What suspension and steering do

It is the second week of March, and Calgary is in the middle of the season every front-end technician knows. A warm chinook melts the snow by day, the water runs into cracks in the pavement, and it freezes again at night. Each freeze pries the asphalt apart a little more, and by the time the plows have scraped over the softened patches, the city's streets are covered in potholes. Ridgeline Auto Service's phone starts ringing with the same complaints: a clunk over bumps, a pull to one side, a steering wheel that is no longer straight, a vibration at highway speed.

Kiran Dhaliwal is nearly a year into her apprenticeship. She has done oil services, tires, inspections, engine work and a winter of brakes. On Monday morning Simone Tremblay puts a familiar repair order on the board: Gord Haney's pickup, with a note that says “clunk and squeak over bumps, front, getting worse.” Kiran remembers the truck. Twice last year Ridgeline recommended new front sway bar bushings, and twice Gord said they could wait. “Well,” says Marty Kowalchuk, reading over her shoulder, “this is the term you learn suspension and steering. Let's start with what all those parts are actually for.”

This lesson explains what a suspension and a steering system have to do, how they carry the vehicle's weight and the forces of driving, what is meant by sprung and unsprung mass, how the tires' contact patches tie everything to the road, and how the parts are grouped into the systems the rest of this course covers. It ends with the safety rules for suspension work and with the first inspection of Gord's truck.

Five jobs at once

A suspension connects the wheels to the body or frame. It has five jobs, and every design is a compromise among them. First, it carries the weight of the vehicle and its load and holds the body at the right height. Second, it keeps the tires in contact with the road, pressing them down with as even a force as possible, because a tire that lifts or bounces cannot brake, steer or accelerate. Third, it isolates the passengers and cargo from the bumps, so that a wheel can move up and down over a pothole without the whole vehicle following it. Fourth, it controls how the body moves: how far it leans in a turn, how much it dives under braking and squats under acceleration, and how quickly bouncing dies away. Fifth, it holds each wheel at the angles the engineers chose, so that the tires roll straight, wear evenly and let the vehicle go where the driver steers.

The steering system lets the driver turn the front wheels, and on some vehicles the rear wheels as well, through a controlled angle, with a sensible effort, while giving the driver a feel for what the tires are doing. It must return to straight ahead on its own when the driver relaxes, and it must not jerk the wheel when one tire hits a bump. Steering and suspension are tightly linked: the steering parts are attached to the same knuckles the suspension moves, so wear or damage in one shows up as a symptom in the other.

Wheel alignment is the setting of the angles at which the wheels sit: whether they lean in or out, whether they point in or out, and how the steering axis is tilted. Alignment is not a separate system; it is the result of how the suspension and steering parts are made, fitted and adjusted. Lessons 10 and 11 cover it in detail.

Sprung and unsprung mass

Everything supported by the springs is called sprung mass: the body or frame, the engine and transmission, the passengers and the cargo. Everything that is not carried by the springs, but moves up and down with the wheels, is unsprung mass: the tires and wheels, the brake rotors and calipers, the hub and knuckle, and part of the mass of the springs, the dampers and the control arms that connect to the body. On a solid rear axle, the whole axle housing, differential and axle shafts are unsprung.

Engineers want unsprung mass to be as low as possible. When a wheel hits a bump, the spring has to push the wheel back down onto the road. A light wheel follows the road quickly; a heavy wheel keeps moving upward and takes longer to come back, so the tire's grip falls for an instant. The ratio of sprung to unsprung mass matters: the higher it is, the less the body is disturbed by the wheel's motion and the better the tire follows the road. Aluminum control arms and knuckles, lighter wheels and independent rear suspensions are all ways to reduce unsprung mass.

# Sprung and unsprung mass of four shop vehicles (one corner shown)
UNITS = "metric"                        # "metric" or "US"
cars = [
    # name, total mass kg, unsprung mass of one corner kg (wheel, tire, brake, hub, half of arms and spring)
    ("Gord's pickup, front", 2220, 62),
    ("Ines's hybrid, front", 1380, 36),
    ("Dr. Singh's EV, front", 1690, 41),
    ("courier van, rear axle", 2700, 95),
]
for name, total, corner in cars:
    unsprung = corner * 4
    sprung = total - unsprung
    ratio = sprung / unsprung
    if UNITS == "metric":
        print(f"{name:<24} unsprung about {unsprung:4d} kg, sprung {sprung:5d} kg, ratio {ratio:4.1f} to 1")
    else:
        print(f"{name:<24} unsprung about {unsprung * 2.205:4.0f} lb, sprung {sprung * 2.205:5.0f} lb, ratio {ratio:4.1f} to 1")
print("a higher ratio means the body is disturbed less when a wheel moves")

Gord's pickup, front     unsprung about  248 kg, sprung  1972 kg, ratio  8.0 to 1
Ines's hybrid, front     unsprung about  144 kg, sprung  1236 kg, ratio  8.6 to 1
Dr. Singh's EV, front    unsprung about  164 kg, sprung  1526 kg, ratio  9.3 to 1
courier van, rear axle   unsprung about  380 kg, sprung  2320 kg, ratio  6.1 to 1
a higher ratio means the body is disturbed less when a wheel moves

The figures are illustrative, but the pattern is real. The van's heavy solid rear axle gives it a low ratio, which is part of why an empty van rides so harshly over broken pavement: the axle bounces and the light body follows it. The compact hybrid, with its light wheels and aluminum parts, has a much higher ratio.

The contact patch

Every force that moves, stops or turns a vehicle passes through four small areas where the tires touch the road. Each one, the contact patch, is roughly the size of a hand. Its area depends mainly on the load the tire carries and the air pressure inside it: to a first approximation, the load divided by the inflation pressure gives the area, because the air inside the tire is what holds the load up. A tire that is underinflated has a larger, longer patch that flexes more, runs hotter and wears on its shoulders; one that is overinflated has a smaller patch, less grip and more wear in the centre.

In this rough relation, A is the area of the contact patch, F is the load on the tire and p is the inflation pressure. The real patch is a little larger, because the tire's carcass carries some of the load, but the relation explains why the placard pressure matters so much and why a heavy electric vehicle needs tires built for higher loads.

# Approximate contact patch area: load divided by inflation pressure
UNITS = "metric"
tires = [
    # name, load on one tire kg, pressure kPa
    ("pickup front, placard", 645, 380),
    ("pickup front, 25 % low", 645, 285),
    ("hybrid front", 400, 250),
    ("EV front", 460, 290),
]
for name, kg, kpa in tires:
    force = kg * 9.81                 # newtons
    area_m2 = force / (kpa * 1000)    # m2
    if UNITS == "metric":
        print(f"{name:<24} about {area_m2 * 10000:5.0f} cm2")
    else:
        print(f"{name:<24} about {area_m2 * 1550:5.1f} square inches at {kpa * 0.145:3.0f} psi")

pickup front, placard    about   167 cm2
pickup front, 25 % low   about   222 cm2
hybrid front             about   157 cm2
EV front                 about   156 cm2

A drop of a quarter in pressure makes the patch about a third larger. That is why the shop's tire pressure monitoring systems are set to warn at about that level, and why Kiran checks and sets pressures on every vehicle before she road tests it for a handling complaint. A soft tire can cause a pull all by itself.

Forces on a moving vehicle

A suspension carries more than the vehicle's weight. When the driver brakes, weight moves forward and the front springs compress, a motion called dive. When the driver accelerates, the rear squats. In a turn, the body leans outward, or rolls, and load moves from the inside tires to the outside ones. A bump pushes a wheel up suddenly, and the spring and damper must absorb that motion and then return the wheel. The suspension's links and bushings also carry the forces that push the vehicle forward, stop it and turn it: every braking force, for example, passes from the tire through the knuckle and the control arms into the body.

Engineers describe body motion with a few words that Kiran will hear for the rest of her career. Heave is the whole body moving up and down. Pitch is the body rotating nose up or nose down, as in dive and squat. Roll is rotation about a line running front to back, the lean in a turn. Yaw is rotation about a vertical line, the vehicle turning; stability control, from AUTO-130, measures yaw to tell whether the vehicle is turning as much as the driver is steering.

The parts and how they are grouped

The springs carry the load: coil springs, leaf springs, torsion bars or air springs. Lesson 2 covers them, along with the dampers, often called shock absorbers, which control how fast the springs compress and extend. Without dampers a vehicle would bounce for a long time after every bump.

The linkages locate the wheel: control arms, struts, trailing arms, links and the bushings and ball joints that let them pivot. They decide the path the wheel follows as it moves up and down and how its angles change on the way. Lessons 3 and 4 cover front and rear designs, including the stabilizer bar, also called a sway bar or anti-roll bar, which links the left and right wheels to resist body roll. Lesson 5 covers electronically controlled dampers and air suspension, which more and more vehicles use.

The steering system includes the steering wheel and column, a steering gear, usually a rack-and-pinion, and the tie rods that connect it to the knuckles. Lesson 6 covers those parts and their geometry, and Lesson 7 covers the hydraulic and electric power assist that makes steering light. Lessons 8 and 9 cover tires and wheels in more depth than the introduction in AUTO-110: how tires are built and rated, what their wear tells a technician, and how to find and cure a vibration. Lessons 10 and 11 cover alignment, and the capstone in Lesson 12 puts it all together on a pull and a vibration.

Dependent and independent suspensions

In a dependent suspension, the left and right wheels are joined by a solid axle or beam, so when one wheel rises the other is tilted with it. Solid axles are strong, simple and keep the wheels at a fixed track and camber as the vehicle is loaded, which is why pickups like Gord's and the courier vans use one at the rear. Their drawbacks are high unsprung mass and the way a bump on one side disturbs the other.

In an independent suspension, each wheel moves on its own linkage, so a bump on one side does not tilt the other wheel. Independent designs give a better ride, lower unsprung mass and more control of the wheel's angles, at the cost of more parts. Almost every car and light truck now has independent front suspension; most cars and many SUVs have it at the rear as well. In between is the twist beam, a rear design used on many small cars and some minivans, in which two trailing arms are joined by a beam that twists; Lesson 4 explains it.

Safety in suspension and steering work

Suspension work carries hazards that oil changes and brakes do not. A coil spring holding up a corner of a vehicle stores a large amount of energy, and a spring that is released carelessly can throw a strut or a control arm with enough force to kill. Springs are compressed only with a spring compressor designed and rated for that type of spring, inspected before each use, with its hooks or plates fully seated, and never with an impact wrench on the compressor's screw unless the manufacturer allows it. A strut assembly is never taken apart with the spring still under load.

Lifting points change, too. When a control arm or axle is unbolted, the vehicle's weight may shift on the lift, so Kiran learns to support the arm or axle with a jack before removing a bolt that carries load, and to use the lift's safety locks before working under the vehicle, as she learned in AUTO-110. Ball joints and tie rod ends are separated with the correct puller or a fork, never by heating the knuckle, and new self-locking nuts and cotter pins are used where the service information says. Every steering and suspension fastener is torqued to specification, many of them with the vehicle's weight on the wheels, because rubber bushings tightened with the wheels hanging will be twisted when the vehicle is lowered and will tear early.

Air suspensions and electronic dampers add their own steps: the air system must be put into service mode before the vehicle is lifted, so the compressor does not run and the control module does not try to level a vehicle with its wheels in the air. Lesson 5 covers that.

The dry-park check

Marty's first lesson on Gord's truck is a dry-park check, a quick test that finds a surprising amount of wear before the vehicle is even lifted. With the truck on the floor, wheels straight ahead and the engine running so the power steering works, Kiran rocks the steering wheel back and forth through a small arc while Marty lies under the front and watches the steering linkage. Any movement in a joint before the wheel itself moves is looseness that the driver feels as play. She then bounces each corner of the truck hard and lets go: the body should rise once and settle. More than about one and a half oscillations points to tired dampers.

Then the truck goes up on the four-post rack in bay 3 with the front wheels free on slip plates. Kiran grabs each front tire at the top and bottom and rocks it in and out to check for play in the wheel bearing and the ball joints, then at the front and back to check the tie rod ends. Marty shows her how to tell which part is moving: a helper watches each joint while she rocks the tire, and a finger placed across a joint and the part next to it feels movement that the eye cannot see. On a short-and-long-arm suspension like Gord's, the ball joint that carries the weight must be unloaded before it is checked; Lesson 3 explains how.

This time the noise is easy to find. The right front stabilizer bar end link's lower bushing has pushed out of its sleeve, and the link knocks against the control arm every time the right wheel moves. Both bar bushings at the frame are cracked and squeak. “That's what he heard last fall,” Marty says. “The bushings let the bar move, the bar beat on the link, and now the link is done too.” Simone builds an estimate for both end links and both bushings, and Gord, this time, says yes. Lesson 3 follows the repair.