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Swaybar Info

Old 10-08-2009, 09:19 AM
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Join Date: May 2001
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Vehicle: 2010 Genesis 2.0T
Default Swaybar Info

This info is direct from the Whiteline website:

Swaybar Chart:

How big a bar do I need and how will it work?
Lets start first by assuming that every vehicle has a certain optimum "anti-roll" value, typically expressed in pounds or kilos. Don't ask me the ideal for the WRX as there are many formulas that have to take into account the COG (centre of gravity), RC (roll centres), the resultant roll couple and roll axis front to rear. We at Whiteline establish this optimal amount thru experience and testing though we are starting to use more computer modelling software to fast track development. With this amount in hand, the next issue is to determine where these amounts are needed and under what circumstances.

Best next to jump to the issue of cornering and how much we need where and when but its important to first identify that we need to split any given corner into at least 3 segments. Corner entry is that first phase where you initially turn in, you are generally decelerating either under brakes or on a trailing throttle. Needless to say the weight shift is moved toward the front. Mid corner is when you start aiming for the apex proper with either a neutral pitch (for-aft weight or movement mode) stance or slight power application. Corner exit is once you seriously start applying power and can be either just before the apex or after. Either way, the main steering line has been established and corner exit power is being applied to maximise forward speed. Accepting the above, its equally important to then segment handling bias according to corner position. That is, a mid-engine car with factory setup will typically understeer on "corner entry" and "middle" stages but very easily oversteer on exit if too much power is applied.

Understanding that everything is a compromise, it is useful to have a disproportionately high rear roll rate on corner entry to deliver more front roll to improve initial turn-in "bite" through the front outer. A heavier front spring like a lot of front anti-roll, will act to oppose the momentary (as little as a 1/10 sec) weight transfer required to give that "bite" which can result in more turn-in understeer. On the other hand, corner exit is all about maximising grip in longitude and latitude with throttle steering used to optimise outcome. Here, disproportionately more rear roll control acts to load the inside front (particularly useful on a fwd and WRX) as the outside rear is held more upright.

The correct total amount of "anti-roll" is inclusive of spring rate, which complicates the issue but it is still safe to assume that it is better to start with less spring rate than too much. The object is to maximise grip through adequate suspension travel at each wheel, a heavier spring rate makes it difficult for an individual wheel to react to changes in road surface under roll so contact is lost. Having said that, there is still no absolute correct number or recipe with two different racecars in any 2x car leading race team using largely different setups depending on the driver's preference. However, in most cases it will still function within a certain optimal pie of anti-roll force.

Multiple adjustment holes on Whiteline Blade swaybars.
A swaybar is actually a torsion spring not unlike a coil spring. Imagine you could uncoil your coil springs, and then hold each end and twist. This is pretty much what happens with the coil when it does its work as the shape forces the material to twist through out its length as it's compressed.

Going back to our straightened piece of spring steel, with your arms holding each end, imagine that each of your arms represent the swaybars "arms" and the spring steel rod represents the "back" or centre portion of the bar between the arms. As you try to twist the ends, a certain amount of flex happens in your arms but most of the action happens across the back of the bar. Hence, the formula calculating swaybar rates requires a value for the length of the back of the bar as well as the length of the arms.

The formula does not allow for a left or right arm value, just a total that acknowledges that the total arm length vs. the back length is the key calculation affecting the rate. We know that shortening the effective length of the arm by choosing a hole closer to the back of the bar will reduce the leverage ratio hence increase the rate but this affects the total arm length so can be done on one side only. In this way, a 2 hole per arm Blade adjustable bar does actually have 3 different settings; a 3-hole bar has 5 different settings. Additional mounting holes on the chassis end multiply the options further.

There are some assumptions and exceptions to these examples, specifically the issue of ultra short arm lengths and its effect on suspension preload. That is, asymmetric adjustment can preload the suspension if the arms are very short. The other issue is that the bars arms also deflect so that is taken into account by the formula but the amount of deflection is governed by the size and shape. Friction and deflection in mounting bushes will also affect the bars outright behaviour.

You may also notice that some of our adjustable use a longer Blade (flattened area) than others. We use this to fine-tune the adjustment range as the Bladed portion deflects a lot less than the simple round bar. (Think of a structural steel I-beam). Even the height and width of the Blade is used to tune the final outcome when designing new bars.

As for different materials, our XRD swaybars use 27mm base material machined down to 24mm outside the chassis D mount points coupled with a specific Blade length to deliver a very broad range of adjustment through a more progressive working curve. This delivers a hybrid outcome tunned for a particular result and is another design tool we can use.

It's therefore important to understand that a 22mm physical diameter swaybar can be made to behave like something totally different just by changing the shape of the bar and its ends. Adjustable bars are a very useful suspension balance tuning tool and a better understanding of how they work should help you get a better result.
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