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    Swaybar Design

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    Sweet Seventy9
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    Swaybar Design

    Post  Sweet Seventy9 on Tue Sep 09, 2014 6:17 pm

    Another article I found on the web.


    Swaybar Design
    By Scott Mueller.
    I have made swaybar function and design a sort of pet project of mine for many years. I have been sticking bigger bars on my cars since the '70s, and currently have non-stock bars on every one of my 5 vehicles except the '96 Jeep GC, and that is only because those are being custom made for me as we speak (nobody offers them off the shelf) and I don't have them back from the shop yet! What will follow is a sort of mini-course in swaybar design, practically applied to the Impala.

    There are several factors important in stabilizer bar (also called anti-roll or anti-sway bars, or even just swaybars for short) design. These factors control the spring rate and effectiveness of the bar, and would include the following:

    -Bar thickness
    -Bar length
    -Swing arm (lever) length
    -Bar material
    -Attachment method (end link design)

    Let's analyze the stock, John Hotchkis, and Herb Adams bars with respect to all of these important specifications and see how they measure up. First we will do the front bars, then the rears.
    BAR THICKNESS - As has been noted the stiffness of a bar is proportional to the 4th power of the diameter. A bar that is twice as thick as another will be 16 times stiffer! This means that small differences in thickness make large differences in rate. This calculation can also take into account whether the bar is solid or hollow, by subtracting the rate from a bar matching the ID or hollow space within.

    The stock front bar is 30mm in diameter, and is solid. We know that the Herb Adams (HA) front bar is 1-5/16" (1.3125") in diameter and is solid. We know that the John Hotchkis (JH) front bar is 1-7/16" (1.4375") in diameter and is hollow, with 0.25" tube wall thickness. This gives an Inner Diameter (ID) of (1.4375 - 0.25*2) or 0.9375". Based on thickness alone, we can therefore calculate the relative rate (stiffness) of the front bars as follows: OD^4-ID^4 = Relative Rate.
    OD ID Relative Rate % over Stock
    Stock 1.181 0.0 1.946 0.0%
    JH 1.4375 0.9375 3.498 79.7%
    HA 1.3125 0.0 2.968 52.5%
    F-car 1.2500 0.0 2.441 25.5%
    This shows that the larger, hollow tube JH bar is nearly 80% stiffer than the stock bar, while the HA bar seems to be only 53% stiffer than the stock bar, that is if we take only the diameter of the bar into account. However, as has been said somewhere before, "size isn't everything", as we soon shall see!

    BAR LENGTH - The stiffness or rate of a swaybar is inversely proportional to it's length. Given two bars of the same diameter and other specifications, one that is twice as long will have half the rate or stiffness. For the front bars, they all have relatively the same length, which is dictated by the width of the vehicle frame at that point. All of
    the front bars are about 35" wide between the pivot points. Therefore no adjustment in the relative rate calculations are necessary between the different bars.

    SWING ARM (LEVER) LENGTH - A swaybar is a torsion bar, and force is applied to the bar via a lever on each end. The length of this lever is very important! The rate of the bar is inversely proportional to the length of the lever you use to exert force on it. This means that given two bars with identical properties, if you make the lever arms on the end of one twice as long as the other, you will also cut the relative rate (stiffness) in half.

    Here is where there are some differences between the bars. I measured the lever arm length of the stock front bar at 14" from the center of the pivot point to the center of the end link attachment hole. If the JH bar is contoured the same as the stock bar as they have indicated, then it should also have the same 14" lever length as the stock bar. The Herb Adams bar, however has a much shorter lever length of only 11", which has a marked effect on the rate. To calculate the relative rate now considering the lever length, merely divide the rate of the bar alone by the length of the lever arm.
    Bar Rate Lever length Relative Rate % over Stock
    Stock 1.946 14" 0.1390 0.0%
    JH 3.498 14" 0.2498 79.7%
    HA 2.968 11" 0.2698 94.1%
    F-car 2.441 13" 0.1878 35.1%

    As you can now see, the HA front bar is really 94% stiffer than stock, and the JH bar is only 80% stiffer than stock. If we did not take into consideration the lever arm length, we would never have realized this!
    Also consider that the thicker (but weaker) JH bar will be more likely to contact or rub the tire during a full lock turn than the thinner (but stronger) HA bar.

    BAR MATERIAL - A swaybar is a spring, and as such should be made of spring steel, or some form of hardened steel. Unlike a coil spring which is wound from spring steel wire, a swaybar is normally formed from regular steel stock and then must be heat treated (hardened) to increase the tensile strength. If the bar is not made from hardened steel, it will yield if bent and subsequently may not return completely back to it's original shape.

    To make a hardened swaybar, you normally have to start with a high carbon steel such as 4340 alloy, which is much more expensive than mild steel. Then the bar is heat treated, which hardens it and dramatically increases the tensile strength. The heat treating process used in hardening a bar leaves the bar with a rough, scaly surface, which is clearly evident on the stock bars as well as the HA bars.
    The JH bars have a smooth finish, which may indicate that they are made from mild steel and not properly heat treated. That is not to say that one cannot grind a smooth finish on a bar that has been heat treated, but generally even if the bar is finished, it will not be smooth without extensive grinding and polishing.

    Because of the rough surface of the heat treated stock and HA bars, I recommend that they be Jet Hot (800-HEADER-9) coated instead of powder coated. The Jet Hot coating is also much more durable than powder coat, as it will not ever chip, flake, peel or corrode. You can have the bars Jet Hot coated in several different finishes, and the cost is $8 per foot. I was charged for 11 feet (or $88) when I sent both of my bars in for coating last year.

    ATTACHMENT METHOD - In order for a swaybar to work, it has to be attached to the lower control arms of the vehicle, and sometimes also to the frame at the pivot points. Most front swaybars have some form of end link that attaches the bar to the control arms. Stock factory type end links usually consist of a long, thin bolt, with washers and bushings where the bolt passes through the bar and the lower control arm.
    Non-performance vehicles often use soft rubber bushings in the end links, but these offer very little resistance to bar movement. With rubber end link bushings, the bar lever ends can move up to 1" or more before actually transferring the full force of the bar to the control arm. Performance vehicles like the Impala substitute urethane bushings in the end links which offer more resistance than soft rubber, and allow less bar movement before transferring the load to the control arm. However even urethane end link bushings give way, and make the bar much less effective than if it were solidly attached. The best setup by far is to have a solid attachment to the control arm,
    which instantly transfers the force of the bar. This is exactly how all of the rear swaybars (including stock) attach to the Impala, that is they bolt solidly to the rear lower control arms. Unfortunately one cannot normally bolt the front bar directly to the control arms, because the front suspension is independent, and allows more movement than a solid mount would allow.
    To solve this problem, the Herb Adams bars use special end links made from tie rods which directly attach the bar to the control arms, but also allow for full suspension movement. Tierods are completely quiet in operation like the stock type end link, but they are MUCH stronger and instantly transfer the bar force to the control arm. They can handle loads far greater than the stock thin bolt end links used in the stock front swaybar, and make any bar much more effective in operation as they take
    the bushings out of the equation.
    The stock front bar uses a bolt-type end link with urethane bushings, as does the JH front bar. The HA front bar uses spherical bearings (Heim joints) or optionally can be equipped with greasable tierod ends. The function of the Heim joints and the tierod end links are the same, but the tierod versions will be quieter and last longer since they are greasable.
    To attach the bar to the frame in the front requires the use of a bushing and clamp. This is to allow the bar to rotate, but also to give it a leverage point. The stock front bar uses a soft rubber bushing, while Herb Adams offers an installation kit with a urethane bushing. JH includes a greasable urethane bushing from Energy Suspension (714-361-3935) with their front bar, which can also be purchased separately and used with theHA bar if desired.

    For the center attachment, I actually prefer a rubber bushing, but not the stock one. The problem with urethane bushin s in my experience is that they eventually wear out,and also make noise (squeaking, rattling). The Energy Suspension bushings do help solve this problem since they are greasable. Even so, I feel there may be a better solution. The factory makes high durometer (hard) rubber bushings for F-car applications which also have a special low friction high strength fabric inner liner that
    allows for easy and smooth bar rotation without noise. They also never need to be greased. The 36mm bushing from an '82-'92 F-car will work withthe JH front bar, while a 32mm bushing from a '93-present 1LE or WS6 F-car will work with the HA front bar.

    Note that it can sometimes be difficult to compress the clamp if you are using the stock bolts which thread directly into the frame. The threads strip very easily, in that case I recommend getting a 3/8" nutsert, which allows you to install much higher strength threads inside the frame, and then you can use high strength 3/8" socket head bolts to attach the bars. I will post more about this in the bar installation instructions I am writing.

    Now let's analyze the rear bars:

    BAR THICKNESS - All of the rear bars are solid, so the rate is simply proportional to the OD to the 4th power. The stock bar is 26mm (1.0236"), the JH bar is 1.25", and the HA bar is 1.5" in diameter.
    OD Relative Rate % over Stock
    Stock 1.0236 1.098 0.0%
    JH 1.2500 2.441 122.4%
    HA 1.5000 5.063 361.2%

    As you can see, the HA rear bar is a whopping 361% stiffer than stock!! The JH rear bar is only 122% stiffer than stock, if we take into account only the diameter of the bar.

    BAR LENGTH - As stated earlier, the rate of a bar is proportional to it's length. This was not important when evaluating the front bars, as they are all the same length between pivot points. However, unlike the front bar, the rear bars here do differ in length due to differences in shape.
    The Herb Adams rear bar goes straight across, and is therefore much shorter than the stock or JH bars, which are both contoured to rise up along the axle tubes. The stock rear bar is 54" long between the pivot points, and so would be the JH bar if it follows the stock contours. The HA bar, on the other hand, is only 46" from end to end, which has a dramatic effect on the rate as we can demonstrate.
    Bar Rate Bar length Relative Rate % over Stock
    Stock 1.098 54" 0.0203 0.0%
    JH 2.441 54" 0.0452 122.4%
    HA 5.063 46" 0.1101 441.4%
    Wow! As you can see, the HA rear bar must now be factored at 441% stiffer than stock, due to it being a straight across (shorter) design. This bar is quite a bit stiffer than the JH bar, and is one of the reasons the HA bar package is much more balanced to neutral handling compared to the stock or JH setups.

    SWING ARM (LEVER) LENGTH - All of the rear bars have a 13" swing arm or lever length, which is dictated by the standard mounting to the lower control arms. Since all of the bars are the same, no relative rate adjustment is needed.

    BAR MATERIAL - The stock and HA rear bars are heat treated. It is not known if the JH bar is heat treated or not.

    ATTACHMENT METHOD - All of the rear bars bolt directly to the rear lower control arms. They do not use any form of end link nor do they use a center frame attachment or bushings like the front bars. Since they all mount the same, no consideration needs to be made here comparing one against the other.

    To summarize the swaybar situation, based on this analysis, I would have to confirm my recommendation of the Herb Adams swaybars over the JH bars. The HA front bar is actually 94% stiffer than stock due to the increased size as well as the shorter lever arm length. The HA bar is also heat treated and uses very high strength solid tierod end links. The current JH front bar is only 80% stiffer than stock (assuming the same lever arm length as stock) plus it uses stock bolt type end links which reduce the
    effective rate of the bar even further through the use of bushings, and which offer questionable strength.

    The HA rear bar is also 441% stiffer than the stock rear bar, while the JH bar is only 122% stiffer than stock. The much stiffer rear bar is one of the keys to the balance and superior handling the Herb Adams swaybars achieve. The stock bars as well as the JH bars (based on these calculations) are calibrated so that the car will understeer (plow), while the HA bars have been designed to remove this understeer tendency, and make the car neutral in handling. Note that using the large HA rear bar
    with the weaker stock or JH front bars will result in an oversteer situation, which would be dangerous.






    _________________
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    1989 Caprice Sedan 9C1 x4

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