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IAUTOINFO.COM: AUTOMOTIVE FRONT SUSPENSION

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Up |Front Suspension |Rear Suspension 


There are three basic types of front suspension systems: coil springs (double wish-bone), torsion bar and strut. Coil springs and torsion bars were the traditional suspensions used on American cars until the recent popularity of strut systems. Strut suspensions are lighter weight, which helps save gas; and they take up less space in the engine compartment, which is very important with the transverse-mounted engines used in most front wheel drive cars.
All three types are independent front suspensions. When a wheel rolls over a bump or into a hole, the road shock is absorbed by that wheel's suspension. The other wheel is only minimally affected. This provides much greater stability and directional control than the old solid axle designs.
In all three suspension systems, each front wheel is connected to a steering knuckle and wheel spindle assembly.




SUSPENSION TYPES




Front suspension - dependent systems
The front wheel's suspension systems are physically linked. There is only one type of dependent system you need to know about. It is basically a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. It's still common to find these on trucks. This suspension tends to shimmy, be heavier than newer suspension systems and are unable to be aligned properly.
Front suspension - independent systems
The front wheel's suspension systems are independent of each other (except where joined by an anti-roll bar) These came into existence around 1930 and have been in use in one form or another pretty much ever since then.
MacPherson Strut or McPherson strut
The system basically comprises of a strut-type spring and shock absorber combo, which pivots on a ball joint on the single, lower arm. At the top end there is a needle roller bearing on some more sophisticated systems. The strut itself is the load-bearing member in this assembly, with the spring and shock absorber merely performing their duty as oppose to actually holding the car up.
The steering gear is either connected directly to the lower shock absorber housing, or to an arm from the front or back of the spindle. When you steer, it physically twists the strut and shock absorber housing (and consequently the spring) to turn the wheel. The spring is seated in a special plate at the top of the assembly which allows this twisting to take place.
Rover 2000 MacPherson derivative
The suspension was derived from a normal MacPherson strut but with an added bellcrank. This allowed the suspension unit to sit horizontally along the outside of the engine bay rather than protruding into it and taking up space. The bellcrank transferred the upward forces from the suspension into rearward forces for the spring / shock combo to deal with.



Double wishbone suspension systems
The following three examples are all variations on the same theme.


Coil Spring type 1
This is a type of double-A or double wishbone suspension. The wheel spindles are supported by an upper and lower 'A' shaped arm. In this type, the lower arm carries most of the load. If you look head-on at this type of system, what you'll find is that it's a very parallelogram system that allows the spindles to travel vertically up and down. When they do this, they also have a slight side-to-side motion caused by the arc that the wishbones describe around their pivot points. There are two other types of motion of the wheel relative to the body when the suspension articulates. The first and most important is a toe angle. The second and least important is the camber angle.
Coil Spring type 2
This is also a type of double-A arm suspension although the lower arm in these systems can sometimes be replaced with a single solid arm. The only real difference between this and the Coil Spring Type 1 system is that the spring/shock combo is moved from between the arms to above the upper arm. This transfers the load-bearing capability of the suspension almost entirely to the upper arm and the spring mounts. The lower arm in this instance becomes a control arm.
Multi-link suspension
This is the latest incarnation of the double wishbone system described above. It's currently being used in the Audi A8 and A4 amongst other cars. The basic principle of it is the same, but instead of solid upper and lower wishbones, each 'arm' of the wishbone is a separate item. These are joined at the top and bottom of the spindle thus forming the wishbone shape. The spindle turns for steering, it alters the geometry of the suspension by torquing all four suspension arms. They have complex pivot systems designed to allow this to happen.
There are a lot of variations on this theme appearing at the moment, with huge differences in the numbers and complexities of joints, numbers of arms, positioning of the parts etc. but they are all fundamentally the same.


Trailing-arm suspension
The trailing arm system is literally that - a shaped suspension arm is joined at the front to the chassis, allowing the rear to swing up and down. Pairs of these become twin-trailing-arm systems and work on exactly the same principle as the double wishbones in the systems described above. The difference is that instead of the arms sticking out from the side of the chassis, they travel back parallel to it. This is an older system not used so much any more because of the space it takes up, but it doesn't suffer from the side-to-side scrubbing problem of double wishbone systems.
Twin I-Beam suspension
Used almost exclusively by Ford F-series trucks, twin I-beam suspension was introduced in 1965. This is a combination of trailing arm suspension and solid beam axle suspension. Only in this case the beam is split in two and mounted offset from the center of the chassis, one section for each side of the suspension. The trailing arms are technically leading arms and the steering gear is mounted in front of the suspension setup. Ford claim this makes for a heavy-duty independent front suspension setup capable of handling the loads associated with their trucks. In an empty truck the truck can be quite bumpy.
Moulton rubber suspension
Also known as cone suspension and trumpet suspension, this suspension system is based on the compression of a solid mass of rubber. It is named after Dr. Alex Moulton - one of the original design team on the Mini, and the engineer who designed its suspension system in 1959. Moulton rubber suspension is now used in a lot of bicycles - racing and mountain bikes.
Transverse leaf-spring
This system combines independent double wishbone suspension with a leaf spring like you'd normally find on the rear suspension. Used on the Corvette, it involves one leaf spring mounted across the vehicle, connected at each end to the lower wishbone. The center of the spring is connected to the front sub-frame in the middle of the car. There are still two shock absorbers, mounted one to each side on the lower wishbones.



COIL SPRING suspension



In a coil spring suspension, the steering knuckle and wheel spindle assembly is connected between upper and lower control arms. The control arms are connected to the vehicle frame, and the coil spring is positioned either between the upper control arm and the car's body or the lower control arm and the vehicle frame. In either case, a shock absorber dampens the up and down oscillations of the spring. The weight of the front half of the car rests upon the coil springs.

The ball joint connects the steering knuckle to the control arms, allowing the steering knuckle to pivot between the control arms when the car is steered. They also permit up and down movement of the control arm. One ball joint is called the load carrier and the other is called the follower. Which is which, depends on the location of the shock and spring.


Conventional shock absorbers, on the other hand, do carry weight. The purpose of shock absorbers is to control spring action and hold tires firmly on the road.
Tires are really just an air spring. And, like any spring, it will continue to bounce until the bounce energy is absorbed. Springs dissipate some of this energy, but it's the shock that absorbs excess energy from the spring. Shock absorbers transfer this motion energy into heat energy and dissipate it into the atmosphere.
Like ball joints, shock absorbers require frequent inspection.
When a shock stops dampening spring oscillation:
  • Steering and handling becomes more difficult
  • Braking action may be affected
  • There is excessive bouncing after stops
  • Springs can bottom out
  • Cupping begins to appear on tires


Other front suspension system components include the stabilizer bar and stabilizer links. The stabilizer bar and links join the two lower control arms to transmit cornering force from the outside wheel to the inside wheel during a turn. This helps equalize wheel loads and prevent the car from leaning or rolling outward when cornering. The stabilizer bar is also called the sway bar.
A strut rod is used in some systems to restrict the forward and backward movement of the lower control arm. It is attached between the lower control arm and the frame. Often, both ends of the strut rod are threaded to allow for the caster angle adjustment when performing a wheel alignment.
Stabilizer bars and strut rods do not wear out - they need to be replaced only when damaged or bent. Wear occurs at their connections, at the bushings. When the bushings wear, the connection is loosened. Vehicle handling gets a little sloppy and the wheels cannot remain in proper alignment.

When the shock and spring is positioned between the control arms, their bottom connects to the lower control arm and their top is connected to the vehicle frame.
When the shock and spring is ride on the upper control arm, their top is connected to the vehicle body.
In both cases, the weight of the car is transmitted through the spring to the control arm at its bottom...and through the control arm to the ball joint. That ball joint is the load carrier. Load carrier ball joints bear approximately one-half of the total vehicle weight. They are subject to severe wear and it is important to periodically inspect them.



Torsion bar system



There are no coil springs in a torsion bar suspension system. Instead, a torsion bar supports the vehicle weight and absorbs the road shocks. While a coil spring performs these functions by compressing, the torsion bar acts by twisting.
The torsion bar is connected between the lower control arm and the frame. The bar can be connected either longitudinally or transversely. Torsion bars can be used to adjust vehicle riding height. They are not interchangeable from side to side, because the direction of the twisting (or torsion) is different between the left and right sides.
Other than the difference between the torsion barand the coil spring, this type of suspension is the same as the coil spring variety. Because the torsion bar is connected to the lower control arm, the lower ball joint is the load carrier. A shock absorber is connected between the lower control arm and the frame to dampen the twisting motion of the torsion bar.



Strut suspension



In a strut assembly the shock absorber is a structural part of the vehicle's suspension. Strut suspension systems are more compact and weigh less than conventional suspensions, making struts ideal for the smaller, more fuel efficient cars of today and tomorrow.
With a strut suspension system, in most cases only the lower control arm is used. The upper control arm is replaced by the strut assembly, which supports the weight of the car. The strut is directly connected to the steering knuckle on one end. Sometimes, in fact, the strut assembly includes the steering knuckle and wheel spindle. On its top end,
the strut is attached to the car's body through a strut bearing and rubber mount. The strut bearing acts as the upper pivot when the wheel turns. Since the strut carries the vehicle weight, the ball joint connecting the lower control arm uses the steering knuckle as a follower.
The similarity to conventional systems can be seen clearly to the modified strut suspension. Here, the coil spring is not part of the strut assembly; instead, it is connected between the lower control arm and the frame. In this case, the lower ball joint is a load carrier.







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Automotive Information for the consumer