Some cars dispense with the torsion rod, by incasing the shaft in a torsion tube. Such a form of construction is shown in Fig. 36. _The torque tube_ A, as it is called, is rigidly secured to the housing B, of the rear axle, the forward end being pivoted to a cross piece C of the frame. [Illustration: Fig. 36. Torque Tube.] The radius rods D D, have their forward ends attached to a sleeve E, located near the forward end of the torque tube A, and the rear ends are secured to the axle housing F at the spring seats. Some manufacturers avoid the use of these radius rods by such a construction in the springs as will prevent any forward and rearward movement of the axle. CHAIN DRIVE.--The chain drive machines require the radius rods, or some other means to counteract the movement of the axle when it meets an obstruction, particularly where the chain transmits the power to the differential on the wheel shaft. JACKSHAFT.--With the double chain drive no differential is used on the axle, but, instead thereof, it is placed on the jackshaft which carries the small driving sprocket wheels. The chain transmits the power direct to each wheel, and a radius rod is necessary to hold the shaft of the drive sprocket wheel the proper distance from the rear axle. [Illustration: Fig. 37. Chain Drive.] Such an arrangement is shown in Fig. 37, in which the drive, or jackshaft A is mounted transversely across the vertically-movable frame B, and the torque bar C, therefore, serves as the means for keeping the jackshaft and the axle D the proper distance apart, and it is also arranged to serve as a radius rod to prevent any undue tension on the chain when a wheel strikes an obstruction. The wheels of such a truck turn freely on the axle stubs of a dead axle. OBJECTIONS TO CHAINS.--Few pleasure cars are now equipped in this manner, as the shaft drive is more desirable for several reasons: The use of chains is always objectionable, as the efficiency decreases with wear quicker than the shaft drive, and requires the jackshaft, sprocket chains and sprocket wheels, besides the noise and excessive wear, by stretching of chains, which are always inherent in the use of chains. It is impossible to prevent dirt, sand and grit from adhering to the chains, unless they are inclosed, a thing which is difficult and expensive. If they are not so protected the lubricant only serves to catch the grit and retain it, so that when it is carried around by the chain, the wheel and chain are both worn out. Another difficulty in the use of chains is due to the inability to keep them at a proper tension at all times. All chains will stretch in use, consequently the tension will change, and when wear takes place, the distance of the centers of driving and driven sprockets has to be adjusted, calling again for another mechanical complication.