WHY DOES A CAR SKID?

Hawera Star, Volume LIII, 10 March 1934, Page 12

WHY DOES A CAR SKID?

HOW IT GAN BE CORRECTED CHECK ON SIDE MOVEMENT. * INTELLIGENT ANTICIPATION REQUIRED. Simple questions are often the most difficult to answer. It is easy to say that a car skids because the tyres have lost adhesion on a slippery surface, but just why skidding takes the various forms with which painful experience has made us all familiar is a far more interesting and complex subject. ...... Why, for example, does a skid Usually commence with a sideways motion of the back wheels, the car behaving as though the rear end were trying to get to the front? Why, again, do the rear wheels lose their grip more readily if the brakes are applied (or, alternatively, if the engine throttle is opened wide) than when they roll freely. - The answers to these questions are rather important because, if fully grasped, they enable a cautious driver to escape a bad skid by intelligent anticipation, says ‘‘The Motor.” A skid cannot commence unless the force tending tyres to slip is greater than the adhesion between the treads and the road. The slip-promoting force mav act sideways—as on a corner when a car tends to fly outwards—or may act in fine with the wheel as when braking or accelerating. It follows that if a big proportion of the available adhesion is being used up in preventing a wheel from slipping or slithering in its own plane, the ‘‘grip” available to overcome any tendency towards sideways movement is materially reduced. On a dry road the margin is very considerable, but on many modern surfaces rain reduces (the cohesion to such an extent that even the engine torque required to maintain a moderate cruising speed may bring the wheels very close to the limit at which spinning commences BRAKES ON WET DAY.

Similarly, the use of the brakes on a wet day, by tending to lock the wheels, “uses up” a great deal_ of the available adhesion, and leaves little to resist sideways movement. For several reasons a front-wheel skid is not nearly so common as a rear-wheel skid. This is a fortuitous circumstance, because it is far more difficult to control a car when the front wheels begin to slide. Usually it is only by fast cornering on a really slippery road that a front-wheel skid can be initiated. It is most unfortunate that there should be-so; great a variety of road surfaces -employed throughout the country, some of'‘which ' are tolerably safe, while others, when wet, are extremely" dangerous. At night it is particularly difficult to distinguish by eye the change from one type of surface to another. • • . The sensitive driver, who holds the wheel firmly and yet in a way which enables him to sense the degree of control, can often “feel” the amount of adhesion which is actually available. He is also in a much better position to correct a skid in the earlc -C' steering gear which works smoothly* and does not require an excessive movement of the wheel is of great assistance. The tyres are also important-, new treads with a sharply defined pattern providing better adhesion than can be obtained with worn covers lor safety in muddy roads chains should alwavs bo employed. In’ conclusion it may he remarked that the differential is often to blame for the initiation of a skicl by allowing one of the rear wheels to spin.

RADIO IN MOTOR=CARS | AN ENGLISH OPINION. I DRIVER’S ATTENTION DISTRACTED. The installation of wireless sets in ] motor-cars and other vehicles is likely to become a big problem for discussion bv motoring organisations m New Zealand in the near future. Already one South Island organisation (the South Canterbury Association) has expressed disfavour of the scheme, oil the ground that it does not make for safety in driving. Hence the following opinion, that of the ‘‘Commercial Motor.an English publication, is of interest: „. , , “While commending the enterprise of operators who have installed wireless receiving equipment in their coaches, wo have always reserved judgment on its safety, from the point of view of concentration on driving. In New York the police have the removal of radio sets from about taxicabs on the ground that they cause ‘a public nuisance’ and divert the attention of drivers. Tt is said that '£loo.ooo have been invested in the installation of wireless apparatus in the taxicabs, and tho facility ha school much appreciated by the public.”

A notable' feature of the English models for this year is the diminishing use of magneto ignition. Although some five firms still cling to it, 39 use single coil,' one- employs double eoil. and four use dual ignition, with both eoil and magneto. It would seem, too. that overhead valves are well established, for they are used by 06 firms, most of' them on all their modelsi' ’ Sixteen other manufacturers offer cars with side-by-side valves. Tn this field, however, there are ears in which overhead inlet and side exhausts are used. One or wo still show the sleeve valve The importance of correct tyre pressures in relation to the steering of a motor-car is not fullv realised by many motorists, and not infrequently the steering mechanism is blamed for any stiffness and “wobble” or “wander.” when uneven or under-inflated front tyres are the real cause. A difierence of three or four pounds in the pressure, of the front tyres will affect the steering.