MadSci Network: Engineering |
David, This was a perplexing question for me as well, so I decided I needed expert help of my own. I was able to talk with Steve, the head engineer at Pioneer Trucking in Salem, OR, one of the leading manufactures of dump truck bodies and pup trailers. He stated simply that the drawbar length is a factor of state and federal regulations on weight per axle and total length between axles for bridge safety. Bridges are built (or repaired) to a certain standard and this is used to dictate how much weight a bridge can safely support. If the load is distributed over a greater length of the bridge span, it will be able to support more weight. I was able to locate the following equation from the Federal Highway Administration's websight: Bridge Gross Weight Formula. The standard specifying the relationship between axle (or groups of axles) spacing and the gross weight that (those) axle(s) may carry expressed by the formula: W=500((LN/N-1)+12N+36) where W=overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds, L=distance in feet between the extreme of any group of two or more consecutive axles, and N=number of axles in the group under consideration. Steve said that some of the dump combinations are carrying upwards of 100,000 lbs. loads, which make them Longer Combination Vehicles allowing them to carry greater loads than the common maximum of 80,000 lbs. So say we are carrying 100k lbs. and have 7 total axles (3 on the truck and 4 on the pup trailer). Now solving for L (and assuming I interpreted this correctly) we get a length of 68.5'. I believe this will be the required length from the front axle to the rear axle, but I may be wrong in that assumption, so don't quote me on it. However, it does give you an idea of how engineers solve for the length needed in the drawbar. There are also other factors to consider such as weight limits per axle and per tandem axle and probably some others that I don't know about. There is a lot of information on the page that I got this formula from at the NTHA websight: ht tp://www.fhwa.dot.gov/////legsregs/directives/fapg/cfr0658b.htm I hope this answers your question. I especially want to thank Steve at Pioneer Trucking who took the time to call back a complete stranger and help me answer this question. His willingness to help epitomizes what Mad-Scientist is all about. BK
Try the links in the MadSci Library for more information on Engineering.