ACKERMAN PRINCIPLEThe creation of toe-out when turning to minimize tire wear. To create the proper geometry, the steering arms are angled to turn the inside wheel at a sharper angle than the outside wheel. This allows the inside wheel to follow a smaller radius circle than the outside wheel.
ACTIVE SUSPENSIONS A type of computerized suspension system that uses hydraulic "actuators" instead of or in conjunction with conventional springs and shock absorbers to support the vehicle's weight. Sensors monitor ride height, wheel deflection, body roll and acceleration so the suspension can handle bumps and irregularities in the road. The system can also limit body roll when cornering to improve handling.
AIR SHOCKS A type of shock absorber that contains an air bladder. By adding air to the bladder, the shock can maintain stock ride height and increase the suspension's load carrying ability. Used primarily for towing applications.
AIR SPRINGS Air-filled rubber or elastomer bags that are pressurized to provide support to the suspension. Air springs are used in place of conventional coil springs on some vehicles. Aftermarket air springs can be installed inside coil springs or between the axle and frame to provide additional lift support for handling overloads or towing.
AIR SUSPENSION A type of suspension that uses air springs instead of conventional steel springs. Computer operated vents on the air springs, suspension sensors and an onboard air compressor allow the system to maintain ride height and vary the suspension's ride characteristics.
ALIGNMENT The process of checking and adjusting steering and suspension components to position wheel geometry for optimum tire wear, handling, traction and stability. The process typically includes checking and adjusting (as needed) toe, camber and caster.
ALIGNMENT SHIMS Metal or plastic spacers used in the alignment process to alter camber, caster and/or toe. On rear-wheel drive applications, shims may be added to or removed from stacks of shims on the front control arms to change camber and/or caster. On front-wheel drive applications, partial shims or full contact shims may be positioned behind the rear axle spindle to vary rear toe and/or camber. Camber shims are also available for 4x4 axle applications. Some shims are adjustable or can be indexed various ways to provide incremental alignment corrections.
ALL-WHEEL DRIVE (AWD) A vehicle (usually a car) where all four wheels are driven. Most are fulltime systems for year-round driving, and use a viscous fluid coupling center differential instead of a transfer case to route drive torque to all four wheels. This allows the front and rear wheels to turn at slightly different speeds when turning on dry pavement.
AXLE, FRONT A crossbeam that supports the weight of the vehicle (typically a truck) and is connected to the spindles with king pins or ball joints.
AXLE,REAR May refer to the drive axles that connect both rear wheels to a center differential in a rear-wheel drive vehicle, or a crossbeam that connects both rear wheels and supports the rear of the vehicle in a front-wheel drive application.
BACKSPACING The distance from the back edge of a wheel rim to the back of the center section.
BALL JOINT A movable coupling in the suspension that connects the control arm to the steering knuckle and allows the wheels to steer. A ball joint is so named because of its ball-and-socket construction. Some support the weight of the vehicle ("loaded" ball joints) while others do not ("unloaded" or "follower" ball joints). Ball joints with steel bearings usually require greasing while those with hard plastic bearings may be sealed or require greasing.
BODY ROLL The leaning or tipping of a vehicle's body to one side when turning. This reduces traction and increases tire scuff due to undesirable alignment changes. Body roll is controlled primarily by a sway bar, but the stiffness of the springs and shocks also play a role.
BUMP STEER Page 1 The tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road. The condition is caused by uneven toe changes that occur as a result of the steering linkage or rack not being parallel with the road surface. This causes the wheels to change toe unevenly as the suspension undergoes jounce and rebound.
BUMP STOPS Rubber bumpers (often cone or wedge shaped) on the chassis that limit suspension travel. "Bottoming out" the suspension means hitting the bump stops.
BUSHINGS A liner, grommet or sleeve made of rubber, plastic or metal that fits around a bolt or bar to support, position and in some instances cushion the part. Bushings are used around the pivot bolts that attach the control arms to the chassis. They are also used around sway bars, the links that connect the ends of the sway bar to the control arms, and on the ends of strut rods. Rubber or soft elastomer bushings provide "compliance" in the suspension to help dampen road noise, vibrations and feedback. Hard plastic (usually polyurethane) bushings "firm" up the suspension for improved handling but also increase ride harshness.
CAMBER A wheel alignment angle that refers to the inward or outward tilt of the wheels as viewed from the front. Outward tilt is called "positive" camber while inward tilt is called "negative." Ideally, wheels should have zero rolling camber (perpendicular to the road) when the vehicle is loaded. Camber changes as the vehicle is loaded and the suspension sags. To compensate, static camber specifications may call for a slight amount of positive or negative camber depending on the type of suspension. On vehicles with independent rear suspensions, excessive negative camber often results when the vehicle is overloaded. Excessive camber can cause uneven tread wear on the tires (one side will be worn more than the other). Camber can be affected by worn suspension components such as control arm bushings and ball joints, or by bent parts such as a MacPherson strut or spindle.
CAMBER ROLL The change in camber that occurs when the front wheels on a vehicle with an independent suspension are steered to either side. The amount of camber change that occurs is affected by the amount of caster. Some camber change is good because it causes the tires to lean into a turn for better handling and traction. But too much camber change can accelerate shoulder wear on the tires.
CAMBER WEAR Tire wear that occurs on one side of the tread because the tire is leaning in or out. The underlying cause may be worn control arm bushings, a weak or sagging spring or a badly worn ball joint.
CAM BOLT A bolt fitted with an eccentric that is turned to change a wheel's camber setting. Camber bolts are typically used on control arms and lower strut mounts.
CASTER A wheel alignment angle that refers to the forward or rearward tilt of the steering axis of the front wheels. A forward tilt of the steering axis is called "negative" caster while a rearward tilt is called "positive." The caster angle has no direct affect on tread wear but it does affect camber when turning. It does have a direct effect on steering return and stability. Most vehicle have a certain amount of positive caster. The higher the caster angle the more steady the car feels at high speed. Many European cars have high caster settings. But too much caster can increase steering effort. The caster angle on many strut suspensions is fixed with no factory adjustments provided.
CASTER SHIMS Small wedge shaped shims that fit between a leaf spring and solid axle to change caster. Used primarily on trucks with a solid front axle or four-wheel drive.
CENTER BOLT The bolt that maintains the alignment of the leaves in a leaf spring, and the position of the axle on the springs.
CENTERLINE The geometric centerline of the suspension is a line that runs the length of the vehicle and intersects the midpoints of the front and rear axles. It is used as a reference line in alignment for measuring toe and thrust angle. The vehicle centerline is a line that runs down the center of the body or chassis. The geometric and vehicle centerlines may or may not coincide.
CENTER LINK The center bar or link in a parallelogram steering system that connects the pitman arm and idler arm. Also called a "relay rod."
CHASSIS The frame or undercarriage of a vehicle. On unibody vehicles, the lower structure to which the suspension is attached.
COIL SPRINGS Page 2 A type of spring made of wound heavy-gauge steel wire used to support the weight of the vehicle. The spring may be located between the control arm and chassis, the axle and chassis, or around a MacPherson strut. Coil springs may be conical or spiral wound, constant rate or variable rate, and wound with variable pitch spacing or variable thickness wire. Coil springs sag with age, and sometimes break. Replacement in pairs is recommended to maintain even ride height side-to-side.
COMPLIANCE The "give" or flexing that occurs in the suspension and steering due to the compression of rubber bushings and joint play. A small amount of compliance is desirable because it absorbs shocks and dampens vibrations to reduce steering feedback and harshness. But too much compliance can make the steering feel vague and mushy (unresponsive), while also contributing to toe wear by allowing excessive changes in toe alignment.
CONTROL ARMS Suspension components which connect the steering knuckles to the chassis or subframe, and allow the knuckles to move up and down.
CRADLE A structural member used in many front-wheel drive cars that supports the engine and transaxle. The cradle is bolted to the subframe, and is also connected to the lower control arms. The position of the cradle is important because it affects camber and caster.
CROSS CAMBER The difference side-to-side between camber settings. More than half a degree difference may cause a steering pull toward the side with the most (positive) camber.
CROSS CASTER The difference side-to-side between caster settings. More than half a degree difference may cause a steering pull toward the side the least (negative) caster. Caster on the left front wheel is sometimes decreased to compensate for high road crown, but adding camber is the preferred approach.
CROSSMEMBER A structural component that bolts between the frame rails or attaches to the subframe of a unibody. The lower control arms may be attached to the crossmember. The position of the crossmember is important because it affects camber, caster and setback.
DEGREE A unit of measure equal to 1/360th of a circle. Camber and caster angles and sometimes toe settings are specified in degrees.
DIRECTIONAL STABILITY The ability of a vehicle to steer straight with minimal driver input. This requires proper alignment as well as steering and suspension parts that are in good condition.
DOG TRACKING Also called crabbing, this refers to a condition where the rear wheels do not follow straight behind the front ones because of rear axle or rear toe misalignment. The rear wheels track off to one side, which produces off-center steering and contributes to front toe wear.
DYNAMIC BALANCE Wheel balance that results from the equal distribution of weight on both faces or sides of a wheel. Achieving dynamic balance requires spinning the wheel to identify the heavy spots on each side. A wheel that lacks dynamic balance will shimmy back-and-forth.
FLEXIBLE COUPLING A rubber or fiber coupling or donut in the steering column that acts like a U-joint to provide flexibility and/or to accommodate changes in shaft angularity.
FOLLOWER BALL JOINT A ball joint that is not loaded or does not support weight.
FOUR-WHEEL ALIGNMENT Aligning all four wheels, not just the front two, to predetermined specifications.
FOUR-WHEEL DRIVE (4WD or 4X4) The use of all four wheels (front and rear) to drive a vehicle (usually a truck or sport utility vehicle). Four-wheel drive provides improved traction on marginal surfaces and for off-road driving, but also creates more friction which can reduce fuel economy. Some applications are full-time four-wheel drive while others are part-time four-wheel drive.
FOUR-WHEEL STEERING A system that uses all four wheels to steer the car. Only used on a limited number of models (primarily Japanese). Turning the rear wheels in the opposite direction to the front at slow speeds can allow faster maneuvering and a much tighter turning radius. Turning the rear wheels in the same direction as those at the front at high speed allows sudden lane changes with much greater stability. Turning the rear wheels in the same direction as the front when parking makes parallel parking much easier.
FRAME ANGLE The angle of a truck's frame with respect to the ground. The angle affects front caster. For every degree of change in the frame angle, caster changes one degree. Raising the rear of a truck increases the frame angle (positive) and decreases caster while lowering the rear decreases the frame angle (negative) and increases caster.
FRONT-WHEEL DRIVE (FWD) Using the front wheels to drive a vehicle instead of the rear wheels. This usually requires the front wheels to be slightly toed-out to compensate for suspension compliance that occurs when the wheels pull the vehicle and want to toe-in. Most rear-wheel drive vehicles, by comparison, have the front wheels slightly toed-in to compensate for compliance that causes the wheels to toe-out as the vehicle is pushed down the road. The primary advantages of FWD are more compact packaging of the vehicle's powertrain and drivetrain, and improved directional stability on snow and ice. FWD places more weight over the front wheels, though, which increases the load and wear on the front brakes and suspension. FWD can also induce "torque steer" causing the vehicle to veer or pull to one side when accelerating.
GAS SHOCKS A type of shock absorber that is pressurized with nitrogen gas to reduce fluid foaming and cavitation. This reduces shock fade for improved ride control. Used as original equipment on many vehicles today,and also sold as an upgrade shock for older vehicles or those without gas shocks.
GEOMETRIC CENTERLINE A reference line that runs the length of a vehicle and intersects the mid-points of the front and rear axles.
HEEL AND TOE WEAR A diagonal wear pattern that forms on the rear tires when rear toe is not equal. This type of wear can produce an annoying vibration that feels like a bad wheel bearing. The cure is to reset rear toe and replace the tires.
HELPER SPRINGS Auxiliary springs that increase a suspension's load carrying capacity. These are typically bolt-on springs with a progressive action that do not come into play until the vehicle is loaded or the suspension deflects past a certain point. May be leaf or coil springs.
HUB CENTRIC A wheel that is centered or located on the hub by a machined center hole -- as opposed to "lug centered" wheels that are located by the position of the lug nuts alone.
IDLER ARM A pivot point in a parallelogram steering system that follows the motions of the pitman arm. A worn idler arm bushing typically causes steering wander (looseness) and toe wear.
INCLUDED ANGLE The sum of the camber and SAI angles in a front suspension. This angle is measured indirectly and is used primarily to diagnose bent suspension parts such as spindles and struts.
INDIVIDUAL TOE The toe setting of each individual wheel with respect to the suspension's centerline. It is the difference in distance from the centerline to the leading and trailing edges of the tire.
JOUNCE Compression of the suspension caused by the upward movement of the suspension or the downward movement of the body or chassis. Jounce occurs when hitting a raised bump. This is followed by "rebound" as the suspension springs back.
KINGPIN A pin that serves as the pivot or hinge for the steering knuckle, used primarily on trucks with I-beam axles and older vehicles that do not have ball joints.
KINGPIN INCLINATION (KPI) The angle formed by a line that runs through the king pin in the steering knuckle on a truck with an I-Beam axle. It's the same as the steering axis inclination (SAI).
LATERAL RUNOUT Also called axial runout, it is the amount of sideways motion or wobble in a wheel or tire as it rotates. It is usually measured by holding a dial indicator against the face of the rim or tire sidewall. A wheel with too much lateral runout will wobble back and forth as it rotates creating a shimmy that feels like dynamic imbalance problem.
LEAD The tendency of a vehicle to drift to one side as it is being driven. Possible causes include road crown, cross camber, cross caster, rear axle steer, uneven ride height side-to-side, uneven tire pressure or mismatched tire sizes side-to-side.
LEAF SPRINGS A type of spring made out of a flat strip or individual leaves. Most are steel, but some are made of lightweight composite materials.
MACPHERSON STRUT A suspension component that combines the functions of a shock absorber with the upper control arm and spring. Named after Earl S. MacPherson who invented it in the 1950s, the strut serves as the upper pivot for the steering knuckle and also supports the weight of the suspension. Used on both front- and rear-wheel drive cars, strut suspensions save weight and bulk but cannot provide as precise a control over wheel alignment as SLA suspensions.
MATCH MOUNTING Mounting a tire on a rim so the low spot of the rim lines up with the high spot on the tire. This reduces overall runout for a smoother running tire and wheel assembly.
MEMORY STEER A condition where the front wheels remember and seek a set position rather than returning to straight ahead. This may cause a steering pull or drift to one side.
MODIFIED STRUT A type of strut suspension where the coil spring is mounted between the lower control arm and chassis instead of around the strut. Typical applications include late model Mustangs and Camaros.
MOTORIST ASSURANCE PROGRAM (MAP) A voluntary program administered by the Automotive Maintenance and Repair Assn. for promoting certain standards of ethics and uniform inspection and repair procedures for the automotive service industry.
NEUTRAL STEERING A vehicle that neither understeers or oversteers. It responds predictably and evenly to steering inputs when cornering.
OFF-CENTER STEERING A condition where the steering wheel is not centered or is crooked when traveling straight ahead. The condition contributes to toe wear because anytime the wheels are steered off dead center, they toe out slightly which increases side slippage and scrubbing. The underlying cause of off-center steering is often rear axle or toe misalignment, but it can also be caused by failing to center the steering prior to adjusting toe. If accompanied by a lead or pull to one side, the underlying cause may be cross camber, cross caster, uneven tire pressure or mismatched tire sizes side to side.
OFFSET The position of the backside of the wheel center section with respect to the centerline of the rim. If the center is closer to the back of the wheel, is has "negative" offset. If the center is closer to the outside face or front of the wheel, it has "positive" offset. Most wheels on FWD cars have positive offset.
OUT OF SPECIFICATIONS The condition of having alignment readings are beyond the minimum or maximum range of tolerances allowed by the vehicle manufacturer. This calls for corrective action by realigning the wheels.
OVERLOAD SHOCKS A type of shock absorber that's equipped with a helper spring to keep the suspension from sagging when a vehicle is heavily loaded.
OVERINFLATION A condition where a tire contains more air pressure than the recommended amount for the tire size and load. Overinflation reduced rolling resistance but also increases ride harshness and the risk of tire damage.
OVERSTEER A condition that occurs when cornering where the rear wheels start to break loose and slide to the outside. Also used to describe the handling trait of a vehicle where the suspension tends to overrespond or overreact to steering changes. Rear-wheel drive vehicles tend to oversteer rather than understeer.
PARALLELOGRAM STEERING A type of steering linkage that uses a pitman arm, idler arm and center link to steer the front wheels. Used primarily on trucks and older rear-wheel drive cars, the system is so named because the center link always moves parallel to the axle.
PITMAN ARM The arm connected to the steering box sector shaft that moves side to side to steer the wheels.
POWER STEERING A steering system that typically uses hydraulic pressure to boost the steering effort of the driver (a few new vehicles use electromechanical assist). Hydraulic pressure is generated by a pump, and routed to the steering rack or power cylinder by control valves in the steering rack or on the steering gear.
PREALIGNMENT INSPECTION A procedure that is conducted prior to an alignment check or adjustment to identify worn or damaged steering and suspension parts. It's an absolutely essential step because worn parts can't hold proper alignment.
PREFERRED ALIGNMENT SPECIFICATIONS Alignment specifications for camber, caster and toe that are in the middle of the acceptable range. For most applications, these are considered the "ideal" settings for maximum tire life and optimum handling.
PRELOAD A thrust load applied to wheel bearings to minimize axial or sideways play. The amount of preload is critical because too little can contribute to steering wander while too much may cause premature bearing failure.
PULL The tendency of a vehicle to steer to one side while being driven. To keep the vehicle going straight, the driver may have to apply constant pressure on the steering wheel. Alignment factors include rear axle steer, cross camber and cross caster. Nonalignment factors include uneven tire pressures or mismatched tires side-to-side, spring sag, brake drag or mechanical binding in the steering or suspension. An imbalance in the power steering system can also cause a pull. If the pull only occurs when braking, the cause is brake imbalance and is not steering or alignment related.
RACK & PINION STEERING A type of steering gear that uses a pinion gear to drive a horizontal bar (the rack). The system is simpler than a recirculating ball steering gear and is used in most front-wheel drive cars and minivans. The rack may be manual or power-assisted.
RADIAL RUNOUT Variation (out-of-round) in the radius or circumference of a wheel or tire. It is measured by placing a dial indicator on the inside edge of the rim or tire tread. Too much radial runout can cause up-and-down vibrations similar to those caused by a static imbalance.
RADIAL TIRE A type of tire that's constructed with the reinforcing belts positioned sideways under the tread rather than lengthwise. This increases flexibility in the sidewall which reduces rolling resistance for improved fuel economy. A radial tire can be identified by looking for the letter "R" in the size designation on the tire's sidewall.
REAR AXLE STEER A steering pull or lead to one side caused by misalignment of the rear wheels or axle. Misalignment creates a thrust angle that causes the vehicle to lead to one side resulting in an off-center steering wheel and accelerated toe wear in the front tires.
REAR-WHEEL DRIVE (RWD) A method of driving a vehicle whereby engine power is applied to the rear wheels. Power from the engine flows through the transmission, down the driveshaft, through the differential to the rear axles and wheels.
REAR TOE The toe setting of the rear wheels. Rear toe is not adjustable on rear-wheel drive cars with solid axle housings but is adjustable on many front-wheel drive cars and minivans. If rear toe is unequal, it can produce a diagonal wear pattern (heel and toe wear) on the rear tires.
REBOUND Extension of the suspension as the springs expand after being compressed. RECIRCULATING BALL STEERING A type of steering gear normally used with a parallelogram steering linkage. So named because of the ball bearings that are recirculated in the gear box between the worm and sector gears to reduce friction.
RIDE HEIGHT The distance between a specified point on the chassis, suspension or body and the ground. Static ride height is when the vehicle is at rest. Dynamic ride height is the average ride height that normally occurs as a vehicle travels down the road. Measuring ride height is an indirect method of determining spring height, which is important because it affects camber, caster and toe. Low ride height indicates weak or sagging springs. Ride height should be within specifications before the wheels are aligned.
ROAD CROWN The slope of a road surface to the outside for proper drainage. Excessive road crown can cause a vehicle to lead to the right. Reducing caster on the left front wheel is sometimes used to compensate for road crown.
ROAD TEST Test driving a vehicle to determine its steering, handling and ride characteristics. Road testing a vehicle is a good way to identify and confirm customer complaints, and/or to verify that problems have been corrected.
RUNOUT The about of variation or wobble in a wheel or tire. See lateral and radial runout.
SCRUB RADIUS The distance between the extended centerline of the steering axis and the centerline of the tire where the tread contacts the road. If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative. Rear-wheel drive cars and trucks generally have a positive scrub radius while FWD cars usually have zero or a negative scrub radius because they have a higher SAI angle. Using wheels with different offset than stock can alter the scrub radius.
SETBACK The amount by which one front wheel is further back from the front of the vehicle than the other. It is also the angle formed by a line perpendicular to the axle centerline with respect to the vehicle's centerline. If the left wheel is further back than the right, setback is negative. If the right wheel is further back than the left, setback is positive. Setback should usually be zero to less than half a degree, but some vehicles have asymmetrical suspensions by design. Setback is measured with both wheels straight ahead, and is used as a diagnostic angle along with caster to identify chassis misalignment or collision damage. The presence of setback can also cause differences in toe-out on turn angle readings side-to-side.
SHACKLE A link that connects a leaf spring to the chassis or frame. The shackle allows the length of the spring to change as the suspension moves up and down.
SHIMMY A back and forth vibration that is felt in the steering wheel, sometimes violent. It can be caused by a bent wheel, excessive radial runout in a wheel, a dynamic wheel imbalance or loose steering parts.
SHOCK ABSORBER A device that dampens spring oscillations and the up-and-down motions of the suspension. The shock uses a piston to displace hydraulic fluid. Valving controls the amount of resistance and the dampening characteristics of the shock.
SHOCK FADE A condition where loss of dampening action occurs because of fluid foaming inside a shock absorber. The rapid oscillations of the piston moving through the fluid churns it into foam, which reduces the amount of resistance encountered by the piston. This causes the dampening action to fade, resulting in loss of control, excessive suspension travel and reduced handling. Pressurizing the fluid camber inside the shock with a gas charge can minimize foaming and prevent fade.
SHORT ARM\LONG ARM (SLA) SUSPENSION A common type of suspension that uses upper and lower control arms of unequal length. The upper arm is usually shorter than the lower arm to control camber changes during jounce and rebound.
SLIP PLATES Movable plates on an alignment rack that go under a vehicle's wheels that allow the suspension to settle prior to an alignment.
SPINDLE The component on which the hub and wheel bearings are mounted.
SPRING COMPRESSOR A tool for compressing and holding a coil spring so it can be removed or replaced, or to allow the disassembly of a MacPherson strut.
SPRING A suspension component that supports the weight of the vehicle. Basic types include coil springs, leaf springs, air springs and torsion bars. Spring height affects ride height, which in turn affect wheel alignment. Weak or sagging springs should be replaced in pairs to restore and maintain proper ride height and wheel alignment.
STATIC BALANCE Wheel balance that depends on an equal distribution of weight around the circumference of the wheel and tire assembly. Static balance can be achieved without spinning the wheel by using a bubble balancer. A wheel that lacks static balance will shake or tramp up-and-down.
STEERING ANGLE (See Toe-out on turns)
STEERING ARM The arms on the steering knuckles (or struts) to which the tie rods are attached to steer the wheels.
STEERING AXIS INCLINATION (SAI) The angle formed by a line that runs through the upper and lower steering pivots with respect to vertical. On a SLA suspension, the line runs through the upper and lower ball joints. On a MacPherson strut suspension, the line runs through the lower ball joint and upper strut mount or bearing plate. Viewed from the front, SAI is also the inward tilt of the steering axis. Like caster, it provides directional stability. But it also reduces steering effort by reducing the scrub radius. SAI is a built-in nonadjustable angle and is used with camber and the included angle to diagnose bent spindles, struts and mislocated crossmembers.
STEERING EFFORT The amount of driver input or muscle it takes to turn or steer the wheels. Excessive effort can be caused by loss of power assist, binding in the steering gear, worn upper strut bearing plates or binding in ball joints or tie rod ends. Excessive caster can also increase steering effort as can underinflated tires.
STEERING DAMPER (STABILIZER) A hydraulic device similar to a shock absorber attached to the steering linkage to absorb road shock and steering kickback.
STEERING GEOMETRY A general term used to describe the angular relationships between the wheels, steering linkage and suspension.
STEERING KNUCKLE A forging that usually includes the spindle and steering arm, and allows the front wheel to pivot. The knuckle is mounted between the upper and lower ball joints on a SLA suspension, and between the strut and lower ball joint on a MacPherson strut suspension.
STEERING RETURN The ability of the steering wheel to self-center after turning. Causes of poor returnability include excessive caster or binding in the steering column, steering gear, ball joints, upper strut bearing plates or tie rod ends.
STRUT TOWERS The panels or structural members in a unibody to which the upper strut mounts are bolted. The position of the towers is important because it affects camber and caster readings.
SUBFRAME The lower frame rails and structural members that comprise the lower elements of a unibody. Steering and suspension components may be attached directly to the subframe, or to a "cradle" or "crossmember" that bolts to the subframe.
SUSPENSION The part of a vehicle that supports the weight and locates and steers the wheels. This includes the springs, control arms, ball joints, struts and/or shock absorbers.
SWAY BAR A horizontal bar (also called a "stabilizer" bar) that connects the right and left suspensions to control body roll. A sway bar may be used on the front and/or rear suspension to help keep the body flat as the vehicle rounds a corner.
THRUST ANGLE The angle between the thrust line and centerline. If the thrust line is to the right of the centerline, the angle is said to be positive. If the thrust line is to the left of center, the angle is negative. It is caused by rear wheel or axle misalignment and causes the steering to pull or lead to one side or the other. It is the primary cause of an off-center or crooked steering wheel. Correcting rear axle or toe alignment is necessary to eliminate the thrust angle. If that is not possible, using the thrust angle as a reference line for aligning front toe can restore center steering.
THRUST ANGLE ALIGNMENT Aligning front toe to the rear thrust angle instead of the vehicle's centerline to compensate for rear axle steer.
THRUST LINE A line that bisects total rear toe. It defines the direction the rear wheels are pointed. The thrust line should correspond to the centerline for the vehicle to steer straight.
TIE ROD A part of the steering linkage that connects the steering arms on the knuckles to the steering rack or center link.
TIE ROD END A flexible coupling in the steering linkage that connects the tie rods to the steering knuckles. Worn or loose tie rod ends are a common cause of toe wear on tires as well as steering looseness and wander.
TIE ROD SLEEVES A part of the tie rod assembly that is threaded internally and is turned to shorten or lengthen the tie rod to adjust toe alignment.
TIRE RATINGS Information on the sidewall of a tire about size, maximum load rating, maximum inflation pressure, tire construction and performance standards. Tread wear is a comparative rating of how long the tire will last compared to other tires. The higher the number, the longer the predicted life of the tread. The traction rating is a measure of the tires ability to stop on wet pavement. An "A" is best, "B" is average, and "C" is the lowest acceptable rating. The temperature rating is an indication of how cool the tire runs as highway speeds. Again, an "A" is the best while "C" is the lowest acceptable rating. Performance tires also carry a speed rating: "H" rated tires are good for speeds up to 130 mph, and "V" rated tires are certified for speeds above 130 mph.
TIRE ROTATION Changing the relative positions of the tires on a vehicle periodically to even out tread wear. Rotation is recommended every 5,000 miles for optimum tire life. When tires are not rotated, they can develop wear patterns particular to their wheel location that shortens tread life and may cause vibrations or a rough ride.
TOE An adjustable wheel alignment angle that refers to the parallelism of the tires as viewed from above. Toe has the greatest effect of any alignment angle on tire wear. Ideally, a vehicle should have close to zero running toe (perfect parallel alignment) when being driven. If the wheels are toed-in or toed-out, the tires will scrub as they roll producing a feathered wear pattern on the tread. Front toe is adjustable on all vehicles by turning the tie rods or tie rod sleeves to shorten or lengthen the steering linkage. On FWD cars, rear toe can often be changed by adding shims behind the wheel hub, or by changing the pivot position of the control arms. Rear toe is also adjustable on RWD cars with independent rear suspensions.
TOE-IN Toe-in means the leading edges of the tires are closer together than the rear edges. A small amount of toe-in is usually specified for rear-wheel drive vehicles to compensate for suspension compliance that allows the wheels to toe-out slightly as the vehicle is pushed down the road. Too much toe-in accelerates tire wear and causes the outside edges of the tread to wear more quickly.
TOE-OUT Toe-out means the leading edges of the tires are farther apart than the rear edges. A small amount of toe-out is often specified for front-wheel drive cars to compensate for suspension compliance that allows the wheels to toe-in slightly when the front wheels pull the vehicle down the road. Too much toe-out accelerates tire wear and causes the inside edges of the tread to wear more quickly.
TOE-OUT ON TURNS The change in toe that occurs when the wheels are steered to either side. The change in toe allows the inside wheel to follow a smaller circle than the outer wheel to reduce tire scuffing and wear. The toe angle is nonadjustable and is determined by the geometry of the steering arms and linkage. A toe-out on turn angle is usually specified for the outer wheel when the inner wheel is turned 20 degrees. If the angle is not within specifications, it usually means a steering arm is bent.
TOE WEAR Wear across the face of the tire tread caused by slippage or scrubbing as the tire rolls along. Toe wear can produce a feathered wear pattern (bias ply tires primarily) as well as shoulder wear on radial tires. It results from too much toe-in or toe-out, which in turn may be caused by toe misalignment, worn tie rod ends, a worn idler arm or a worn or bent center link.
TORQUE STEER An annoying tendency in FWD cars with unequal length driveshafts to pull or veer to one side during hard acceleration. This is caused by unequal toe changes that occur as engine torque loads the suspension. Compliance allows the wheel with the longer driveshaft to experience less toe-in change than the wheel with the shorter driveshaft, causing the vehicle to veer towards the side with the longer driveshaft. FWD cars with equal length driveshafts usually do not experience this condition. TORSION BARS A steel bar that is twisted to support the weight of the vehicle. Torsion bars are used in place of coil or leaf springs on some vehicles, and allow ride height to be adjusted to compensate for sag that occurs over time.
TOTAL TOE The combined toe reading of a pair of wheels on a given axle. Total toe is the difference between the leading and trailing edges of both tires with respect to one another. It may be specified in inches, millimeters or degrees.
TRACKING How the rear wheels follow the front wheels. For proper alignment, they should follow the same path. If the rear wheels don't track straight and follow slightly to one side due to rear axle or toe misalignment, the result can be off-center steering and accelerated tire wear.
TRAILING ARMS Components in the rear suspension that connect the rear axle or spindles to the chassis.
TURNING PLATES Plates on an alignment rack that go under the front wheels and allow the wheels to be steered 20 degrees to either side to measure toe-out on turns.
TURNING RADIUS The diameter of the smallest circle in which a vehicle can complete a U-turn. Turning radius depends on the wheelbase of the vehicle (longer vehicles usually need more space to turn around), and maximum steering angularity.
TWIN I-BEAM A type of independent front suspension used on Ford pickup trucks that used two parallel I-beam axles (one for each wheel). The design combines the superior strength of an I-beam suspension with the flexibility and ride comfort of an independent suspension.
U-BOLT A bolt in the shape of a "U" that attaches an axle housing to a leaf spring.
UNDERINFLATION A condition where a tire contains less air pressure than the recommended amount. This increases rolling resistance (which may contribute to a steering pull or lead), tire wear and the risk of tire failure due to overheating from excessive flexing of the sidewalls.
UNDERSTEER A condition that occurs when turning where the front wheels tend to plow to the outside and the vehicle does not respond quickly to steering changes. Front-wheel drive vehicles tend to understeer.
VARIABLE ASSIST POWER STEERING A type of power steering that allows the amount of hydraulic boost to change with vehicle speed or driving conditions rather than remaining constant. Most variable assist systems provide maximum assist at low speed to aid parking and low speed maneuvers, and reduced boost at higher speeds to increase stability and road feel.
VARIABLE RATE SPRINGS A type of spring that changes stiffness as it deflects. A variable rate spring uses coils of varying thickness or spacing to provide a soft ride when the vehicle is lightly loaded, but a firmer ride when the load increases.
VEHICLE CENTERLINE A reference line that runs lengthwise down the center of a vehicle body or chassis. WANDER An instability or looseness in the steering that causes the vehicle to drift from side to side while traveling straight. Causes include worn tie rod ends or an idler arm, but can also include loose wheel bearings, play in the steering column couplings, a worn steering gear or loose steering rack or gear box mounts or bolts, or insufficient caster.
WHEEL ALIGNMENT RACK Equipment that supports and positions a vehicle so that wheel alignment may be checked and adjusted as needed. Racks are usually drive on lifts that raise the vehicle to a convenient working height, and has turn plates and slip plates as well has hydraulic jacks to raise and support the suspension for a prealignment inspection.
WHEEL BALANCE The even distribution of weight around a wheel so that it rotates without vibrating or shaking. See static and dynamic balance. It is achieved by positioning weights on the rim that offset heavy spots on the wheel and tire assembly.
WHEELBASE The distance between the centers of the front and rear wheels. Measuring and comparing the wheelbase on both sides of a vehicle can identify rear axle misalignment or front wheel setback.
WHEEL BEARINGS Bearings located inside a hub that support a wheel and allow it to turn with minimal friction. The bearings may be roller or ball, and sealed or serviceable. The serviceable variety should be cleaned and repacked with fresh grease periodically (typically every 2 years or 24,000 miles). Adjustment is also critical to achieve the proper play or preload. Worn or loose wheel bearings can cause steering wander.
WHEEL WEIGHT A weight used to balance a wheel and tire assembly. Most are metal (usually lead) and clip to the wheel rim. Wheel weights come in various sizes and styles, and must be properly attached to the rim so they don't move or fall off. Different style clips are available for various types of rims. Self-adhesive stick-on weights are also available that mount to the inside face of alloy wheels.
WHEEL TRAMP The up-and-down bouncing motion of a wheel or spindle due to static imbalance or an out-of-round tire or wheel.