Anti Roll Bar Moment Arms

[KJ]

Hey!
We're a first year team from Mangalore, South India. We're off to Michigan in May!
I'm designing the suspension systems for our car, and i've run into a rough patch. I'm hoping I can get some help here.
I'm done with the entire kinematic design, and i've also gone ahead with calculating my spring rates. I followed the tech tips from OptimumG for my anti roll bar calculations. So I have calculated the roll stifness that my springs provide, and based on a ballpark figure of TLLTD, I've estimated Anti roll bar rates.
Now, is it just a simple matter of using the torsion equation to arrive at my Anti Roll Bar dimensions (length, ID, OD) ?? Where does the moment arm calculation fit into the entire picture? I'm basically confused as to what condition of the car i design that moment arm for, and how i can estimate the forces causing torsion of the ARB. Ive read that ARB's provide an adjustable roll stiffness to the car, but I really havent been able to find an explanation on how those values are arrived at.
Thanks a ton!

[KJ]

Hey!
We're a first year team from Mangalore, South India. We're off to Michigan in May!
I'm designing the suspension systems for our car, and i've run into a rough patch. I'm hoping I can get some help here.
I'm done with the entire kinematic design, and i've also gone ahead with calculating my spring rates. I followed the tech tips from OptimumG for my anti roll bar calculations. So I have calculated the roll stifness that my springs provide, and based on a ballpark figure of TLLTD, I've estimated Anti roll bar rates.
Now, is it just a simple matter of using the torsion equation to arrive at my Anti Roll Bar dimensions (length, ID, OD) ?? Where does the moment arm calculation fit into the entire picture? I'm basically confused as to what condition of the car i design that moment arm for, and how i can estimate the forces causing torsion of the ARB. Ive read that ARB's provide an adjustable roll stiffness to the car, but I really havent been able to find an explanation on how those values are arrived at.
Thanks a ton!

[Adambomb]

Sounds like you've already "got it," perhaps just don't realize it yet. http://fsae.com/groupee_common/emoticons/icon_wink.gif Yes, for the bar, just use torsion stiffness equations. Then you will probably want some method to adjust torsional stiffness at the track without an excessive amount of work, and some method to package it all and connect it to the rest of the suspension. (HINT: this is where motion ratios come in)

Best place to start for ideas for that is to look at as many pictures as you can of what other people have done. Figure out how theirs work, and you should be able to combine some elements of their designs to what you already have figured out on your car. And now you have begun the dreadful task that is Packaging...

[KJ]

Hey Adambomb,
Thanks for your reply. Looks like i had already "got it" as you suggested. I do have my design complete for now, but my values seem a little absurd to me. Since i've fixed my ARB length from packaging issues, my OD and ID dimensions seem really low. Can I put the 12 mm OD, and 9 mm ID i'm getting, down to the 500mm length that im taking? I'm now worried if this is feasible structurally, with such a thin OD, such a huge length.

[Adambomb]

Those numbers are similar to ones we've had in the past. However, it is true, even though they have the right torsional rate, structurally you might have a problem. Make sure to calculate the stress in the bar in the worst case scenario. You will probably want to use 4130 and heat treat it (shouldn't be a problem, that's generally about the only material you can get with that OD and wall thickness).

I've had to change suspension packaging before because I couldn't get the bars to meet structural requirements. It's just another constraint that makes suspension packaging a pain in the butt.