Motorcycle Interactions with Potholes, Roadway Deterioration, and Debris - Part 1 (Longitudinal Edges)
A motorcycle fall is sometimes attributed to interaction with a pothole, pavement deterioration, a pavement edge, or roadway debris. Some of these can cause a motorcyclist to lose control and capsize. However, many are traversable or avoidable without causing significant control or stability issues. In evaluating these crashes, the reconstructionist may need to evaluate if a pothole, area of deteriorated road, or debris on the road was an adequate mechanism to cause a loss of control, and if these were traversable or avoidable by the motorcyclist. This evaluation will need to consider the characteristics of the pothole, deterioration, or debris, along with the characteristics of the motorcycle, the rider, and the maneuver required to traverse or avoid. The visibility of the pothole, deterioration, or debris, and the time and distance it would take for the motorcyclist to maneuver around it, may also need to be considered. As with any other issue of causation, this is a determination that will have to be made on a case-by-case basis. This post begins developing principles and data that might be helpful in evaluating the role of a particular roadway irregularity in causing a motorcyclist to capsize.
One example of a roadway feature that could contribute to a fall by a motorcyclist is a longitudinal pavement edge. A longitudinal edge could be encountered by a motorcyclist as one side of a lengthy pothole. Another example occurs on a roadway that is being repaved, particularly when one lane has already been repaved and the adjacent lane has not. In such instances, the repaved lane may sit higher than the adjacent lane. A motorcyclist that maneuvers from the lower lane to the higher lane could encounter a longitudinal pavement edge of sufficient height to contribute to a loss of control and a capsize.
Passenger vehicle drivers sometimes encounter longitudinal pavement edges, as well. A classic scenario is when a driver allows their passenger side tires to exit the paved portion of the roadway and encounters the pavement edge when they attempt to steer back onto the roadway. While there are clear differences between a motorcycle and a car encountering a longitudinal pavement edge, significant research has been conducted related to the loss of control that can occur for passenger vehicle drivers in these situations [References 1-9]. There are aspects of this research that can inform an evaluation of a motorcycle’s interaction with a longitudinal pavement edge.
These passenger-vehicle-related studies have revealed the following scenario that can lead to a loss of control:
The vehicle exits the roadway at a shallow angle, and the passenger side tires drift onto the right shoulder.
The driver steers to the left to reenter the roadway with a shallow attack angle relative to the pavement edge.
The passenger’s side front tire fails to reenter the roadway and begins scrubbing on the pavement edge.
Realizing the front tire is not climbing, the driver may increase their steering input until the tire climbs the pavement edge.
The steering input that was just enough to accomplish the climbing of the pavement edge now quickly becomes more than what is necessary to return to the driver’s initial lane of travel. This steering input may send the vehicle back to the left beyond the boundary of the lane.
The driver may respond with a countersteer, but may still lose control or travel beyond their lane boundary where they can collide with other traffic or fixed objects on the far side of the roadway.
A key feature of this scenario is the scrubbing that occurs between the car tire(s) and the pavement edge. Several studies have shown that it is when such scrubbing occurs on an edge of sufficient height and severity that control problems can result. When scrubbing does not occur, the pavement edge typically will not cause control problems for the driver, though a driver can still lose control due to an overcorrection that is independent of the pavement edge. The following factors will determine if a tire scrubs along the pavement edge: (1) the height and shape of the pavement edge; (2) the tread and sidewall heights of the tire; (3) the tire pressure (which could affect the shape of the tire); (4) how worn the tire is; (5) the angle at which the tire approaches the pavement edge (the attack angle), (5) and the speed of the vehicle. An edge with a 90-degree square profile will be more severe and likely to produce scrubbing than one with a rounded or beveled transition from the shoulder to the pavement. The greater the height of the pavement edge, the more likely the tire is to scrub along the edge. The smaller the tread and sidewall heights of the tire, the more likely the tire is to scrub. A worn tire climbs sooner than a new tire. This may be partly because a worn tire has reduced tread height, bringing more of the tire sidewall into contact with the edge. Klein and Johnson [2] also attribute it to a larger cornering force generated by a tire when it is worn. The lower the angle of approach between the tire and the pavement edge the more likely scrubbing is to occur. And finally, the higher the vehicle speed, the more likely the driver is to experience difficulty controlling the vehicle after the scrubbing tire finally mounts the pavement edge.
Some of these findings are conceptually applicable to motorcycles. Consider a motorcyclist traveling on a straight section of roadway and entering a lengthy pothole or area of roadway deterioration. Assume the following:
The pothole is longitudinally traversable without instability, as long as the tires of the motorcycle do not interact with one of the longitudinal edges (on the left or the right).
The pothole is of sufficient width that the motorcycle tires will not simultaneously interact with both the left and right edges.
The depth of the pothole is such that, if the tire interacts with either longitudinal edge, the tire will be engaged on its sidewall.
Now, assume that the front tire of the motorcycle does interact with the left edge of the pothole and begins scrubbing along that edge. This would apply a lateral reaction force to the side of the tire and a longitudinal scrubbing force that would tend to steer the tire to the left. A leftward steer would induce a rightward lean that could destabilize the motorcycle and cause it to capsize. On the other hand, the rider could respond to the leftward steer by inputting a rightward steer. If the scrubbing is of short duration, and the rider’s clockwise steering response is of the necessary magnitude, a fall could be prevented. The rider could also respond more severely than what is needed and the rightward steer could induce a lean back to the left and the motorcycle could capsize onto its left. If the pothole is of sufficient length, this process could repeat itself, with the rider eventually becoming unable to control the motorcycle and capsizing could occur.[1] It is also possible in this scenario that the longitudinal edge itself could prevent the rider from countersteering sufficiently to counteract the scrubbing-induced steer.[2]
This example also illustrates that, for a motorcycle, the instability from interaction with a longitudinal pavement edge would likely occur in roll rather than yaw (as it would for a car). For a car, the roll moment applied to the vehicle by the lateral force from the pavement edge will typically be offset by the opposing roll moment applied about the contact point by the vehicle’s weight, since the center of gravity of the vehicle is laterally distant from the contact area. For a motorcycle, the steering induced by scrubbing can induce roll instability.
The ultimate outcome of an interaction between the front tire of the motorcycle and the left longitudinal edge of the pothole will depend on the length and depth of the edge, the shape of the tire, the speed of the motorcycle, and the rider’s skill level. For a reconstructionist analyzing a situation like this, the following questions might be worth asking and exploring via the physical evidence:
Does the pothole exhibit a longitudinal edge?
What is the shape and depth of the longitudinal edge?
What is the shape of the tire and where is the tire likely to interact with the edge?
What forces would this apply to the tire and would this induce steering?
What speed was the motorcycle traveling when it interacted with the edge?
How long (in terms of time) would the interaction have persisted?
Is there evidence on the front or rear tire of the motorcycle of scrubbing on the longitudinal edge?
Where is that scrubbing? Does it makes sense dimensionally with the pavement edge?
Is there material transfer from the tire onto the pavement edge?
These questions will not necessarily be answerable. Oftentimes, by the time a reconstructionist is involved, the pothole or roadway deterioration has been repaired and the precise shape and depth cannot be determined from photographs. If the pothole was not photographed, historical aerial and streetview photographs on Google can sometimes be helpful for quantifying the shape and dimensions of the pothole. State, city, and county departments of transportation also sometimes document their roadways photographically at regular intervals and these photographs can be helpful for quantifying the shape and dimensions of roadway deterioration.
References
Nordlin, E.F., Parks, D.M., Stoughton, R.L., Stoker, J.R., “The Effect of Longitudinal Edge of Pave Surface Drop-Off On Vehicle Stability,” CA-DOT-TL-6783-1-76-22, California Department of Transportation, March 1976.
Klein, R.H., Johnson, W.A., “Vehicle Controllability in a Pavement/Shoulder Edge Climb Maneuver,” SAE Technical Paper 780620, 1978, doi:10.4271/780620.
Zimmer, R.A., Ivey, D.L., “Pavement Edges and Vehicle Stability – A Basis for Maintenance Guidelines,” Research Report 328-1, Texas Transportation Institute, September 1982.
Ivey, D.L., Sicking, D.L., “Influence of Pavement Edge and Shoulder Characteristics on Vehicle Handling and Stability,” Transportation Research Record 1084, 1986.
Olson, P.L., Zimmer, R., Pezoldt, V., “Pavement Edge Drop,” The University of Michigan Transportation Research Institute, UMTRI-86-33, July 1986.
Glennon, J.C., “Effect of Pavement/Shoulder Drop-Offs on Highway Safety,” State-of-the-Art Report 6, Transportation Research Board, 1987, ISSN: 0892-6891.
Rudny, D.F., Sallmann, D.W., “Analysis of Accidents Involving Alleged Road Surface Defects (i.e., Shoulder Drop-offs, Loose Gravel, Bumps and Potholes,” SAE Technical Paper 960654, 1996, doi:10.4271/960654.
Deyerl, E., Cheng, L., “Computer Simulation of Pavement Edge Traversal,” SAE Technical Paper 2009-01-0464, 2009, doi:10.4271/2009-01-0464.
Ivey, D., Zimmer, R.A., Julian, F., Sicking, D.L., Johnson, W.A., Nordlin, E.F., “Pavement Edges,” Article contained in Influence of Roadway Surface Discontinuities on Safety – State of the Art Report, Transportation Research Circular Number E-C134, Transportation Research Board, May 2009.
Endnotes
[1] Broker and Hottman give a good description of this scenario for bicycles, referring to it as a front wheel diversion. See: Bicycle Accidents, Crashes, and Collisions: Biomechanical, Engineering, and Legal Aspects, Second Edition, by Jeffrey Broker and Megan Hottman, Lawyers and Judges Publishing Company, Inc., 2017, ISBN 9781936360581.
[2] Hough has observed: “Two-wheelers are particularly vulnerable to pavement edges or grooves. Remember, a two-wheeler is balanced mostly by steering the front wheel…For example, if a motorcycle starts to fall over to the left, you can steer the front wheel more to the left to rebalance. The term for this balancing act is countersteering…Countersteering explains why edge traps are so hazardous to two-wheelers while only a jarring, wheel-bending inconvenience to other vehicles. A car or a side-car rig can slide sideways without losing balance, but if the rider of a two-wheeler loses steering for more than a couple of seconds, it becomes very difficult to maintain balance. Easing up to a curb, you can maintain balance right up to the point where the front wheel contacts the edge. After that, with the tire scrubbing along the edge of the curb, you can’t countersteer to maintain balance…the trick is to cross it aggressively at a maximum angle rather than attempting to ease over, and to use a little power to bounce the front wheel up.” See: Hough, David L., Proficient Motorcycling: The Ultimate Guide to Riding Well, 2nd Edition, Fox Chapel Publishers, 2008.
Photo by Ian Taylor on Unsplash