Motorcycle tires are designed and constructed to support dynamic loads and forces on varying road surfaces and weather conditions. The type of tires used, how they are maintained, and environmental conditions all contribute to how a tire wears and its expected lifespan.
In this article, Tire Engineer Richard Sherman and Vehicle Engineer Roland Hoover provide an overview of motorcycle tires and discuss the factors that contribute to motorcycle crashes.
MOTORCYCLE TIRE FAILURES – EXPERT ARTICLE
Motorcycles are more susceptible than conventional 4 wheeled vehicles to sliding during a crash, which can lead to scrapes and other impact related damage to the tires. Our experts specializing in motorcycle dynamics and tire performance are frequently retained to discern if damage to the tire was a contributing cause to the crash, or a result of the crash forces.
To the untrained eye, tire damage that results from a crash may be indistinguishable from damage that contributes to the cause of a crash. However, these issues can be reliably determined through forensic examination by a qualified expert who understands the construction of motorcycle tires, factors that affect optimal tire performance, and how different defects and conditions present in the evidence following a crash.
Tire failures can occur suddenly, as in the case of a puncture, or over time, as in the case of excessive wear that weakens the tire’s structure. The nature of the failure can typically be understood through examination of the tire, the motorcycle, and the environment in which the crash occurred. A reconstruction of the crash is often necessary to calculate the speeds, forces, and various points of impact throughout the incident.
Motorcycle Tires & How they Differ from Automobile Tires
Motorcycle tires are shaped differently than automobile tires because they function differently. The cross-section diagram below demonstrates that motorcycle tires are cone shaped, where an automobile tire is flatter across the tread surface.
The cone shape of the motorcycle tire controls turning as the rider varies the lean of the bike. To travel straight, riders keep the bike upright and utilize the center of the tread. The outer area of the tread or “shoulder” is used for turning, as demonstrated below.
The footprint of a motorcycle tire on the road surface is relatively small, about the size of a tennis ball. This footprint or “contact patch” travels across the tread face between the shoulders of the tire based on the motorcycle’s angle of lean. Passenger car tires, on the other hand, have a larger contact patch and rely on the vehicle’s steering and suspension inputs to transmit traction across the entire tread width of the tire, whether travelling straight or turning.
For any type of vehicle, tires provide the interface through which dynamic forces, such as accelerating, braking, and cornering, are transferred from the wheel to the ground (road surface). An understanding of tire performance as it relates to specific vehicle dynamics is essential for reliable forensic investigations.
Modern motorcycle tires are highly engineered products consisting of multiple layers and materials. Tires available on the market today are described as either bias ply or steel belted ply; this is determined by the internal components that wrap around the lining. Bias ply tires are more common, while steel radial tires tend to be specified on high performance and higher end motorcycles. The construction and anatomy of the tires in question during a forensic investigation will dictate the possible failure modes, in consideration of the performance characteristics of that tire and the expected wear patterns.
Tire Damage vs. Tire Defect
Motorcycle tires are chosen for a specific riding application and environment; and with normal usage and maintenance will experience wear consistent to those conditions. It is important to discern the difference between damaged and defective tires.
- Damaged Tires - A damaged tire may exhibit abnormal wear patterns or surface conditions caused by impact, abuse, or improper maintenance. Damage can occur from problems with the motorcycle’s suspension setup, incorrect tire pressures, or worn out bushings or bearings. Tread patterns that cup or exhibit localized areas of wear may indicate separations between the internal components of the tire. Damage can also be the result of improper modifications or repairs to the motorcycle.
- Defective Tires - A defect in a tire may exhibit unusual wear differences or surface irregularities similar to a damaged tire, but caused by a defect in materials or construction. Externally visible signs of defects may include bubbles or sidewall undulations. The separation of internal tire components or the failure of individual components are other possible failure modes and may present differently based on the construction and intended purpose of each tire.
Red flags in a tire failure investigation may include tires that are inappropriate for the intended use or tires that are heavily worn, affected adversely by environmental conditions, or improperly inflated. Any of these situations should be readily apparent to a qualified mechanic and could negatively affect tire performance.
External Factors to Consider in Tire Investigations
There are a number of unusual circumstances that are known to affect tire performance. While unusual, our forensic experts are familiar with these issues and can address them if they apply to your case.
- Sprays & Polishes - Overspray from polishes and cleaners used to enhance the appearance of a motorcycle can significantly reduce the available friction between the tire and the roadway.
- Bolt-on Components - The fasteners used with bolt-on components, such as lights and luggage racks, can leave a protruding bolt or other sharp edge capable of causing a puncture of tear in the tire.
- Custom Wheels - Clearances for custom wheels must be checked at all suspension travel positions to ensure the tires do not rub or have the potential of catching or getting cut by the motorcycle frame, fenders, bolts, or other accessories.
- Climate conditions - Constant exposure to the sun and ambient temperatures (Northern vs. Southern states) will affect the useful life of a motorcycle tire.
- Overloading - Both the motorcycle and the tires have a predetermined load capacity, and the Gross Vehicle Weight Rating of the bike will include the weight of stock tires. If this weight limit is exceeded, the suspension, handling, and overall performance of the bike will suffer. Overloading also accelerates tire wear and can lead to tire failure.
VEHICLE ENGINEERING INVESTIGATIONS
The vehicle experts at Robson Forensic examine crash sites, use field measurements, scrutinize crash scene photos, and review vehicle design documents to determine if vehicle or environmental factors or road conditions contributed to the cause of motorcycle crashes.
For more information, contact the authors of this article or submit an inquiry.
Automotive Engineer & Tire Expert
Richard Sherman is an automotive engineer specializing in tire design and failure analysis. He applies his expertise to the investigation of vehicle crash incidents, with a particular focus on those involving failed or damaged tires. Richard approaches his forensic casework with experience gained working in engineering positions for tire, vehicle, and automotive component manufacturers.
Prior to joining Robson Forensic, Richard worked for the Goodyear Tire & Rubber Company where he was engaged in the design, manufacture, and quality assurance of passenger vehicle tires. As a tire designer, Richard was charged with the development of new tires as well as the tuning of existing product lines to ensure consistent, reliable performance. In a later role as Quality Team Leader, Richard performed root cause failure analyses related to product quality concerns.
Automotive Engineer & Motorcycle Expert
Roland Hoover is a mechanical engineer with broad experience in automotive and motorcycle engineering. His career includes more than 20 years of engineering experience at OEM vehicle manufacturers, aftermarket parts manufacturers, racing teams, and specialty performance tuners.
Prior to joining Robson Forensic, Roland was a Research & Development Engineering Manager at an aftermarket motorcycle parts manufacturer; in this role, he was responsible for the development and testing of wheels, drivetrain, steering, braking, suspension, and electronic systems components. In addition to new product development, Roland was also responsible for root cause and finite element analysis investigations into product failures and consumer complaints.