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CLUMSY

5th October 1962, Page 61
5th October 1962
Page 61
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Page 61, 5th October 1962 — CLUMSY
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Which of the following most accurately describes the problem?

IBy A. A. Townsin,

_THOUGH the A.E.C. Reliance 590 coach chassis for

36-ft.-long bodywork is large, its versatility is such -L kthat it could confidently be taken on almost any coach run in the country, subject only to the barest allowance for its physical dimensions. This applied not only to its capabilities in terms of speed al-id hill-climbing, but also to its general handling characteristics. Although care had to be taken to avoid " kerbing " the rear wheels at corners or roundabouts, it gave no impression of being unmanageably clumsy or awkward to drive.. The fuel consumption figures Would have been no disgrace to a lighter 30-ft-long model.

Only two faults of any significance were noted. Brake fade could be provoked without great difficulty, although a sensible driver might never be aware of this weakness, at any rate in this country_ Some difficulty experienced in gear selection was irritating to a degree much larger than the natiti-e of its cause or possible cure would suggest The Reliance 590 is now available with wheelbases of 17 ft. 6in. and 18 ft. 7 in. for vehicles having overall lengths of approximately 33 ft. and 36 IL respectively. Left-hand steering is also available on the shorter version„ but the test chassis was an 18-ft. 7-in.-wheelbase right-hand model with ZF six-speed synchromesh gearbox. It was loaded, to a gross weight of 11.5 tons, which, with John Knight and Lew Slipper of A.E.C. and myself, represented that of a typical fully-laden 51-seater luxury coach with unladen weight of 8 tons, allowing 20 lb. of luggage per passenger. The A.E.C. AH590 engine was in its high-rated form, giving a maximum of 153 b.h.p. at 2,000 r.p.m. The axle ratio was the standard one of 4.7 to 1.

The problem of providing a sufficiently low bottom gear to give adequate maximum gradient-climbing ability and a high enough ratio for economical and fuss-free high-speed running without resorting to unduly wide ratio steps is considerably eased by the use of a six-speed gearbox. Some potential purchasers may wonder whether such a unit is necessary, but after a day out with the vehicle, would strongly recommend it for most types of coac operation. The ratios are generally very evenly spaced, ti ratio steps between second and fifth speeds all beir between 1.55 and 1.59 to 1. That between first and secon is slightly greater (1.79), and the overdrive sixth speed t 0.82 to I represents a step-up of 1.22 to 1 from the dire fifth. This seems rather close by comparison with II others, and when conditions called for a change down froi overdrive it seemed more often than not to be necessat to drop to fourth almost immediately.

The overall sixth speed ratio, giving 30.9 m.p.h. pi 1.000 engine r.p.m., seems well chosen. It gives theoretical maximum of 68 m.p.h. with 10 per cen governor run-up, and in practice 70 m.p.h. was achieve on level stretches of motorway on a number of occasiot when travelling in both directions. The gradient perforn ance is naturally limited in this ratio, the maximui theoretically climbable at steady speed with the load, tested being 1 in 38 on a basis of 25 lb./ton rolling resis ance. In practice, although motorway gradients of .t1type found on MI pulled the speed down to 50 m.p.h overdrive was not found to be too high to prevent it bein in frequent use even on undulating main roads.

Direct fifth, giving 25.3 m.p.h. per 1,000 engine r.p.ndoes not seem likely to be used for long periods becat.n of its closeness to overdrive. However, as well as affordin a maximum of 56 m.p.h.. it proved to be an unexpected] flexible ratio. Although it would hardly be the ideal choic for full-throttle acceleration from 10 m.p.h. in norm; service, the maximum pull (enough for a 1-in-28 climl being theoretically produced at 28 m.p.h., tests of this wet not as unrealistic as might have been expected, for th engine pulled smoothly and relatively vigorously, even .1 the beginning of each run. Speeds attainable in the km indirect ratios were 9, 15. 23 and 37 m.p.h. respectively.

Taken in general, the performance figures in the data able can be left to speak for themselves. What they do not onyey is the pleasantly easy-going manner in which they vere produced. No engine noise could be detected above he rush of the ,vind when travelling at the front of the 'pen chassis at anything much over 40 m.p.h., even under u11-throttle conditions.

Testing an open chassis is, of course, unrealistic in this .s in many other respects. I did, however, have an

■ pportunity of riding in a complete Reliance 590 coach hortly before the test and can confirm that a very satisactory standard of quietness is achieved. Helping appreiably in this, the ZF gearbox is almost certainly the quietest iox ever fitted to any A.E.C. production model. Helical ;ears do not always justify the complications they entail, )ut in this case the almost complete lack of gear noise in .11 but the two lowest ratios is a major contribution to efinement of running.

Vibration was almost negligible at all speeds. The fitting if a torsional vibration damper, although mainly intended o protect the crankshaft from the effects of torsional iscillations at about 2,000 r.p.m., may help here. Steady tiling has long been a characteristic of A.E.C. diesels and his one was no exception.

Performance without fuss might be expected to imply xtravagarice in terms of fuel consumption. Although road works on M1 enforced a reduction of speed to about 35 n.p.h. for about half a mile during the course of what was ntended to be a full-throttle run, it is evident that highpeed motorway running is unlikely to drop the m.p.g. igure much below 10.

The undulating route test was carried out on a 25-mile oute, largely over the Chiltern Hills, commencing at the New Ground A.A. box on ;the A41 road and running via Berkhamsted and Amersham to the Polish War Memorial near Northolt Aerodrome. The speed was kept near 40 m.p.h. wherever possible. Traffic delays in the Berkhamsted and Amersham areas made the first 12 miles not unlike country bus work in hilly territory, an intermediate check giving a figure of 12.5 m.p.g. at an average speed of 24 m.p.h. over this stretch. More steady running was possible on the second stretch, although several hills and roundabouts and two traffic lights at " red " made the figure of 17.9 m.p.g., obtained at an average speed of 29.7 rn.p.h., particu larly commendable. • Performance on more severe gradients was checked on Bison Hill, near Dunstable. The 0,75-mi1e stretch with an average gradient of I in 10.5 frequently used for test by

The Commercial Motor was climbed in 2 min. 17 sec., second gear being called for on the steepest section with a gradient of 1 in 6.5 and remaining in use for I min. 11 sec. There was no change in the water temperature of 81°C. (177.8°F) as recorded by the gauge fitted to the chassis (a stick thermometer could not be used because of the layout of the cooling system). The ambient temperature was 22°C. (71.6°F.) and the cooling system obviously has generous reserve capacity. An easy and smooth restart on the steepest part of the hill was subsequently made in first and reverse gears, and although it was thought that a restart in second would be too much to expect, a change up with the vehicle barely rolling was successfully achieved, accompanied by some clutch judder.

As the data panel shows, good stopping distances were recorded in the brake tests, and these were also notable for a complete absence of any tendency to pull to one side, there being very little marking of the road surface. In normal use, a trace of the delay effect generally associated

with direct air-pressure systems could be detected, but this was slight and the response to pedal pressure was nicely graded. The handbrake proved capable of holding when facing both up and down a 1 in 6.5 slope, and it was found to be just able to stop the vehicle without assistance from footbrake or engine when it was allowed to roll down this slope at rather less than walking pace.

The first attempt at a brake fade test, also carried out on Bison Hill, had to be abandoned due to the proximity of holiday-makers' cars when the full brake application was about to be made after using the brakes to maintain a steady 20 m.p.h. down the hill. I had coasted down most of the hill but engaged direct drive and applied full throttle to keep the brakes working on the gentler slope. However, something not far short of a full application was found to be necessary to make what was intended to be a normal stop. Appreciable drum expansion became apparent when the handbrake was applied, as the lever came up its stop without appreciable resistance, although it held e chassis on a comparatively gentle slope.

A repeat run was more successful and gave a Tapley eter reading of 62 per cent. at the foot of the hill. Other iffic again caused some difficulty, however, and it seems obable that the brakes had partially recovered as a result bringing the vehicle momentarily to rest during the :ter part of the .descent. Rapid recovery of the linings is confirmed by the fact that the successful test of handake holding power on the 1 in 6.5 gradient was made imediately after this run.

Appreciable loss of brake efficiency is not likely to be perienced in normal service in Britain, and some of that perienced on the test was due to a drop in reservoir essure caused by my inability to maintain a constant foot osition during the descent, coupled with the idling engine ing incapable of making up the resultant loss of air. is had been made up by the time the Tapley meter test Li made. None the less, a lining with better anti-fade aracteristics might he desirable for Alpine touring, ■ ssibly with a larger reservoir capacity. The air system of the split-circuit type.

Suspension characteristics of a complete coach are rticularly difficult to assess from the chassis in the case an underfloor-engined model, although in this case the ;k was aided by the way the load-frame extended forward just behind the driving position. This undoubtedly counted for the front-end ride being the best I have perienced on a chassis of this type, not excluding one th air suspension, since frame deflection is the most 'portant factor in this condition. With bodywork, the !fiance 590 does, in fact, give a ride of the usual good indard associated with this vehicle layout. Air suspenni is available as an optional extra.

ability at Speed

Steering and handling characteristics are among the most 'portant items in determining whether a vehicle is pleasant drive. The Reliance 590's noteworthy lack of clumsiness feel, despite its size, is largely due to its precise and, r the most part, comparatively light steering. Particurly praiseworthy was the way in which a true course Li maintained at speed without the need for concentram on the part of the driver. At 70 m.p.h. on MI full tuition could be devoted to the road ahead and other hides. Responsiveness does not always go hand in hand th this sort of characteristic, but normal main road bends could be taken with surprisingly small movements of the wheel. At the other end of the scale, slow-speed manoeuvring did not call for undue effort provided the wheels were rolling.

The choice of steering ratio seems just about right. This model has a 40 to 1 steering box and although not usually in favour of lower-geared steering, I found it preferable to another A.E.C. of similar weight with a 33 to I ratio. The only circumstance in which the Reliance 590 seemed a little heavy was the negotiation of sharper corners or roundabouts, when the castor action seemed a little over-generous.

The problem of providing an adequate steering lock on a 36-ft.-long model has been solved by the addition of a relay lever between the drop arm and the axle. This enables the "inner" front wheel on each lock to turn through an angle of some 48 degrees. The steering geometry is probably also improved; certainly no more than a trace of reaction could be felt through the wheel. The turning circle diameters recorded show that the use of 10.00-20 tyres is by no means necessarily an insuperable obstacle to giving a good lock on a chassis of this type.

The temporary seat was rather higher than I would have chosen, another drawback of testing a vehicle of this kind in chassis form, The driving position, so far as could be judged, was comfortable, perhaps the only criticism being the need to lift one's right foot rather excessively when transferring from the accelerator pedal to the brake.

The gear-change action would be excellent, apart from the fault already mentioned. The gate is entirely logical, with the six forward ratios in progressive order, higher gears being towards the right. Reverse, on the one hand, and fifth and sixth on the other, are protected by very weak springs, and it proved all too easy to select the wrong gear. A change up from second to third could and did all too often become one into fifth, for instance. Apart from those changes which were directly fore and aft, one could never be quite sure of what to expect when the clutch was re-engaged.

Stronger springs would help, but personally I would much prefer a trigger rod device to give position location of each ratio position and would consider its cost well worth while. The synchromesh was effective and the effort required to move the lever very moderate for a box capable of handling torque of this order.

To sum up, the Reliance 590 has a distinct "quality " character, providing comfortable and rapid travel for its passengers at reasonable running cost to the operator and giving the impression of being able to go on doing so for long periods without trouble.

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