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the weight with minimum side sway, serve to hold the front axle steady, and while it does not permit much motion up and down it still possesses sufficient flexibility so the mechanism is not jarred enough to harm it.

Q. Describe a compound spring and state its advantages.
A. A compound spring is a form in which various members are

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Fig. 196.—Rear End of Heavy Motor Truck Frame Showing Substan

tial Platform Spring Suspension.

joined together so they may be brought in action progressively as the loads increase and as greater spring strength is desired. The construction of the rear end of a typical three-quarter ellipticspring of the compound form is shown at Fig. 197. Under light loads the more flexible member of the lower spring, which is the longest, and the shortest member of the upper spring are utilized and when the loads increase the other members are brought into service progressively. A compound spring is said to produce as easy riding when a car is not fully loaded as when it is filled with its full complement of passengers. With the ordinary spring suspension designed to carry maximum weight stiff action may result if the car is not loaded to capacity. With a compound spring only certain portions of it

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Fig. 196A.-Franklin Wood Frame Supported by Full Elliptic Springs

at Front and Rear. are brought in action when the loads are light, the entire spring being used only under conditions where maximum strength is needed.

Q. How is spring strength obtained?

A. The strength of leaf springs is determined by the width and ühickness of the laminæ, the number used and the material of which they are composed.

Q. How is spring flexibility secured?

A. A spring is made flexible by using long and thin plates and by employing as small a number as possible.

Fig. 197.—Compound Spring Structure.

Q. How are springs attached to axles?

A. The method of securing semi-elliptic springs to front axles is clearly shown at Fig. 198. They are held by means of U clips which straddle the spring at its center and which bolt it firmly to spring pads formed integral with or supported by the axle. On rear axles the springs are secured to special fittings movable on the axle housing tube in the same manner

Q. Why are movable spring blocks needed at the rear axle?

A. It is desirable to mount the rear spr ngs on spring blocks which are not rigidly secured to the axle, though kept from end movement by collars, because it is considered good practice to relieve the springs of all driving and braking torque stresses. When

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Fig. 198.—View of Front Axle Showing Method of Attaching Semi

Elliptic Spring.

the brakes are employed the tendency of car movement is to turn the axle around in the direction in which the rear wheels are moving. It is desirable to resist this tendency to turn by means of a special restraining member instead of depending upon the springs to do so. Obviously if the spring blocks were securely fastened to the axle housing tube, the tendency of axle rotation would be resisted by the springs which would have to be of heavier construction than if depended only to support the car. Q. What is driving torque?

Driving torque is similar to braking torque and is a tendency the axle has to turn owing to ground resistance to rotation of the driving wheels.

Q. What means are provided to resist these stresses?

A. The rear axle is usually restrained from movement by a special torque tube or rod which runs from the axle to which it is rigidly secured to a suitable restraining member carried by one of the frame cross members. The torque stresses may be resisted by a torque tube as shown at Fig. 199. This member acts as a housing for the drive shaft as well as a torque tube, and owing to its length the turning force present at the differential case will be reduced ap

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Fig. 199.—Rear Axle Assembly Showing Torque Tube and Brace Rod.

preciably at the upper end. In some driving systems the drive shaft is provided with two universal joints and is not protected by a tube or housing. Axles of the type illustrated at Fig. 200 utilize a separate torque rod which runs parallel to the driving shaft and having its center line on the same plane as that of the drive member. This torque rod is securely attached to the differential housing at the rear end and to a special carrier member adapted to be supported from one of the frame cross braces at its front end.

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Fig. 200.—Rear Axle Showing Application of Torque Rod and Radius

Members to Take Braking and Driving Torque.

Q. What are functions of radius rods?

A. Radius rods or distance members are employed to keep the axle always at the same distance from the frame side, and to act as bracing members to prevent the tendency of one end of the axle to advance more than the other under certain conditions where the driving wheels encounter varying resistances. The brace rods may be of the form shown at Fig. 199 where they are attached to the front end of the torque tube or they may be separate members as shown at Fig. 200 adapted for attachment to the frame at the front end and to the axle at the back.

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