Suspension Definitions
From SCION-TECH : The Scion Resource : tC, xA, xB, xD
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[edit] Also See
[edit] General Terms
[edit] Camber
Camber angle is the angle made by the wheel of your Scion vehicle. More specifically, it is the angle of verticle tilt of the wheels and tires when viewed from the front or rear of the vehicle. Camber must be taken into consideration in the design of steering and suspension components. If the top of the wheel is further out than the bottom (that is, tilted inwards), it is called positive (+) camber; if the bottom of the wheel is further out than the top, it is called negative (-) camber.
Camber angle alters the handling qualities of the suspension. Negative (-) camber improves grip during cornering maneuvers. The negative angle provides the outer tire which is taking the greatest proportion of the cornering loads, at a better angle to the road, increasing the contact patch of the tire and transmitting the forces perpendicular to the tire, rather than through a shear force across it. On the other hand, for maximum straight-line acceleration, obviously the greatest traction will be attained when the camber angle is zero (neutral) 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 great geometric models, but also real-life behavior analysis of the components such as flex, distortion, elasticity, etc. What was once an art has now become much more scientific with the use of computers, which can juggle all the variables mathematically instead of relying on the designer's intuitive feel and experience, and as a result the handling of even low-priced automobiles has improved significantly.
In older cars with double wishbone suspensions, camber angle was usually adjustable, but in newer models with McPherson strut suspensions it is normally fixed. While this may reduce maintenance requirements, if the car is lowered by use of shortened springs, this changes the caster angle (as described in McPherson strut) and can lead to increased tire wear and even impaired handling. For this reason, individuals who are serious about modifying their car for better handling will not only lower the body, but also modify the mounting point of the top of the struts to the body to allow some fore/aft movement for caster adjustment. Aftermarket plates with slots for strut mounts instead of just holes are available for most of the commonly modified models of cars.
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 contact patch. By applying negative camber, this effect is reduced, thereby maximizing the contact patch.
[edit] Toe
[edit] Suspension Components
[edit] Camber Arms
See aftermarket Scion xD Camber Kits for: xA, xB, xD, tC
[edit] Coilovers
See aftermarket Scion xD Coilovers for: xA, xB, xD, tC A coilover is an automobile suspension device, short for "coil (spring) over strut". It consists of a shock absorber (British: damper) with a coil spring encircling it. The shock absorber and spring are preassembled as a unit prior to installation, and are replaced as a unit when the shock absorber has leaked. This provides for optimal damping without torsional loads. Some coilovers are adjustable for ride height and hardness as well using a simple threaded spring perch similar to a nut.
The coilover is a basic component of the MacPherson strut suspension system, which is distinguished from other arrangements by employing a particular design of anti-roll bar as a longitudinal constraint. This was the first widespread use of the coilover in automobile suspensions. But there are other designs; the word coilover should not be considered a synonym for the MacPherson strut arrangement.
[edit] Shocks
See aftermarket Scion xD Shocks for: xA, xB, xD, tC In a vehicle, it reduces the effect of traveling over rough ground, leading to improved ride quality. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps. They also, along with hysteresis in the tire itself, damp the motion of the unsprung weight up and down on the springiness of the tire. Since the tire is not as soft as the springs, effective wheel bounce damping may require stiffer shocks than would be ideal for the vehicle motion alone.
Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars can be used in torsional shocks as well. Ideal springs alone, however, are not shock absorbers as springs only store and do not dissipate or absorb energy. Vehicles typically employ both springs or torsion bars as well as hydraulic shock absorbers. In this combination, "shock absorber" is reserved specifically for the hydraulic piston that absorbs and dissipates vibration.
[edit] Springs
See aftermarket Scion xD Springs for: xA, xB, xD, tC A Coil spring, also known as a helical 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.
The two usual types of coil spring are:
- Tension coil springs which are designed to resist stretching. They usually have a hook or eye form at each end for attachment.
- Compression coil springs are designed to resist being compressed. A typical use for compression coil springs is in car suspension systems.
Metal coil springs are made by winding a wire around a shaped former - a cylinder is used to form cylindrical coil springs.
Many types of coil spring are wound in an annealed (soft) condition and then tempered to achieve their strength as a spring. Over time, this tempering can be lost and the spring will sag because it can no longer withstand the loads applied. Such springs can be re-set by annealing, returning to their original length (or deliberately setting them to a different length) and then re-tempering. Damage to springs, such as using oxy-acetylene to cut the end off a car suspension spring to lower a vehicle's ride height, can destroy the tempering in localised areas of the spring.
[edit] Strut Tower Bar
See aftermarket Scion xD Strut Bars for: xA, xB, xD, tC
[edit] Sway Bars
See aftermarket Scion xD Sway Bars for: xA, xB, xD, tC In a turn, the sprung mass of the vehicle's body rotates around its roll axis. The roll axis is a line that joins the front and rear roll centers (SAEJ670e). If the vertical distance between the roll axis and the center of gravity is not zero, a torque (roll moment) equal to the centrifugal force times the distance between the center of gravity and the roll axis will be exerted on the sprung mass, causing the body to lean towards the outside of the turn. This force is called the roll couple. One effect of body (frame) lean, for typical suspension geometry, is positive camber of the wheels on the outside of the turn and negative on the inside, which reduces their cornering grip (especially with cross ply tires).
Roll couple is resisted by the suspension's roll stiffness, which is a function of the spring rate of the vehicle's springs and of the anti-roll bars, if any. The use of anti-roll bars allows designers to reduce body lean without making the suspension's springs stiffer in the vertical plane, which allows improved body control with less compromise of ride quality.
The spring rate of an anti-roll bar is based on the fourth power of the torsion bar's diameter, the stiffness of the material, the inverse of the length of the lever arms (i.e., the shorter the lever arm, the stiffer the bar), the geometry of the mounting points, and the rigidity of the bar's mounting points. Some anti-roll bars, particularly those intended for use in auto racing, are adjustable, allowing their stiffness to be altered by increasing or reducing the length of the lever arms. This permits the roll stiffness to be tuned for different situations without replacing the entire bar.
Anti roll bars provide 2 main functions:
The first is the reduction of body lean. The reduction of body lean is dependent on the total roll stiffness of the vehicle. Increasing the total roll stiffness of a vehicle does not change the steady state total load (weight) transfer from the inside wheels to the outside wheels, it only reduces body lean. The total lateral load transfer is determined by the CG height and track width.
The other function of anti roll bars is to tune the high g / limit understeer behavior of the vehicle. The limit understeer behavior is tuned by changing the proportion of the total roll stiffness that comes from the front and rear axles. Increasing the proportion of roll stiffness at the front will increase the proportion of the total weight transfer that the front axle reacts and decrease the proportion that the rear axle reacts. This will cause the outer front wheel to run at a higher slip angle, and the outer rear wheel to run at a lower slip angle, which is an understeer effect. Increasing the proportion of roll stiffness at the rear axle will have the opposite effect and decrease understeer.
[edit] Characteristics
[edit] Chassis Flex
As a vehicle is driven, various forces are applied by the suspension on the chassis. This occurs under braking, cornering, driving surface variations, or any other vehicle movement. In higher these loads are, the more is demanded of a chassis. The chassis obviously has to be strong enough not to fail under these loads, but beyond that it must not deflect appreciably. Suspensions are carefully designed to position the wheels and tires of the vehicle for optimum performance under all conditions of vehicle use. If the chassis deflects when the forces are high, it causes suspension mounts and attachment points to temporarily shift, which destroys the careful suspension design when it is needed most. Chassis stiffness is most important in high performance or racing cars, where suspension loads are at their highest and suspension adjustment is most critical.
[edit] Understeer
Understeer is a term for a car handling condition during cornering in which the circular path of the vehicle's motion is of a markedly greater diameter than the circle indicated by the direction its wheels are pointed. The effect is opposite to that of the oversteer and in simpler words understeer is the condition in which the front tires don't follow the trajectory the driver is trying to impose while taking the corner, instead following a more straight line trajectory.
This is also often referred to as pushing, plowing, or refusing to turn in. The car is referred to as being 'tight' because it is stable and far from wanting to spin.
As with oversteer, understeer has a variety of sources such as mechanical traction, aerodynamics and suspension.
Classically, understeer happens when the front tires have a loss of traction during a cornering situation, thus causing the front-end of the vehicle to have less mechanical grip and become unable to follow the trajectory in the corner.
In modern race cars, especially open wheel cars, understeering is caused mainly due to the aerodynamic configuration. In this respect, the lack of a heavy aerodynamic load (downforce) in the front side prevents the front tires from gaining enough traction. At the same time understeer can be caused by having a heavier aerodynamic load at the rear end of the car giving the rear tires more traction than the front tires. Also, suspension balance should take into account the types of surfaces being driven - differing levels of friction in each surface influence the potential understeer behavior. Camber angles, ride height, tire pressure and centre of gravity are important factors that determine the understeer/oversteer handling condition.
Understeer covers several different phenomena, in particular, there is a big difference between linear range understeer, typically between 0 and 0.4g, and limit handling understeer, which is at higher lateral accelerations, and is what racing drivers are talking about.
