The Performance oj
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AL-ENGINED BUS CHASSIS
WE are able to give, in this road-test report of a Daimler double-deck bus c ha ssis equipped with a Gardner engine and a Fluid Flywheel, first impressions of an oil engine driving through an hydraulic transmission system. The characteristics of units of this type and of this form of transmission are, separately, well known, but a combination of the two strikes something of a new note and is, therefore, all the more interesting. Whilst it must be admitted that the latest designs of internal-combustion engine operating on the heavy fuel have made enormous strides in the direction of silent and vibrationiess running, the "
smoothing" characteristics of the hydraulic coupling (as embodied in the Fluid Flywheel) help to make this type of unit even more attractive than it has already become when used in conjunction with "solid" transmission.
Outstanding Chassis Points.
Before describing the behaviour of the vehicle on the road, we will outline its specification. Broadly speaking, the chassis is the same as the petrol-engined machine, with the B24 exception of the engine and the gearbox. On the vehicle tested the lastnamed is a five-speed pre-selective unit, and a fairly high axle ratio is employed, in order that the high torque characteristics of the engine at low and moderate speeds may be used to the best advantage.
The box is not of the over-drive variety, fifth gear being direct and, although the lowest ratio'is moderate —approximately 28 to 1—we did not find a sufficiently severe gradient, during the course of an extended trial in the Midlands, to warrant its use We satisfied ourselves, how ever, that the comparatively high overall ratios of the other four gears, coupled with the flexibility provided by the Fluid Flywheel, give all the essentials necessary for an attractive road performance generally. The Gardner five-cylindered engine starts from cold easily, without preheating and, when warm, an instantaneous start can always be obtained without using the decompressing device. So soon as the unit is running it is ready for work, but in the interests of proper lubrication the engine should be allowed to idle for a few minutes before setting off on a journey. Once under way, however, full power is developed without any suspicion of misfiring or other signs of refractoriness, as often experienced with cold petrol engines.
Behaviour of the Fluid Flywheel.
The characteristics of the Fluid Flywheel must, of necessity, be mentioned repeatedly, for the behaviour of the vehicle is largely influenced by the hydro-dynamic qualities of the coupling. Up to about SOO r.p.m. there is a high degree of slip, but the rapid interchange of energy generated by the fast-moving fluid starts to build up the " capacity " of the coupling until the loss by slip is, in the extreme case, only about 2 per cent,
It is fairly obvious that, at very low road speeds, the available torque is much higher with a Fluid Flywheel than with a solid drive. Although any advantage in this direction disappears at a speed of about 10 m.p.h., the larger amount of available power which the slipping characteristic of the Fluid Flywheel provides, gives a degree of flexibility which is highly desirable in a vehicle used for general
passenger-service purposes, where frequent stops are usual.
On level roads the bus will move away from rest rapidly when the second gear of 21.79 to 1 is used. It is possible, of course, to get away with " top " engaged, but there is no material advantage in doing this, because gear-changing is, in itself, a simple and straightforward opera= tion. When using the pre-selective arrangement intelligently a really outstanding performance can be obtained. For the purpose of our acceleration graphs we used second gear to start from rest.
As the chassis under review is essentially a town vehicle there is not much point in quoting the average speeds, over relatively long distances, attained during our test. As a matter of general interest, however, we may remark that, without seemingly forcing the pace, we averaged. 26 m.p.h. over a distance of 10 miles on a main road. A far more important test was that instituted to check the capacity of the vehicle for city work.
Useful Average Speed.
Allowing five stops to the mile as a general figure for service operation, we timed a run of exactly one-fifth of a. mile. Starting ,away from rest in second gear, a speed of 25 m.p.h. was attained in the fifth gear before the brakes were applied, the time occupied, from the moment that the chassis began to move to the time when it was brought to rest at the end of the distance, being ss secs.
We put the Daimler through a severe hill-climbing test by tackling the long rise of Edge Hill (near Kineton), with its 1-in-7 gradient and steep protracted approach. Although the chassis was fully laden the climb v.ras made in second gear. At no time was there any indication that first gear would be required, for although the vehicle speed fell to alaout 7 m.p.h., the engine always seemed to have power in hand.
Performance on Edge Hill.
The first climb was made at the conclusion of a run of about 20 miles, completed more or less non-stop. At the top of the hill the cooling water was below boiling point by several degrees, despite the presence of a shield over the lower part of the radiator.
Subsequently, another climb was attempted, a stop-and-restart test on the steepest part being included. This, naturally, added to the difficulties of the test, and in consequence there was slight boiling on reaching the summit of the hill. Using first gear, the vehicle moved away from rest on the 1-in-7 section smoothly and easily, and it was possible to change up to second gear in about 20 yds.
The hydraulic coupling between the engine and the gearbox is un questionably of value when restarting on a severe gradient. The procedure is to hold the machine (in gear with the engine running slowly) by the hand brake, then to depress the accelerator pedal, the load being picked up off the brake in a perfectly straightforward manner. The need for the driver to exercise finesse in • the matter of letting in the clutch is removed.
While manceuvring on Edge Hill we were able to assess the value of the brakes. Both hand and foot systems operate smoothly and with good effect, whilst the fact that they seemed equally efficacious with the vehicle running backwards is a definite point in their favour. Thanks to the Dewandre three-cylinder servo system, only a light pedal pressure is needed to pull up, even in an emergency. The hand brake, although naturally less efficient than the pedal system, as it applies only the rear brakes, nevertheless affords powerful retardation.
We have already dealt at length with the characteristics of gearchanging and the Fluid Flywheel, but little mention has been made of other main features, for example, the steering and suspension systems. As will be seen from the accompanying data panel, it takes but 21 turns of the steering wheel to bring the front wheels from lock to lock. Consequently, sharp turns require a fair amount of effort to be exerted by the driver.
On the other hand, steering is delightfully sensitive, and if the wheel be moved gradually there is little indication of heaviness. Suspension, too, is excellent, the chassis showing little or no tendency towards pitching or rolling, and we opine that, with a properly equipped bus body, road shocks would be practically eliminated.
Altogether, then, this Daimler is a vehicle of which the maker may justly be proud. It is beautifully finished and of thoroughly modern design, whilst the special transmission system may be considered as marking a definite advance.