THE SOCIETY OF AUTOMOBILE ENGINEERS 5-ton worm-driven truck, as indicated in my paper. These figures have been taken from a very successful and exceptionally well designed heavy American truck. In braking on a down-grade the bulk of the load is on the front axle, and the torsion moment due to braking can never be as great as when the truck is rolling backwards down an incline with the bulk of the load cn the rear axle. This in almost every case constitutes a greater strain on the torque members than could have been brought on by the reaction of the engine, and accounts for their failure in an upward direction. I think Mr. Taylor has also overlooked the fact that there is no compari­ son between the energy stored up in a 5-ton machine with a 50 per cent overload descending a steep incline at 12 miles per hour, when the j ack­ shaft brakes are instantly locked, and the reaction from the energy de­ rived from the acceleration of the driving mechanism. In his remarks in regard to. the accelerometer he admitted a greater retardation from the brakes than acceleration in handling a car, no matter what you do, unless you spin a heavy flywheel and jerk the clutch in. I maintain, as I said before, that it is impossible to put the same tor­ sion moment on the drive-shafts of a 5-ton worm-driven truck with a flywheel of normal dimensions, no matter how you may jerk the clutch in, or from the braking strain wi'th a propeller-shaft brake, when figuring with a 50 per cent overload. In regard to Mr. Taylor's comment as to not being able to lock the rear wheels under these conditions, it is well to note that if you havc a movement of not less than 6 inches on the pedal, you can easily stop a 5-ton truck, with about 150 pounds pressure on the pedal, using a 2O-inch drum on a 4o-inch road wheel, even figuring &:> per cent. on the rear and 50 per cent. overload. A MEMBER.-What kind of brake friction surface? A. M. LAYcocK.-The figures were estimated with a coefficient of friction of .23. MOTOR TRUCK WHEELS By ARTHUR J. SLADE (Member of the Society) This paper is the outcome of the writer's tcmerity in offering a sug­ gestion to the Commercial Car Wheels Division of the Standards Com­ mittee, that, in considering the standardization of metal wheels, along lines similar to the standardization of wood wheels, certain practices, as to material and design, 'should be recommended for the benefit of the motor truck designer. A precedent has been established by the Iron and Steel Division of the Standards Committee, in recommending good engineering practice; as for ~ample: New TextNew TextDownloaded from SAE International by Brought to you by RMIT University, Sunday, October 21, 2018MOTOR TRUCK WHEELS273 "Steel castings foraxles, crankshafts, andsteering spindles areused only atagreat risk." The chairman ofthe Commercial Car Wheels Division, and the Secretary andGeneral Manager oftheSociety, didnot,however, view the proposal with approval, butsuggested apaper forpresentation atthis meeting, with aview tobringing outavailable information astosuitable material anddesigns fortruck wheels, through those members bestquali fiedbyexperience toexpress themselves onthesubject with authority. Itisperhaps natural that wheels built ofwood should have been de veloped toahigh state ofperfection inthiscountry where such alarge andvaried supply ofwood isavailable. The rapid development ofthe West called forvehicles ofrugged andsturdy type forthepioneer's use inopening upacountry without roads ofanysort. Later thetransporta tion companies, operating stage coaches over roads little more than trails, asnew sections were settled, andeventually theagriculturist, who even now often operates hisfarm wagons over roads unworthy thename, de manded thehighest quality ofwheels. The majority ofthemotor trucks built inthis country have been equipped with wooden wheels; theearlier wheels conforming closely to thedesign ofhorse-drawn wagon wheels, e