Sway Bars (FAQ)
From Nissan 350Z & 370Z Wiki
This page covers frequently asked questions about sway bars. These are also known as anti-roll bars, stabilizer bars, etc.. Read this page to get a good understanding of your suspension's characteristics and how upgrading your 350Z's stabilizers may affect handling and feel. There are links to installation tutorials, aftermarket sway bar options and more, so read up and ask more questions.
Contents |
Also See
- Suspension FAQ
- Front Control Arms (FAQ)
- General Suspension (FAQ)
- Lowering Your 350Z
- Suspension Definitions
GENERAL
What does a swaybar do?
The purpose of a swaybar (sometimes referred to as an anti-swaybar, rollbar, anti-rollbar, or stabilizer bar) is to keep the body of the car parallel with the wheels during a turn; this helps to reduce body roll.
Body roll is undesirable; the more the body rolls, the more weight is shifted to the outside wheels. When the weight is distributed more evenly, the inside tires can do some of the work that the outside tires were doing before, allowing for more "grip" while turning.
This also has the effect of altering the load caried by an end of the car. Stiffening the rear swaybar increases the load carried by the back end of the car while cornering. Stiffening the front swaybar increases the load of the front of the car while cornering. The more load an end of the car carries, the harder it is for that end of the car to turn. Using a stiffer swaybar in the back of the car causes the back to carry more load which in turn allows the front of the car to turn better -- reducing understeer (or increasing oversteer). You're basically manipulating the car's center of gravity while turning.
The torsional force that a sway bar can take is limited by the Bulk Modulus of the material (don't quote me). If you twist the bar too much it then looks like a pretzel and doesn't do squat for you.
The diameter of the bar and the length (and angle) of the "Arms" determine the "stiffness" of the bar. Assuming the rest of the geometries to be the same.. i.e. overall width, etc.
The "stiffness" goes up with the
- fourth power of the bar diameter
- inverse power of the Arm length
- Cosine of the Arm angle (depends on which way it is measured...etc..)
The counter force of a sway bar is essentially like a spring for small twisting motions, i.e. proportional to the twist angle.
Some things to note are....
- Going over a speed bumps does not use the capabilites of the sway bar. Both ends rotate up and down simultaneously. There is no twisting involved.
- Having a large bar gives up the independent suspension travel of the wheels, i.e. driving may be worse on "wash board" roads. If one wants better performance for these conditions, one must move to looser springs and sway bars, but stiffer shocks.
- sway bars don't help in the winter on slipery surfaces...
What does a sway bar do? (simple)
A swaybar is like a moderator of a talk show... It tries to "even out" the forces on either side of him/herself by transitioning conversation one way or the other, trying to give equal coverage of both sides of the issue.
You will probably note that there aren't many good talk show hosts out there.... and similarly, there are not too many good sway bars out there. That is why you should do your research before purchasing.
Aa very stiff moderator will not dive far into the issues (the supporters on either side of the issue are never given much time to talk at all..) It is the same with sway bars. Too stiff a sway bar, and the cornering ability (ability to see rational issues on both sides) tends to go down. Too small a sway bar and you tend to be overwhelmed by emotional gibber (precurser to rolling the car over :).
Sway bars essentially connect to either suspension arm and the sway bar is "grounded" to the frame (as a talk show host must be grounded with some basic skills and has be assertive in order to manipulate the often rowdy guests).
Sway Bar Formulas
An equation for the strength of a stabilzer bar:
R = 5e6 x D^4 / (0.4244 x A^2 x L + 0.2264 B^3)
where:
- R = Torsion rate of bar
- D = Bar diameter
- A = Effective length of lever arm
- L = Length of bar
- B = Length of lever arm
What happens if the Sway Bars are too stiff?
The limiting condition is when there is infinite roll stiffness. At this point, you have lost side-to-side independence in the front wheels.
The stiffer you make the anti-sway bars, the flatter the car's cornering response will be. In race cars, the limiting factor often becomes the tires. Well tuned racing Porsche 911's often have so much front roll stiffness that they pick up the inside front wheel.
One obvious problem that can occur is that you can lift up more then one inside wheel. Frank Bov complains that to be competitive in his AutoX class on a national level, he is pushing his BMW to the point where he has 2 wheels off the ground. A little bit hairy, because it is the first step to rolling the car. I have even had reports that I got the inside wheels of my 145 off the ground at an AutoX on a go-kart track. I beleive them, considering I drove _over_ (not on) the "speed bumps" in the apex of the corner and didn't feel them.
The other, less obvious problem is that you start imparting forces on the suspension that were not part of the design intent. Old 140s, 122s, and 1800s run the risk of bending control arms. This is usually only a problem in car that are driven really hard over rough surfaces (can you say 'rally cars?'). The problem arises when one front wheel is forcefully asked to move up while the other is not (say, by hitting a hole, rock, log, etc.). The sway part does its work and imparts a resisting force on the control arm of the wheel trying to move up. If the wheel is really being forced up hard, it will still go up, and the control arm will bend at the sway bar link. (That's why I run rubber sway bar bushing with the IPD bars now. The poly-eurothane ones got taken out when I replaced the bent a-arm and fitted the R-Sport springs.)
One other important note: The springs, shocks, swaybars, and tires & wheels all effect ride and handling. Only when these are carefully tuned to work with each other do you get the best solution. Suspension tuning is a real science where not all the answers are applicable to all purposes. Also remember, especially with the tall, boxy volvos (120s, 140s, [54]44s, 200s) that the car has such a high center of gravity for its width that there is going to be body roll, whether or not all 4 wheels stay on the ground. That's why the real sports cars are a lot shorter then they are wide.
Phyics and sway bars
By using larger sway bars than standard, you are able to resist the suspension motion that much more, thereby lessening the amount of body roll (lean).
True.
That ultimately decreases the amount of weight transfer from side to side thereby lessening the amount of centrifugal force on the car
False. The laws of physics still apply. The roll couple is not reduced. Centrifugal acceleration (V^2/R) is not reduced. But the increased roll stiffness reduces the angle change (body roll) caused by the roll couple. Decreasing the roll angle has at least two beneficial effects.
- In steady state roll (a long corner) the suspension geometry change is reduced. Most suspension geometries except a solid axle produce some camber change as a result of roll. Camber change alters the tire's footprint, potentially reducing lateral traction.
- Since steady state roll doesn't go as far, the duration of the transient while entering a curve is reduced. I think this is whatwe mean when we refer to "crisper" response. My 1985 Ford Crown Victoria SW has such a long transient that I could eat lunch while waiting for steady state cornering behavior.
350Z SPECIFIC
Who makes sway bars for the 350Z?
There are quite a few companies manufacturing sway bars for the Nissan 350Z. See our list of available sway bars to compare the different options.
Does the 350Z have OEM sway bars?
Yes it comes with both front and rear sway bars in stock form.
What size are the OEM sway bars?
See our OEM sway bar overview for a full explanation of the differences and sizes.
INSTALLATION
Can I install sway bars myself?
You or a small time mechanic should be able to bolt the front bar in. All the parts and peices fit in the OEM holes. And you will only need a ratchet set. It should take the dealer less than 20 minutes.
The front bar is easily accessed once the plastic underpanels is removed. The rear bar is a bit more tricky, because it involves clearing the rear exhaust.
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