Have an idea for an adjustable rear swaybar. Does this make sense?

Discussion in 'Fiesta ST Autocross' started by Couldahadav8, Mar 15, 2014.

  1. Couldahadav8

    Couldahadav8 Active Member

    In looking at options for rear sway bars, I found this one that was designed for the B-spec Mazda 2:


    Since I have generally been a mustang guy, I just happen to have a couple of left over adjustable panhard rods from the 2011 and 2013 mustangs. Those panhard rods look a heck of a lot like that swaybar above. This is the one I have from Maximum Motorsports:

    In looking underneath the fiesta, there are holes near the springs on both sides of the twist beam. These are where the red bar brackets from the Tri-Point bar attach. I put the MM panhard bar under the car and low and behold, it is almost the perfect length to attach the two smaller holes on either side of the beam.


    Another angle:


    And another:

    This is another angle of where it could attach:


    This is just mocked up. Wanted some feedback about it. From what I can tell, the rear swaybars out there stiffen the twistbeam in various ways. I would think this would work. The ends are adjustable. I would expect that making the rod shorter would stiffen it up. Not sure that the bracket where this would potentially attach would need to be reinforced or not. That would seem easy enough to do.

    Any thoughts would be appreciated.


    Attached Files:

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  3. Sekred

    Sekred Active Member

    Panhard rods are designed to stop lateral movement of the suspension while still allowing it to move vertically. They are not designed to resist twisting motion like a sway bar which uses heated treated spring steel. I'd say that if you bolted that panard rod on in the position shown, it would simply twist or bend and stay that way the first time you drove around a corner hard.
    The Tri-Point image looks a bit like a strut brace to me?.
    reddog99 likes this.
  4. wash

    wash Active Member

    With heim joints, that bar would do nothing.

    I'm thinking about rolling my own with a hollow splined bar like a stock car setup.

    I also have this crazy idea of replacing the twist beam bushings with Johnnie Joints...
  5. Couldahadav8

    Couldahadav8 Active Member

    The tri-point part pictured above is their b-spec (i.e., racing) rear sway bar.


    Here is a picture on a car:


    I am going to go out on a limb and say that the bar above would be more bendable than the part I have. The panhard bar on a mustang has to deal with both lateral and fore-aft motion with weight amounts much greater than on the back of the fiesta. The attachment points on the Mustang use rubber bushings that allow a shocking amount of movement in the back. Stock mustang panhard bars are known to bend and break at times. Maximum motorsports builds parts for guys that race cars. This is a stiff aluminum piece that does not have the reputation for failing. It might bend, but I would be surprised. I think it is more likely that the attachment points would bend.

    The heimjoints will allow some movement, but they won't cause the bar to do "nothing." Most swaybars have heimjoints on the links. The comparison to a strut tower brace makes sense. The tri-point piece is more of a brace than a true sway bar.

    The term swaybar for the rear on this car seems a misnomer to me. Most of the pieces out there are braces or stiffeners. They are not true, torsion spring type swaybars.

    From Grassroots Motorsports:
    The Fiesta’s rear suspension doesn’t technically have a rear anti-roll bar. The car has a twist beam rear suspension, which basically means that the rear wheels are connected by what looks like a piece of guardrail when viewed from below. This beam flexes as part of the suspension, providing roll resistance. This suspension setup is used on many B-class cars, as it is fairly cheap and lightweight.

    This is a picture of the steeda rear sway piece on the grm car. Its just a brace that stiffens the beam.


    Here is similar one from a different manufacturer:


    There are traditional set ups for the twistbeam type axle, but I haven't seen one for the Fiesta ST (not saying there isn't one, just haven't seen it). I think the below one is on an older fiesta or similar car.


    Thanks for your thoughts. I am going to think more about this. Might just buy the tri-point piece.

  6. wash

    wash Active Member

    Aluminum is definitely the wrong material for an anti-sway bar, the fatigue life is all wrong and the yield strength is measured differently than any other material. Its just about the worst thing to make a spring with.

    If you look at the Steeda bar it bolts through the beam so when the beam twists, the bar twists. If you mount it with heim joints the beam will twist and the heim joint will allow it to do so without adding any stiffness from your panhard bar, it will do nothing.

    The Whiteline in the bottom picture is a more traditional anti-sway bar and closest to the way I would do it. The arms on the sway bar create a lever for the suspension to apply torsion loads to the anti-sway bar. Also the end links are far enough from the centerline that they are not highly stressed (that's my biggest complaint with the Steeda style bar).

    The Tri-point and UR seem designed to mostly separate the driver from their money. I doubt they are very effective.

    The automotive aftermarket is littered with wildly ineffective products. They only thing worse than buying one is copying one.

    That panhard bar is a well made panhard bar, its just a lousy anti-sway bar.
    reddog99 likes this.
  7. Couldahadav8

    Couldahadav8 Active Member

    I can see your point about aluminum and appreciate the feedback. Thanks.

    Tri point produces that sway bar for Mazda motorsports' b-spec package. It is not designed for the masses. It is a race piece designed by race engineers who have won at the highest level of road racing. That design has advantages that none of the other designs out there have in terms of adjustability. They could have easily done the steeda or white line approach but didn't. There is probably a reason for that. I have a tendency to trust designs from shops who work with Randy Pobst and have a bunch of world challenge trophies. But that's just me.
  8. WestcoastST

    WestcoastST Active Member

    I take it you are not a metallurgical engineer by the way you throw terms around which you think are correct. Yield strength of aluminum is not measured differently than any other material. Yield strength, is a measurement of the onset of plastic deformation, and is typically measured by the offset method. Typically 0.05% offset yield strength is reported. This method of reporting yield strength uses a stress strain curve (created from a load deflection curve), where the linear elastic portion of the stress strain curve (the straight line portion of the stress strain curve) is offset by 0.05%. The location where this offset line crosses the stress strain curve is called the yield strength (0.5% offset). It is measured the same for all metals.

    6061-T6 aluminum would have a yield strength of 40,000 psi, similar to some low or medium carbon steels.

    Swar bars are not made from spring steel, as "spring steel" (AISI 1070-1090) would be too brittle to withstand the operating forces. They are typically made from 8620 or 4130 steel. Not normally classified as spring steel.
    Last edited: Mar 17, 2014
  9. wash

    wash Active Member

    Well I'm not a metalurgical engineer but aluminum is tested differently, in a metal like steel yield strength is the strain it can withstand before sustaining a permanent deformation. In aluminum its the strain it can withstand before permanently deforming more than a set amount. That means every time the strain even gets near the "yield" point, the aluminum is deformed a little.

    The effect is cumulative and in my layman understanding, probably why aluminum has such a poor fatigue strength and undoubtedly why it isn't used for springs.
  10. WestcoastST

    WestcoastST Active Member

    No, you are incorrect, and I am a metallurgical engineer. You are mixing up yield strength and fatigue strength. I test aluminum steel, superalloys and titanium for a living. The difference between aluminum and other alloys is in the response of the material to externally applied loading. The straight portion of the stress strain curve is not straight, but slightly curved. This does not require one to test aluminum differently. This is why I mentioned 0.05% yield strength, tested by the offset method.

    Aluminum does not have a defined endurance limit, like steel does. That is why it is not used for springs. It is used for landing gear, however, which shows it has a defined yield strength.

  11. wash

    wash Active Member

    I think you are splitting a very thin hair here.

    I have a layman's understanding and I know that any aluminum part with a cyclic load has to be designed with fatigue in mind.

    What's a fatigue failure if not yield?

    Getting back to the test, I could be mixing up fatigue and yield tests but not so much that aluminum springs are a good idea.

    And for airplanes, they use aluminum in the skin and there was a fairly well know example of fatigue failure there in a jet making short hops between the Hawaiian islands: the top ripped off due to the strain of altitude change (atmospheric pressure).
  12. WestcoastST

    WestcoastST Active Member

    You stated that aluminum was tested differently, which I disagreed with. Cyclic failures differ from yield failures (for all metals) in that it takes less stress to fail in fatigue than it does to fail in one cycle. For aluminum, there is no limiting stress where, if you tested an aluminum part below this stress, it would not eventually fail. This fatigue limit, or endurance limit does exist for most other alloys, and is the reason why aluminum gets inspected more often than steel does, especially in aircraft applications. The curve I referenced in the wiki link shows the difference between aluminum and steel under fatigue loading. Usually, if a part survives one million cycles, it is deemed to be fatigue resistant. You are correct saying that aluminum springs are not a good idea. The fatigue failure in the Hawaiian air fuselage was due to disbonding of multiple sections of the aircraft, which resulted in explosive decompression, and loss of lives.
    Last edited: Mar 17, 2014
  13. jasyatz

    jasyatz Member

    I was able to get to 1/16" toe out with the TriPoint bar. The mounting flanges are 1/8" steel plate and prevent tearout of the mounting holes in the lower spring bucket. I also thought of making my own, but by the time I cut the plates, bought the bar, Heim joints and hardware, It was about the same price. It kills the muffler though, a straight pipe exhaust is needed as the bar and the muffler occupy the same space.
  14. wash

    wash Active Member

    I know there is some test that is different. I don't know what you think it different and what you think is the same but any test where the results are interpreted differently depending on what you are testing, that's a different test, it doesn't matter that you use an Instron for both.

    My point about fatigue failures being yield failures is that after fatigue reduces your yield strength below your operating stress, you get a yield failure because your stress is higher than your strength. I would not be surprised if the fractures looked identical under microscope.

    My knowledge might not be text book perfect but its functional for dealing with the common materials I fabricate.

    Getting closer to the topic, despite the material choice, I don't think the design is any good at increasing roll stiffness.

    Jasyatz has an interesting comment about rear toe, it could be the bar is designed to destabilize the rear with toe-out and make the car rotate that way. You could do the same thing with axle shims if that is the case.
    WestcoastST likes this.
  15. jasyatz

    jasyatz Member

    The only problem with the toe/camber shims is that the shim thickness to REDUCE negative camber/increase toe out makes the ABS sensor clearance too large and sets an ABS code deactivating the ABS. Lapp Racing has a template for the shims but they designed them to INCREASE negative camber.
  16. AlanBDahl

    AlanBDahl Active Member

    Are we convinced that all these designs meet the definition of sway bar per the SCCA Solo rule book? Are there precedents from other cars?
  17. Couldahadav8

    Couldahadav8 Active Member

    The little make-my-own-swaybar fantasy of mine is dead.

    Alan is right on target with his comment. I believe the Tri-Point design is great because it would allow for toe changes that might aid turning without having to stiffen the rear excessively. How great that would be is moot at least in reference to using the sway bar in G street. I think the tri-point would be illegal because it could be used to change toe. You could say that the bar is designed to prevent toe changes, but the part might also be used to change toe, which seems prohibited.

    From the rule book:


    A. Substitution, addition, or removal of a single anti-roll bar and supporting hardware (brackets, endlinks, bushings, etc.) is permitted. The use of any bushing material is permitted. A bushing may be implemented as a bearing.

    B. Substitution, addition, or removal of anti-roll bars may serve no other purpose than that of an anti-roll bar.

    C. No modification to the body, frame, or other components to accommodate anti-roll bar addition or substitution is allowed except for the drilling of holes for mounting bolts. Non-standard lateral members which connect between the brackets for the bar are not permitted.


    I should probably do just the easy thing and buy something that is already out there. I like Wash's idea with the Speedway bar. I am also going to look for sway bars out there for cars with similar twist beam designs.
    Last edited: Mar 19, 2014
  18. jasyatz

    jasyatz Member

    The toe change is a benefit of design. It's a sway bar marketed as such and has already been approved as such. Technically a sway bar attaches to 4 points on a chassis, so that's the only thing I could see grounds for questioning. Wit the plates installed, it does just that. In the past, designs like this have been classified and approved sway bars. As for it's " purpose", it was only found by dumb luck that by increasing rate, the toe also changes.. And rear toe adjustment is allowed by shims.
  19. AlanBDahl

    AlanBDahl Active Member

    "approved as such" by whom? Has someone confirmed this with the SEB? Just because I call a cat a dog doesn't make it a dog :). I'd really like to know what alternate sway bar designs rather than the traditional U-shape have been approved.
  20. wash

    wash Active Member

    Does 13.7(a) mean we only get to change one end of the car?

    But getting back to the bar and thinking about the motion in roll, I think the twisting has the potential to increase the toe out as the body rolls on to the outside wheel. That could be a little rear steer. At the same time, as you compress the suspension up in to the body, some of the camber is going to change to toe in.

    Its complex just trying to figure out what it does in a corner...
  21. jasyatz

    jasyatz Member

    It's been approved for Bspec and alternately these cars are legal for SP as prepared, ( not sure whether it's CSP or FSP though can't remember) and since STF (and STX) have the same swaybar rule as SP, leads me to believe its a legal part.

    But sent a letter to Doug Gill last night to gain some clarification. I urge you to do the same if you have questions.

    @Alan, what are your thoughts on the design, Does it meet the intent of 13 ?

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