The Dynamics and Aero-Dynamics of the Front-Wheel-Drive Car

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The driver is conscious or the behaviour of his car when :
1: Driving in a straight line on a level road.
2. Cornering.
3. Climbing a hill.
4. 2 and 3 in combination.
For cases 1 and 2 some degree of understeer is desirable to confer stability.

“To ensure understeer it is desirable to load the front tyres more than the back tyres in order to diminish their cornering power, and this is commonly done by placing the mass of the engine and gearbox well forward in the frame. The transverse engine mounting fulfills this requirement, and the act of putting torque through the carcase of the front tyres also diminishes their cornering power, and thus enhances the under-steer effects obtained by carrying 60% of the weight on the front wheels. Hence the Morris Mini Minor is inherently dynamically stable.

“The transverse engine mounting, coupled with the general configuration of the vehicle, also plays an important part in securing aero-dynamic stability. With increase in road speed the centre of wind pressure moves steadily forward, and if it advances sufficiently ahead of the centre of gravity, a side wind acts through a long lever to prise the car off the straight course.

“On the Morris Mini Minor cars the centre of gravity is naturally far forward and the relatively long, parallel, sides of the body behind the c. of g. act in themselves as a strong stabilising influence. Hence so far as stability on the straight, is concerned a front engine driving the front wheels is superior to any other combination.

“In strong contrast, with a rear engine, driving the rear wheels, the rear tyres at once support the greatest weight and transmit torque, and therefore have the least cornering power, and the c. of g. is far back and the lever effect of the forward centre of air pressure thereby enhanced. The rear-engined car is thus inherently unstable both dynamically and aero-dynamically. The former condition can be ameliorated by low front tyre pressures and a strong anti-roll bar; the latter by fins if they are made large enough, but very few stable rear-engined vehicles have yet been produced, some critics might say none.

“On corners the front-drive car understeers with power applied. Contrariwise, shutting the throttle at once slows the car and reduces the cornering radius, and the natural instinct of the average driver when going too fast is thus turned into a valuable safety measure.

“In the hill-climbing condition weight is transferred from the front to the back of the car, and hence load is taken off the front, and added to the rear wheels. If, however, the front wheels are initially loaded more heavily than the back they continue to carry adequate weight for traction on dry roads up to a gradient of 1 in 3, and thus have an ample margin for all normal motoring in any part of the World.

“On slippery surfaces some of these assessments of merit must be revised. When cornering, or when on a straight road in the presence of external excitations such as side winds or road camber, the exceptional traction ability of the rear-engine car is offset by difficulty in control if slippery surfaces cause the rear wheels to spin and the car to side-slip. In this event the spin must be suppressed by reducing power and, at the same moment, steering control restored by a counter movement. If overdone this results in a slide in a sense opposite to the original and, unless the driver be skilled, to a further series of tail slides which may increase until the vehicle becomes completely out of control. But whereas wheel-spin on a rear-drive car promotes an already inherent disposition to oversteer (which can only be corrected by shutting the throttle), on the front-drive car wheel-spin exaggerates an existing understeer condition which can easily he countered by laying on more steering lock whilst at the same time maintaining traction by keeping the throttle in some degree open. So on slippery roads driving a front-driven car is relatively easy and it may, with no great skill on the part of the driver, ascend hills of sub-critical gradient by rising superior to adverse external forces to which rear-driven cars. and particularly rear-driven cars carrying more weight on the front wheels rather than on the back, all too easily succumb.

“We see then that the rear-engined rear-drive car has outstanding traction ability if the driver is sufficiently skilled to avoid wheel-spin or to take the right action if this occurs. The conventional layout favours cornering at limiting speeds by a driver able to use the throttle to help the steering. In all other circumstances the well-designed front-drive front-engine car shows to advantage to a degree which lessens manifestly the burden of the average individual and thereby adds to the sum of road safety.

“Conventional cars have a front engine which drives the rear wheels through a long propeller shaft and a back axle which is mounted on leaf springs. This arrangement, commonly called the Hotchkiss drive, has considerable elasticity which cushions irregular impulses from the engine or a roughness in take-up through the clutch.

“When the engine and transmission is in one unit, be it at front or back, and drives the adjacent wheels, this elasticity is lost and roughness in the drive is often observed although it can be diminished by the use of very flexible engine mountings. But these are in themselves objectionable as the movement of the engine upon them may be felt as a disagreeable phenomenon by the driver, and for this reason on some cars of this kind a resilient coupling is included in the exposed half-shafts.

“On the Morris Mini Minor the required cushioning effect has been obtained in a most elegant manner by the use of flexible elements in the inner universal joint, and this (patented) scheme has proved so successful that comparatively rigid engine mounting points can be used. The driver and passengers thus receive the full benefits of the very extremely stiff body structure with its reinforcing sub-frames at each end.

“The outer universal joint on any front-wheel-drive car presents a complex and critical problem for it has to;
(a) have large angular displacement;
(b) give constant velocity as between the input and the output shaft irrespective of angular displacement;
(c) be compact;
(d) sustain high loading without rapid wear;
(e) be inexpensive.

“The Birfield joint has been developed from the Rzeppa design to meet these requirements. As the centre-line of the steering is projected through the neutral point of the joint, neither full power nor over-run, nor turning the wheels on the lock, result in any reaction being fed into the steering mechanism.

“The joint fits neatly between the upper and lower wishbones, and as these are of unequal length there is a very slight swing axle effect which gives small gyroscopic reactions which are fed into the steering gear. This is of simple, positive, rack-and-pinion type, but the design in general, and the angle at which the teeth are cut in particular, results in a big difference between the “forward” and “reverse” efficiencies. Steering-wheel shake is thus exorcised without diminishing the sensitivity and feeling of direct control which makes a definitive contribution to safety, as well as being features that are greatly enjoyed by experienced drivers.”

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