Steering Geometry Diagnostics
When you’re in the wheel alignment trade, you will inevitably have a vehicle come back with steering quality complaints or uneven tire wear after having the alignment adjusted. Tire casing problems aside, the fault would almost always be found in defective steering geometry caused by a bent steering knuckle assembly. Bent steering knuckle assemblies are easy to ignore simply because they do require extra time and effort to measure and evaluate in today’s fast-paced undercar service market but, the symptoms of bent steering knuckles are easy to spot, if we do a thorough pre-alignment inspection.
Camber, Caster, and Toe
Let’s begin with a recap. Positive caster angle is best illustrated by the rearward tilt of the steering fork on a bicycle. Positive caster obviously places the front wheel ahead of its pivot point and most vehicles are designed with positive caster angle. A negative caster angle is best illustrated by the casters on a tool box trailing their pivot points. When weight is applied to the two front wheels of a vehicle, positive or negative caster forces the front wheels to a centered position which helps reduce steering wander. Camber is the vertical position of the wheels in relation to the road surface. Negative camber results when the tops of the two front wheels tilt inward toward the chassis centerline. Positive camber results when the tops of the wheels tilt outward from the chassis centerline. Positive camber works in conjunction with king pin or steering axis inclination (SAI) to reduce steering effort. On older vehicles with individually replaceable wheel bearings, positive camber places the vehicle weight squarely on the larger inner wheel bearing. Toe is the most critical tire-wearing angle. Wheels pointed inward from the centerline are “toed” in, wheels pointed outward from centerline are “toed” out. A slight amount of toe-in is required to prevent the front wheels from following ruts or contours in the road, and compensates for flexing and wear in the tie rods and tie rod ends, as well as for minor changes in suspension height and geometry.
SAI And Steering Radius
Two steering geometry angles, SAI and steering radius, are built into the steering knuckle and are non-adjustable. In the real world, defects in SAI and steering radius often go unnoticed if the vehicle is driven on interstate-style highways and not through turns on city streets. The upper ball joint or strut support bearing on a front suspension is closer to the chassis centerline than the lower ball joint. An imaginary line drawn through the upper strut support bearing or ball joint and lower ball joint should theoretically intersect with the centerline of the tire at the point of road contact. SAI consequently allows the wheel to pivot on its centerline. If the SAI is incorrect, the tires begin to swing in a radius around this theoretical pivot point. Incorrect SAI caused by bent struts, bent spindles or excessively offset wheels results in greater steering effort and accelerated suspension system wear. SAI also tends to return the front wheels to center because, when combined with caster angle, SAI to apply more weight on the inside front wheel by lifting the chassis an inch or two. At center, SAI acts in combination with the caster angle to reach equilibrium on both wheels reducing steering wander. SAI and caster angle generally increase the positive camber angle of the inside tire and decreases positive camber angle of the outside tire during a turn. This camber change counteracts the tendency of the tire tread to lift from the road surface during a turn.
Because the inside wheel turns through a shorter radius than the outside wheel, the steering system must change from toe-in to toe-out to reduce tire scrub when navigating a sharp corner. The portion of steering knuckle responsible for turning the inner wheel through a sharper turning radius is the steering arm. The angle of the steering arm intersects the vehicle centerline at approximately the length of the vehicle’s wheelbase. The angle of the steering arms allows the rear wheels to track more closely with the front when turning a corner. The actual process of going from toe-in (or toe zero) to toe-out when navigating a turn is known as the Ackerman Effect. The Ackerman Effect is always a compromise between different driving conditions. A NASCAR Toyota, for example, might have very little Ackerman angle because the car is driven through long, sweeping curves, often at a slight drift angle. In this case, two degrees of Ackerman would increase tire wear and negatively affect the driver’s control of the vehicle. Most NASCAR vehicles feature a slotted steering arm that allows Ackerman angle to be adjusted on each steering arm to meet track conditions. At the other extreme, a metro-area delivery van steering around 90-degree street corners and into very confined turn-around areas would need at least two degrees differential in Ackerman angle to make precise turns and reduce front tire wear. Without a sufficient Ackerman angle, the vehicle would tend to “push” going around a sharp corner, which would result in poor steering response and accelerated tire wear.
Old bias-ply designs from the 60s produced very high rolling friction and were very sensitive to incorrect camber and toe angles. The bias-ply belted tires of the 70s and 80s were an improvement on the original design, but developed problems when used on vehicles with steering geometries designed for bias-ply tires. Current passenger tire designs use a flexible sidewall and a firm tread belt to deal with the negative camber and high caster angles used in modern steering geometries. Negative camber angles and increased caster angles increase the tread contact patch at high cornering speeds, which improves steering quality and response. Toe angles have been reduced because modern suspension produces much less toe variation. Always inspect tire pressure, matching casing design, size and tread pattern before road testing. Test for loose steering components by rocking the steering wheel key-on, engine off. Badly worn steering shaft couplers and tie rod ends will generally make a knocking sound. The steering should be checked hands-off when starting the engine. If the steering wheel rocks as the engine starts, the pressure metering system in the power steering gear might be defective.