Virtual Ball Jont Location

[marks14]

Hello,

When using a multilink suspension that simulates a SLA suspension I understand how to find the virtual ball joint of one of the "a-arms" when the two links are in the same plane. I've noticed with most production car suspension it isn't that easy, and these links do not stay in the same plane. I'm getting confused on how exactly to find the location for the virtual ball joint when the two links that make up the virtual a-arm are offset vertically. Can some one point me in the direction of somewhere I can find more information on how to determine the virtual ball joint location for these instances? I found a nod from GregLocock about this issue HERE.
Forgive me if this is an ignorant question, I'm pretty new to all of this.

 

[GregLocock]

I strongly suggest for your FSAE car that you don't do anthing so stupid.

However the process is either to draw the suspension in some position and move it in small steps of angle and displscemdnt to the final location, checking that all arm lengths stay the same and BJ to BJ lengths on the spindle stay the same,

or do the whole thing as a kinmatics solution.

Cheers

Greg Locock


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

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[marks14]

Hey Greg,

Thanks for the reply. This actually isn't a question regarding a FSAE car. I'm trying to completely understand how the suspension on my car works before I go making big changes to the spindle. Thank you for the idea of getting down to the roots and treating this like a kinematics problem. Embarrassing as it may be, I didn't think of going back to the fundamentals. I'm going to do some more research on the subject and then I'm sure I'll have more questions.

Thanks again

 

[dynatune4xl]

Why would you go to two virtual lower ball joints on an FSAE car ? The only reason why this is done on (rwd/awd) production cars is to get the steering rack out of the way enigne (allowing this to be mounted lower in the chassis) and because it has to be mounted low for best compliant behaviour (and package restrictions too) the double ball joints are only necessary to make a decent amount of Ackermann possible. Nothing of that is important for an FSAE car.

Cheers,

dynatune4xl

 

[marks14]

I think there is a misunderstanding here. This is not for a FSAE car. I'm trying to understand how the rear suspension (A-Arm and 3 links) on my rwd production car works before I modify the uprights.

 

[BrianPetersen]

What is the car ... and do you have some good pictures of the suspension?

 

[marks14]

It's a 1995 Nissan 240sx. This image I found online is very similar to what I'm working with:

http://i901.photobucket.com/albums/ac213/uncommondenominator/S13 Build/DSCN0108-1.jpg

The only difference between this sub-frame and mine is mine has a little bit less anti-squat built into it. It's a little hard to see, but both inner and outer pickup points on the link closest to the photographer are lower than their counterparts on the more rearward link that makes up the virtual a-arm. I can take some and post some more pictures of the links I'm trying to understand tomorrow if needed.

EDIT: After looking at that picture again, I think I need to explain my concern. When looking at the two links in plan view they look like they intersect each other much further from the center-line of the car than if one was looking at the two links from the rear view.

 

[dynatune4xl]

This type of suspension is a derivative from a double wishbone where the upper wishbone has became a "virtual" wishbone with two links creating a virtual "steering axis", most likely to reduce the "KPI" offset trying to affect the compliant suspension behavior in a favorable way. Since this kind of suspension configuration is falling under the category "multi-link" I would suggest that you use a proper 3D suspension software to analyze the kinematic (and possibly also the compliant "elasto kinematics" ). Working it out on a piece of paper will only cost you a lot of time and does not make you understand what happens when forces are applied at the contact patch. There are some good tools available on the net.

Cheers,
dynatune4xl

 

[marks14]

Thanks dynatune. I understand the basic configuration of this suspension and 3D suspension software is defiantly the best way to go. I'm just trying to understand the fundamentals before blindly entering this information into a program.

 

[GregLocock]

One way to do that is to set up a very simple example, and see how it compares to the single bsll joint version

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[dynatune4xl]

Greg is right about using a simple example to start understanding the basics, at least from a kinematic point of view. However, these "virtual" steer axis suspension types have been developed almost entirely for their compliant behavior and that makes things a bit more complex and without a computer model you will struggle to understand what's happening (probably also with a computer model one will struggle but one has to start somewhere...)

Cheers,
dynatune4xl

 

[BUGGAR]

Doesn't the steering arm prevent rotation of the upright and corresponding rotation at the "two" ball joints? The two upper joints don't look like they're designed for rotation anyway. I see why you recommend a 3d program. I just tried Front Suspension Geometry Pro 4.0 and sent it back. What program do you recommend?

 

[GregLocock]

Doesn't the steering arm prevent rotation of the upright and corresponding rotation at the "two" ball joints?

No, but you are right the tie rod is an integral part of the mechanical configuration.


I'd be a litle wary of any program that doesn't work the kinematics out from scratch even if it uses those calculations to build a look up table.

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[dynatune4xl]

I doubt whether you will find a freeware suspension tool that is capable of handling a virtual steering axis suspension and providing 100% correct results. In order to do so the software needs to be based on an vectoralgebraic algorithm (=3D) and most freeware tools are based on the classical line draw (2D) principle. I would also be wary about such programs on this suspension.
Besides that in order to understand the compliant behavior the suspension tool should allow some kind of deformations (bushing/rose joints/links) and the most commonly used commercially available tools do not have that either (setting aside of course all the MBS tools that are hugely complex and expensive). Do a search on the internet and you will certainly find one or two tools that can do the job.

Cheers,
dynatune4xl

 

[GregLocock]

Or, as we did with our first on, you use 3D CAD and are very careful

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[cibachrome]

A double ball joint suspension is produced for anther major reason. The "compliance management" issue isn't one of several. The extra ball joint's friction degrades it's main purpose if not done properly.