A recent case raised the question of what dynamic vertical front wheel loads are encountered by an ATV during “normal” and “abnormal” use. To answer that question, Safety Engineering Associates designed and fabricated an instrumented ATV front suspension strut and conducted numerous tests to measure wheel loads while riding. An on-board data acquisition system recorded the vertical wheel force, front suspension travel, vehicle speed and other parameters of interest.
“Normal” riding conditions of approximately 15mph on flat off-road terrain produced wheel loads approximately 1.5 times the static wheel loading of the ATV and rider measured on laboratory scales. Hard cornering on level ground produced approximately twice the static wheel load. Descending a 20 degree downhill slope at slow to moderate speeds resulted in a peak dynamic front wheel load about 2.5 times higher than the static load.
Representing the “abnormal” or more extreme riding conditions, fast riding through a rough pasture at speeds of 25 to 30 mph produced wheel loads approximately three and a half to four times the static load. Hitting a rectangular “bump” approximately 5 inches high by 8.5 inches wide at 15 mph generated wheel loads nearly five times greater than the static load.
These answers assisted our client in defending a product liability case where a component supporting the front wheel fractured and the wheel separated from the ATV. Testing of exemplar components demonstrated that a force equivalent to approximately 100 times the static wheel load was required to fracture this part. Knowledge of front wheel loads during normal and extreme use of the ATV and other case facts led to the conclusion that the fractured component broke during the crash landing and subsequent tumble that occurred after the rider became airborne while going over a steep embankment at high speed.