Truck Weights by Loadometer To have full data regarding overloads on highways, some method must be adopted of weighing trucks anywhere on the highways. Two weighing jacks known as "Loadometers," manufactured by Black & Decker, Baltimore, Maryland, were procured. One of the principal items of interest in the investigation is the proportion of load thrown on the rear axle by different loadings. It was found that the average rear axle loading is 75.3% of the gross load. This figure applies to all capacities of trucks. In the five-ton class, the minimum and maximum percentage of weight on the rear axle was found to be 69% and 87%, respectively, of the total gross load, the lighter relative loading on the rear wheel being pig iron and hay the heavier. One 6-ton truck was weighed which had a total load of 44,292 pounds, 80% of which was on the rear wheels or a weight per real wheel of 17,916 pounds. 10,000 lbs. maximum load on any wheel was used by the Committee on Slab Design in computing stresses in slab as shown by their accompanying report. Description of Loadometer The Loadometer used is in the form of a screw jack, the nut portion of the jack being set in an 8-inch cylinder filled with oil, this cylinder forming the base of the jack. As the jack is screwed out and takes the load, the pressure is transmitted through the oil into an ordinary hydraulic pressure gauge. This gauge is graduated to 20,000 pounds. The jack stems have a travel of four inches and with two different sized heads give them a wide enough range so as to be put under and lift any truck found. The jacks weigh 45 pounds each. Description of Their Use The Loadometers were taken to the scales at the Los Angeles Ice & Cold Storage Plant at 4th Street and Central Avenue, Los Angeles, and tested for accuracy. Both Loadometers were correct to within less than one per cent. Of three trucks on which Loadometers were tried out the results were as follows: Loadometer weights were 11,210 lbs., 15,780 lbs. and 14,540 lbs. Scale weights for same were 11,250 lbs., 15,840 lbs. and 14,660 lbs. The conditions for weighing at the scales were, of course, very good-much better than could be found on the road. The work was started on San Fernando Road in the vicinity of Newhall Tunnel. Permission was secured from the Board of Supervisors of Los Angeles County for a regular traffic officer to be present with the men at all times to stop trucks and order the driver to submit to the weighing. It was found that this officer could not be dispensed with as truck drivers in general resented being stopped, especially if they were conscious that they carried an overload. The truck that was to be weighed was first trailed for a short distance, say one quarter of a mile, to obtain the speed at which it was traveling. If there was any variance in speed on curves or up or down a grade this fact was noted. The speed meter of the machine used on the work (a Nash Chummy Roadster) was checked up and found to be correct before work was started. The machine then ran around the truck, going ahead until a comparatively level spot in the road was located. At this point the truck was stopped and the actual weighing took place. One jack was put under the rear axle close to one wheel and the other under the front axle close to the wheel on the same side of the truck as the rear jack. Both jacks were raised at the same time and their readings recorded. The distance was measured from the center of the tire on the opposite side of the truck to the point at which the jack was applied and also the entire distance between the centers of the two tires for each axle separately. Let S equal the total distance from center to center of tires; s equal the distance from the center of the tire opposite the point at which the jack was applied; L equal the jack reading; W equal the total weight. From these quantities and the use of the formula W-2 Ls the loads carried on the S axle were determined. Each axle is thus separately weighed. The sum of the two results gave the total gross weight of the truck. In order that the results from the above method be absolutely correct, the road must be level transversely; the load must ride evenly between the wheels; and it must be a load that will not shift when one side of the truck is raised as with high loads. The results obtained, however, are reasonably correct. In no case were they farther than 4% from correct, the average error being 2.50% for the trucks checked on standard scales. The method and results previously described were used because the work had to be done with reasonable accuracy in the shortest possible time. Greater accuracy could be obtained by using the two jacks under the rear axle and determining the weight thereon, the load being very slightly lifted from the pavement. After weighing one end in this manner, both the jacks could then be moved to the front axle which could be weighed in a similar manner. A still more perfect method of weighing loads with loadometers, and one which would entirely remove any feeling of uncertainty on the part of the truck operator or the court, would be to use four jacks, putting one under each wheel and completely lifting the load off the pavement. Under the method first described it took about twenty minutes to weigh a truck. Other Field Data Taken Relative to Trucks 66 The number of inches of the tires in contact with the road as well as the manufacturer's rating thereof in inches was noted for comparison with the Vehicle Act which fixes a maximum of 800 lbs. per 1 inch of tire in contact with the road." The condition of the tires was observed. A tire presenting a smooth surface to the road and not too thin to have lost its resiliency was graded good; a tire presenting a rough surface to the road but still retaining its resiliency was called fair; one which was worn down so thin that it had lost all resiliency or which was so cut and torn as to be of insufficient width to support its load was called poor. The character of the load was recorded as well as the type of the body used. The kind of drive, whether chain or gear, was noted. The distance was measured from the center of gravity of the load to both the front and the rear axle. A load of oil-well casing is distributed on its wheel base in a manner differing from a load of pig iron on a similar truck. The license number and name of owner of the truck were taken and measurements were made of the spread of the tire under its load by placing pieces of clean paper on the pavement. The truck was then driven over them. The width of the track was then compared with that of the tire not in contact. The results varied widely, depending on the make and quality of the rubber. The greatest spread encountered on any old tire was 3 inches for a 7-inch tire under a large load. The average spread on all old solid tires is about 1/4 inch. One hundred ninety-two trucks were weighed and the other data obtained regarding them. These are representative of all classes of loaded trucks traveling on the State and County roads. Tables No. 16 and No. 17, pages 60-62, are self explanatory and are in fair agreement with the information compiled from data obtained by weighings on the public scales. These records are not included in the previous tables. TABLE NO. 16 OVERLOADS CARRIED BY TRUCKS WEIGHED ON THE ROAD WITH LOADOMETERS Comparison With Legal Loading |