OF GREAT IMPORTANCE

Sun (Auckland), Volume 1, Issue 64, 7 June 1927, Page 11

OF GREAT IMPORTANCE

TYRES AND MODERN STRESS UNAPPRECIATED VIOLENCE One of the most illuminating methods of studying automobile progress is provided by an examination of the varying complaints which follow changes in design. The modern motor-car, although differing only in detail construction from those manufactured 20 years ago, provides both driver and passenger with an entirely different type of general service. The ever-increasing demand for improvements in comfort and speed has made the modern car efficient, but it is efficient in a totally different manner to the most reliable cars of 1914. MISUNDERSTOOD, BUT IMPORTANT Probably the most misunderstood, and at the same time one of the most important, links in the chain of reliability is the pneumatic tyre. Very few motorists have the slightest conception of the conditions under which a tyre operates, and the circumstances most liable to produce wear. Tyre manufacture has reached a stage of extraordinary perfection. The chemical and physical investigation of the properties of rubber, together with an unending series of experiments, has improved tyres out of all recognition. It is a fact, however, that motorists will be heard to remark that “the modern tyre does not always equal in milage the older type,” particularly referring to those which are within the. high-pressure classification. SUBJECTED TO VIOLENCE Now the old-fashioned car, which was incapable of rapid acceleration or deceleration, subjected the tyre to a far nearer approach to pure rolling than can ever apply to the modern vehicle, which usually weighs under 20cwt., has powers of violent acceleration, and can maintain an average of 30 m.p.h. over good or bad roads, with an engine of 1,500 c.c. It has recently been established that, Under rolling contact alone, tyres are free from practically all signs of abrasion and wear; they will last up to 100,000 miles, and show but little effect through flexion, even when artificial road inequalities are included in the test. DRIVERS' RESPONSIBILITY It is the conditions of use that affect the life of a tyre, and of these many conditions a certain number are definitely within control of the driver. When a car is used all the year round the tyres are subjected to strains of

varying nature, but few drivers trouble to consider that tyre stresses to-day are quite different from those encountered by tyres of three, or even two, Quite apart from any increased vibration resulting from the general adoption of high speed engines, road speeds have enormously increased, and it is well known that this is a prime cause of tyre wear. With an increase of touring speeds it cannot be said that springing has improved proportionately. A bouncing contact between road and tyre gives every opportunity for tyre wear, allows slip to take place, and tears particles from the rubber surface as the car progresses. The modern motorist expects far more than mere reliability from his tyres. He demands a vastly increased degree of comfort, which has naturally led to the re-designing of tyres to suit various makes of cars and different conditions of loading or spring action. REDESIGNING NECESSARY Until quite recently motor manufacturers regarded the tyre as an afterthought: it would have appeared extraordinary to them if they had been requested to vary any part of the design or wheel loading to suit the tyres alone. Yet this is a perfectly logical attitude. The low-pressure tyre is certainly more sensitive to alterations to “toein,” speed, violent braking and springing. Not one motorist in ten will trouble to realise that a small alteration in pressure has little effect upon a high-pressuer tyre, but may cause a low-pressure tyre, with its increased area of contact, to work under totally different conditions to those for which it was designed. If motor users required tyres for speed alone or for comfort, each of these conditions would be easy to satisfy. The modern car must be both comfortable and speedy at the same time. The touring conditions of to-day are strenuous, and the damage which mechanical or driving faults can cause is cumulative. A wheel which is liable to wobble does not necessarily render a car unsafe, but it implies that a straight track is never maintained at speeds far below those liable to produce any danger or peculiarity of actual steering. A tyre with a quarter inch error in the amount of “toe-in” provided is pushed 16 miles sideways for every 1,000 miles of its normal forward travel.

The nuts and threads on the rim lug bolts should be oiled every time a tyre is changed so that they will tighten up readily. If they become too dry it may be thought that the nuts are too tight when they are not, with the result that the driving lug, which holds the rim in position, may pull out, allowing the rim to shift and cut off the valve stem.

Sparking plugs of the detachable mica-insulated variety should never bo cleaned by scraping the inner portion of the mica with any sharp instrument, because the scratches which will be left on the surface of. the mica around the central electrode allow carbon to form readily, and, possibly, to set up a short-circuit. The correct procedure is to wipe off all loose soot with a rag moistened with petrol and then to polish the mica with ordinary liquid metal polish; thehigher the sheen obtained the greater will be the carbonresisting powers of the insulation surface.