Cause And Effect: Steering Geometry, Alignment, And Tuning
The various components of a typical straight-axle steering system are basic but require maintenance and tuning in order to function properly. From the rotation of the steering wheel to the meshing of the steering box's gears and on through the movement of the various links and levers that comprise the steering system, there is much to know. While this overview of steering systems, steering geometry, wheel alignment, and steering system tuning won't make you an instant steering expert, the information contained herein will make you a better - and more knowledgeable - off-road enthusiast.
The Principles of Suspension Geometry and Alignment
Since 4 Wheel Drive & Sport Utility is all about straight axles, we're going to concentrate on the type of steering systems and geometry inherent to such setups. Therefore, don't expect to see the term camber in this story, since the only time a straight axle shows any camber is when the housing is bent.
The aspect of steering that has the most effect on a solid axle's steering response and performance is the caster angle. Also known to oldsters as kingpin angle, caster angle refers to the vertical angle of the steering knuckle.
Usually, the tops of the steering knuckles are angled slightly to the rear for reasons of steering stability. With 2 to 6 degrees of caster set into the steering knuckles, the steering will exhibit a self-centering action, keeping the front wheels tracking straight ahead when the vehicle is driven in a straight line. Caster's centering effect also causes the front wheels to return to a straight-ahead position when the vehicle exits a corner. When the caster angle is near zero, drastic changes to the steering occur. With zero caster, the steering will tend to be twitchy, especially at highway speeds, and the front wheels won't self-center when driving in a straight line or when exiting a corner. On the other hand, adjust the steering knuckles with maximum caster and the opposite effect is seen, with huge increases in stability but less responsive steering.
That, of course, is a basic overview of caster changes, but it's important to familiarize yourself with how caster effects a straight axle-equipped 4x4's steering, especially if you plan to install - or have already installed - a suspension lift. Here's why: Compared to an OE leaf spring and the amount of caster it sets, an aftermarket set of spring packs may position the axle slightly ahead of or behind the original location. Imagine the axlehousing being attached to a high-arch, aftermarket spring pack at a location closer to the front hanger or shackle; such a position would rotate the axlehousing - and the top of the steering knuckle - rearward because of the curvature of the spring pack, creating increased caster. The opposite holds true if the axle is moved rearward on the curve of the spring pack; the top of the steering knuckle is rotated forward, and a reduction in caster is the result.
Sticking with caster changes, many front leaf-spring packs employ a thin 1- to 3-degree shim between the axlehousing and the spring packs that is intended to keep the driveshaft's U-joints from binding. However, when the thick end of the shim is placed at the front of the vehicle, the axlehousing and its steering knuckles are rotated forward, with a resulting reduction in caster.
Changes in caster angle also occur when a multi-link coil suspension is modified or lifted. If the axle is simply moved directly down - as with a lift - no caster changes result. However, if new locating links are installed, or if the OE locating links are reused with taller springs, there are definite changes in caster angle, which must be addressed to retain acceptable handling traits, since the locating links are responsible for positioning the axlehousings in both later and longitudinal planes, as well as in a vertical plane as it relates to the steering knuckles.
A steering system's toe setting is the adjustment that controls the angle of the front wheels. Viewed from directly above the front tires, toe is the distance between the leading and trailing edge of each tire. When the front of the tires are closer together than the rear of the tires, the steering is set toe-in. If the rear of the tires are set closer than the front of the tires, toe-out is the result. On a solid-axle 4x4, the toe setting is crucial to straight-line stability. Improper toe adjustment will result in noticeable drifting, especially at highway speed, as well as uneven tire tread wear.
Another interesting aspect of steering geometry is the Ackermann Angle. When the front tires are steered around a corner, the outer tire must travel a longer path than the inside tire. Thus, the angle of steering for the outer tire must be less than the inside tire, which is steered noticeably tighter. Setting the steering geometry to produce a proper Ackermann Angle is crucial to cornering, even at low speeds and on dirt, rock, or hard-packed surfaces. Fundamentally, Ackermann greatly reduces a cornering vehicle's tendency to scrub or skid its tires sideways. The degree of Ackermann is set at the factory and is not truly changeable without reengineering the entire steering system.
Caster is a measurement - in degrees - of the steering axis angle. As shown, the line drawn through the upper and lower ball joints is the caster angle. For accurate and stable steering and straight-line tracking traits, there should be several degrees of positive caster set into the steering knuckles. Positive caster also helps the steering system's self-centering effect when exiting a turn.
Steering and Alignment Tuning
When the steering system on your 4x4 went through the factory's research and development phase, very little time was spent considering the aftermarket suspension modifications that most enthusiasts make to their 'wheelers. In other words, an OE steering system simply isn't designed to function properly in a moderately or severely lifted application. In fact, as previously stated, even small changes to a suspension's ride height can drastically alter the steering characteristics. With that in mind, aftermarket performance steering manufacturers have developed specific products intended to make a lifted 4x4's steering system perform as the OEM designed it.