THOSE JELLY JUGGLES

NZ Truth, Issue 1174, 31 May 1928, Page 17

THOSE JELLY JUGGLES

When the Car Gets the Shudders QNE of the things which haunt the dreams of the designer is engine vibration.

ALTHOUGH this bugbear has been jt\ largely beaten into the background, even now the trouble is one which is extremely difficult to overcome. Vibration m any moving machinery is an insidious foe which lurks m the background and makes its presence felt sometimes at most unexpected movements. Vibration is extremely difficult to reduce to mere facts and figures, and it is only by constant experiment that this enemy of smooth working can be tracked down and destroyed. One peculiarity of vibration which makes it extremely difficult to conquer is that any -particular- throbbing may not arise from one cau&, but by a combination of several smaller shakes which at certain periods get into step and make the best of engines behave almost like stone-crushing machines. Seeing that vibration m the engine cannot be totally eliminated, designers have come to the conclusion that tjie next best thing is to isolate the other parts of the chassis and the body. To this end the modern engine is mounted usually either on springs or on rubber cushions. Each maker has his own ideas on the subject, and there are many and widely varying systems; but reduced to tin-tacks they all come down to the same thing. /■':■ One interesting method is that used oil the Chrysler, where the front bear* ing of the engine on the four-cylinder car is carried on a transverse laminated spring. : On the larger Chryslers the brackets holding the engine are insulated from the frame by sheets of live rubber, which serve to damp out vibration before they reach the frame, and m turn the body. The Rolls-Royce people carry the system still further by adding friction type shock absorbers on each side of the front end of the engine. The vibration arising from a four-

cylinder, engine is different from that of a six-cylinder power unit. In the four-cylinder engine the quivering consists mostly of an easily detectable up-and-down movement, whereas m the six-cylinder engine the up-and-down shaking is not so noticeable, but it is complicated by a secondary vibration caused by tortional stress m the crankshaft, which leads to a whipping movement. In forcing a six-cylinder car over the last few yards of a fairly steep slope, when the rate of speed has fallen to about 15 miles an hour, this peculiar shuddering whip can be felt, unless the insulating system is exceptionally good. Another cause of vibration, and a very frequent one, which cannot be provided against. by the makers, is that arising from unequal explosive forces m the cylinder. This might, -of course, be due to any one of a number of causes, such as a leaking valve, faulty plug, bad compression, or bad distribution of gas due to fouling of the valve ports. «■ The 1928 cars now arriving are much better protected against vibration than any of the previous models, and it is quite easy to see that makers have devoted a very great amount of thought to this difficult subject. The compromise between a rigidly mounted engine and one which rolls about on its mountings has m most cases been admirably arranged, and \he 1928 cars, particularly the closed models, should be found to be considerably improved compared with those of last year.