Like a motorcycle, a bicycle is a single-track vehicle, and both rely on the balance of the rider to remain upright; however, bicycles and their riders are more vulnerable to crashes caused by weather and surface conditions. This article provides a detailed description of how a rider's balance and a bicycle's uprightness are maintained in the course of the safe operation of the bike. It then identifies factors that can disrupt the uprightness of the bike and the rider's balance, causing a crash. Crashes result from a sudden and unexpected movement of the handlebar or the front wheel. This occurs if the front tire strikes an object or contacts a condition that suddenly diverts the alignment of the front wheel. Weather also poses a risk for bike crashes. Rain or snow lessens tire traction, and riding into a headwind makes it difficult to hear vehicles approaching from the rear. Thus, when investigating a serious bike crash, it is important to determine the wind speed and direction. When a bike crash involves a collision with a motor vehicle, it is important to determine environmental factors that may be involved, such as the presence of a shoulder suitable for bike travel, whether the cyclist was attempting a left turn across a traffic lane, the width of the traffic lane, the width of the motor vehicle, and whether the travel lane for the motor vehicle and the bike would allow three feet between them when they are alongside. Also, any environmental factors that may have caused the bike to turn unexpectedly into the motor vehicle's path should be investigated.