Re: Why do dump trucks have long tow bars separating front and rear containers

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