The Traffic Accident Reconstruction Origin -ARnews-

Re: Deceleration by downshifting

Mike Wegrzyn (wegrzyn@chickasaw.com)
Sat, 1 Mar 1997 08:12:52 -0500 (EST)

Mr. Smith:

Having done hundreds of truck collisions, one factor that always remains constant is that no two trucks will be 'typical'. In lieu of finding the accident truck's twin or 'sister' (loading it identically) and conducting your own deceleration tests, your only other option must be to consider the engine and it's operating r.p.m. range, transmission type, transmission manufacturer and specific 'gearing', differential ratio(s), drive axle tire sizes, gross vehicle weight and weight found on each wheel set.

After obtaining model number and serial number of the truck's transmission, contact the transmission manufacturer who can then give you gearing ratios for each gear set within this transmission. This information coupled with operating engine r.p.m., differential ratios, tires sizes, etc., can then be utilized to establish range of velocities throughout the gearbox/engine r.p.m. range. Simple math should then be able to allow you to establish acceleration changes from each gear set. If you were fortunate enough to establish transmission gear at the time of the collision, and no physical braking was occurring, and you utilize the above technique, you will be within 2 - 8 mph of pre-impact velocity - even closer if you can firmly establish an engine r.p.m.. If this is a late model truck, the motor carrier may even be able to supply engine r.p.m. from computer data downloaded from the wreckage.

The truck driver and his skills (or absence of such) is the wildcard in this scenario. A good interview (if he survived) as to exactly how/when and in what manner downshifting occurred (and why) is a must. Some drivers will skip one or more gears in downshifting.

Mike Wegrzyn
Mike Wegrzyn
wegrzyn@chickasaw.com

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