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Front suspension - dependent systems


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Front suspension - dependent systems

So-called because the front wheel's suspension systems are physically linked. They are, in a word, shite. I hate to be offensive, but they are. There is only one type of dependant system you need to know about. It is basically a sold 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, but if you find a car with one of these you should sell it to a museum. They haven't been used on cars for donkey's years for three main reasons:

Shimmy - because the wheels are physically linked, the beam can be set into oscillation if one wheel hits a bump and the other doesn't. It sets up a gyroscopic torque about the steering axis which starts to turn the axle left-to-right. Because of the axle's inertia, this in turn feeds back to amplify the original motion.

[suspension4]Weight - or more specifically unsprung weight. Solid front axles weigh a ton and need huge springs to keep their wheels on the road.

Alignment - simply put, you can't adjust the alignment of wheels on a rigid axis. From the factory, they're perfectly set, but if the beam gets even slightly distorted, you can't adjust the wheels to compensate.

Front suspension - independent systems

So-named because the front wheel's suspension systems are independent of each other (except where joined by an antiroll bar) These came into existance around 1930 and have been in use in one form or another pretty much ever since then.

MacPherson strut

This is currently, without doubt, the most widely used front suspension system in cars of European origin. It is simplicity itself. Unfortunately, the image on the left doesn't show you all the components - I'll put a new one there as soon as I can. However, the system basically comprises of a 'coil-over-oil' 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 (purple in this image), or to an arm from the front or back of the spindle (in this case). When you steer, it physically twists the shock absorber housing (and consequently the spring) to turn the wheel. Simple. The spring [suspension1]is seated in a special plate at the top of the assembly which allows this twisting to take place. If the spring or this plate are worn, you'll get a loud 'clonk' on full lock as the spring frees up and jumps into place. This is sometimes confused for CV joint knock.

The following four types of system are all essentially a variation on the same theme.

Coil Spring type 1

This is a type of double-A arm suspension. The wheel spindles (purple) are supported by an upper (green) and lower (blue) 'A' shaped arm. If you look head-on at this type of system, what you'll find is that it's a very basic lever 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 which the levers scribe around their pivot point. This side-to-side motion is known as scrub. Unless the links are infinitely long the scrub motion is always present. 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 (steer angle). The second and least important, but the one which produces most pub talk is the camber angle, or lean angle. Steer and camber are the ones which wear tyres. Also note that the springs/shocks in this example are in a so-called 'coil over oil' arrangement whereby the shock absorbers (yellow) sit inside the springs (red).


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 single solid arms. The only real difference between this and the type 1 system mentioned above 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. This particular type of system isn't so popular in cars as it takes up a lot room.

Double Wishbone

So-called because the lower and upper arms are the shape of wishbones. The spindle is a highly complex construction in this system, as are the wishbones themselves. This rapidly becoming one of the most favoured suspension types for new cars as it gives excellent roadholding capabilities whilst taking up very little room under the car. This allows for smoother lines on the bodywork, and less intrusion in to the engine bay. A 2D diagram such as that on the right does not do this system any justice. To really appreciate it, you need to get your head in a wheel well and have a look. And I know a few mechanics who've still not been able to figure it out even then.

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. Car manufacturers claim that this system gives even better road-holding properties, because all the various joints make the suspension almost infinitely adjustable. There are a few variations on this theme appearing at the moment, with differences in the numbers of joints, numbers of arms, positioning of the parts etc. But they are all fundamentally the same.

Trailing-arm suspension

Unfortunately, no image for this yet. The trailing arm system is literally that - a shaped suspension arm is joined at the front to the chassis, alowing the rear to swing up and down. Pairs of these become twin-trailing-arm systems and work on exactly the same principle as the arms in the coil spring type systems described above. The difference is that instead of the arms sticking out from the side of the chassis, they travel back along it. If you want to know what I mean, find a VW beetle and stick your head in the front wheel arch - that's a double-trailing-arm suspension setup. Simple. It's used mostly in older cars and beach buggies now.


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