The basic job of a suspension system is to keep an even tire weight / force over all road conditions while maintaining proper steering angles to enhance handling while minimizing tire wear. To accomplish this, the spring rates, sway bar rates, tire sidewalls and damping of the shocks should be matched within certain ranges. The change in suspension angles, meaning wheel geometry or alignment angles, during turning and compression / rebound needs to be controlled within certain limits. The pitch, or tilt front to rear, during acceleration and braking also needs to be controlled.
Caster is the angle formed between a vertical line and the steering axis when viewed from the side of a car. The steering axis is a line connecting the top suspension pivot point to the lower ball joint, i.e.; the line around which the suspension pivots. Caster is a functional angle in that the higher it is, the more stable the steering while turning. High caster makes the steering “want” to return to center after turning. The further the steering is turned, the more the tires lean into a curve. The camber of the outer tire changes more towards negative and the camber of the inner tire goes more positive the further the steering is turned. Caster also works to cushion the effect of bumps while turning. Most vehicles use a caster setting between +3 and +7 degrees.
Caster is not adjustable on most vehicles, although the subframe might be shifted to correct a side to side variance. Caster is more commonly found as an adjustment on larger vehicles such as Vanagons and Eurovans. It cannot be directly measured by an alignment machine, and is calculated using the camber change between when the wheel is turned inwards vs. outwards to a specific angle. Rear caster is rare, since it only exists on 4 wheel steer vehicles.
The steering axis inclination (SAI) is the angle between the centerline of the steering axis and a vertical line from center of the tire when viewed from the front. SAI urges the wheels to a straight ahead position after a turn. By inclining the steering axis inward at the top, it causes the tire to rise as the wheels are turned, roughly describing an arc. As the tire is in its lowest position at center steer, the steering naturally wants to return to center. Think of caster being the twisting angle front to rear, and SAI being the twisting angle side to side. A proper range of combination of these two angles provides stability without as much of the tire edge wear that occurs due to high caster.
Camber is the angle of the tire in and out at the top versus the bottom compared to a vertical line. Camber functions for side to side stability. Higher negative camber corners better, but wears the inside edge of the tire during straight line driving. It also reduces the traction of the inside wheel. At zero camber, the tire patch has just a sideways shear force component, and the inside edge wants to lift, while the outside edge wants to curl under. Angling the outside tire against the side force of cornering maximizes the contact patch area. Ideally, one would think that the inside wheel should change to positive camber during a turn. To some extent, that is the job of the caster / SAI combination of angles. Since the weight is partially lifted off an inside tire, the traction loss of having the wheel at some camber angle less negative than the outside tire is minimized. Conversely, for maximum traction during acceleration and braking, camber would be at zero. Most cars have front wheel camber set around -1 to -2 degrees. The rear wheel camber is generally about the same or higher than the front.
Toe, in alignment terms, is the in and out angle of the front of a tire in relation to the rear of the tire. Most cars the tires toe in very slightly at the front, called positive toe in. This assists the vehicle to stay straight on the road. If the toe gets too high, the tires scuff as you drive, and rapid tire wear will result. The resulting force of toe is related to camber. Camber mildly causes a tire to want to roll in an arc, inwards for negative camber, outwards for positive camber. This happens due to the drag of the tire patch on the road, and changes with road speed and power or braking. A small amount of toe in will compensate for this effect with negative camber, vice versa for toe out and positive camber. Generally, total toe, combining both front wheels, is less than +.1 degree. Rear toe is generally higher to help stability.
The exact length of the steering rack and each tie rod assembly, plus sometimes having the ends of the tie rods angled, make it so that the inside wheel turns more than the outside wheel in a curve. This difference in toe from side to side compensates for the fact that the inside wheel is turning on a smaller radius on the road. This avoids the two front tires fighting each other during sharp turns.
There are two other measurements that concern the suspension angles versus the contact area of the tire patch. The front to rear dimension is called trail. The side to side dimension is called scrub radius.
Positive caster yields a overall steering pivot line that touches the road some distance in front of the tire contact patch. This distance is called trail. Think of the tire contact patch is a point of drag against the movement of the vehicle. It tends to want to pull itself into a straight line with the caster angle, helping the steering return to center. The length of the trail at rest is shorter than when the vehicle is being driven. The lateral forces now act at some distance behind the center of the contact patch due to an effect of the tire called pneumatic trail. Nominally at freeway speeds, pneumatic trail is about one inch. Large differences occur due to tires, inflation pressure, weight, speed and temperatures, but the overall effect is always similar. Caster versus trail yields good steering feel and even control of varying steering angles.
Scrub radius is the distance between the steering axis inclination line and the tire contact patch center line at the road. Zero scrub radius is when the lines intersect at the road surface. Positive scrub radius is when the SAI line is inside the tire line. Negative scrub radius is when the SAI line falls outside of the tire line. At zero scrub radius, the tire tread wants to squirm, causing some instability and tread wear. Most cars have negative scrub radius, which helps reduce torque steer, which is steering force of a tire under power. When scrub radius gets too high, whether positive or negative, the steering effort increases and changes in steering require more effort, i.e.; poor road feel. One can visualize that the effect comes from having to force the tire traction point ahead or behind the pivot point while steering. Changing rim offset and incorrect camber adjustment both contribute to altering scrub radius, sometimes detrimentally.
To put it all together, the real test of suspension angles is how the wheels respond during driving. Most important is that the toe not change significantly during steering, even when the vehicle hits a bump or a pothole, even when accelerating or braking. Camber changes during steering should be calculated to assist traction and control. Modern 5 point suspension technology (the tie rod end is included as a point), has the amazing ability to move the effective top suspension pivot point in all three dimensions to provide a degree of control well beyond what was possible last century.
Springs, Shocks and Sway Bars
Sprung weight is the total weight of a vehicle that is suspended: Engine, transmission, body etc. Unsprung weight is the total of the weight that is moving up and down with the suspension. This includes the rim and tire, brakes (on most cars), the spindles with their bearings, hubs and balljoints, the lower section of the shocks or struts, about half the weight of the control arms and springs, and part of the weight of any driveshaft.
Roll centers are the points about which the vehicle’s sprung weight will rotate or roll during a turn. In general, a higher roll center is better, as the center of gravity of the sprung weight is more dangling under the roll centers. The converse will cause more roll for the same amount of side force, and in extremes is not particularly safe. Reference early army jeeps with control arms and rear leaf springs that would roll over without a moments notice. Let’s not mention some of the early SUV’s that followed suit. A roll axis is the center line that runs between the front and rear roll centers. Controlling roll is the job of the springs and sway bars.
Most spring are wound evenly, meaning a constant angle between each coil section. As weight is put on the vehicle, the springs compress in a fairly linear manner. Within its working range, if the first 100 lbs compresses a loaded spring 1″, then 200 lbs compresses it 2″, i.e.; the spring rate. Progressive spring are wound by varying the angle and diameter between coils. The tighter coils touch each other during compression, which effectively shortens the spring, making it stiffer. This helps control the higher forces during a big bump by helping limit the overall compression. It also helps control movement as a spring returns from extension so the suspension does not go way past center as it returns to neutral. The tighter coils may contact somewhat with just vehicle weight, still allowing a progressive spring rate as the suspension goes down to droop and back to neutral. Progressive springs also work to reduce body roll or sway.
The actual force versus up and down movement on a wheel, or wheel rate, depends on the location of the spring in relation to the wheel. Think of it as the effect of spring rate directly on the wheel. In a strut configuration, the spring is almost directly attached to the wheel, and the wheel rate is equal to or very close to the spring rate. If the spring is mounted on a control arm, then the geometry determines the wheel rate, and it is always less than the spring rate by some factor.
Springs rarely fail, as by design they are more than capable of dealing with a vehicle’s weight. The only sign of a failing spring is a change in ride height. Overloading may damage springs, but more commonly failed springs come from poor design and/or metallurgy in aftermarket springs.
Shocks control springs, including the sway bars, as sway bars are basically side to side springs. Shocks / struts also help control pitching, the front to rear movement caused by braking or sharp acceleration. Shocks work by capturing the energy of the movement of the unsprung weight by controlling the shock fluid through internal valves. In the process, the kinetic energy is converted to heat, and the fluid warms up.
Start with the concept of critical damping, which is the amount of damping force that it take to bring a suspension back to neutral ride height after an upward compression or downward droop without allowing any movement past the neutral point or oscillations. The best soft damping is about half critical, meaning one movement past neutral, then back to neutral without further oscillations. One quarter critical would allow two oscillations, which is poor control. Any shock worn or damaged to this state must be replaced. Damping at the critical level may already slightly unweight a tire during rebound. Damping higher than critical can lead to the vehicle skittering across the road when driving over bumps through curves, as the tires unweight and loose traction over bumps. And the ride is felt as too stiff for most drivers. Shocks typically damp compression less than rebound to reduce jarring over bumps. During rebound, the shock must control the energy stored in a spring from compression.
Sway bars control the tilt of a vehicle during the side force of turns. They do so by pushing down on the outside tire while lifting up the inside tire. Too large of a sway bar on a light vehicle will unweight the inside tire enough to loose traction. Bigger is not better. Proper range is important. The heavier a vehicle, the larger the sway bay that can be used without issues.
Most cars now use sway bars front and rear. The ratio of stiffness is calibrated to control handling balance, which is the tendency of a vehicle to understeer or oversteer. Understeer is when the front end breaks loose first and “plows” across the road. Increasing rear bar stiffness or decreasing front bar stiffness decreases understeer. Oversteer is when the back end kicks out under hard cornering force. Since either condition under neutral power is related to the sprung weight, the front to rear weight ratio is the biggest component.
Some SUV’s are equipped with sway bars that disconnect for off road use, allowing free movement without unweighting tires. Some higher end vehicles have variable shock absorbers connecting the ends of the sway bars to the chassis, allowing a variable amount of roll control.
Think of a tire sidewall as effectively being an air spring, with a slight bit of damping from the tire carcass. A short sidewall and/or high tire pressures makes that spring stiffer, and limit side to side movement between the rim and the tread. Tall sidewalls and/or lower pressures make the spring softer, and allow more sideways movement. Tire sidewalls need to somewhat matched to the vehicle suspension for stiffness to operate correctly. There are other factors in the load rating and rolling characteristics of tires, but stiffness is definitely the most critical.