Connections between motorcycle accident reconstruction and rollover and pedestrian crash reconstruction
High-speed, single vehicle rollovers can often be analyzed in three phases: the loss-of-control phase, the trip phase, and the roll phase.
Analogously, a single-vehicle motorcycle crash can often be analyzed in three phases: the loss-of-control phase, the capsizing phase, and the sliding and tumbling phase.
The motorcycle and the rider will often separate and each will carry out their own sliding and tumbling phase. The rider’s sliding and tumbling phase may be preceded by an airborne phase.
A pedestrian crash can often be analyzed in three phases: the impact phase, the airborne phase, and the sliding and tumbling phase.
If we’re willing to neglect air resistance (we usually are), then there is little difference between how we would analyze the motion of an airborne motorcyclist, an airborne pedestrian, and an airborne passenger vehicle.
From an equation standpoint, there is no difference between these scenarios:
A motorcyclists that has been thrown into the air as a result of a high-side motion of the motorcycle;
A motorcyclist that has been thrown into the air as a result of his motorcycle impacting a passenger vehicle;
A pedestrian that has been thrown into the air as a result of being struck by a car;
A passenger vehicle occupant that has been ejected and thrown during a rollover crash.
A passenger vehicle that has been driven into a ravine and become airborne.
Once the person or vehicle in these scenarios is airborne (meaning not in contact with the ground or any other object), then gravity and air resistance are the only forces acting on them. They will eventually re-engage with the ground. Speed will be lost when they strike the ground and speed will be lost as they slide and tumble after landing.
Of course, the process by which any of these people or vehicles became airborne matters.
The significance of a calculated takeoff speed will differ, depending on if we are talking about:
A vehicle that has driven into a ravine without first contacting any fixed objects;
A pedestrian that has been impacted by a low-fronted car;
A motorcyclist that has become airborne after he and his motorcycle struck a car;
A motorcyclist that is thrown as a result of the motorcycle completing a high-side motion.
The connection of the takeoff speed to the initial speed, and our ability to reconstruct the initial speed, will differ in each of these instances.
A rolling vehicle will, on average, decelerate at a rate of 0.44g, give or take about 0.06g.
That’s not a bad estimate for the rate at which a sliding and tumbling motorcycle will decelerate on pavement.
A sliding and tumbling motorcycle on pavement will decelerate at a rate of about 0.48g, give or take about 0.1g [1].
There are many exceptions and nuances, of course:
First of all, a motorcycles deceleration will depend on how deeply the component are digging into the surface.
Second, a wet road can reduce the rate at which a sliding and tumbling motorcycle decelerates.
A motorcycle sliding on a wet road while lightly scratching the surface will probably decelerate at a rate below 0.48g, and below 0.44g for that matter.
A motorcycle sliding on gravel while furrowing deeply into the surface will probably decelerate at a rate above 0.48g, perhaps well above 0.48g.
Still, these decelerations harken back to Thomas Bratten’s Tumble Number [2]: “The Tumble Number is an empirically derived deceleration rate applicable in cases where a reasonably compact mass decelerates from a finite velocity while undergoing random and undefined rotation in multiple planes and making irregular ground contacts until it finally comes to a rest…The developed Tumble Number is strikingly close to 1/2 g, a value which appears in certain theoretical formulas having nothing to do with tumbling objects.”
References
Rose, Nathan, Motorcycle Accident Reconstruction, Society of Automotive Engineers, ISBN 978-0-7680-9507-4, 2019, https://www.sae.org/publications/books/content/r-483/.
Bratten, T., "Development of a Tumble Number for Use in Accident Reconstruction," SAE Technical Paper 890859, 1989, https://doi.org/10.4271/890859.
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