Steering Concepts
2
Steering Concepts Why Ackermann? In case of parallel geometry, each wheel will try and rotate about a different centre. This will cause tyre slip. To avoid this, we make both the wheels rotate about the same point – the turning center. We introduce Ackermann to the geometry by inclining the steering arm to the wheel centreline. This way, the steering arms are not always parallel to each other. For the wheels to rotate about the same point, one tyre will turn by a greater angle than the other one. Understeer and Oversteer Understeer results when the slip angle of the front tires is greater than the slip angle of the rear tires. A greater steering angle is then required in order to maintain the turn. When the steering angle reaches full lock and the turn cannot be maintained, the vehicle drifts to the outside. In an understeer condition, the driver is attempting to negotiate a turn, but the vehicle mushes ahead refusing to cooperate. Oversteer produces just the opposite condition. During oversteer, the slip angle of the rear tires is greater than the front. Consequently, the turn-rate increases automatically and the driver therefore reduces the steering angle to compensate. During severe oversteer, the steering angle may reach full lock in the opposite direction while the vehicle continues on into the turn. The vehicle is then said to "spin out." A vehicle that understeers is considered safer in the hands of the average driver. The weight bias of the vehicle determines its inherent oversteer/understeer characteristics. A vehicle that is heavier at the front will tend to understeer and one that is heavier at the rear will oversteer. A vehicle in which the weight is equally distributed between the front and rear axles tends to exhibit neutral steer characteristics. Although the inherent
description
Why is Ackermann Geometry used in steering?What is understeer and oversteer?
Transcript of Steering Concepts
Steering ConceptsWhy Ackermann?In case of parallel geometry, each wheel will try and rotate about a different centre. This will cause tyre slip. To avoid this, we make both the wheels rotate about the same point the turning center. We introduce Ackermann to the geometry by inclining the steering arm to the wheel centreline. This way, the steering arms are not always parallel to each other. For the wheels to rotate about the same point, one tyre will turn by a greater angle than the other one.Understeer and OversteerUndersteer results when the slip angle of the front tires is greater than the slip angle of the rear tires. A greater steering angle is then required in order to maintain the turn. When the steering angle reaches full lock and the turn cannot be maintained, the vehicle drifts to the outside. In an understeer condition, the driver is attempting to negotiate a turn, but the vehicle mushes ahead refusing to cooperate. Oversteer produces just the opposite condition.During oversteer, the slip angle of the rear tires is greater than the front. Consequently, the turn-rate increases automatically and the driver therefore reduces the steering angle to compensate. During severe oversteer, the steering angle may reach full lock in the opposite direction while the vehicle continues on into the turn. The vehicle is then said to "spin out." A vehicle that understeers is considered safer in the hands of the average driver.The weight bias of the vehicle determines its inherent oversteer/understeer characteristics. A vehicle that is heavier at the front will tend to understeer and one that is heavier at the rear will oversteer. A vehicle in which the weight is equally distributed between the front and rear axles tends to exhibit neutral steer characteristics. Although the inherent understeer/oversteer characteristics of a vehicle are determined by its weight distribution, the design of the suspension and the selection of wheel and tire size can enhance or moderate those characteristics.An obvious solution to the negative effects of understeer and oversteer would seem to be that cars ought to be designed for neutral steer. Neutral steer is the theoretical ideal in which the slip angle of front and rear tires increase in unison throughout the range of steering angles. Unfortunately, the factors that influence vehicle dynamics are not so precisely manageable. With the slightest encouragement, a car with neutral steer characteristics can easily cross over into an oversteering condition. Consequently, designers prefer to create some degree of understeer in order to avoid oversteer.
