By Tim Schewe
In what may be one of the ultimate Tonka toy dreams, I was lucky enough to participate in a training session where we were tipping heavy trucks over on purpose. The adventure, spearheaded by Eric Brewer, took place on a dormant runway at the Boundary Bay airport that is used by government agencies for driver training and collision reconstruction testing.
The aim of the exercise was to record video of the combination as it tipped over and use the measurements that we obtained to validate the formula for calculating the tipover speed. Events like this are included in the collision investigator’s curriculum vitae to support qualification as an expert witness for court purposes.
One of our test vehicles was a conventional tractor and two flat deck trailers loaded with bundles of two-by-four lumber. The steering linkage had been modified by adding a remote controlled brake chamber that would steer the vehicle hard to the left when activated.
A tow truck pulled the truck and trailers down the right side of the runway and let go when the speed reached 60 km/h. The remote control was triggered and the sudden steering change induced a tipover. Think “crack the whip” on a grand scale!
In this case, the physical forces are greatest at the back of the last trailer. Due to the high center of mass, these forces tip the trailer over instead of causing the tires to slide sideways on the pavement as they would with a car or light truck. The twisting force propagates along the combination and eventually the whole unit is laying on its side.
Here’s a good example of what happens courtesy of a dash cam:
The tires of the rear trailer leave a curved black mark on the pavement that end in a sharp hook when the tire first lays flat on the pavement. The videos of this showed smoke coming off of the tire as the tremendous forces between it and the pavement generated heat.
As you might imagine, there were two-by-fours everywhere!
Incidentally, if you are able to show where the truck is laying completely on its side and where it slides to after that, you can do a slide to stop calculation to corroborate the tipover speed calculation.
Get out your measuring tapes. What is needed now is the height of the centre of mass of the load on the trailer, the track width of the trailer and the radius of curvature of the tiremark. Plug that data into the formula and you know how fast the truck was going when it tipped over.
We were doing this on a flat surface and did not have to measure the grade to account for it’s influence in the result.
Testing sessions are a regular part of a collision investigator’s career and one that I found both interesting and challenging. The RCMP’s collision reconstruction program in E Division (British Columbia) brings together the police, engineers and academics to research and refine the process of investigating serious collisions on our highways.
Tim Schewe is a retired constable with many years of traffic law enforcement. To comment or learn more, please visit DriveSmartBC.ca
