Design for acceleration
By J. Lowrey, B.Sc.
Once upon a time, before the last war, someone or other built the world’s first sports car. I don’t propose to join in the argument as to just who was responsible for it, but all the early sports cars had Performance, which justified the use of a capital letter. They may have been brutes to handle, but in speed and acceleration they were well ahead of production cars of the period.
Thirty years has changed the sports car quite a lot—for the better in most respects, hut not in all. For it is certainly a fact that most modern sports cars are little faster than the brisker touring models, their attraction lying less in what they do than in the way that they do it.
Acceleration is supposed to be the subject of this article, and in this, as in other factors of performance, the gap between popular sports and touring cars has tended to close: Sports cars have adopted well-behaved engines and are equipped with “all modern conveniences”, while touring cars have pepped-up their engines and, by wheelbase abbreviation and spineless construction, have reduced their weight quite a lot.
There still remains, however, a certain number of true sports cars, born of racing experience, and capable of real performance. These cars can still show a clean pair of heels to such fast tourers as V8s and Super Snipes, giving the owners something to think about, and it is to be hoped that after the war there will still be a reasonable supply of fast sports cars.
The fundamental source of good acceleration is, of course, a high power-to-weight ratio. A vital accessory to the fact is sufficient wheel adhesion to enable the power to be employed effectively.
As always in engineering, however, the problem has its subtleties and complications, so that a good power-to-weight ratio does not always provide the acceleration it should.
The power unit, for example, is important, to the extent that something more than good power at peak r.p.m. is needed. An inflexible and unresponsive engine may provide a very high maximum speed when correctly geared, but it will not provide such good acceleration as a lower-powered but better-behaved engine.
Just how much flexibility is required of an engine depends to a considerable extent on the transmission used. If an infinitely variable gear was used, with a constant speed governor to maintain the engine at peak speed, flexibility wouldn’t count at all. But most sports cars, having not more than four gears and with high ratios to provide lively acceleration for overtaking medium-fast traffic, need a decent power output at low engine speed to get away from rest at all quickly.
This question of flexibility is often shown up on the road, when a generously powered touring car may show better initial acceleration than either a sports car or a sports motor-cycle, though over any distance the latter are the most accelerative things on wheels.
A suitable transmission system is essential to the best acceleration with any normal power unit. The main requirements, apart from freedom from serious friction losses, are an adequate number of gear ratios, suitably spaced, and quick changes between ratios. The Austin-Hayes gear has proved that an infinitely variable (between limits) gear is capable of working without trouble, and such gears were used on G.W.K. cars and Zenith, Budge, and Ner-a-Car motorcycles, but they have not been seen on any modern sports car.
The ordinary sliding-gear or sliding dog-clutch gearbox is light, and is popular with a few drivers simply because it is a little difficult to use properly. But upward changes must be brutal if the utmost acceleration is needed, unless the aid of friction is sought. The latter aid to rapid gear changing may be used either in the form of a clutch stop or between the cones of a synchromesh device.
An epicyclic gearbox is sometimes used, gears being selected by the operation of friction clutches and brakes. Such gearboxes can provide really quick gear changes, but they are apt to be heavy, and the fierce changes possible may lead to an abnormally strong (and heavy) rear axle being needed. The various friction surfaces may be brought into action mechanically (Wilson), hydraulically (de Normanville), or electrically (Cotal) to taste, but performance is much the same with all methods of actuation. A minor trouble with epieyclic gears is that close ratios are hard to obtain, but the rapidity of the gearchange usually compensates for this.
Getting the power delivered at the gearbox tailshaft converted to useful thrust on the car depends on adequate load on the driving wheels and a high coefficient of friction between tyres and road. Hitherto, most really accelerative vehicles have depended on rear-wheel drive only, but the present war has resulted in four-wheel drive becoming popular again, and I think it will be used on some future racing and sports cars.
If four-wheel drive is used, weight distribution need not greatly affect acceleration, but with two-wheel drive it is essential to concentrate enough weight on the driving axle. With small cars wheelspin may only be a serious problem for a few yards after starting from rest, but with larger cars the trouble is more serious, modern G.P. cars being liable to spin their wheels at speeds of over 150 m.p.h., and such cars as “Bluebird” at speeds of the 300 m.p.h. order.
The amount of weight that can be concentrated at the back of a car is usually limited mainly by the need to maintain good handling qualities. A motor-cycle gains adhesion, since the rider can slip back from the saddle to the rear mudguard pad, and also since, with a short wheelbase and high centre of gravity, load is transferred from front to rear wheel during acceleration; in fact, with sprint machines it is only too easy to lift the front wheel off the ground when starting from rest. A short and high car, such as the old type E.R.A., must gain appreciably in acceleration from this transfer of load from front to rear wheels.
One component of a car which can reduce acceleration, particularly on poor surfaces is the differential. It is desirable for good cornering, unless a very narrow rear track is used, but not for good acceleration. It can be done without, as on a Frazer-Nash, or locked, as on some trials cars. A modern development is the self-locking differential.
Popular in racing is the Z.F. differential, which locks up as soon as the difference in speed between the two wheels exceeds about 15 per cent., this device being used on modern G.P. cars and on some E.R.A.s. A progressive device, which might affect cornering less seriously, has been patented by L. M. Ballamy, but has not been used in racing, I think. There is, of course, the de Lavaud system of fitting a separate free-wheel in the drive to each rear wheel, but free-wheels are not popular for high-speed motoring.
The type of rear springing used affects acceleration for two reasons. One is the obvious fact that a wheel which is bouncing on a poor surface will inevitably spin, the other is the fact that with a normal rear axle, torque reaction tends to reduce the load on the off-side wheel. Prevention of wheel bounce depends on proper spring and shock absorber design and on a high ratio of sprung to unsprung weight. Independent rear-wheel springing, or a de Dion-type rear axle, thus helps, by letting the bevel gears and even the brakes be sprung. It also prevents torque reaction upsetting the even loading of the two wheels. The latter advantage, however, is perhaps less important if a self-locking differential is used. It should also be remembered that most of the newer rear springing systems increase total weight, and so reduce acceleration.
I have said little about weight reduction, except that it is desirable. How it may best be achieved on any particular car is outside the scope of this article, but most British sports cars could be improved considerably in this direction.
A final factor in acceleration is streamlining. At high speeds, large amounts of power are used in stirring up the air, and streamlining can improve acceleration considerably. There is the point, however, that if streamlining raises the maximum speed it will be necessary to raise the gear ratios, and acceleration from rest may suffer accordingly. In the case of a sports car, some sacrifice of low speed performance in the interests of better characteristics on the open road is probably justified. As an alternative, a five-speed gearbox might provide for the increased range of speeds.
There is no reason whatever why our post-war sports cars should not have really good acceleration, and acceleration which can be used whenever it is wanted, without noise attracting the unfavourable attention of passers-by. It is presumably too much to hope that years of listening to tanks and aircraft will have made the great British public appreciative of noisy gears and crisp exhaust notes! The Bentley and Lagonda folk seem to have the right idea, in producing tractable and quiet cars capable of good acceleration at low speeds or high. Let us hope that other sports cars will acquire some of the desirable characteristics of these two types of “express carriage”.