The Traffic Accident Reconstruction Origin -ARnews-

Re: Directional stability of bobtail tractors

Bob Cole (rpcole@ican.ca)
Wed, 28 Jan 1998 12:32:01 -0500 (EST)

Never mind the testing - there are lots of accidents to support this. Tractors have much less braking capacity on the steering axle than on the drive axle. Unless you are on a surface with a very low 'f', you probably will not lock up the steering axle - the drives on a bobtail will lock up very easily. Take any vehicle (in a parking lot - without anything around - try it on wet pavement or snow covered roads if you have snow) and lock up the rear wheels by applying the parking brake, put in a little bit of steering input and then hold on - your going to be facing the opposite direction. Even without steering input, if their is slightly different 'f' values on one side compared to the other, the vehicle will come around.
Trucks that have a bobtail proportioning valve or ABS will not have this problem and should stop straight (and in a much shorter distance than a regular bobtail).
The Ontario region of CATAIR did some tests on a split mu surface with bobtails, ABS on and then turned off. With ABS on it stopped straight as an arrow, without the ABS, the tractor spun around violently.
I do not have a lot of 'bobtail' driving experience but I did experience a few months ago how easy it is to lock up the rears. I was travelling in a town at fairly low speeds (lower than normal because I was bobtailing) when a traffic light changed. It was not a panic stop but a little harder than normal. I came to a stop at the light and then smelled burning rubber and a cloud of smoke going by the cab - looked in my mirror and saw about 20 feet of skid marks, I did not realize or feel that I had been skidding. At higher speed or on wet pavement, I would have probably started to loose steering control before I would have even realized that I had locked up the rears (remember that air brakes do not give you any 'feel' that you experience with hydraulic brakes).
Bob Cole
rpcole@ican.ca

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