SUSPENSION MISCONCEPTIONS


If the typical 4x4 vehicle owner is honest with themselves, they have to admit that the majority of their driving is still on road, even if they go wheeling every single weekend. At that point a truly dual purpose suspension should be a must. Due to a lack of engineering know how, most manufactures simply don't design their suspension systems with a balance between on and off road performance. Instead they simply shift the spectrum towards off road performance and let the customer suffer the on road consequences. Like so many markets, off road aftermarket suspensions suffer from a fair amount of creative inertia. That is, once something is accepted as 'the way to do it' on one application, many falsely assume that the entire recipe applies equally well to other platforms. Or perhaps a company may prefer to convince its customers of this because it has become their specialty. The long-arm legacy from the Jeep TJ is a perfect case in point. This is because the stock short arm allows for the correct geometry, but degrades rapidly with excessive lift height. This means that the long arms are not central to, or even necessary for correct geometry on TJ's lifted up to and not exceeding 3 inches of lift.

In the often mail order world of suspension kits, quality is all too often confused with value, often resulting in additional purchases and even replacement purchases that far negate the original hoped for savings.


Roll Center Geometry

Roll center is the imaginary point around which the body leans in a turn and also around which it moves when the suspension flexes. There is one roll center (RC)for the front and rear suspensions. The location of each roll center for most solid axle suspensions is defined by the geometry of the track bar or pan hard bar. The actual roll center is found by drawing an imaginary vertical line down the center of the middle of the vehicle and another straight line between the bolts at the ends of the track bar, ignoring the bends. The intersection is the roll center.

Roll center is important to suspension engineers because its correct placement relative to the center of gravity (COG) is central to the managing both body lean and weight transfer in turns. The further apart they are, the more lean, and along with it geometry effects that degrade handling. This is one of the critical geometry parameters that must be right before you tune or lift the vehicle. When properly located relative to the COG, the roll centers defined by track bar placement will allow the engineer to further optimize the overall vehicle performance via spring and shock tuning, without the burden of having to attempt to compensate for poor geometry.


Control Arm Geometry

Control arms are the links in the suspension that connect the axle to the frame. On most solid axles suspensions there are two arms, one above the other, on each side of the vehicle. In stock form they are more or less running parallel to the ground/frame. The reason for having two is to keep the axle wrap under control, but their length and the positions of their ends relative to the axles also define some important imaginary geometry points called instant center (IC). Like roll centers, these points and their relationship to the COG determine most of the 'automatic' handling behaviors that happen during bumps, turns, acceleration, and braking. Some of these behaviors have names that you may be familiar with such as anti-squat, which is geometric resistance to the rear end dipping during acceleration. Or anti-dive which is similar to anti-squat but for the front of the vehicle during braking, and roll steer which occurs when the lean of the vehicle in a turn causes the control arm geometry to actually 'turn' similar to the way a skateboard turns, producing it's own vehicle direction change without drivers input, via steering. Depending on how the geometry is set up and the ground clearance considerations make this nearly impossible on a lifted 4x4. Bad roll steer is caused by any lift kit that increases the control arm geometry too much, adding dangerous amounts of roll steer. This effect is most dramatic in rear suspensions because they have no driver input via steering. Roll steer is caused by the fact that non horizontal arm also move the the axle forward and back when they move up and down due to body lean, and the steeper the arms, the greater the movement.

To some extent, longer that stock arms can improve all parameters by reducing control arm angles and relocating the instant centers, but only if the upper and lower control arms are properly angled toward each other at the chassis end, if not their 'long-arm' benefit is wasted. A good suspension engineer must know how to locate control arms for the best possible combination of all these effects, which requires them to consider everything from handling priorities, driver preference, and other suspension factors including ride quality via springs and shocks. Done properly, correct suspension geometry is the basis for a safe enjoyable and highly versatile suspension.



Control Arm Bushings

For factory vehicles, the bushings in the control arms seem boringly simple. In fact the factory bushings were seen as 'binding' and was figured as the key to less suspension flexibility. Several off-road suspension companies have staked their names on kits that revolve in large part around replacing these boring rubber bushings with the more perceived acceptable urethane bushings. The problem is that those rubber bushings are just as much a key tuning element of the overall suspension as the springs, shocks and stabilizer bars are. The purpose of the stock bushing is to provide a delicate balance between providing enough give for low ride harshness while remaining durable enough to last for an acceptable mileage (which they do quite well). For example, track bar bushings that are too soft may result in vague steering and tendency to shimmy (also known as death wobble), but too stiff can cause the bar, or more importantly the stock brackets to fail. Meanwhile the control arm joints that are all metal or have hard plastics in them provide no isolation, and invariably result in a harsh ride, (especially in a uni body vehicle), and bracket failures. Yet the reason they exist in lift kits is because once the off road aftermarket discovered that the stock suspensions have some inherent bind at large amounts of flex, they replaced the bushings (that were in fact coping with the bind quite well) with joints that seek to eliminate the bind altogether, ignoring or more importantly unaware that the bushings were also absorbing parts of the impact forces from bumps which is actually their primary function.

The reason why this impact shock absorption is so important is not just ride comfort, it's also there to keep the brackets alive. With no soft bushings in the suspension, it begins to self destruct even from seemingly mild on road impacts. Brackets, or the welds that hold them, start to crack slowly, called fatigue, and eventually fall apart if left unchecked. Often this sort of failure of the stock brackets is blamed on the original vehicle maker, which is simply unfair and incorrect because the elimination of isolation from the bushings is the culprit! Thus the challenge with control arm bushings design for dual on and off road suspensions is add a tolerance for 'misalignment' that comes from increased articulation while preserving the isolation that keeps the chassis together and ride comfort.

In some cases such as the Jeep TJ, the factory control arm bushings are actually very good at isolation that also serves to tolerate a considerable amount of articulation. But all to often the replacement arms come with non-isolating joints that sacrifice ride comfort and durability that could have been had by keeping the bushings in the factory arms. In other cases such as the Jeep JK, the arms are longer and strong enough for even hard off road use, so no replacement for the sake of off road performance is necessary, though the aftermarket often appears to suggest otherwise, mainly out of habit rather than a sound engineering assessment.


Steering Geometry

Since any street legal vehicle must have a mechanical steering connection from driver to tires, this system is critically affected by any suspension height change. Most enthusiasts are by now aware that for solid axles, the track bar and steering drag link must be parallel to avoid 'bump steer', but that's just the beginning of the considerations. Roll Steer is caused when the steering linkage doesn't pass through the Roll Center of the suspension geometry. Meaning that every time the vehicle leans or articulates, there is steering input that the driver didn't intend. This happens because there is a small


lateral shift of the axle relative to the frame end of the steering which effectively steers the vehicle. Note that there can be both roll under steer or over steer depending on the specifics of the geometry.

On twisty roads, bump steer and roll steer along with the larger problem of rear roll steer also known as rear steer keeps the driver very busy trying to maintain the intended course because the vehicle is always doing 'extra' things that the driver never intended. Some may even know that a horizontal track bar and drag link are also preferable to minimize problems, but this is mainly to keep the handling symmetrical when turning right vs left. Even with out 'flat' geometry, it's still possible to have negligible bump or roll steer. All to often when the tires hit bumps it is mistaken for bump steer, but it's actually the opposite because the driver does feel a steering kick but the vehicle doesn't change direction. A high steer system can not only be a benefit to off road obstacle clearance, but the improved steering geometry also pays the additional benefit of allowing for a higher front roll center.


 
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