Adjustable Blade Anti-Sway Arms ???

[rabieng]

Hello everyone. Just wondering if anyone could help me out here. I am an automotive engineering degree student and as part of a design project I would like to model adjustable blade sway arms in conjunction with a interchangable hollow sway bar (the type similar to those used in stock car racing) for a touring car application. To give you and idea of what I am talking about I have found only a few actual applications of this design on the web and have included the links below. Now I know it may be all "theoretical" but I have to treat the project as something real and design a working solution with real calculations, data etc. Preferably I would be interested in sway bars that are adjustable by the technician during a pitstop i.e. have to be adjustable in seconds.
Does anyone have any experience with adjustable blade type sway bar arms? Can anyone offer any help, advice or recommend literature on this type of anti-sway system. Any help offered would be greatly appreciated.



http://www.jaytorborg.com/anti-roll_bars.htm
http://www.hrpworld.com/index.cfm?fo...action=product

[trackrat79]

It is not just a matter of studing the sway bar in a touring car aplication. A sway bar is a tool. A tool does not know what it is being used for. You need to have an understanding of the materials involved to understand how it works. Being an automotive engineering student a good reference of metalergy would do you good. That would give you more insite in to how the adjustable sway bar works. But hear is a quick discription of how it works. The main tube is your maximum posible stifness. The blades are shaped in that fashion to put mor give into the system or soften the efects of the bar. The blade starts in a vertical posision or on edge, that is applying +100% of the sway bars power to the suspension. As you rotate the blade to a flat position the material gets thinner and adds flex to the system ther for reducing the overall strength of the bar being used. Some of the vertical forces being applied to it from the suspension are absorbed into the blades. You can not actually adjust the sway bar itself. You cannot adjust the metal of the bar so you have to change the amount of force that gets to the bar.

Someone else may come along and say I am completely wrong and I may be. I am not an engineer by any meens. Every thing I stated is from my own first hand expieriance and self studies. I have been racing since I was a little kid. It does not matter how big or little the peices are physics and math don't change.

[Norm Peterson]

Actually that's a fairly accurate description of how it works.

In a little more structural detail, the stiffness of a sta-bar can be expressed as the sum total of a whole collection of individual stiffness terms, and the stiffness of the assembly (the bar) is the result of springs-in-series math. Or more simply, by working with flexibilities and a simple summation of same before taking the reciprocal.

Simplified a bit, most of the flexibility in any sta-bar comes from twist in the central torsional section and bending in the arms. What the blade adjusters do is vary the flexibility of the arms, so all you need to know in order to plot a curve of bar stiffness vs arm adjustment is the stiffness (flexibility) of the arms at various points of adjustment and the torsional stiffness of the center section. Being pickier would include one or more of the "little" effects.

Actually, it isn't particularly difficult to make such sta-bar adjustments possible from the driver's seat, assuming that there is no rule prohibiting it.

Norm

[GBodyGMachine]

Ill post a short response. I think this would be cool. But as far as how often it would be used im not so sure. Swaybars for me are the last think I figure when building a chassis. So, it works with the springs, shocks, and other suspension components. Im not sure if I would ever have to change it on the fly. Keep us updated.

Jeff

[CarlC]

Norm and track have it. From an engineering perspective the deflection of the blade is a simple beam deflection calculation. However, as the blade rotates the effective moment of inertia of the blade changes. From a theoretical standpoint, the stiffest cross-section is an I-beam when loaded in parrallel and in-line to the web. If the loading angle is changed, the effective MOI changes, and the beam will deflect more.

Another, easier way to think about it is to take a sheet of 3/4" thick plywood and carry it flat. It's very weak. Turn it on end and it is very strong. MOI is based on the distance from the bending axis to the outermost fiber. On the sheet of plywood carried flat that distance is 3/8". When it is carried on end it is 24". It gets more complicated due to materials and exponentials, but that's where your research comes into play.

[zbugger]

I believe ATS is coming out with just what is being talked about here. I wonder if Shane or Tyler would like to pop in and show off a little. In fact, I'd be interested in one for my '77 Camaro. Hell, I may just do it with a stock car bar.

[rabieng]

Hey Folks,
Thanks a whole lot for the posts so far. How these bars work makes perfect sense and that is why I am interested in this simple but effective system. I must say that prior to studying I was a Automotive Technician (what am I talking about, I still am a mechanic at heart :o) ) but this is like specialized racing stuff that you don't find on "normal" passenger road vehicles. I have never heard of these rotary blade type adjustable sway bars on stock cars.
Now for the folks who have experience with such blades, a valid point was made earlier by trackrat79 about materials. I am thinking along the lines of Aluminum alloys for the blades. I'm thinking about overall weight, cause I wanna get that bar as light as possible in the design but still need the strength. I am concerned about the fatigue that may be displayed by the part over extended use. Any other suggestions for realistic materials? I will have to check them out with a Materials expert before I can start designing and all the spreadsheet calculations associated with this stuff.
Since I am not from the "scene" who is ATS that zbugger mentioned?

[David Pozzi]

I'd like to install one on my wife's car (rear bar) but I'm not up to doing the math to figure it all out.

The adjustable arm should have a triangular shape but the taper needs to be such that the arm bends uniformly throughout it's length.

I've heard of the arms bending sideways when adjusted half-way, giving unpredictable results. Rault used guides on the ends to prevent this from happening, now such formula cars use T bars.

If you look at the Cobra arm in the first link given, that one goes way too soft when full soft. I don't see much use for an arm that can go from 100% to 20%.

I'm flat guessing that at full soft you'd want something like 10% or 15% loss of the total bar rate, at least for the rear of a Camaro. Most of these adj bars are used to compensate for substantial fuel burn-off. A Trans Am car carries something like 40 gallons (260lbs) of fuel just behind the rear axle! Many of them have adjustable front and rear bars, but only one side is adjustable.

Someone sells an adjustable bar for a Mustang with toggle switch control. I think a servo for aircraft trim might work, or there are cable adjusters.

I'd stick with steel for material. Maybe Titanium if you want to get fancy. Aluminum just isn't going to have enough spring. Go with a hollow bar for less weight.

[derekf]

Since I am not from the "scene" who is ATS that zbugger mentioned?

ATS is American Touring Specialties. Their website is www.t56kit.com (but it's not current) or their usernames around here are Teetoe Jones, wickedmotorhead, and a couple of others I can't remember off the top of my head.

[rabieng]

Thank you David for those valid points concerning material selection, arm profile etc.
You mentioned Rault using guided arm ends to prevent bending of the arms. Good stuff. Do you know how I can get in touch with Rault or if they have a webiste?

Any other known manufacturers of adjustable blade type sway bar arms? Where or who would you go to if you wanted to get your hands on such a piece of equipment?

[trackrat79]

If you must run something other than steel your best bet is probly carbon fiber. As was stated before aluminum would not be a good choice as the blade mite just bend when turned on its side as the thickness of the material gets thinner. I am not sure about titanium. I was under the impresion that titanium is strong to a point but very brital. I have seen carbon fiber ones so I know it will work, I wish I had specs of the actual blades to give you.

[David Pozzi]

Here's a formula.
Race Car Engineering by Warren J Rowley is a good source for info. http://www.rowleyrace.com/

Most blades don't have guides but Rault felt they were needed on their Indy cars. Suspension movement on an Indy car (at Indy) can be as low as 1/4", so everything counts in that application.

[rabieng]

Thanks trackrat79 and David. Excellent. Will be following up those ideas and leads... hopefully when I get it done I can post a pic of what I modelled.

Thanks again for all your help.

[BB69]

Something nobody else mentioned is a coupling device between the two halves of the swaybar. American Axle and Manufacturing (make the axles, driveshafts, suspension forgings and other parts for GM, Dodge, etc) introduced a swaybar for the H3 Hummer I believe (could also be the Dodge Power Wagon) that is able to switched on and off. When you go off road, the bar is switched off so the wheels can fully articulate. The switching is done at the middle of the bar with a remote controlled locking mechanism. Basically, they can detach one side from the other eliminating the affects of the bar.

While this system is purely on/off, it wouldn't take much to make it adjustable. Think about joining the two halves of the bar with a spring. By changing the preload on the spring, you would change the bar rate in the same manner as twisting the bars mentioned in the other posts. If you really wanted to get trick, you could use a viscous coupling filled with the same fluid that's used in modern adjustable shocks. The fluid has metal particles in it, and it's viscosity can be changed by applying a magnetic field. Being an automotive engineer myself, this kind of mechanism is a little mroe cutting edge. The adjustable mechanisms described in the other posts are great because they are simple and work, but they don't take advantage of the awesome microprocessor power available. With an adjustable viscous coupling, you could adjust the sway bar rate for anything: road conditions, fuel load, passengers, tire pressure, anything.

Something to think about.
Ken

[Norm Peterson]

I think BMW has also used a switchable front sta-bar, presumably of similar design (645 models?).

You'd need to be able to change the rate of any spring separating the bar halves, not its preload, in order to effect a variable-rate adjustment for the bar as a whole. Mathematically, this adds yet another "spring in series" to the mix that's flexible enough to have a meaningful overall effect. A preload setting would only affect the location of the "knee" point in a bi-linear load-deflection curve, not the rate on either side of such point. You might be able to ultimately get to the same point (in terms of load and deflection) either way, but the bi-linear path would likely not "feel" as secure as you pass through the knee.

Off the top of my head, providing infinitely variable adjustment over some range in something that looks anything like a conventional bar (read: will fit in generally the same space) involves moving something to vary either the lengths of members in torsion or the inertias of elements in bending.

I'm not at all sure that a magnetic fluid connection is going to do what you want from a sta-bar, unless the intent is to provide really heavy damping in roll as such motion is in progress rather than by adding any additional roll spring stiffness that limits the amount of roll ultimately developed at some lateral acceleration. Unless there's something about this fluid that I am unaware of, a fluid device only offers resistance when there is some relative velocity across it. But if you're counting on a sta-bar to provide part of your steady-state balance it had better be able to resist load when nothing in the suspension is moving.

I suppose the standard "viewpoint = hard-core cornering performance before ride comfort" disclaimer applies here . . .

Norm

[BB69]

Norm,
I read your post a few times to make sure I was comprehending everything you said. I have a few questions. Please explain your "knee" concept in more detail. I can see a knee in the load deflection curve if the rate of the overall bar changes during deflection. Is that what you mean? Basically, if whatever adjustable spring you were using suddenly changed its rate, you would get a knee. However, if the rate were changed before the deflection, the rate of the bar should stay the same throughout the deflection. What am I missing here?

As for the preload, you're right. Adding preload will not change the spring rate of the bar, but it will change the force the bar is exerting. Maybe that concept could be used for some tuning. The fluid will only work with a relative velocity, but there is always going to be some relative movement. If you combined the fluid with a variable orifice, you could tune the dynamic response and then shut the orifice and depend solely on the rest of the bar for steady state.

Obviously, I haven't designed this or thought it all the way through. My point for bringing it up is that mechanical engineers have to be able to combine mechanical and electronic features in today's world. With the kind of processing that is available these days, we can't afford to limit ourselves to purely mechanical devices. Of course, if you are designing for a pure race car where your environment is much more defined, that is a different story. Most of us mechanical engineers don't get that opportunity.

Ken

[David Pozzi]

I don't have a problem with the discussion going where it may, but blade adjusters work very well and it seems any alternative should be as light and simple to make and use, -unless it provides superior results in some area.

[Norm Peterson]

Norm,
I read your post a few times to make sure I was comprehending everything you said. I have a few questions. Please explain your "knee" concept in more detail. I can see a knee in the load deflection curve if the rate of the overall bar changes during deflection. Is that what you mean?

Yes. And that will occur with a preloaded spring arrangement as the preload is exceeded by an opposing force that will develop as the bar is loaded.

Basically, if whatever adjustable spring you were using suddenly changed its rate, you would get a knee.

In this case, I’m looking at the bigger picture, where the overall rate of the bar would depend on the condition of the preload spring. As long as the adjustment spring is under any preload it will not be contributing any flexibility to the system and the bar’s rate would be based only on the torsional and bending elements of the bar. Only after the preload is overcome will that spring deflect in accordance with the amount of load it sees, and only then would the bar’s overall rate include the effects of the adjustment spring (read: you’d then have a softer overall rate).

However, if the rate were changed before the deflection, the rate of the bar should stay the same throughout the deflection. What am I missing here?

If you're changing the rate before applying any deflection, you're not adjusting preload. In order to change the overall bar rate over its entire range of deflection, you need to be varying one or more of the individual stiffnesses that comprise the bar, not any of the initial force conditions. That’s precisely what rotating blade shaped arms or moving the attachment points of endlinks along the arms accomplishes, as should a rather more complicated method for varying the effective torsional resistance of the center section. All start deflecting from their zero stress state at zero point infinitesimal applied bar end displacement.

Adding preload will not change the spring rate of the bar, but it will change the force the bar is exerting. Maybe that concept could be used for some tuning.

This can already be done quite simply with conventional bars and existing hardware, i.e. endlinks adjustable for length or shims under the chassis bushings. I suppose that you could use this as part of a cornerweighting procedure in lieu of accomplishing the entire adjustment via the springs.


This is in no way intended as a discouraging comment, but I also think a discussion that includes such things as fluid flow and electronic controls is getting close enough to active suspension and will involve enough additional considerations - some being quite subjective - to justify its own separate thread.

Norm

[BB69]

Norm,
That was a great explanation. I have a different thought in my head, but I have to think about it so I can fully explain what I mean. If I think about it long enough, I may realize that my idea is just plain flawed.

Anyway, I will leave the active stuff alone, I think I at least planted a seed for a developing mechanical engineer, which was my intent.

Ken

[442 Ragtop]

How about other mechanical ways of making a sb adjustable?

I can think of a few:

1. An arrangement that lets the arms slide horizontally on the torsion bar. Put them close together, and you effectively have a stiffer arm; move them apart, and its looser.
2. An arrangement that lengthens or shortens the arms. Short arms effectively stiffens the bar.
3. End links with springs in them, with an arrangement to lock out coils of the spring. Lock out all the coils, and you have a stiff bar; lock out just a few and it's looser.

I like 3. the best, since it would work w/ a bar that fits in the stock location.