Introduction
When a vehicle turns, a common misconception is that the front and rear wheels trace out identical arcs. However, this is not the case. Understanding the differences and the reasons behind these differences is crucial for both drivers and automotive enthusiasts. This article delves into the concepts of turning radius, slip angle, and Ackermann steering geometry to provide a comprehensive guide on why the rear wheels do not necessarily follow the same arcs as the front wheels during a turn.
Key Concepts in Vehicle Turning
Turning Radius
The turning radius is a critical factor in determining the path a vehicle takes during a turn. The front wheels typically have a smaller turning radius compared to the rear wheels. This difference arises because the front wheels are responsible for steering the vehicle. When a car turns, the front wheels pivot around a central point, usually closer to the front axle. This results in a tighter and more direct path for the front wheels. The rear wheels, on the other hand, follow a wider and more circular path.
The Path of Travel
The path of travel for the front and rear wheels during a turn is distinctly different. In a vehicle with a long wheelbase, the rear wheels have to travel a longer path to follow the turn. This means that the rear wheels describe a wider and less tight arc compared to the front wheels. The tightness of the arc is more pronounced in vehicles with longer wheelbases, making the difference between the front and rear wheel paths more apparent.
Oversteering and Understeering
The differences in the turning paths of the front and rear wheels can lead to specific driving phenomena, such as oversteering and understeering. Oversteering occurs when the rear wheels lose traction and slide outward, causing the vehicle to fishtail or spin. This often happens when a driver turns the steering wheel too sharply. Understeering, on the other hand, occurs when the front wheels lose traction, making the vehicle continue straight instead of turning as intended. Both oversteering and understeering can be dangerous and require careful handling to regain control of the vehicle.
Ackermann Steering Geometry
To address the differences in turning paths and improve driving stability, most vehicles employ a steering geometry called Ackermann steering. This design ensures that the inner front wheel turns at a sharper angle than the outer front wheel. This geometry helps in ensuring that the rear wheels follow a complementary path. Ackermann steering is particularly effective in smaller vehicles where the difference in turning paths between the front and rear wheels is more pronounced.
The Influence of Speed on Tire Behavior
The behavior of the tires during a turn is significantly influenced by the speed at which the vehicle is traveling. At low speeds, the rear wheels track as described by Brelsford, where the rear wheel axis is perpendicular to the tangent at the point of intersection with the circle. However, as the speed increases, the tires experience a phenomenon known as a "slip angle." This is where the tires do not point in the same direction as the vehicle's direction of travel but instead at an angle to it. This slip angle is a result of the tread and carcass of the tire deforming to take the turn.
The force generated by the deformation of the tire tread and carcass provides the cornering force necessary to make the vehicle follow a curved path. As the tire points at an angle to the direction of travel, it forces the vehicle to change direction toward the way the tire is pointing. For vehicles with a long wheelbase, the effect of this is particularly noticeable. In such vehicles, the rear wheels will describe a wider path, while the front wheels will roll inside the rear wheels' path, resulting in a smaller turning radius.
Conclusion
Understanding the differences in the turning paths of front and rear wheels is essential for safe and efficient driving. The varying turning radii, the effects of slip angles, and the design of Ackermann steering geometry all play crucial roles in how a vehicle negotiates turns. While the front and rear wheels are interconnected during a turn, the subtle differences in their paths are significant and can affect driving dynamics.