In this article, the bicycle experts discuss bicycle frame and component failures, with an emphasis on the failures that are most commonly associated with a loss of control or crash.
Bicycle Frame & Component Failures
The components that make up a bicycle have a useful service life. Bicycle frames, forks, handlebars, wheels, brakes and other parts may fail due to a design or manufacturing defect, overloading, or simply wear out over the life of a bicycle. Design factors such as function, light weight, durability and cost dictate the material used for a component. All of these considerations can play a role in the likelihood and nature of a component’s failure.
The Critical Nature of the ‘Control Surface’ Components
The frame and fork of a bicycle are the most obvious and visible parts of the structure, but the points that the rider interacts with to control movement are also very important to safety. To control speed and direction the rider interacts with the handlebars, brake levers, bicycle seat and pedals. These components are what the rider’s body touches and in the event of a failure to one or more of these parts the rider no longer has full control of the speed and direction of the bicycle. Control surface components are colored orange in the image below:
- Frame failure: Most bicycle frames are made of metal, but carbon fiber composites have become very popular especially in lightweight road bicycle construction. While metal frames often give warning before a failure in the form of cracks, bends and bulges, carbon fiber failures are much more difficult for the average rider to identify before a catastrophic event.
- Head tube failure can be very serious
- Bottom bracket area can creak before failure
- Cracks sometimes form in areas hidden behind other components
- Fork failure: Bicycle forks not only hold up the front of the bicycle but they are critical for steering. A common failure with carbon fiber composite forks occurs when the lower legs become separated from the steer tube, which is the section of the fork that passes through the bicycle’s frame and is clamped by the handlebar stem. In a fast descent, a failure of this type typically sends the rider face first to the pavement.
- Steer tube separating from fork crown can be very serious
- Single fork leg failure with damage on backside can indicate something became caught in spokes
- It can be difficult for an untrained observer to determine whether a fork failure caused a crash, or if the fork was damaged during a crash event
- Pedal/crank arm failure: A portion of the rider’s weight is supported by the pedals, especially when standing to accelerate or climb. A failure of the pedal (or the crank arm to which it is attached) can send the rider crashing onto the top tube of the frame or over the handlebars.
- Inexperienced mechanics may not know that the left pedal is reverse-threaded, leading to improper assembly and failure
- Composite components such as crank arms can fail without prior visible indications of problems
- Handlebar/stem failure: The handlebars are connected to the bicycle fork by the stem. The bars and stem support some of the rider’s weight and also receive the steering inputs from the rider. When the integrity of this assembly is compromised the rider loses the ability to steer and may lose the ability to stay upright on the bicycle.
- Composite components like handlebars and stems must be carefully assembled to proper torque values
- Improper threaded fastener torque can cause cracks in aluminum and carbon composite components
- Seat/seatpost failure: The rider’s weight is supported by the seat, but it is also the pivot point while pedaling and steering. Fasteners that break or are improperly tightened can lead to a loss of control of the bicycle.
- Composite components should be assembled with torque wrenches and inspected regularly
- Improper threaded fastener torque can allow seats and seatposts to slip under the rider’s weight
- Brake failure: Brake pads wear out, as do control cables. Both are ‘wear items’ that must be checked and replaced regularly. Without robust components, proper installation and regular inspection a rider can lose the ability to control speed.
FORENSIC BICYCLE INVESTIGATIONS
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