Roll Stiffness

[The Beast]

I have measured roll stiffness measured for few vehicles in K&C Machine & Tried to correlate to the axle / part stiffness.
It does not correlate to the rear torsion beam roll part stiffness, However it is correlatable in Front Stabi Dia/Span ratios.
Could some one explain why ?

Roll Stiffness Calculation is Based on
Roll Stiff = Roll Moment / Roll Angle
Roll Moment = Tread / Load Change Fz
Roll Angle = Tan-1 ( Change in Wheel center Z / Change in Wheel Center Y) * pi()/180

 

[GregLocock]

Are you including the springs in your calculation?

Cheers

Greg Locock


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[The Beast]

Yes i do.
The System is a FWD Front Engine Vehicles with McPherson Strut + Coil over Torsion Beam Suspension system in Rear

Thanks in Advance

 

[GregLocock]

Well, that's the sort of thing I get involved in all the time, and my models correlate to the K&C rig reasonably well.

So unless you can reveal more, all I can say is that you are doing something wrong.

Cheers

Greg Locock


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[The Beast]

Please find the File i have uploaded it includes 8 vehicle data roll stiffness per unit load ploted
i concluded with the data that the red boxes are having stiffer front suspension & weaker rear beam stiffness.
However rear case i could not conclude, where the parts are actully stiffer than the blue ones.
Would like to understand where am i wrong.

 

[NormPeterson]

Are you considering vertical tire stiffness? How are you dealing with sta-bar stiffness effects?


Norm

 

[The Beast]

Tire Radial Stiffness is considered.
Stabi Bar Diameter & Span is considered.

 

[GregLocock]

The pretty plot you've supplied merely shows different roll stiffness distribution for different cars.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[The Beast]

Dear Greg
Added Axis so that the differences can be seen.
Units in kNm/Deg.

Thanks in Advance

 

[The Beast]

Graphs Added

 

[NormPeterson]

Stabi Bar Diameter & Span is considered.

I wasn't particularly questioning if you were considering sta-bar stiffness. More about how you were modeling the sta-bar contributions. Let's just say a sta-bar, or an axle in torsion pretending to be part of one, may not behave exactly the same as an axle's pair of suspension springs.

How are you accounting for geometric load transfer effects? A high enough geo RC would provide a zero-roll answer, but you'd still have the Fz load changes. Geo RC heights that are significantly different probably complicate the math a bit, or at least disallow simplifying assumptions.


On "Roll Moment = Tread / Load Change Fz", I'm having a units problem (length*force vs length/force). Perhaps you meant to type "Roll Moment = Tread * Load Change Fz"?


Sorry for all the edits. It's been a while since I tried tackling something similar.


Norm

 

[cibachrome]

Sorry, I still don't fully understand the situation.

You have 'K&C' test data on a real vehicle ? [Probably FWD if it has a twist axle].

You have a suspension model with representative values for tire and suspension bushings and springs with the correct descriptive units ?

You are adding to this conglomeration the roll stiffness value of the twist axle [Which probably has an additional roll bar within it ?]

Your base twist axle roll stiffness value is probably wrong if you did not model the twist axle cross beam correctly. The cross beam by itself (not including the extra roll bar) is actually 2 beams: one of them has warping characteristics due to its shear center location. A properly executed FEM would tell you this.

Otherwise provide the vehicle description. Depending where the "K&C data" came from, K&C might really stand for Kitchy and Crappy.