Stabilizer bar

[suspguy]

Hello everybody

i was looking up some formulas for calculating stabilizer bar roll stiffness, given things are:

Bar Diameter
Material properties
Lever arm length
Torsional section length

I found some data about it in thread108-40691

But the problem I was having the most formulas available dont take into effect the distance between mount bushings
To get more accurate results I was trying to take into effect the deflection due to rubber bushings as well

The methodology i am using is

1) Calculate the bending moment and torsional moment at bushing center due to vertical load at bar end.

2) Calculate bushing deflection due to bending and torsional load taking into account conical and torsional bushing rates.

3) Add thoose defections to the total deflection at bar end which can then be used to calculate roll stiffness

I am assuming a D-shaped bushing

If I did that would i be wrong??


I am a little confused if I should use radial deflection as well or my line of thought is totally messed up
In short i want to include bushing deflection also to calculate stabilizer bar roll stiffness

Any IDEAS or CORRECTIONS??

Thanks for your help!!!!!

 

[GregLocock]

1) D block stiffness

Some d blocks are grippy, so they add a bit to the sta bar, and rather more important, they add to the ride rate. In pure roll they don't change the overall roll rate much.

some d blocks are slippy so they don't.

2) D block location

Good luck

You are right, this is important. I don't know that a hand calculation will get you to a good solution, we use a crude FE model.





Cheers

Greg Locock

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[suspguy]

Greg

Thanks for your reply

i know it wont be a very accurate solution but I am just trying to get into the 15% ball park, ignoring the bushing defection gives me a 50% error

one other thing that I am trying to understand is how does the mount bushing actually deflect during pure roll, and it is a slippery kind of bushing so i can probably ignore torsional defection

Does it deflect conically or radially ??

Thanks for your help

 

[GregLocock]

conically -no

radially, well it obviously depends on the rate. I would say that it is well inside your 15% margin though.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

The commonly published formulas generally overlook a number of smaller bar deflection components that can be hand-calculated as well. Probably the largest of these is bending deflections that show up in the center torsional section due to arm angularity and chassis bushing offset from the junctions or bends between the torsional section and the arms. Just this one can reduce the rate as computed by the simpler formulae by 5% or so, depending on bar dimensional specifics.

Norm

 

[anmartin]

Hello,

You can find a formula for rollstiffness of stabilizer bar in SAE AE-21


Martin

 

[carnage1]

could you replace the d blocks with Delrin to increase the stiffness of the stabar assembly and decrease ride rate?
or would a solid plastic be to stiff and cause problems?

 

[GregLocock]

No, go as stiff as you can /if/ you are using a sliding joint between the bar and the D block. If you are using a grippy D block then you'll need some compliance obviously.

Cheers

Greg Locock

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[patprimmer]

If you choose plastic you can use in order of decreasing compliance, Polyurethane, or Ultra High Molecular Weight Poly Ethylene, or Nylon or Acetal (one trade name being Delrin)

Regards

eng-tips, by professional engineers for professional engineers
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[suspguy]

I had another question regarding calculating sta bar roll stiffness, since most of the static formulas available for calculating roll stiffness do not take into account bushing stiffness at body attachment point
I thought of deriving a formula which takes into account bushing stiffness as well I used a spring attachment at the bushing instead of a rigid support, i am assuming half sta bar fixed at the center and calculated the Reaction force at the point using the Deflection method

Is it possible that the reaction force at the bushing is actually greater than the applied force at the free end (Thats what I am getting) and also once i know the reaction force at the bushing hence know the deflection how can I include it into overall bar deflection

Thankf for your help

 

[GregLocock]

Is it possible that the reaction force at the bushing is actually greater than the applied force at the free end ?

Yes

once i know the reaction force at the bushing hence know the deflection how can I include it into overall bar deflection

Add the deflection multiplied by the appropriate ratio to the deflection at the end of the bar. That's not quite right, I suspect, but good enough. The ratio is width of sta bar/width of d blocks.




Cheers

Greg Locock

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[suspguy]

Greg

When you say width of the bar do you mean the bar diameter or the moment arm length

And also the reaction comes out in the opposite direction to the applied force hence the deflection due to the reaction force is in opposite direction, so do I have to subtract the deflection at bushing to deflection of bar end

Because the way I am calculating the deflection at bushing end is R/k where R = Reaction force and k = spring stiffness

Thanks

 

[NormPeterson]

If you already have a value for the bushing stiffness, you should be able to roll that term into an overall formula without having to worry about the bushings' own loads or deflections (unless you're investigating the effects of nonlinear bushing properties). I think you'd be looking at a roll stiffness term due to bushing effects that looks something like

[BushingRadialStiffness (lb/in)] * [BushingSpacing (in)]^2 / 114.58

where the resulting roll resistance is in units of in-lbs/deg and would be in series with the bar's own roll resistance of

[BarEndRate (lb/in)] * [TotalBarWidth (in) ]^2 / 114.58

Rates as applied through endlinks or other bushed bar and connection would include the effects of stiffnesses for those bushings (also as springs in series).


Norm

 

[GrzegorzD]

Hello All,
I am working on rubber bushing of stabilizer bar. It's for car suspension. The radial stiffeness should be 4kN/mm (+/- 15%), torsional 1 to 1.5 Nm/deg and conical < 1.5 Nm/deg.
Greg - you have told above that the there no conical deflection:
"GregLocock (Automotive)
15 Dec 05 16:16

conically -no

radially, well it obviously depends on the rate. I would
say that it is well inside your 15% margin though."

Are you sure that the bushing haven't any deflection in conical direction? Only pure roll? I suppose that sometimes the bar can bend - the requirements told us that conical stiffeness lesser is better.
So I wonder what you are thinking about it.

Thanks for your help

Greg

 

[NormPeterson]

Bushing torsional stiffness will have negligible effect on the rate of the bar, which was the initial direction of this particular discussion. Simply put, a bushing stiffness of 1.5 Nm/deg (about 9 in-lbs/deg) isn't going to materially affect the deformation of any reasonable sta-bar.

But if you're looking into loads carried through the bushings and their brackets for the purposes of designing THEM, there will be some conical bushing deformation involved, and the brackets as well as the bushings must obviously be designed to accommodate the induced loads reliably over time & number/severity of cycles. Somewhere I saw a photograph where bending deformation of the torsional portion of a sta-bar was clearly present. Unfortunately, I can't find it any more, but going off memory there might have been something like 5* of bar angularity going through the bushings.

Norm

 

[GregLocock]

What Norm said - the coning rate of a typical D block does not /significantly/ affect the roll rate of the car, to the accuracy of a hand calculation.

Many practical sta bar installations have D blocks in the wrong place, and angled outer arms, and many have kinked torsional runs. Once you do that bending will always come into play. If your D block has a high coning rate then this will tend to prise the D block mounting apart.

I still wouldn't get madly upset by high coning rates - after all plenty of people run nylon D blocks, if only on circuit cars.




Cheers

Greg Locock

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[suspguy]

Hello all

I have developed a Finite element tool that uses beam elements to model a stabilizer bar and have also built in the bushings.
I am able to get the deflections for all the beams, my question is what is the most accurate way (use which deflections) to get roll stiffness number for the stabar system (Sta bar system means stabilizer bar and bushings only)

Thanks for your help

 

[GregLocock]

Is it a non linear model?

You actually need to model the effect at the wheel end so that will include the motion ratio and the effect of the angle of inclination of the sta bar link, assuming you have one.


Cheers

Greg Locock

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[suspguy]

Yes it is a non-linear model but the non-linearity is only in the bushings (3D springs) and in the links and am using the beam element with 6 dof/ node
I am not modeling the control arms but i know the motion ratios through my kinematics model and I know the deflections
but how do i calculate the stabilizer bar stiffness at the link end and not the wheel I will worry about the wheel later
I am aiming at less than 5% error from an ADAMS model

Appreciate any help

 

[GregLocock]

If you apply the force in the axial direction of the link, and measure the motion in the axial direction, you'll probably get the stiffness that you can use.

it's actually quite tricky if the links are inclined and the sta bar is not horizontal (etc) which is why ADAMS was invented.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.