Z31/Z32 suspension systems

[MikeMan]

Ok here we go. Apologies in advance as this will get quite technical.
Glossary
Camber:
The leaning of the wheels either into or away from vertical. Camber is often used with respect to the chassis of the car, but in situations such as heavy cornering and when travelling over bumps, this can be misleading as it may differ from the camber angle with the road (which is a true representation of the behaivour of the wheel) Simply put, negative camber (where the wheel leans 'into' the car) tends to cause oversteer while positive camber tends to cause understeer. Generally speaking, it is good to have a small amount of negative camber as this can provide better traction in cornering anyway.

Toe:
This is the angle of the wheel in respect to the 'forward' difrection. In the front wheels, toe-out will generally cause the car to understeer while toe-in will cause the car to oversteer. This is because when the car turns into a corner, more weight is transferred over the outer wheels rather than the inner wheels, causing the outer wheel's angle to have more effect. In the rear wheels, the opposite is true. A toe of zero causes the car to lose straight-line stability at higher speeds as the car will tend to 'wander'. Too much toe in or out will cause the rolling resistance to be too large and more power will be required for cruising, decreasing top speed and fuel efficiency.

Caster (castor): This is the angle of the vertical line about which the front wheels steer. (The caster angle 'leans' towards the front of the car). This tends to produce a self-centreing effect not unlike a shopping trolley wheel, where the wheel wants to go straight rather than wanting to turn. The main reason for castor though is to affect the camber of the wheels when cornering. Excessive caster in front wheel drive cars will produce higher amounts of 'torque steer'.

Mechanical trail: This is the horizontal distance between the contact patch of the tyre (tire for you north americans) and the axle. This is the exact analog of a shopping trolley wheel. However, excessive trail can (like a shopping trolley wheel) cause wild oscillations in the steering angle as the wheel tries to 'find' centre. Generally the mechanical trail and the caster affect this self-centreing ability in unison, and a small amount of each is a Good Thing (tm).

Pneumatic Trail: This is the distance between the centre of the contact patch of the tyre and the point which all lateral forces are transferred from the ground to the tyre and vice versa. This is mainly affected by tyre pressure but also tyre compound and even tread height.

Introduction
First before we start to get into specifcs we need to understand why there is a need for differing suspension systems. Depending on the way a suspension system is designed, various aspects of the geometry can change through the suspension's travel, or through the steering angles. The suspension needs to ensure each wheel maintains proper contact with the road, at the proper camber and toe angle, while still providing adequate ride comfort.

A major drawback of simpler independant suspension systems is that the camber constantly changes throughout the suspension's travel. This can cause undesirable results especially in cornering such as oversteer or understeer. Most advanced suspension systems designed to counter this generally take up far more space. Hence, the choices most manufacturers make for the suspension systems used is generally a tradeoff between ride comfort, cornering stability and space constraints.

The 300zx suspension systems
The z31 in particular uses a MacPherson Strut suspension system for the front wheels. This type of system takes up very little lateral room which is well suited to the z31's cramped engine bay. Perhaps if another suspension type had been used the Z may have remained inline-6. This type of suspension system provides very good ride comfort and adequate cornering stability. Due to the design however, the camber does in fact change through the suspension's travel, leading to slight oversteer. Thankfully oversteer isn't as much of a problem at the front wheels as it is at the rear wheels and this is why the z31 can still maintain relatively good handling This type of suspension is still used today in many FWD cars for space reasons.


In the rear of the z31 is a semi-trailing arm design. A trailing arm system is a type of independant suspension where each wheel is free to move up and down about a pivot in front of the wheel. This would theoretically seem like a perfect type of suspension as the camber of thew wheel never changes with respect to the car's body. However, in body roll situations, the wheel's camber with respect to the road changes because the car's angle with the road changes. This causes very strong oversteer, especially if you encounter bumps whilst cornering hard.

In an attempt to counter this oversteer, the semi-trailing arm suspension was developed which tried to counter this action by incorporating an angle on the pivot of the arm with the body. Doing this allowed the camber with the road to remain quite good in body roll situations. However, the camber angles with the road still changed through the suspension travel, and ride comfort was quite bad due to the sprung weight of the trailing arms (which in turn caused minor bumps to be transferred to the car's chassis). This particular type of suspension works very well with sway bars (also known as anti roll bars) as these attempt to eliminate body roll.



The z32 on the other hand uses 4 wheel multi-link suspension. Perhaps the best suspension system in widespread use, this type of suspension involves complex angles and geometries in order to maintain a near perfect camber and toe angle in all situations. Technically speaking it comprises of multiple pivoting control arms attached to various pivot points on the car's chassis. These types of suspensions are designed with complex mathematical modelling programs. As such, they can be designed for specific applications and shouldn't really be modified without the aide of these types of simulation programs.

Because through all these suspension types the camber changes with suspension travel, it is a good idea to get a camber adjustment kit if you decide to lower the car.

Sway Bars
Sway bars attempt to trade off a little ride comfort for a much better cornering ability by trying to eliminate body roll. Sway bars work via linking left and right wheels through a thick metal bar attached behind or in front of the wheels themselves. In this way, the axle connecting the wheels stays more or less parallel to the angle of the car. This attempts to solve many problems encountered when the chassis of the car experiences body roll.

The problem with perfectly rigid sway bars is that if one wheel goes over a bump, the sway bar will restrict its movement (as the other wheel is linked to it. This will cause the car to behave as if it had no suspension at all! This is obviously a bad thing so manufacturers allow the sway bar to have a little 'give' so that it can bend and twist slightly. The bushings used for the sway bar's pivots also have a fair amount of give in them too and these are commonly replaced by urethane or solid metal bushings. Upgraded sway bars (more rigid) will offer much better cornering stability at the expense of ride comfort and road noise.


Springs and shocks
Springs are great for the absorbtion of bumps when attached to fairly heavy things (such as cars) as the car's momentum will generally want to keep the car still while the spring has no worries expanding and contracting. The major problem with using a spring alone is that the spring tends to keep oscillating between expanding and contracting, making the car continue to jump up and down well after the bump has been passed. This is where dampeners come in. Typically consisting of a loose fitting piston or other type of moving bottleneck in a fluid filled cylinder, the shock absorber's job is to provide a small amount of resistance to moving so that the spring will stop bouncing.

A common modification is to install adjustable coil-overs. Consisting simply of a spring and shock unit, these allow you to adjust various aspects of the suspension's behaivour. On the best models, you can adjust the spring preload (which effectively lowers/raises the car at the same time as slightly adjusting spring stiffness) and the compression and rebound damping (which affect how much resistance the shock provides to the compression and expansion of the spring). Some extremely good models even alow this all to be done electronically from within the car, effectively creating a sports car suspension at the flip of a switch.

Strut braces
Strut braces are quite literally metal bars used to brace the tops of both strut towers. Often seen on many types of upper end sports cars (such as the 350z) these strut braces contribute to the overall structural rigidity of the car. Often body flex is not an important issue and even small modifications such as upgrading sway bar bushings will give a much more positive result. Often used in excess by ricers, all sorts of blinged up strut braces can be purchased, often going at insanely expensive prices for what they are (a solid bar of metal). While actually performing an important job, strut braces are often not worth the money, as that money could be better spent on other suspension modifications yielding a much better result. Of course, if your suspension has already been modified to provide very good performance, a strut brace will eventually be necessary.


Any questions?

-Mike

Sources:
http://z31.com/suspension/
http://www.zilvia.net/faqs/specs/z32specs.asp
http://www.autozine.org/technical_sc...uspension2.htm
http://www.chris-longhurst.com/carbi...ion_bible.html

[Zgringo]

Your my hero....Thanks Mike......tried to call you last night (day your time)

[k3smostwanted]

wes or red...sticky this!!!

i think broke covered a little bit in his sticky...

[DeleriousZ]

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