Suspension: designs, shapes, sizes

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by: 123pugsy, Alittle1, Aosborn, Chevythunder, Cobalt327, Crashfarmer, Crashtech, Crosley, Jon, Metzijndrie, Schnitz, Techinspector1
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Contents

[edit] Basic information

[edit] Front suspension

[edit] McPherson strut assembly

This is the most common suspension used on new cars. The strut houses the coil spring.

[edit] Over/under A-arm

[edit] Leaf spring and solid axle

Otherwise know as Hotchkiss suspension. This arrangement is most commonly used in 4x4 vehicles and very old cars. The concept here is that two parallel leaf springs are attached longitudinally to the frame. One end of the leaf spring must have a shackle which acts as a pivot and allows the spring to change in length while compressing or rebounding. The center of the leaf springs are attached to the axle typically with U-bolts. The springs also act as a lateral locator for the suspension, though a Panhard bar can be added for more lateral stability. Many vintage cars and trucks incorporated a single transverse leaf spring in their suspension system.

[edit] Rear suspension

[edit] Solid axle

Solid rear axle with leaf springs

[edit] Leaf spring

Leaf spring rear suspension is much like a front leaf spring suspension minus the steering linkage and knuckles. Two leaf springs are mounted longitudinally in the chassis and attach at their ends to the frame both at the front and back. Normally, one end of the spring is solid-mounted with a bolt/nut through a bushed eye to a bracket on the frame and the other end is connected through a shackle arrangement which allows no binding as the length of the spring changes during upward and downward movement of the differential. A solid axle differential is then mounted somewhere near the center of the springs. The springs also act as a lateral locator, though a Panhard bar can be added to alter the roll center and to more positively locate the axle.

[edit] Triangulated 4 Link

Triangulated 4 link rear suspension

Two longitudinally straight lower control arms and two angled upper control arms hold the rear end in place horizontally while allowing it to move vertically. This setup allows for the use of coilovers or coil springs and shocks. This "triangulation" of the upper control arms keeps the rear axle centered, thus no Panhard bar is necessary with this set-up.

[edit] Parallel 4 Link

Two straight lower control arms and two straight upper control arms provide superior axle roll control during a hard launch. This set-up is commonly found in drag racing applications. This suspension design has no provision to keep the rear axle centered so a centering device like a Panhard bar or Watt's link must be used.

[edit] 3 Link

3-Link suspension is a design of rear suspension which has 3 trailing (longitudinal) arms on a solid rear axle connecting it to the frame. The two bottom lower links typically mount toward the outboard end of the axle on the underside and are parallel in top view. The third link can be centered on top of the third member on center, or is commonly mounted toward the right side of the car to help counteract engine torque. This type of linkage arrangement is excellent for handling as it allows for much freedom of movement in rotation. This system must utilize a lateral locator such as either a Panhard bar or Watt's link for centering the rear axle. Coil springs and shocks are used in this setup or a coil-over setup can be used.

Custom 48 inch ladder bar rear suspension

[edit] Ladder bar

V-shaped bars which attach the front point of the bars to the frame toward the front of the car and the other end to brackets which are welded to a solid axle differential housing. At the housing brackets, the differential may be "clocked" or rotated to change the pinion angle. At the front, or Vee point of the bars, they may be moved up or down in their brackets to change the way the load of the differential is fed into the frame structure of the car.

Ladder bars are a low-cost way of controlling differential rotation, although they may be overly harsh in a street application. Where the car has to negotiate an uneven entry or exit such as angling into or from a driveway, there is no compliance in the bars like there would be in an OEM system. You could find one of the front tires being pulled off the surface of the roadway due to chassis twist and non-conformity of the bars. Another peril is the bending or breaking of brackets or welds from the flexing, a good reason to limit ladder bars to a race-only vehicle.

At least one manufacturer has addressed this non-conformity. Auto Weld Chassis has developed a compliant urethane member in the Vee-point. This is a low-cost, new generation of ladder bars for Pro-Street use. They place a heavy duty urethane bushing in the front with a heavy duty outer sleeve welded directly to the ladder bars for extra strength. The bushing has an inner steel sleeve which accepts a drag legal 3/4" bolt. Stainless steel adjusters and double shear plates are employed at the back of the bars. The flexibility of the urethane bushing allows the rear to twist and float slightly to soak up road irregularities and bumps for a better handling, quieter, maintenance-free ride that is still able to supply the necessary traction.

Ladder bars may be used to control axle rotation with a leaf-spring arrangement or may be used with coil-overs or coils/shocks using a Panhard bar or Watts Link to control lateral body movement in relation to the differential housing.

[edit] Truck arm

A truck arm suspension uses two I-beam section links that mount wide at the axle and close together at the center of the car. The arms mount solidly (with U-bolts) at the axle and on bushings at the frame. This suspension was originally used on '60s-early '70s Chevrolet/GMC light trucks. Junior Johnson was the first car builder to use a truck-arm suspension in NASCAR. It was so successful, it is now the mandated standard suspension.

The GM arrangement was compromised in several respects. The Panhard bar was too short for the long suspension travel inherent in light trucks, resulting in excessive lateral body movement in relation to the axle housing. The shock absorbers were mounted at the wrong angles. This arrangement is over constrained and relies on deflection of the links and bushings in order for the body to roll. Because of this it acts like a built-in anti-roll bar. However, the rate cannot be adjusted without replacing the bushings at the front mounting point of the arms. This is something to be aware of when upgrading from factory to heavy duty aftermarket arms. A Panhard bar is traditionally used for lateral restraint, although a Watts Link could also work.

[edit] Torque arm

A torque arm setup utilizes two lower control arms while having a unique arm which attaches near the center of the rear end near the pinion. This third torque arm extends all the way to the transmission tailshaft or crossmember. It can use either coilovers or a coil spring and shock setup. Also this setup (like the 3-link) requires either a Panhard bar or Watts link for centering the rear axle. This system is known to be a good handling set up, however during hard launches the flexing of the torque arm can change pinion angle. One solution to this problem is to buy a sturdier aftermarket torque arm.

[edit] Solid axle centering devices

[edit] Panhard bar

Panhard bar on solid rear axle
A Panhard bar is a lateral connection between a point on a cars frame and the opposite end of a solid rear axle. Both connection points are allowed to pivot, allowing the vehicles rear axle to move up and down while limiting side to side motion. The advantage that the Panhard bar has over other types of lateral restraint devices is its simplicity which allows for both easy initial fabrication and adjustment.

The primary disadvantage of this system is the small amount of arc that is inherent when the bar swivels around either pivot point. To minimize the side to side movement a Panhard bar should be both flat and as long as possible. The ideal Panhard bar will be long enough to span just shy of the backing plates left to right on the vehicle. In order to accomplish this, a bracket should be welded to the differential housing which would place the bar just short of interference at the backing plate on one side of the car. On the other side, a bracket should be welded to the frame or frame member which would place the bar just short of interference on the other side of the car.

Since the Panhard bar is the lateral locating device for the suspension, it plays a major role in determining the roll center for the suspension. Generally speaking the roll center can be made lower more simply with a Panhard bar than other forms of lateral locating devices (Watt's link, etc.).

[edit] Watts link

Watts link

A Watts link centers a rear differential and prevents lateral movement of the body of the vehicle in relation to the differential housing through two rods attached to a plate which pivots on the center of either the rear axle housing or the chassis. With a axle-mounted pivot, the locating arms attach to the chassis. When the pivot is attached to the chassis, the locating arms attach to the axle housing.

This setup allows up and down motion of the rearend through the rotation of the steel plate while the bars attaching to opposite sides of the rotating plate prevent the rearend from moving side to side as they oppose each other's motion. While a Watt's link is very effective in controlling lateral axle movement, they need to be well built, especially the center pivot, as it is subject to high lateral loads.

[edit] Mumford link

This linkage allows the roll center to be lower than possible with either a Panhard or Watts arrangement. The advantage is that it allows the use of high rate suspension springs, which can provide some advantages with aerodynamic loads without seriously affecting the distribution of the roll couple.

[edit] Solid axle traction devices

[edit] Traction bars

Usually a single bar with one end attached to the U-bolts holding the differential housing to the leaf springs and the other end attached to a bracket attached to the front spring eye. The idea of this arrangement is to prevent leaf spring "wind-up" and resultant wheel hop on acceleration.

When power is applied to the pinion, it attempts to climb the ring gear in the differential housing. This rotates the differential housing, changing the pinion angle in relation to the vehicle and also twists the leaf spring into an "S" shape. The spring will only twist so far, then will attempt to unwind in the other direction, then wrap up again, then unwind again, etc., etc. This wrap-unwrap of the spring results in wheel hop and upsets traction of the tire. It's that chirp-chirp-chirp-chirp sound you hear from the tires as they bounce up and down upon acceleration from a stop.

Another style of traction bar anchors the bar at the spring like the aforementioned bar, but has no connection at the front. Instead, the front of the bar is fitted with a large egg-shaped rubber bumper that comes up against the bottom of the leaf spring when the spring tries to wrap. At rest, the bumper is adjusted so that is is just a fraction of an inch off the bottom of the spring on each side of the car.

This arrangement should be the same length as the spring section and the rubber snubber should make contact at the front spring eye to be most effective.

[edit] Semi-independent rear suspension

[edit] Swing axle

[edit] Conventional swing axle

The most familiar form of this suspension was found in the early Corvair and early Volkswagen. The differential unit was solidly mounted to the chassis and each wheel assembly pivoted at a universal joint near the differential. Major disadvantages were poor camber control and the possibility of "jacking," a situation where the outside wheel "tucks under," often resulting in the overturning of the vehicle. An alternate design eliminates one universal joint, with both wheels pivoting about the remaining universal joint.

[edit] Low-pivot swing axle

This modification of the swing axle described above provided a single pivot point for both wheels located below the differential, with provision for axial motion of the axles to prevent binding.

[edit] De-Dion

This design combines the excellent camber control of the beam axle, with the fully independent rear suspension's reduction of unsprung weight and elimination of driveshaft torque. The differential unit is solidly connected to the chassis and two universal joints and a splined connector are used on each of the two axles. A single assembly, usually consisting of a large diameter tube bent to avoid interference with the differential unit, connects the wheel uprights.

[edit] Independent rear suspension

[edit] Spring types

[edit] Coil spring

A large coil-type spring which sits in perches on both the suspension components and the frame or frame member. This type of spring acts like any small coil spring and can have a progressive or linear spring rate through the use of different spring overall length, number of coils, wire diameters and coil spacing.

[edit] Leaf spring

Curved steel "leaves" are packed together to achieve one unified pack of curved leaves which resists being bent further or straightened. With the weight of the vehicle on them, this resistance to being straightened is what supports the vehicle and keeps the wheels on the ground when going over bumps.

[edit] Coilover

A coil spring and shock absorber combined in one unit, often made adjustable to allow suspension feel and/or ride height fine tuning.

[edit] Torsion bar

The spring action is created by a beam that (usually) runs across (left to right) the vehicle. Within this beam is usually two spring steel rods that are joined to the housing beam in the middle and have each end connected to a pivot arm that joins onto the wheel hub of both sides. Therefore as the weight is put on the suspension the spring steel rods will 'twist'. This method is used with a shock absorber mounted separately and often means that the ride height of the vehicle can be adjusted.

[edit] Methods of improving suspension

[edit] For performance

[edit] Benefits of a rigid platform (chassis)

A rigid chassis allows a suspension system to operate more consistently. It does this by preventing geometry changes from occurring outside of the intended pivot point in the suspension. Also as a chassis flexes, points such as the center of gravity and roll center will move. By stiffening up the chassis it ensures that these points will remain consistent and the suspension can function as it was designed.

[edit] Methods for quantifying suspension performance

  • 60 foot ET or lap times
  • Skid pad performance

[edit] Resources

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