the Al road much heavy traffic was encountered, but it was possible to use the overdrive gear at speeds down to 25 m.p.h. on reasonably level stretches. As acceleration in that ratio was good, Overmuch gear changing was not required. On traffic-free sections— which occurred infrequently—a speed of 38-40 m.p.h. was kept up. This is a comfortable cruising speed in overdrive, the maximum speed in this • gear being about 53 m.p.h.
The turn-round point just north of Sawtry was reached 1 hour 40 minutes after leaving the works, at an average speed of 29.2 m.p.h. Even more traffic was met on the return to Luton; and the time taken was 1 hour 46 minutes, making a total running
time of 3 hours 26 minutes for the 97.2-mile round trip. To top up the tank 6.9 gall, of fuel were required, giving an average consumption rate of 14.1 m.p.g. at an average speed of 28.2 m.p.h.
When comparing this result with the fuel-consumption figures obtained on my usual short undulating route between Barton and Clophill, it becomes clear that the true advantage of the overdrive ratio when laden is shown on a long run containing long stretches of fairly level road, particularly if a speed of at least 30 m.p.h. can be maintained. This applies overseas particularly.
The Barton-Clophill circuit does not permit such speeds to be kept up for more than a minute or two at a time, hence the heavier fuel figures obtained and the slightly lower average speeds, despite hard driving. Another point that arises from these short-distance results is that, when laden, the oVerclrive ratio makes relatively little difference to the consumption rate, although, as the runs • made without the semi-trailer show, a marked difference occurs when running unladen. From these results the following conclusions can be drawn: (1) The overdrive gear (which adds only £41 5s. to the cost of the chassis) is beneficial if the vehicle is to be operating on long-distance services over reasonably level main roads.
(2) If the gear is used on vehicles running short trips, particularly along hilly or twisting routes, the improvement in con sumption rate will not be marked.
(3) If a large proportion of unladen running is contemplated, the gear will be an advantage, irrespective of distance or terrain.
After these somewhat lengthy fuel-consumption trials, the normal short-distance trials were conducted. The first of these was the braking test. This was at first disappointing, but a check at the Commer works showed the semitrailer brakes to be sadly out of adjustment.
When the fault had been rectified reasonable retardation figures were obtained, the brake mal-adjustment making a difference of 15 ft. when stopping from 30 m.p.h. This merely serves to add emphasis to the necessity of ensuring that on any articulated unit the semi-trailer brakes are kept in tip-top condition. The braking figures recorded are decidedly better than would have been obtained had the outfit been fitted with a vacuum-hydraulic layout, but there was evidence of a long time-lag in the system. If this could be reduced, even better retardation rates would be obtained.
When making "trash" stops from 30 m.p.h. the outfit was perfectly stable and no wheel locking occurred, although the tractor rearaxle wheels were on the point of skidding, because of their relatively light loading.
It was with difficulty that a long enough stretch of level road was found for the acceleration tests, several attempts on surfaces with only slight gradients producing very uneven results. Eventually satisfactory figures were obtained on a quarter-mile section of the North c12 Orbital Road, near Hatfield. These showed the Commer to have an acceleration performance above average for this class of outfit.
With such a relatively low powerto-weight ratio, however, it is to be expected that even small gradients will slow down. acceleration appreciably, although under normal road conditions the Commer can hold its own with other traffic.
For the hill-climb and brake-fade tests the outfit was taken to Bison Hill, which is 1 mile long and has an average gradient of 1 in 101, the steepest section measuring 1 in 6. The climb was made in an ambient temperature of 63 F. and took 6 minutes 10 seconds, 2 minutes 50 seconds being spent in bottom gear. The temperature of the coolant in the water-filler neck rose by 13° F. during the climb, but, as with all Commer forward-control chassis, it is impossible to take a true headertank temperature reading because of the shape of the neck. The rise recorded, however, is sufficient ,indication that the engine cooling system is adequate for any abnormal operation in this country.
A descent of the hill was then made, the gear lever being placed in neutral. The foot brake was used to keep the speed down to 20 m.p.h. and the descent lasted 2 minutes 35 seconds. For 43 seconds top gear was engaged and full throttle was applied to compensate for the reduced gradient towards the bottom.
This is a very severe test for brake fade, but an emergency stop from 20 m.p.h. revealed that the braking efficiency had been reduced by less than half, a Tapley meter figure of 29.5 per cent. being obtained. This compares favourably with the 50 per cent. maximum retardation obtained earlier in the test with cold drums and is the lowest degree of fade ever recorded on this hill by The Commercial Motor with a vehicle of more than 3 tons gross weight. The tractor had moulded brake facings.
Hand Brakes Held Safely
Returning to the steepest section of the bill the outfit was stopped and held safely with the tractor and semitrailer hand brakes applied, even though all the brake drums were still smoking slightly from the fade test made immediately before. Neither brake by itself would hold the vehicle, but the tractor brake might have done so had the drums been cool.
A successful bottom-gear restart was then niade.without undue clutch slipping, but from the engine's performance during this test it is clear that a 1-in-6 gradient would be the steepest on which a restart could be made at over 16 tons gross train weight.
The Commer handles well on the .road and is not tiring to drive, the steering being light but direct, the pedals light to operate and the engine quiet. On previous Com m e r forward-control models I have had to criticize the remote-control gearchange linkage, so I was relieved on -this occasion to find that the linkage had been redesigned, with certain beneficial results.
-More Positive Movement Gear changing is now Iflare definite, although it was a little stiff on the test vehicle. There is no possibility of inadvertently trying to engage reverse when changing down into second gear. Similarly, the overdrive engagement catch has been dispensed with as being unnecessary.
On main-road operations the engine pulls well, and on most roads it will rarely be found necessary to have to use bottom. gear (other than when starting, of course), second providing ample power for most trunkroute hills. The brake treadle has a sufficient degree of" feel " to preclude the possibility of braking too fiercely when running light.
No dampers were fitted to the front axle of the vehicle tested, although they are available as optional equipment. Nevertheless, the ride in the cab was not rough when laden, although a certain amount of "trailer tugging" could be felt at times. Without the semi-trailer, no great discomfort is caused to either driver or passengers by bouncing.
Maintenance follows the usual lines for Commer forward-control oilengined models, a particularly good feature of the design being the engine accessibility, which is much better than with most forward-control chassis, The power unit is reached by removing the seat cushions and seat-box lids, and, short of a tilting cab, it would be difficult to provide better access.
The 10-11-ton prime mover with Scammell coupling gear has a basic price of £1,560, the purchase tax being £316 13s. 2d. Without the Scam mell gear the chassis and cab are £130 cheaper. The price of the S.A.E./S.M.M.T. or,fifth-wheel coupling gear is decided by the semi-trailer manufacturer. The air hydraulic brakes add £65 12s. 6d. to the total price. The Searnrnell. 11-ton 25-ft. drop-sided semi-trailer, without spare wheel and carrier, costs £706.