According to the Centers for Disease Control and Prevention, there were close to 500,000 nonfatal injuries in 2013 involving cyclists, and most of these occurred in urban areas. Bicycle ridership has increased in many U.S. cities, a trend encouraged by the addition of dedicated facilities such as protected bike lanes or shared-use paths in urban settings. New bicycle sales have remained relatively steady in the U.S., and the National Bicycle Dealers Association points to an increasing trend of more bicycle retailers selling used bicycles.
It is possible this rise in bike ridership will lead to a rise in bike-related accidents that will require investigations. A forensic investigation can benefit from multiple viewpoints and expert backgrounds. It makes for a stronger case and allows the forensic team to better foresee the opposing side’s strategy and perspective. Let’s look at an investigative approach to a case study from three different perspectives: mechanical engineering, accident reconstruction, and biomechanical engineering.
Getting Into Gear
The case study involves a bicycle crash. While the focus of many forensic investigations involves bicycle-vehicle collisions, investigations into other potential hazards are worth sharing. One such example is the case of a front-wheel fender that was poorly mounted, had inadequate clearance, or was not designed properly and what can occur when a failure mechanism materializes. In this case, the injured rider’s insurance company is pursuing subrogation to recover losses from other parties, including the bicycle manufacturer, seller, service shop, electric motor/battery manufacturer, and city in charge of the facility where the incident occurred.
The collision occurred when the rider of an electric bicycle was riding on a designated bicycle trail. The front tire allegedly stopped suddenly, launching the rider over the handlebars and causing serious injuries. The rider was approximately 250 lbs. and was reportedly traveling close to 15 mph.
This case was investigated while providing technical expert consultation to one of the defendant parties. The scientific method was followed while pursuing a multidisciplinary investigation. Medical records and depositions for the involved parties were reviewed, bicycle manuals and specifications were researched, and the subject bicycle was inspected.
The leading theory was that the bike’s front fender, which attaches to the back side of the fork crown, loosened from its main upper tab connection while in motion and caught the front wheel. The mechanism is initiated by a front fender upper tab bolt/nut that loosens due to vibration. When it comes to stopping the wheel, the concept is not unlike an old-fashioned spoon brake, where a metal or leather pad is pressed onto the top surface of the tire to create friction and convert the kinetic energy of the rider into heat to be dissipated.
In the mechanics of a bicycle, the fender stays are usually mounted behind the rotational axis of the front tire. If the fender were to rotate freely, about the stays’ mounting point, the initial tire-fender gap would increase as the fender moves backward and decrease as the fender moves forward. The mechanism in question comes up when the fender rotates forward, possibly as a result of the tire’s rotation. A loose nut may become fully detached, and if the corresponding bolt is lost, the fender becomes free. Depending on the overhang length of the fender, the stays may catch on the fork, the fender could catch the tire, or both. Due to gravity, the fender may rotate backward first and bounce a few times before rotating forward. The upper mounting tab may be flexible enough to pass under the fork crown and allow the fender to move forward. Once the fender and tire become wedged against each other, the front tire begins to briefly skid, the back tire lifts off the ground, and the rider is vaulted about the axis of the front tire. The more the fender rotates, the more it digs into the tire, and more friction is created between the fender and the tire by the decreasing clearance. Besides the damage to the bicycle, the rider may suffer bruising, road rash, broken bones, or more serious injuries.
Other hypotheses as to the cause of the crash included sudden, hard application of the front brake; failure/seizure of the electric motor; or the front fender becoming loose, with its upper tab suddenly pushing on the horizontal portion of the linear-pull brakes causing the front brake to be applied.
Bicycle Inspection
A mechanical inspection was performed on the bicycle. The front fender’s upper tab and the right-side stay were found to be loose. The bicycle’s front fender had only one set of stays. At the time of the inspection, it was unclear whether both of these conditions were present when the incident happened. In addition, the related fender mounting hardware (nut, washer, bolt) was missing for the upper tab attachment point. Nondestructive testing on the bicycle consisted of rotating the front tire by hand as it was suspended by a bicycle repair stand.
During the bicycle inspection, a failure mechanism similar to the leading theory was found. However, the fender’s upper tab never rotated past the linear-pull front brake. Nevertheless, road-tire interaction was not captured due to the use of the repair stand. Although some scratches on several areas of the bicycle were found to be consistent with a vault event, no signs of violent energy dissipation were found, such as a broken plastic fender or significantly bent fender stays or wheel spokes. In addition, the fender mudflaps did not show signs of wear or heavy scrapes.
Rider speed at the time of the incident was estimated from gear settings and expected rider cadence. The twist-grip gear shifters of the subject bicycle were found on the one (crankset) and eight (cassette) positions. The chain itself was found on the two (crankset) and eight (cassette) positions. Based on these settings, the speed at the time of the incident was estimated to be in the range of 15 mph as described by the rider. The electric motor-assist feature was not active at the time of the event.
Biomechanical Analysis
Biomechanical analysis began with an evaluation of the injuries, which are used to work backward to the impact position and to the pre-impact body position and speeds. In this case, the rider sustained a lacerated chin, broken right hand, bruises, and abrasions. All injuries were consistent with a possible bicycle fall and gave insight into the kinematics of the fall. The bruises indicated areas that were compressed, while the abrasions identified the bodily locations that slid against a hard surface, such as a paved multiuse path. The lacerations, in the absence of a cutting edge, indicate a compression event, while the broken hand was a telltale sign of an area of high-impact force.
Rider testimony suggested that the front wheel had locked, acting as a pivot point and allowing the remainder of the bike to rotate about the front wheel. Likewise, the body of the rider would have been moving at the riding speed immediately after the front wheel locked. The rider was not rigidly coupled to the bicycle and would vault over the front tires with enough energy to cause a broken hand and lacerated chin. It is likely that the rider vaulted over the front tire, broke his hand trying to break his fall, and impacted the roadway with his chin, making the injuries consistent with the falling mode.
Reconstructing the Accident
Exemplar bicycle testing began as an effort to understand whether the plaintiff’s leading theory was plausible. The same model and comparably sized bicycle was procured brand new, and a test ride was planned along the route taken on the date of the incident. Due to repackaging, the fenders did not come pre-attached to the bicycle, as was the case with the subject bicycle.
The ride included on-road and on-trail portions of the same route followed by the rider the day of the incident. With a nominal width of 10 feet, the incident site was found to be compliant with guidelines such as the 1999 American Association of State Highway and Transportation Officials (AASHTO) Guide for the Planning, Design, and Operation of Bicycle Facilities. No issues were found with the incident site that would lead to liability on the part of the city that owned the path.
Torque settings for the fender-retaining bolt/nut joints were set to minimum values by hand tightening, a worst-case scenario for the clamp force applied to the joint, since specific values were not available from the manufacturer. Two areas were monitored with cameras: the top fender bolted connection and the right lower fender stay connection. A vertical paint mark was placed across the nut/washer/bolt surfaces to record their initial position and monitor the relative movement of these components as the test progressed.
The test revealed that the bolted connection loosened up within the first few miles of riding on the route. Both on-road and on-trail surfaces produced enough vibration input to cause loosening. Initially, as the nut started to back out, the fender was held mostly in place. After some time, the loose fender exhibited more appreciable movement and generated an audible rattling noise that was more pronounced when going over interfaces such as sidewalk breaks, large cracks, or gutter/road intersections. Several miles after, the alleged failure mechanism was experienced, including the rider falling forward. The moment the nut and washer separated was captured on video. Almost instantly, the front wheel locked up, and the rider fell forward. The bicycle was thrown about eight feet and bounced about to its final resting position. The locked front tire left a skid mark on the trail. The fender was broken, and the stays had more obvious deformation, something not observed on the subject bicycle. In addition, the front fork was significantly bent backward.
From a reconstruction perspective, the failure mechanism is a forward vault preceded by a skid mark. Contact marks were visible atop the rack at the rear, indicative of a forward rotation by the bicycle following the front tire seizing. The bicycle’s point of contact after said rotation can be approximated from a splash mark left on the asphalt by a water bottle that was attached to the frame. Working backward from the final resting place of the bicycle and rider and taking into account the skid mark left prior to the vaulting motion, the speed was approximated to be between nine and 11 mph. This is consistent with the bicycle’s gear settings and the test rider’s description of the incident.
Comparison of Findings
The biomechanical analysis further supports the accident reconstruction findings. Upon experiencing the failure mechanism, the rider was vaulted over the front tires, consistent with the kinematics concluded in the subject incident and what has been reported in controlled bicycle crashes. Even with protective equipment, the test rider suffered some contusions, and there were abrasions on the gloves and related padding that would have likely resulted in abrasions, mostly on the left side of the body. The numerous marks on the left side of the body indicate that the rider impacted the road leading with his left side. No broken bones or compression lacerations were reported for this impact. The bruises and abrasions were consistent with the subject incident and confirm the injury mode. The major difference between the test and the subject incident is the lack of bone fractures and compression lacerations for the test rider. This is explained by the reduced energy of the test rider, resulting from slower riding speeds (nine to 11 mph versus 15 mph) and less body weight (150 lbs. versus 250 lbs.), which made the fall less likely to cause a bone fracture or skin laceration.
The leading hypothesis was shown to be possible, as the failure mechanism was replicated. Although the test was completed sooner than expected, there were two key revealing factors related to the test-result time frame that became very helpful to the investigation.
On a macroscopic scale, there was a significant difference in how quickly the failure mechanism was experienced between the two incidents, with the test incident being experienced in nearly a fifth of the distance it took for the subject incident. In the microscopic scale, the incident revealed that the failure mechanism, once the fender was loose, occurred in fractions of a second, leaving the rider with no time to react. Our client was able to discuss these findings with the other co-defendants and show proof that the failure mechanism was, indeed, possible. This, combined with other revelations, proved to be critical in our client’s course of action. Combining all the mechanical, reconstruction, and biomechanical information allowed us to complete a very thorough investigation and gather a comprehensive story to better position our client during mediation.
In summary, forensic experts with varying disciplines were able to rely on their training, education, knowledge, and experience to work together and understand the problem at hand. Some of the key findings of the investigation are highlighted below:
• The fenders were intended to be delivered pre-assembled on the bicycle. There were no torque specifications on the bicycle owner’s manual or even a mention of the fenders.
• An owner’s manual for a different bicycle from the same manufacturer did mention the fender connection, specifying: “Tighten until secure” and/or “Tighten until snug.” These are unclear instructions that do not help the user understand what is correct to avoid under- or over-tightening of the joint.
• The out-of-the-box fender was held on by a regular nut instead of a nylon-insert locknut. Nylon-insert locknuts have a certain amount of prevailing torque and are more suited for situations in which vibrations may loosen a joint. Other solutions are available in the market, such as breakaway stays or release fender tabs.
• The incident trail was in compliance with AASHTO guidelines.
• Video analysis of the test bicycle allowed for a clear physical understanding of the failure mechanism.
• During exemplar testing, the failure mechanism was experienced in a shorter distance when compared to the subject bicycle. A reasonable assumption may be that the subject nut was tightened more than finger tight by the bicycle selling dealer. Still, it is unclear if “snug” or “secure” were met, as these terms are vague.
• While the ultimate failure mechanism is experienced in fractions of a second, the loose fender exhibited appreciable movement and generated an audible rattle noise for a few miles, something that could have served as a warning to the rider.
• Speeds were approximated from gear settings and/or skid marks and post wheel-locking distance to final rest.
• Injuries on the riders were consistent with a vault event, and their relative magnitude could be explained by the physical difference and speed of both riders.
The role and responsibilities of each involved party were understood and put into the context of the subject incident. This multidisciplinary approach to the investigation proved to be efficient and of great benefit in outlining the course of action to bring the subrogation claim to resolution.