THE UNSPRUNG WEIGHT ON VEHICLES.
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The Advantages of Reducing the Proportion of Unsprung to Sprung Weight.
THE IMPORTANCE of reducing the weight of the unsprung portion of the chassis on road vehicles is generally appreciated and, on the whole, designs reveal the fact that this requirement
has been carefully studied. .
We do not hint the matter discussed in detail, as it is so generally accepted as one of-the principles of construction, but recent departures from the usual procedure appear to warrant the closer consideration of the question.
In the case of rear axles, as usually constructed and fitted with differential, it is obvious that when the vehicle is travelling at a good speed over an irregular road, there.is a tendency for one;or both.rear wheels to break contact with the ground immediately after passing over an obstacle.
If one wheel should leave the road, it is free to rotate at any speed which the engine may induce, and it is obvious that, as the other wheel is rigidly held by friction with the road surface, and the differential star piece is made to rotate 'through the crown wheel or worm wheel. the .free. road wheel is caused to rotate at an abnormal rate.
This means that when the wheel again comes in contact with the road a. certain amounttof slip must occur until its speed is made to oonform with that of the other road wheel. As,this slip is certainly detrimental to the tyre anePalso to the differential, it is necessary to pay every attention to convenient methods of avoiding such a fault. As the differential is only required when the vehicle is changing its direction of travel, and at such times the speed is not generally suffieient5to At the same time it'is necessary, so far as possible, to reduce the tendency of the road wheels to leave the ground, and the controlling factor in establishingthis tendency is the weight of the unsprung parts of the chassis. ' At _first examination it would appear that, with a heavy axle, there should be a more secure hold on the road than in the case of a light axle_ and, if this were the case, the advantageeof arranging the gearbox as a part of the axle would be most marked, but further consideration shows•this to be an error. When the rear wheels pass over an Uneven part of the road an upward:impulse is given to the wheels on all the unsprung weight supported by those wheels. It is evident that the flexibility of the springs-will, in a great measure, prevent that impulse from being imparted to the sprung weight which is supported by the rear wheels. The consequence is that the impurse is more with great unsprung weight than with small unsprung weight, and, as it is•the sprung weight over the read wheels.which serves to neutralize to some extent this upward impulse, the advantages of reducing the unsprung weight tota Trninimum are obvious. 'Thus the total weight on the rear wheels in proportion to the unsprung weight is the controlling factor, and a light:axle on a vehicle of a given weight Will hold the road better than a heavy one on the same vehicle. The advantages of minimum unsprung weight are also evident in the smooth working of the vehicle and the consequent safety of the goods being transported. With a light axle, the upward impulse is less and, as this impulse is transmitted to the,chassis frame and body through the springs, the vibration on the latter parts is not so serious as with a,oheavy axle. Considerations of weight:distribution have, in some cases, induced manufacturers to place -the gearbox along with the rear axle ; this method undoubtedly places more weight over the road wheels, and for moderate running on good roads advantages are in some cases gained, as can be observed if we consider the extreme case of a very light two-seater ear in which the gearbox is placed right up to the engine. There is no doubt that the question of unsprung weight is one which calls for serious consideration both by designers and purchasers of road vehicles. HOWARD ENSAW.Tags