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| Word | Description |
| A-Arm | In automotive suspension, a car's a-arm is a nearly flat and roughly triangular member (or sub-frame), that pivots in two places. The broad end of the triangle attaches at the frame and pivots on a bushing. The narrow end attaches to the steering knuckle and pivots on a ball joint. Two such devices per wheel make up double wishbone suspension, while one control arm per wheel makes up a part, usually the lower part, of MacPherson strut suspension or of various other configurations. |
| A-Body | A-Body is the term for the 1964-1972 GM platform that the following vehicles were built on: Chevrolet Chevelle, El Camino, Malibu, Monte Carlo / Buick Skylark, Special, Grand Sport, Regal, Century / Pontiac Lemans, GTO / Oldsmobile Cutlass, 442, F-85 A-Body is the term for the 1973-1977 GM platform that the following vehicles were built on: Chevrolet Chevelle, Malibu, Monte Carlo, El Camino / Oldsmobile Cutlass / Buick Regal, Century / Pontiac LeMans, F85, Grand Prix, Grand Am, Sprint A-Body is the term for the 1978-1987 GM platform that the following vehicles were built on: Buick Regal, Grand National, Century / Oldsmobile Cutlass / Chevrolet Monte Carlo, Malibu, El Camino / Pontiac Grand Prix, LeMans, Grand Am |
| Anti-Squat (Anti-Dive) | Anti-dive and anti-squat are expressed in terms of percentage and refer to the front diving under braking and the rear squatting under acceleration. They can be thought of as the counterparts for braking and acceleration as jacking forces are to cornering. The main reason for the difference is due to the different design goals between front and rear suspension, whereas suspension is usually symmetrical between the left and right of the vehicle. Anti-dive and anti-squat percentage are always calculated with respect to a vertical plane that intersects the vehicle's center of gravity. Consider anti-dive first. Locate the front instant centers of the suspension from the vehicle's side view. Draw a line from the tire contact patch through the instant center, this is the tire force vector. Now draw a line straight down from the vehicle's center of gravity. The anti-dive is the ratio between the height of where the tire force vector crosses the center of gravity plane expressed as a percentage. An anti-dive ratio of 50% would mean the force vector under braking crosses half way between the ground and the center of gravity. Anti-squat is the counterpart to anti-dive and is for the rear suspension under acceleration. Anti-dive and anti-squat may or may not be desirable depending on the suspension design. Independent suspension using multiple control arms can be an issue if the percentage is too high (say over 30%). A percentage of 100% in this case would indicate the suspension is taking 100% of the weight transfer under braking instead of the springs. This effectively binds the suspension and turns the independent suspension into no suspension like a go-cart. However, in the case of leaf spring rear suspension the anti-squat can often exceed 100% (meaning the rear may actually raise under acceleration) yet because there isn't a second arm to bind against and the suspension can freely move. Traction bars are often added to drag racing cars with rear leaf springs to increase the anti-squat to its maximum. This has the effect of forcing the rear of the car in the air and the tires onto the ground for better traction. |
| B-Body | B-Body is the term for the 1959-1996 GM platform that the following vehicles were built on: Chevrolet Caprice, Impala SS / Buick Roadmaster, LeSabre / Pontiac Bonneville, Catalina |
| Ball Joint | In an automobile, ball joints are spherical bearings that connect the control arms to the steering knuckles. More specifically, a ball joint is a steel bearing stud and socket enclosed in a steel casing. The bearing stud is tapered and threaded. It fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. |
| Bump Steer | Bump steer is the term for the tendency of a wheel to steer as it moves upwards into jounce. It is typically measured in degrees per foot. On modern cars the front of the tire moves outwards, as the suspension is raised, a process known as the front wheels "toeing out". This gives roll under steer. The rear suspension is usually set up to minimize bump steer, where possible. A typical value is two degrees, or perhaps more, for the front wheels. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. Solid axles generally have zero bump steer, but still have roll steer, in most cases. That is, if the wheels move upwards by the same amount, they tend not to steer. |
| Bushing | A bushing or rubber bushing is a type of vibration isolator. It provides an interface between two parts, damping the energy transmitted through the bushing. A common application is in vehicle suspension systems, where a bushing made of rubber (or, more often, synthetic rubber or polyurethane) separates the faces of two metal objects while allowing a certain amount of movement. This movement allows the suspension parts to move freely, for example, when traveling over a large bump, while minimizing transmission of noise and small vibrations through to the chassis of the vehicle. A rubber bushing may also be described as a flexible mounting or anti-vibration mounting. These bushings often take the form of an annular cylinder of flexible material inside a metallic casing or outer tube. They might also feature an internal crush tube which protects the bushing from being crushed by the fixings which hold it onto a threaded spigot. Many different types of bushing designs exist. An important difference compared with plain bearings is that the relative motion between the two connected parts is accommodated by strain in the rubber, rather than by shear or friction at the interface. Some rubber bushings, such as the D block for a sway bar, do allow sliding at the interface between one part and the rubber. |
| Camber | Camber angle is the angle made by the wheels of a vehicle; specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or rear. It is used in the design of steering and suspension. If the top of the wheel is farther out than the bottom (that is, away from the axle), it is called positive camber; if the bottom of the wheel is farther out than the top, it is called negative camber. Camber angle alters the handling qualities of a particular suspension design; in particular, negative camber improves grip when cornering. This is because it places the tire at a more optimal angle to the road, transmitting the forces through the vertical plane of the tire, rather than through a shear force across it. Another reason for negative camber is that a rubber tire tends to roll on itself while cornering. If the tire had zero camber, the inside edge of the contact patch would begin to lift off of the ground, thereby reducing the area of the contact patch. By applying negative camber, this effect is reduced, thereby maximizing the contact patch area. Note that this is only true for the outside tire during the turn; the inside tire would benefit most from positive camber. On the other hand, for maximum straight-line acceleration, the greatest traction will be attained when the camber angle is zero and the tread is flat on the road. Proper management of camber angle is a major factor in suspension design, and must incorporate not only idealized geometric models, but also real-life behavior of the components; flex, distortion, elasticity, etc. What was once an art has now become much more scientific with the use of computers, which can optimize all of the variables mathematically instead of relying on the designer's intuitive feel and experience. As a result, the handling of even low-priced automobiles has improved dramatically in recent years. In cars with double wishbone suspensions, camber angle may be fixed or adjustable, but in MacPherson strut suspensions, it is normally fixed. The elimination of an available camber adjustment may reduce maintenance requirements, but if the car is lowered by use of shortened springs, the camber angle will change. Excessive camber angle and can lead to increased tire wear and impaired handling. Significant suspension modifications may correspondingly require that the upper control arm or strut mounting points be altered to allow for some inward or outward movement, relative to longitudinal center line of the vehicle, for camber adjustment. Aftermarket plates with slots for strut mounts instead of just holes are available for most of the commonly modified models of cars. |
| Camber (Control) | Camber changes due to wheel travel, body roll and suspension system deflection or compliance. In general, a tire wears and brakes best at -1 to -2° of camber from vertical. Depending on the tire and the road surface, it may hold the road best at a slightly different angle. Small changes in camber, front and rear, can be used to tune handling. Some race cars are tuned with -2~-7° camber depending on the type of handling desired and the tire construction. Oftentimes, too much camber will result in the decrease of braking performance due to a reduced contact patch size through excessive camber variation in the suspension geometry. The amount of camber change in bump is determined by the instantaneous front view swing arm (FVSA) length of the suspension geometry, or in other words, the tendency of the tire to camber inward when compressed in bump. |
| Caster | Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a car, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations. |
| Center (Drag) Link | A drag link converts rotary motion from a crank to a second crank or link in a different plane or axis. The term is commonly used in automotive technology for the link in a four bar steering linkage that converts rotation of a steering arm to a center link and eventually to tie rod links which pivot the wheels to be steered. A drag link is used when the steering arm operates in a plane above the other links. The drag link converts the sweeping arc of the steering arm to linear motion in the plane of the other steering links. |
| Chassis | In the case of vehicles, the term chassis means the frame plus the "running gear" like engine, transmission, driveshaft, differential, and suspension. A body, which is usually not necessary for integrity of the structure, is built on the chassis to complete the vehicle. |
| Coil Spring | A coil spring is a mechanical device, which is typically used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. They are made of an elastic material formed into the shape of a helix which returns to its natural length when unloaded. Coil springs are a special type of torsion spring: the material of the spring acts in torsion when the spring is compressed or extended. |
| Control Arm | A control arm is a suspension member intended to control wheel motion in the longitudinal (fore-aft) plane. The link is connected (with a rubber or solid bushing) on one end to the wheel carrier or axle, on the other to the chassis or uni-body of the vehicle. Control arms typically are mounted ahead of the wheel. In that position they resist dive under braking forces and wheel hop under acceleration. |
| Damping (Suspension) | Damping is the control of motion or oscillation, as seen with the use of hydraulic gates and valves in a vehicles shock absorber. This may also vary, intentionally or unintentionally. Like spring rate, the optimal damping for comfort may be less than for control. Damping controls the travel speed and resistance of the vehicles suspension. An undamped car will oscillate up and down. With proper damping levels, the car will settle back to a normal state in a minimal amount of time. Most damping in modern vehicles can be controlled by increasing or decreasing the resistance to fluid flow in the shock absorber. |
| Disc Brake | The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion. A brake disc (or rotor) is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon-carbon or ceramic-matrix composites. This is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads (mounted on a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes (both disc and drum) convert friction to heat, but if the brakes get too hot, they will become less effective because they cannot dissipate enough heat. This condition of failure is known as brake fade. |
| Driveshaft | An automobile may use a longitudinal shaft to deliver power from an engine/transmission to the other end of the vehicle before it goes to the wheels. A pair of short drive shafts is commonly used to send power from a central differential, transmission, or transaxle to the wheels. A drive shaft, driving shaft, or propeller shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them. Drive shafts are carriers of torque: they are subject to torsion and shear stress, equivalent to the difference between the input torque and the load. They must therefore be strong enough to bear the stress, whilst avoiding too much additional weight as that would in turn increase their inertia. |
| Drum Brake | A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped surface. The term "drum brake" usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disk brake, it is sometimes called a "pinch drum brake", although such brakes are relatively rare. A related type of brake uses a flexible belt or "band" wrapping around the outside of a drum, called a band brake. |
| F-Body | F-Body is the term for the 1967-2002 GM platform that the following vehicles were built on: Chevrolet Camaro / Pontiac Firebird, Trans-Am, Formula, GTA |
| G-Body | G-Body is the term for the 1978-1987 GM platform that the following vehicles were built on: Buick Regal, Grand National, Century / Oldsmobile Cutlass / Chevrolet Monte Carlo, Malibu, El Camino / Pontiac Grand Prix, LeMans, Grand Am |
| H-Body | H-Body is the term for the 1971-1980 GM platform that the following vehicles were built on: Chevrolet Vega, Monza / Pontiac Astre, Sunbird / Oldsmobile Starfire / Buick Skyhawk |
| Idler Arm | On an automobile or truck with conventional parallelogram steering, the Idler Arm or idler arm assembly is a pivoting support for the steering linkage. The idler arm consists of a rod which pivots on a bracket attached to the frame of the vehicle on one end and supports a ball joint on the other end. Generally, an idler arm is attached between the opposite side of the center link from the Pitman arm and the vehicle's frame to hold the center link at the proper height. Idler arms are generally more vulnerable to wear than Pitman arms because of the pivot function built into them. If the idler arm is fitted with grease fittings, these should be lubricated with a grease gun at each oil change. |
| Instant Center (IC) | Due to the fact that the wheel and tire's motion is constrained by the suspension links on the vehicle, the motion of the wheel package in the front view will scribe an imaginary arc in space with an “instantaneous center" of rotation at any given point along its path. The instant center for any wheel package can be found by following imaginary lines drawn through the suspension links to their intersection point. A component of the tire's force vector points from the contact patch of the tire through instant center. The larger this component is, the less suspension motion will occur. Theoretically, if the resultant of the vertical load on the tire and the lateral force generated by it points directly into the instant center, the suspension links will not move. In this case, all weight transfer at that end of the vehicle will be geometric in nature. This is key information used in finding the force-based roll center as well. In this respect the instant centers are more important to the handling of the vehicle than the kinematic roll center alone, in that the ratio of geometric to elastic weight transfer is determined by the forces at the tires and their directions in relation to the position of their respective instant centers. |
| K-Member (K Member) | A K member is a crossmember in a vehicle with a longitudinally mounted engine and contains the engine mounts. A K Member is a structural section of steel, usually boxed, that is bolted across the underside of a uni-body motor vehicle, to support the internal combustion engine. |
| Motion Ratio | The motion ratio of a mechanism is the ratio of the displacement of the point of interest to that of another point. The most common example is in a vehicle's suspension, where it is used to describe the displacement and forces in the springs and shock absorbers. The force in the spring is (roughly) the vertical force at the contact patch divided by the motion ratio, and the wheel rate is the spring rate divided by the motion ratio squared. This is described as the Installation Ratio in the reference. Motion Ratio is the more common term in the industry, but sometimes is used to mean the inverse of the above definition. Motion ratio in suspension of a vehicle describes the amount of shock travel for a given amount of wheel travel. Mathematically it is the ratio of shock travel and wheel travel. The amount of force transmitted to the vehicle chassis reduces with increase in motion ratio.A motion ratio close to one is desired in vehicle for better ride and comfort.One should know the desired wheel travel of the vehicle before calculating motion ratio which depends much on the type of track the vehicle will run upon. How to decide the motion ratio? It basically depends on 3 factors a)Bending Moment: To reduce the bending moment the strut point should be near to the wheel. b)Suspension Stiffness: The suspension tends to get stiff when its inclination of the shock absorber to horizontal tends to 90 deg. c)Half Shafts: In Rear suspension the wheel travel is constrained by the angle limitations of the universal joints of the half shafts. Design the motion ratio such that at maximum bounce and rebound shocks are the first components that bottom out by hitting bump stops. |
| Panhard Bar Rod | A Panhard rod (also called Panhard bar or track bar) is a component of a vehicle suspension system that provides lateral location of the axle. |
| Rack and Pinion Steering | Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a linear gear that meshes with the pinion, converting circular motion into linear motion along the transverse axis of the car (side to side motion). This motion applies steering torque to the swivel pin ball joints that replaced previously used kingpins of the stub axle of the steered wheels via tie rods and a short lever arm called the steering arm. The rack and pinion design has the advantages of a large degree of feedback and direct steering "feel". A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the only cure is replacement. |
| Rod End | A rod end bearing, also known as a heim joint, is a mechanical articulating joint. Such joints are used on the ends of control rods, steering links, tie rods, or anywhere a precision articulating joint is required. A ball swivel with an opening through which a bolt or other attaching hardware may pass is pressed into a circular casing with a threaded shaft attached. The threaded portion may be either male or female. |
| Roll Center Height (RCH) | This is important to body roll and to front to rear roll stiffness distribution. However, the roll stiffness distribution in most cars is set more by the anti-roll bars than the RCH. The height of the roll center is related to the amount of jacking forces experienced. |
| Roll Couple Percentage | Roll couple percentage is the effective wheel rates, in roll, of each axle of the vehicle just as a ratio of the vehicle's total roll rate. Roll Couple Percentage is critical in accurately balancing the handling of a vehicle. It is commonly adjusted through the use of anti-roll bars, but can also be changed through the use of different springs. A vehicle with a roll couple percentage of 70% will transfer 70% of its sprung weight transfer at the front of the vehicle during cornering. This is also commonly known as "Total Lateral Load Transfer Distribution" or "TLLTD". |
| Scrub Radius | The scrub radius is the distance in front view between the Steering Axis and the center of the contact patch where both would theoretically touch the road. The steering axis is the line between the top pivot point of the hub and the lower ball joint of the hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The inclination of the steering axis is measured as the angle between the steering axis and the center line of the wheel. This means that if your camber is adjustable within the pivot points you can change the the scrub radius, this alters the width and offset of tires you can safely run on your car. If the steer axis intersection point is outboard of the center of the contact patch it is negative, if inside the contact patch it is positive. |
| Shock (Absorber) | The shock absorber's duty is to absorb or dissipate energy. In general terms, shock absorbers help cushion vehicles on uneven roads. |
| Spherical Bearing | A spherical bearing is a bearing that permits angular rotation about a central point in two orthogonal directions (usually within a specified angular limit based on the bearing geometry). Typically these bearings support a rotating shaft in the [bore] of the inner ring that must move not only rotationally, but also at an angle. |
| Spindle | In an automobile, the spindle is a part of the suspension system that carries the hub for the wheel and attaches to the control arms. |
| Spring Rate | Spring rate is a ratio used to measure how resistant a spring is to being compressed or expanded during the spring's deflection. The magnitude of the spring force increases as deflection increases according to Hooke's Law. Briefly, this can be stated as F = -kx \, where F is the force the spring exerts k is the spring rate of the spring. x is the displacement from equilibrium length i.e. the length at which the spring is neither compressed or stretched. Spring rate is confined to a narrow interval by the weight of the vehicle,load the vehicle will carry, and to a lesser extent by suspension geometry and performance desires. Spring rates typically have units of N/mm (or lb/in). An example of a linear spring rate is 500 lb/in. For every inch the spring is compressed, it exerts 500 lb. A non-linear spring rate is one for which the relation between the spring's compression and the force exerted cannot be fitted adequately to a linear model. For example, the first inch exerts 500 lb force, the second inch exerts an additional 550 lb (for a total of 1050 lb), the third inch exerts another 600 lb (for a total of 1650 lb). In contrast a 500 lb/in linear spring compressed to 3 inches will only exert 1500 lb. The spring rate of a coil spring may be calculated by a simple algebraic equation or it may be measured in a spring testing machine. The spring constant k can be calculated as follows: k = \frac{d^4G}{8ND^3} \, where d is the wire diameter, G is the spring's shear modulus (e.g., about 12,000,000 lb/in or 80 GPa for steel), and N is the number of wraps and D is the diameter of the coil. |
| Steering Ratio | Steering ratio refers to the ratio between the turn of the steering wheel (in degrees) or handlebars and the turn of the wheels (in degrees). In motorcycles and bicycles, the steering ratio is always 1:1, while in most passenger cars, it is between 12 and 20:1. Example: If one complete turn of the steering wheel (360 degrees) causes the wheels to turn 24 degrees, then the ratio is 15:1 (360/24=15). |
| Strut | An automotive suspension strut combines the primary function of a shock absorber (as a damper), with the ability to support sideways loads not along its axis of compression. This means that a strut must have a more rugged design, with mounting points near its middle for attachment of such loads. The most common form of strut in an automobile is the MacPherson strut. The MacPherson strut combines a shock absorber and a spring in a single unit, by means of which each wheel is attached to the car body. |
| Suspension | Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's road holding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations,etc. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different. |
| Suspension Types | Suspension systems can be broadly classified into two subgroups — dependent and independent. These terms refer to the ability of opposite wheels to move independently of each other. A dependent suspension normally has a beam (a simple 'cart' axle) or (driven) live axle that holds wheels parallel to each other and perpendicular to the axle. When the camber of one wheel changes, the camber of the opposite wheel changes in the same way (by convention on one side this is a positive change in camber and on the other side this a negative change). De Dion suspensions are also in this category as they rigidly connect the wheels together. An independent suspension allows wheels to rise and fall on their own without affecting the opposite wheel. Suspensions with other devices, such as sway bars that link the wheels in some way are still classed as independent. A third type is a semi-dependent suspension. In this case, the motion of one wheel does affect the position of the other but they are not rigidly attached to each other. A twist-beam rear suspension is such a system. |
| Sway Bar | A sway bar (aka stabilizer bar, anti-sway bar, roll bar, or anti-roll bar) is an automobile suspension device. It connects opposite (left/right) wheels together through short lever arms linked by a torsion spring. A sway bar increases the suspension's roll stiffness—its resistance to roll in turns, independent of its spring rate in the vertical direction. |
| Toe | In automotive engineering, toe is the symmetric angle that each wheel makes with the longitudinal axis of the vehicle, as a function of static geometry, and kinematic and compliant effects. This can be contrasted with steer, which is the antisymmetric angle, i.e. both wheels point to the left or right, in parallel (roughly). Positive toe, or toe in, is the front of the wheel pointing in towards the centreline of the vehicle. Negative toe, or toe out, is the front of the wheel pointing away from the centreline of the vehicle. Toe can be measured in linear units, at the front of the tire, or as an angular deflection. In a rear wheel drive car, increased front toe in (i.e. the fronts of the front wheels are closer together than the backs of the front wheels) provides greater straight-line stability at the cost of some sluggishness of turning response, as well as a little more tire wear as they are now driving a bit sideways. On front wheel drive cars, the situation is more complex. Toe is always adjustable in production automobiles, even though caster angle and camber angle are often not adjustable. Maintenance of front end alignment, which used to involve all three adjustments, currently involves only setting the toe; in most cases, even for a car in which caster or camber are adjustable, only the toe will need adjustment. One related concept is that the proper toe for straight line travel of a vehicle will not be correct while turning, since the inside wheel must travel around a smaller radius than the outside wheel; to compensate for this, the steering linkage typically conforms more or less to Ackermann steering geometry, modified to suit the characteristics of the individual vehicle. |
| Travel (Suspension) | Suspension travel is the measure of distance from the bottom of the suspension stroke (such as when the vehicle is on a jack and the wheel hangs freely), to the top of the suspension stroke (such as when the vehicles wheel can no longer travel in an upward direction toward the vehicle). Bottoming or lifting a wheel can cause serious control problems or directly cause damage. "Bottoming" can be either the suspension, tires, fenders, etc. running out of space to move or the body or other components of the car hitting the road. The control problems caused by lifting a wheel are less severe if the wheel lifts when the spring reaches its unloaded shape than they are if travel is limited by contact of suspension members. |
| U-Joint | A universal joint, or U joint, is a joint in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft. |
| Unsprung Weight (Transfer) | Unsprung weight transfer is calculated based on the weight of the vehicle's components that are not supported by the springs. This includes tires, wheels, brakes, spindles, half the control arm's weight and other components. These components are then (for calculation purposes) assumed to be connected to a vehicle with zero sprung weight. They are then put through the same dynamic loads. The weight transfer for cornering in the front would be equal to the total unsprung front weight times the G-Force times the front unsprung center of gravity height divided by the front track width. The same is true for the rear. |
| Weight Transfer | In wheeled vehicles, weight (load) transfer is the measurable change of load borne by different wheels during acceleration (both longitudinal and lateral). This includes braking, and deceleration (which is an acceleration at a negative rate). No motion of the center of gravity (CoG) relative to the wheels is necessary, and so load transfer may be experienced by vehicles with no suspension at all. |
| Wheel Rate | Wheel rate is the effective spring rate when measured at the wheel. This is as opposed to simply measuring the spring rate alone. Wheel rate is usually equal to or considerably less than the spring rate. Commonly, springs are mounted on control arms, swing arms or some other pivoting suspension member. Consider the example above where the spring rate was calculated to be 500 lbs/inch, if you were to move the wheel 1 in (2.5 cm) (without moving the car), the spring more than likely compresses a smaller amount. Lets assume the spring moved 0.75 in (19 mm), the lever arm ratio would be 0.75:1. The wheel rate is calculated by taking the square of the ratio (0.5625) times the spring rate. Squaring the ratio is because the ratio has two effects on the wheel rate. The ratio applies to both the force and distance traveled. Wheel rate on independent suspension is fairly straight-forward. However, special consideration must be taken with some non-independent suspension designs. Take the case of the straight axle. When viewed from the front or rear, the wheel rate can be measured by the means above. Yet because the wheels are not independent, when viewed from the side under acceleration or braking the pivot point is at infinity (because both wheels have moved) and the spring is directly inline with the wheel contact patch. The result is often that the effective wheel rate under cornering is different from what it is under acceleration and braking. This variation in wheel rate may be minimized by locating the spring as close to the wheel as possible. |