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DOUBLES & FACILITY DESIGN Doubles, and truck. full trailer vehicles do present a peculiar facility design problem in that the vehicles are seldom backed as a whole. Because of this, space must be left at facilities where they will appear, to be used while separating them and for parking one section of the combination while the other is backed into the dock. It should be noted that when both semitrailers of the doubles vehicle have been backed into the dock, space will still be needed to store the converter dolly. Doubles are now used throughout the West, and their use is spreading in the Eastern states. The off track char- acteristics of a 65-foot doubles or a 65-foot truck. full trailer are such that off track is actually less than for a 55.foot tractor.semitrailer. On some special routes doubles 110 feet long are used. Those vehicles do off- track more than 55.foot tractor-semitrailers. DESCRIPTION OF SIZE AND TRACK PLOT PROCEDURE Track Plot Procedure The methods of determining vehicle maneuverability characteristics presented in Appendix I are, of course, only approximations of actual vehicle performance. These methods do not, for instance, allow for wheel slip, and assume there are no irregularities in the roadway which would affect vehicle track. It has also been assumed that the steering wheels are moved from one position to another instantaneously and, hence, that the vehicle enters a turn with the steering wheels turned to their maximum, negotiates the turn with the wheels turned to their maximum, and leaves the turn with the wheels directed straight ahead. In practice, of course, the wheels are turned continually as the vehicle enters, negotiates and leaves the curve. Because of the assumptions made, and error inherent in the incremental advance method used with it, track plots obtained using the procedure described in Appendix I should not be considered to represent the actual path a vehicle will follow. A result obtained using the procedure, therefore, should not be construed to be a standard. Generally. error introduced by the as. sumptions on which it is based results in the final plot being larger than the actual track. A comparison of maximum track obtained using the method presented in Appendix I was made with that determined by (1) SAE J695 (1971 SAE Handbook), and (2) SAE J695a (1973 SAE Handbook). The track obtained for 900 and 2700 turns was compared with that from the Stevens, Tignor and LoJacono method given in SAE paper 650721. The results of those comparisons are shown in the following chart. Off-track For 55 foot Tractor-Semitrailer Shown on Page 26 of APPENDIX I As Calculated By Several Methods Method SAE J695a 1973 SAE Handbook SAE 1695 1971 SAE Handbook APPENDIX I Procedure SAE Paper 650721 Maximum off-track 16.3' 21.9' 23.S' 900 off-track 2700 off-track 16 ' 14.9' 23.2' 21 ' Vehicle size procedure The size determination procedure. as the track plot procedure. will tend to produce a final track plot which is slightly larger than would actually occur. The vehicle size determination procedure presented In Appendix I is almost opposite the procedure a truck operator uses when he determines the length of a vehicle he is going to order. For the procedure presented in Appendix I, weight is given and length is determined. The truck operator wiII usually take vehicle length and from' that determine the weight which can be carried. The reason [or the difference in approach to the problem of vehicle length is basically that an architect trying to find the vehicle requiring the most space to maneuver and an operator developing the size of vehicle he needs for his business have two different problems and have different data available for use in solving their problems. The architect, in solving his problems, knows he wants to find the vehicle taking the most space for maneuvering. The only information he has available to him for solving that problem is state law regulating size and weight. An operator, in setting up the vehicle he is going to operate, is concerned over the space needed for its maneuvering and is also concerned in obtaining a vehicle of proper size (neither larger nor smaller than necessary) to handle the freight he will haul. The opera- tor. in solving his problem, has available to him state laws for size and weight, and information from vehicle 3