Adams/car rc vertical

[student_hsd]

Hi guys,

Currently I am working with Adams/car to improve the suspension of our formula student racing car. There is one particular problem I encounter, when plotting.
I have been using the fsae template to remodel our current suspension. That has worked almost without a problem.
The problem is that, after simulation, when i try to plot wheel travel over roll center location vertical, the roll center does not act as I anticipated.
Also by changing the location of the inner tie rod, the roll center will move.
I have attached the template. Maybe someone knows why that particular problem is happening.

Thanks in advance.

 

[GregLocock]

"when i try to plot wheel travel over roll center location vertical, the roll center does not act as I anticipated."

What did you expect exactly?

perhaps a plot would be useful

"by changing the location of the inner tie rod, the roll center will move."

depends on your definition of roll center. I'm not surprised a force based roll centre would move with changes to ITR. I'll check whether geometric roll centre height moves with ITR location.

what sort of suspension is this?





Cheers

Greg Locock


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[GregLocock]

Small changes in the height of the OTR do not affect GRCH, if the tie rod is the right length. If it is the wrong length then you have bigger problems than RCH. I din't move the ITR as the model was being recalcitrant.

Cheers

Greg Locock


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[student_hsd]

Thank you for your replies.

I'm merely working with what I've got. The steering system is fix, aswell as the upright. All I can change are the control arms.
Maybe I am plotting the wrong? roll center. I only know how to plot "wheel travel" over "roll center center location vertical".
I have attached the plot though.
I have calculated the instant centers in excel, aswell as the roll center height and have used vsusp.com to verify. Both will give me
a rc height of around 25mm above ground, whereas adams will plot a rc height of -14 at equilibrium.

best regards

 

[GregLocock]

That's a very peculiar plot for RCH, normally RCH gain vs wheeltravel is linear. What does your bump steer curve look like?

Cheers

Greg Locock


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[student_hsd]

Indeed. It blows my mind that it is negative at equilibrium too.
I've attached a plot of "toe angle over wheel travel" for parallel travel.

best regards

 

[GregLocock]

That's far too aggressive, in fact if I read it right you've got toe in in jounce, a big no no for a front suspension.

Negative rch is not bad in itself, but I think the bump steer plot shows that you have bigger problems.

Cheers

Greg Locock


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[student_hsd]

I absolutely agree with you. But since the upright is already fix aswell as the chassis hardpoints, there is nothing i can improve.
That's why all I can do is, change the position of the upright and the lengths of the control arms.
It doesn't bother me that the roll center is negative, but the fact that it's not compliant with what i've calculated nor is it coming even close to
what vsusp is calculating.

 

[GregLocock]

You are wasting your time. With a bump steer curve like that you aren't even close to need to worry about RCH. Your tie rod is acting like a wishbone so the classical drawing board method for calculating geometric RCH will fail. If you publish your hardpoints I'll check the kinematics. I haven't used Car in any serious fashion so i don't know how it calculates RCH, it probably uses camber gain. The Help will tell you.

I prefer using force based roll centre heights as that actually measures something I'm interested in.

Cheers

Greg Locock


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

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[student_hsd]

Hi Greg,

thank you for your input, i will attach an excel sheet with my hardpoints.
I know that there is a difference between forced base and geometrical roll center, yet I don't know what the difference is.
Maybe you can link me an article about that, i'd be very thankful for that.

best regards