Miscellaneous aspects of design
Edward Batten, before the war responsible for the Batten-Special Ford V 8 and some “one-off” hybrids, discusses engine and chassis design in general.
Now that we can look toward the future with more confidence, it is pleasant to anticipate the comforts and interests of normal life.
For many of us the possession and use of a good car takes a high place in our scheme of enjoyment. If we use this time of anticipation to obtain a clear picture of the car that we desire and the qualities which make it desirable there will be no regrets when it is bought.
First we must decide on the size of our car, and if competition in trials or speed events is contemplated it should be as near a class limit as possible. The simplest way to ensure that its performance will be good is to see that its power-to-weight ratio is satisfactory. This information may be obtained from the maker’s catalogue, but if this is not available weigh the car, and if the figure obtained approaches closely to one hundredweight per 100 c.c. of engine capacity, a lively performance can be anticipated.
The engine should now be considered, and if of normal design and below 1,500 c.c. a 4-cylinder is best. Larger engines should have six cylinders up to 3,000 c.c. and eight or more above this size. This increase in the number of cylinders is necessary to keep down the weight of the reciprocating parts on which the working strain increases by the square of any addition to their weight at the same revolutions per minute.
Stroke/bore ratio is the next important point. For an overhead-valve engine this should be about 69mm. bore by 100 mm. stroke, though I have known engines with a stroke as short as 82 mm. and a bore of 73 mm. to give good results. Always select an engine with the shortest stroke available. Its lower piston speed will reduce wear and high r.p.m. are more easily attained, also short stroke engines can safely be made lighter.
Compression ratio is our next concern and it should be as high as possible, because power increases directly with an increase of compression ratio. With ordinary fuel a ratio of six-to-one is about the maximum, although with cylinder heads of aluminium alloy or other highly conductive alloys higher ratios are possible. Nine-and-a-half to one is the highest that I have used for road work. This was satisfactory with special fuel and gave great economy and power. The snag was that if one ran out of fuel only pure benzole would bring the car home and benzole was not easily obtainable, although the 1 1/2-litre engine I have in mind actually achieved 45 to 47 m.p.g.
A limiting factor in power development of a well-designed engine is the point where pre-ignition occurs. Assuming that fuel and sparking plugs are suitable, this is caused by the temperature of the exhaust valve rising to a degree sufficient to fire the compressing charge. As practically all the cooling of a valve is through its contact with the valve seat, the importance of using a highly conductive alloy for o.h.v. cylinder head construction can be realised.
Engine cooling should be by pump, drawing water from the radiator and delivering it to head and cylinders – not by attempting to draw water from the cylinder head where, when full throttle is used for a lengthy period, steam is likely to be generated and this condition aggravated by reduction of pressure due to the pump’s action.
The best place for the timing gear is at the rear of the engine where it is least affected by torsional vibration periods of the crankshaft. In my experience magneto ignition is best for sports or racing cars. Battery ignition is often the limiting factor in speed. With magnetos the rise of voltage with increase of revolutions enables them to be effective at a higher speed. Fairly good results can be obtained with double-contact-breaker battery ignition, although the system where a spark occurs with the motion of each contact-breaker alternately makes an engine rough if not maintained in accurate adjustment. The Ford system on the V 8 obviates this difficulty in a most intelligent way. The motion of one contact-breaker causes the spark and the other prolongs the saturation period of the coil, producing long periods of contact and short breaks.
Carburation cannot be discussed adequately in an article such as this, and on new cars designed for competition work is usually satisfactory. If an electric pump is used a second one should be fitted, with a separate switch to permit them to be tested independently.
Engines of six or more cylinders need dual or multiple carburetters to enable them to develop their full power, but three carburetters should be avoided on a six as the suction intervals of adjacent pairs of cylinders are different, and on the pairs with the short and long interval the cylinder following the short interval receives an over-rich mixture. The problem of unequal gas distribution can be overcome by filling a carburetter to each cylinder. This appears to give maximum power but at the sacrifice of docility, owing to the difficulty of maintaining synchronisation of the throttle openings of so many instruments. Another problem arises with slide-type throttles. If a balance pipe is provided to improve slow running, the constant suction then applied to the slides is so great at small throttle openings that it is difficult to open them against the friction of their contact with the carburetter bodies. When the throttles are opened this friction collapses cumulatively, causing a jerk and much wheelspin.
Clutches are usually well up to their duties and nearly all of them emanate from the same manufacturer. If tuning does produce clutch slip, larger plates and linings should be fitted, not stronger clutch springs.
Gearboxes can be either synchromesh or self-change; if of the self-change type make certain by enquiry that the gearbox is large enough for the special demands that you will make upon it. In the past some manufacturers have harmed the reputation of a very excellent product by fitting a box that was too small.
An enclosed propeller shaft is desirable for a sports or racing car with orthodox rear suspension, as the torque tube relieves the springs of the reactions to starting, driving, and braking. The shaft may then be supported by bearings within the tube and thus supported can be light and flexible enough to make torsional vibration unobtrusive. With open propeller shafts of large diameter this vibration is very noticeable when big axle movements occur, it being due, one presumes, to the shaft not rotating at a constant velocity when the ends are not in the same plane.
Most rear axles are satisfactory, but for competition work the bevel pinion should be supported by a roller bearing at the inner end. Straight-cut axle gears are sometimes used on sports cars instead of the more usual spiral or hypoid types. I have never owned a car with a silent axle of this kind, so, in spite of their greater efficiency, I am prejudiced against them.
Suspension is the only feature of a car of which you are constantly aware. It should provide comfort and good roadholding – not one at the expense of the other. There are many systems from which to choose and little help in this problem can be obtained from theory. The system that I prefer, in practice, appears to have two faults. First, varying track width, and, second, the end of the drag link describes an arc in relation to its attachment to the drop arm. The layout that is perfect on the drawing board cannot in practice be made rigid enough for a sports car. Stiffness of the frame is also a factor in good suspension, and I feel that the time has passed when we could be satisfied with something like an iron bedstead. The frame, if we are to have one, should be closely identified with the shape of the car. Braking is adversely affected by frame weakness and tests should be conducted (with circumspection) from speeds of over seventy m.p.h. Some cars develop incurable tramp when braking hard from high speed.
Independent rear suspension is not applied to standard cars in this country, though a few foreign production models are so designed. Bad roads on the Continent have probably forced this development. On the heavier cars one notices that comfort is attained at the expense of tyre wear. Tyre wear on the lighter cars does not appear abnormal.
Now we come to external appearance, and I maintain that the vehicle which is best for its work looks best. This standpoint implies streamlining. A few people dislike streamlining of sports cars, but the benefits in economy and speed are so great that this attitude is foolish. Cleaning up the front into a smooth whole, including lamps, wings and radiator, will take the car from 95 to well over 100 m.p.h.
The qualities that make a good car are, we now realise, first, lightness, high power, and good streamlining, and, second, that happy blending of minor excellences which is the mark of the master designer. There are very few cars in which one feels godlike in one’s command of power and speed, and scarcely conscious of the mechanism that responds so readily to one’s lightest touch. On these dry, sunny days we think of them, open the garage doors and wheel out – the bicycle. When peace returns the demand for such productions should be unmistakable.