Designers often simply assume that 90° angles between the front wheel spindles and the steering levers provide proper steering.
DANIEL D. FRITZINGER
Edited by Paul Dvorak
Small vehicles with inexpensive steering systems often show high tire wear. This is because the front tires most likely do not use the same point for their turning radius. Users may notice that such vehicles do not negotiate corners well and front wheels tend to fight each other.
Designers often simply assume that 90° angles between the front wheel spindles and the steering levers provide proper steering. They do not. The crux of the problem is finding the proper spindle-lever angles. The following procedure does so.
A common steering system for simple vehicles such as grain wagons, toy cars, and low-cost go-carts consist of a spindle-lever arm and a member that moves the tie rod to produce steering action. A wagon tongue connected to a steering column serves the purpose in a towed grain wagon. When a properly designed steering system turns, the centerlines of the rear axle and both front axles intersect a single point, the center point of the turning radius. With such designs, vehicles smoothly negotiate turns.
Before designing a steering system or troubleshooting an existing one, calculate the tie-rod length from:
Tlength = W + 2Lsin(φ)
where W = spindle-to-spindle center distance, in.; Tlength = tie-rod length, in.; L = center-to-center lever arm length, in.; and = angle between lever arm and vehicle centerline, degrees. Estimate this angular value to begin the calculations.
For a required turning radius, calculate the angles through which the inner and outer spindles must rotate for their centerlines to intersect the rear axle centerline at the same point. These angles are not equal and are labeled l and r for left and right. For a left turn, l will be greater than r. Find the angles from:
where M = front to rear centerline length, in.; and R = a turning radius, in.
Determine new positions of points A and B on the steering lever arms from:
Ax = Lsin(φ r)
Ay = Lcos(φ r)
Bx = Lsin(φ+ l)
By = Lcos(φ+ l)
Calculate the length of the tie rod in turn mode, Tt, which is the hypotenuse of triangle ABC in the diagram with
The tie-rod length does not change, but the geometry that defines its length does. Consequently, the best value for lets Tlength = Tt. But it is unlikely that the correct value of will be selected on the first try. Ambitious users can solve for after setting the equation for Tlength equal to Tt. Users can also put the equations into a spreadsheet or other math package and solve for a range of values. From the sample spreadsheet, a usable value for comes when the difference between Tlength and Tt approaches zero.