Bottom line: adjustable end links will not allow you to change the stiffness of your sway bar.

For the OP what you're thinking of is a torsion bar, similar in design to a sway bar but not connected to the opposite side. Its an alternative to coil springs not typically used anymore.

 

The point being, don't have any preload on your sway bar end links unless you want to load one side of the car more than the other, like if you were going on a circle track.

Dan

 

thank you all for the discussion here.

However all of you are talking about the forces that are occuring during a dynamic operation. The stuff that I illustrated is looking at things when the car is static.

By using just simple beam calculations, the math is pointing towards what my model is saying.

So With that being said, if I increase the force at the edge of a beam( my sway bar, there should be an equal force except in the opposite direction. The math behind it is proving that.

So please show me mathematically what the forces are at the bushings just as I illustrated

 

No, lengthening the endlinks against a degree of freedom of your system will not add any static force to the system, it will just change the position. The swaybar bushings are essentially plain bearings and you're applying a moment to them. no way for those forces to be reacted into the chassis.

to some VERY small extent you will shorten the effective lever arm of the sway bar so it would make it slightly stiffer, but not because there is any preload on it.

 

There is no load on the swaybar when the car is static because the only time the swaybar gets loaded is when the car leans or SWAYS. If your swaybar is loaded when the car is at rest then your frame is twisted or your end links are different lengths.

You are thinking of the bushings as a solid mount however they allow the swaybar to rotate so as you lengthen the end links the swaybar does just that it rotates within the bushing. No bending of the bar occurs because the bushings are simply a fulcrum point. Your equal and opposite force happens on the opposite end of the swaybar on the other control arm.

It is difficult to effectively show how the swaybar works with simple 2d illustrations, in order to effectively show how the swaybar works you need to think of it in 3 dimensions.

OK I figured out how you can see what everyone is trying to tell you, go out to your car, remove both endlinks and tell me how much force it takes to move the swaybar up and down. This is essentially what you're trying to tell us is generating a load by lengthening the link.

 

Alright, I took a sway bar I had laying around and bolted it to my table, It rotated...

However the math is telling me something else...

Remember folks, not arguing here, just trying to disprove my own math

This is something that I can visualize but I want to translate it to forces. Maybe I can try and do a reiteration of the forces in 3 dimensions and see what I can come up with.

thank you all for all the interesting opinions and discussion. ( Sometimes it is difficult being an engineer )

Which now leads to the next question, What if instead of bushings, the sway bar is Bolted solid to the chassis restricting its rotation all together.. wouldn't that be creating severe moments about those points ?

 

Whats wrong with your diagram is you pretended the bushings are a fixed condition state on the Bar. That is not true so your diagram is wrong. Now if you had a straight beam in the equation then you would have an accurate diagram.

 

That ruins the whole point of a sway bar and makes your bar essentially a beam spring since theres no rotation in the mid section.

If you want stiffer sway bars you either have to go bigger or change the arm length.

If you want stiffer springs get stiffer springs

 

http://en.wikipedia.org/wiki/Torsion_bar_suspension

 

That torsion bar is a very interesting concept...

I want to thank each and everyone of you, for disproving my math. Which was the intent of the post.


thank you very much for those that provided insight.

 

No problem, I have an MSME and still sometimes over-think myself to the wrong conclusion. But its good that you can see that even though it seems rational to you, there's something not quite right.

 

^ most definitely agree , over think, over engineer, which then leads to poor real world conclusions in some cases

 

^ you guys should apply at Audi.

... masters of the overengineered car.

l8r)

 

Your analytical approach looks correct however i think on the second page, the force applied in your FBD is colinear with the fixed points (constraints). However, if you look at the sway bar, the ends are not colinear and are in fact offset towards the rear of the vehicle as it terminates above the control arms. This FBD should be performed in a three dimensional cartesian coordinate plane rather than a simple 2D model.

 

fixed points/constraints = bushing locations.

also, dont forget to do torsion calculations when you use superposition to solve the three dimensional FBD.