A Road-test Report by Cecil Clutton, with some interesting observations on the Cotal Electric Gear Change
If smug complacency is an accusation which can justly be laid at the door of some of our more drab manufacturers, it is certainly not true of our quality designers. I have had the good fortune to test two post-war models, the Mark V Bentley and the 2-litre Aston-Martin, and I can say without any hesitation that we have in them two machines that can show the Continent the way. The Mark V was, of course, ready to go into production the year the war started, and there are quite a few of them motoring about. Whether or not this particular model goes into production after the war, I certainly hazard the opinion that an expectant eye can be directed towards Derby when the manufacturing of motor-cars sets in once more.
The Aston-Martin is still in a more experimental stage, being the only one of its kind in existence. Many readers of Motor Sport will have seen it when it made its bow to the public at the Chessington Rally last summer.
A full technical description of it, with drawings, has already appeared in the Motor, and a very few remarks on its layout can therefore suffice. The engine is a standard 2-litre o.h.c. .Aston-Martin unit. The compression ratio is 7 3/4 to 1 and the output is slated to be 90 b.h.p. at 5,000 r.p.m. The four 78 x 102-mm. cylinders are fed by two Zenith carburetters, ignition is by coil with automatic control and the engine is three-point flexibly mounted. The front mounting is high up, at the level of the fan-spindle, and the back ones are far back and low. The chassis and body are integral and tubular members play a prominent part in ensuring rigidity. Rear suspension is by semi-elliptic springs and an exceptionally perfect independent layout obtains in front. Each wheel is supported on short parallel trailing bars, somewhat in Porsche style, and the spring itself is the coil variety. This arrangement entirely eliminates gyroscopic action and, of course, the wheels remain vertical when cornering. The advantages of independent suspension are often more than nullified in those cars where the wheels “knuckle under” when cornering, and the exceptional cornering abilities of the Banality layout are undoubtedly due, in large measure, to the fact that the front wheels lie over in the other direction when the body rolls, so as to have a definite “digging-in” effect, against the tendency to slide outwards on a bend.
In addition, the Aston-Martin has a split track rod so arranged as to give perfect geometry, so that the three bogeys of steering have, to all intents and purposes, been eliminated. The steering ratio is fairly high, requiring 2 1/4 turns from lock to lock.
Shock-absorbing is effected hydraulically. Retardation, courtesy of Messrs. Lockheed.
The clutch is a single-plate and the gearbox is a Cotal, giving overall ratios of 4.55, 6.7, 10 and 15 to 1. With 5.50″ x 17″ tyres, this gives maxima of 28, 42, 62 and 92 m.p.h. at 5,000 r.p.m.
Turning now to the body-cum-chassis, it is certain that, when put into production, it will assume a very different shape, and the criticisms which can be levelled at it must not, therefore, be taken as indicating any inherent disability in the car. The “one-piece” front end, with built-in headlamps, is at once pleasing in appearance and aerodynamically efficient. Headlamps are very troublesome to incorporate in a streamlined front end, and I have sometimes wondered if a single strip lamp, say 5″ wide, Would not be easier to accommodate, with its much shallower reflector. The opinion of an expert would be interesting. The rest of the saloon body is not, however, truly streamlined and palpably exerts a lot of drag, as can be judged by the amount of wind noise it sets up (especially when the windows are closed) and the rain tracks upon its surface when the weather is wet. The front seats are of the modern type, with leather stretched on a tubular steel frame. They are quite exceptionally comfortable and support the small of the back in a way which few upholstered seats achieve; but a little more lateral support round the shoulders would be acceptable to the passenger, who has nothing to locate him when high-speed cornering is afoot.
The back seats are very occasional and the present small overall dimensions of the car are likely to be substantially increased in production form. Both in the interests of comfort and road-holding a 4′ 2″ track and 8′ 6″ wheelbase are needlessly small for a car of this type. A truly streamlined body, too, should extend to the full width of the car, there being no visible rear mudguards, and this would substantially increase the seat width.
The driving position is desirably erect, in the Continental fashion, giving a fine sense of absolute control. The floor level should, however, be brought lower, so as to reduce the at present somewhat excessive overall height. So much for coachwork criticisms, but with renewed emphasis that this body is avowedly very much of a first experiment, which would not be perpetuated in production.
On the road this is a machine which convinces you within the first half mile that it is all the way a winner. I have never driven a car which I could handle at higher speeds, or with greater confidence, on a wet road. The steering is the most pleasurable to handle of any i.f.s. car I know, and second only to a Bugatti for sheer delight. On the dry it has a very slight tendency to over-steer, which is as it should be, and becomes perfectly balanced on the wet. Its capacity for “holding on,” even when driven really fast round a bend with a wet, smooth surface and pronounced reverse camber, is simply amazing.
The limitation to the road-holding is at present set by the body, for at 80 m.p.h. the whole outfit seems to begin to lift and steering becomes very remote. This is a serious fault, but one which can certainly be rectified, since the car itself is not exceptionally light, an all-up weight of 24 cwt. being quoted.
On really rough going the springing works equally well, absorbing even the worst bumps with complete efficiency, to the great comfort of the occupants.
So far as the engine itself is concerned nothing much need be said, as it seerns to be a standard Aston-Martin unit. As is to be expected, on a 7 3/4 to 1 compression ratio it does not take kindly to “Pool” petrol, and even with a fairly retarded initial setting for the ignition control, the throttle has to be used with great delicacy, especially in the range from about 2,000 to 2,500 r.p.m. Aston-Martins have always been famous for a high power output coupled with outstandingly smooth running, and this engine is no exception. A mean effective pressure of 117 lb./sq. in. at 5,000 r.p.m. is very creditable, yet the engine is perfectly smooth at all working speeds and exceptionally silent as well. Many high efficiency engines have tiresome power roars and similar manifestations which are insufferable in a saloon motor-car, but there are none of these in the Aston-Martin, which is essentially a machine for fast, effortless, long-distance touring. Only at low speeds does the engine obtrude itself, in the shape of flutter below 20 m.p.h. in top gear, while when ticking over the flexible mounting permits it to rock the car to a disagreeable extent.
Power outputs of this magnitude are not achieved without fairly pronounced valve-timing overlap – about 45˚ in this instance – and while the engine will pull smoothly at 1,000 it does not really start working to advantage until after 2,000 r.p.m.; 5,000 r.p.m. is the maximum permitted engine speed and 4,500 r.p.m. is entirely unobtrusive. For fast driving, therefore, the effective range may be said to lie between 2,000 and 4,500 r.p.m., and to maintain these speeds without exertion a Cotal box is almost a sine qua non. At the same time, it has the inherent disadvantage that it is not, apparently, practicable to employ very close ratios in gearboxes working on the sun-and-planet system. Each ratio on the Aston-Martin represents an increase of some 50 per cent. in r.p.m., which is undesirably wide for anything but large engines giving useful torque at very low speeds. To have the same geometrical proportion (i.e., 1 : 1.5) between each pair of gears also bunches the maximum speeds of the lower gears and leaves an unduly wide gap between the maximum speeds of third and top gear – a difference of 30 m.p.h. in this case. There is no doubt that the best assortment of ratios is that which most nearly distributes the maxima equidistant over the speed range. For instance, taking the same top and bottom gear ratios as we have here, ratios of 4.55, 6, 9 and 15 to 1 would have given the far preferable maxima of 28, 46, 70 and 92 m.p.h. With the Cotal, however, it is particularly difficult to obtain this relatively wide gap between second and bottom gear, since bottom gear is, so to speak, a combination of second and third gear. Thus, on the D.8/120 Delage, which I road tested for Motor Sport in May, 1938, third gear was relatively higher than in the present box, but, as a result, second and bottom gear were exceptionally close together.
Even with this disadvantage, the Cotal is undoubtedly, to my mind, the finest gearbox available, and the rapid changes possible undoubtedly mitigate it to a large extent. All the same, it does remain an undoubted snag, especially with a power curve of this kind, and it is a pity if it cannot be overcome.
These lightning changes might put an undue strain on the transmission in the hands of a brutal driver, and, perhaps with this in mind, the designers have endowed this engine with what would appear to be a truly Edwardian flywheel. A heavy flywheel is a delightful adjunct to fast, easy cruising, but it does give the engine a feeling of sluggishness when gear changing. While it is agreed that a decent sized flywheel is desirable with the Cotal box, I think that something a bit smaller would have answered the purpose. As it is, it is hopeless to wait for the revs. when changing up, and the best technique is to do a straight-through change, just easing the clutch and maintaining a partial throttle opening. This gives a delightfully smooth and rapid change, the magnets absorbing the extra revs, apparently without protest. Changing down an appreciable pause in neutral is necessary, to give time for the revs. to build up. For downward changes the use of the clutch is, of course, unnecessary.
On “Pool” petrol the disadvantages of wide ratios are particularly apparent, and with decent fuel they would not be so obtrusive. My admiration for the Cotal box is unbounded, and I have harped on its shortcoming perhaps rather unduly, because it seems a pity that anything which is already so good should miss perfection by such a narrow margin. Even as things are, Aston-Martins have undoubtedly done the right thing by incorporating a Cotal box in their super-modern motor-car. With proper fuel and a somewhat lighter flywheel there would not be much about which to complain.
The cruising speed of the car is about 65 m.p.h., at which gait it wafts along with effortless silence. This is certainly a good cruising speed, but when the car has a really efficiently streamlined body I think that it will seem inadequate. A somewhat shorter stroke and a higher axle ratio should put the cruising speed comfortably into the 80s, which, after all, is not an unreasonable figure, even on English roads, especially with such superb brakes and road-holding. With a fully streamlined body, 90 b.h.p. should be sufficient to pull the car along at 105-110 m.p.h. without difficulty, and, with the present stroke, this would indicate an axle ratio of about 3.8 to 1, which, in turn, would put the cruising speed up to 78 m.p.h. Probably, however, an even higher axle ratio of, say, 3.5 to 1 would give greater ease of running and longer life, more than justifying the corresponding slight drop in top gear performance, and, perhaps, maximum.
Cruising speed seems rather to have lost its importance of recent years, but with the increased use of streamlining there is little doubt that its importance as a factor of design will shortly return.
With the present body a petrol consumption of 22 m.p.g. is compatible with an average speed of over 45 m.p.h. on an ordinary run, and better streamlining would enable as good a consumption at even higher average speeds. It must be remembered, too, that the reduced tractive resistance of a streamlined car is kind to tyres, though one wonders if the return to higher cruising speeds will not also stage a come-back for larger wheel diameters.
Summing up, it will be seen that I have spoken of the post-war Aston-Martin more in the light of its potentialities than with reference to this isolated experimental machine. Even if it were put on the market practically as it is it would be a very good car, but when the lessons learned from it are incorporated in a perfected production model, then, indeed, we shall have a machine of which British automobile engineering can be proud.