Angular rates in ARB ?[resolved]

[kokalz]

Hey guys!
I've just recently started educating myself with suspension design process and I've been wondering what values of angular rates people have in the front and rear ARBs. I seem to be getting a very low answer for some reason and since the formulas I use (from Allan Staniforth's Competition Car Suspension) are all in terms of in*lbs units I feel like I've done some kind of error in convertion. Thanks!

[billywight]

Try converting everything to a real unit system and re-doing your calculations. Your final rate will depend on many factors, there is no one solution.

[acedeuce802]

What is your process for this?

My process for bars/springs was to make a spreadsheet that calculates ride and roll rates based off of your spring rates, and roll stiffnesses. Then figure out how much roll stiffness you need to add that your springs don't provide. Back-calculate to figure out specifics of bar design.

[MCoach]

Originally posted by acedeuce802:
What is your process for this?

My process for bars/springs was to make a spreadsheet that calculates ride and roll rates based off of your spring rates, and roll stiffnesses. Then figure out how much roll stiffness you need to add that your springs don't provide. Back-calculate to figure out specifics of bar design.

^ This.

[kokalz]

I used the simple formulas from books. We already have an SW design of an ARB, but are thinking of changing it to a different one. So in order to find the geometrical properties of modified ARB I wanted to find the angular rate of the current bar and keep in same for the new bar. I used the following formulas:
I find the 2nd moment of area for a hollow tube as

"J=Pi*(Douter^2-Din^2)/32"

I then find the angular twist as
"Theta = T*L/(G*J)"

and then the "gradient as T/Theta", where T is torque. Take arbitary values of T and compare the new bar for the same values of T.

[onemaniac]

You question is still not clear.
From your last post it sounds like you just want to find out the OD and ID of the new ARB tube that give the same angle of twist for given torque. Am I correct?
If so, you already figured it out yourself what is your question?????

If you were curious about the target roll stiffness for front and back that's another thing.

[BillCobb]

The question is clear. Their results are typical of what happens when you blindly use a 'formula' without the background knowledge of limitations and application.

Its RADIANS, not DEGREES.

[onemaniac]

Originally posted by BillCobb:
The question is clear. Their results are typical of what happens when you blindly use a 'formula' without the background knowledge of limitations and application.

Its RADIANS, not DEGREES.

If that's actually his problem then I'm just speechless.

[acedeuce802]

Originally posted by BillCobb:
The question is clear. Their results are typical of what happens when you blindly use a 'formula' without the background knowledge of limitations and application.

Its RADIANS, not DEGREES.
Exactly. When deriving the formulas yourself, you aren't just ensuring that the formula is correct for your given inputs and expected outputs, but you are also learning in the process. For example, I looked all over for a formula to relate spring/bar stiffness's in lbs/in to ft-lbs/deg in vehicle, but eventually drew an FBD and it all made sense.

[kokalz]

I'm sorry it took so long to reply, difficulties with internet connection.
The problem is not that I don't know difference between radians and degrees. Sorry if I made it a bit confusing, but I convert to degrees later, all of the calculations are obviously carried out using the radians. The confusion I get is that for my calculations I follow the basic formulas for angular twists. The formula I derived can be seen in one of my answers above. However, going through the formulas in, say, susprog, it looks like I need to also include the rollbar arm length. Sorry for over complicating the question, just slightly confused by the amount of formulas that are derived in different sources.

[kokalz]

Nevermind guys, I think I figured where my confusion came from.