Steering System

1 Steering System STEERING SYSTEM The function of steering is to steer the front wheel in response to driver command inputs in order to provide overall directional control of the vehicle. The factors kept in mind while designing the steering system were Simplicity Safety Requiring minimum steer effort Economical Steering geometry Ackerman The Ackerman Steering Principle defines the geometry that is applied to all vehicles (two or four wheel drive) to enable the correct turning angle of the steering wheels to be generated when negotiating a corner or a curve. When a car is travelling around a corner (the red lines represent the path that the wheels follow) the inside wheels of the car follow a smaller diameter circle than the outside wheels. If both the wheels were turned by the same amount, the inside wheel would scrub (effectively sliding sideways) and lessen the effectiveness of the steering. This tire scrubbing, which also creates unwanted heat and wear in the tire, can be eliminated by turning the inside wheel at a greater angle than the outside one. Dept. of Mechanical Engineering 2 Steering System The difference in the angles of the inside and outside wheels may be better understood by studying the diagram, where we have marked the inside and outside radius that each of the tires passes through. The Inside Radius (Ri) and the Outside Radius (Ro) are dependent on a number of factors including the car width and the tightness of the corner the car is intended to pass through. Aligning both wheels in the proper direction of travel creates consistent steering without undue wear and heat being generated in either of the tires. Steering Arm Angles Creating mis-alignment of the wheels is achieved by a combination of the angle and the length of the steering arms. Below a few diagrams are shown that give examples using parallel and angled steering arms to demonstrate why there is a need for using the Ackerman Steering Principle. Parallel Steering Arms The steering arms in the diagram to the left are straight and parallel to the sides of the vehicle, which would create a situation where equal movement of the steering servo would produce equal angular movement of the wheels. As the steering arm pivot point (A) is vertically aligned with the king pin pivot point (B) when the wheel is pointing straight ahead, the same amount of movement to the Left or to the Right moves the steering arm pivot point the same vertical distance forward of its starting point. Angled Steering Arms Dept. of Mechanical Engineering 3 Steering System The steering arms in the image to the left are angled inwards to create a means for the wheel angles to change at a different rate. This is the basis of the Ackerman Steering Principle and creates this unequal angular movement of the wheels. As the steering arms are angled, the pivot point (A) is not vertically aligned and is, in a straight ahead position, part way round the circle. Because of this, a Right movement of the steering arm will cause the pivot point to move a greater distance in the forward direction than a Left movement of the steering arm. An important point worth noting is that this unequal angular movement is exponential, that is, the more you turn the wheel the greater the angular difference between the wheels - otherwise both the wheels would never point forward when the car is not turning. Low Lateral Acceleration At low speeds when the tires have minimal tire shear losses on dry, clean pavement, the true Ackermann steering geometry is beneficial as the tires are in almost a perfect situation of minute slip angle. Parallel or reverse Ackermann in this scenario would push (or under steer) the front of the car away from the desired path. In both situations, the inside tire contributes to this push similarly to a centrifugal force. High Lateral Acceleration At high lateral accelerations, true Ackermann becomes disadvantageous as loads on the outside wheel increase and the greater slip angle of the inside tire creates higher tire temperatures and slows down the car due to tire drag. The inside tire has also surpassed the maximum slip angle of grip assuming the outer tire is already at the optimum slip angle. Parallel or reverse setups are Dept. of Mechanical Engineering 4 Steering System more advantageous in this situation as both the inside and outside tires still have lateral grip. Reverse Ackermann steering can even be more beneficial than the parallel Ackermann geometry since the outside tire (which currently has more load due to weight transfer) is at the optimum slip angle and the inside is at a lower slip angle with less grip. This in turn allows the inside tire to have grip but less than the outside tire, decreasing the effects of under steer. 100% Ackermann is when both the wheels are travelling in concentric circles while 0% is for travelling in equal circles. Forward Ackermann geometry with 60% Ackermann was chosen for our BAJA vehicle. Reasons for the choice: It creates an additional drag force that helps yaw the car. The second is that the slip angle of maximum lateral force changes with vertical load, so to extract maximum lateral force, the outside wheel needs to be a different amount than the inner. Camber Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber. The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. The camber angle taken for Baja car is typically around neg. 1/2 degree as tire develops its maximum cornering force at such a small negative camber angle. This fact is due to the contribution of camber thrust, which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch). Dept. of Mechanical Engineering 5 Steering System Caster Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, then the caster is negative. Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved. The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by Dept. of Mechanical Engineering 6 Steering System imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it. Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable. Toe in/Toe out When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics. For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to Dept.

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