Anti-roll bar calculations

[Alex Hodgkinson]

Ok, now I'm thinking about fabricating new ARB's up for my car. There are lots of questions I could ask about this, but I'm going to start off with just one, the main calculation behind an ARB - quantifying it's resistance to being twisted. Yes there are lots of other factors and I'll get to those in time, but for now I want to make sure the groundwork is done and makes sense.

I did calculate this once while I was at University but I don't seem to have the paperwork for it anywhere.

To begin with I want to calculate how much a given length of steel tube will twist when one end is fixed and the other has a rotational force applied to it.

So, the variables I have are as follows;

- Tensile modulus of the material
- OD of material
- ID of material (thus wall thickness)
- Distance between the fixed point and point about which force is applied
- Rotational force applied

I think those are all of the relevant values, please correct me if they're not.

How do I go about finding out what I want to know? References as well please, not just an equation - I'd like to know why not just how.

Thanks in advance,

Alex

[phoenix]

I have formulae from David Gould, Mike Pilbeam and Fred Puhn. These have all been published along with explanations, so I don't think I should plagiarise them here.

I use a fourth formula, however, using a different modulus for the steel, based on information given to me by the man who makes my springs - who I trust implicitly!

If you want to PM me I will happily share the information with you.

[MGDavid]

sorry if this is basic but aren't (most) a/r bars solid?

[Tim Falce]

Originally Posted by MGDavid

sorry if this is basic but aren't (most) a/r bars solid?

Not necessarily, mine isn't.

[Alex Hodgkinson]

No neither of my car's OEM bars were either - 28.5x2mm up front and 15x2mm rear.

To add to what I was saying before, I can buy "uprated" ARB's for my car, but the springs I'm running and the aero balance compared to the standard car is massively different. That and the fact that no aftermarket ARB's state their spring rates makes me want to make my own to my own spec rather than take a chance on something that "might" work..

[Notso Swift]

If you know the standard rate then some companies (check the Whiteline site) give the % increase for various sizes all based on the same material, they make standard and uprated, but they done list the original poundage, so you just need the reference point

[Belatti]

Originally Posted by MGDavid

sorry if this is basic but aren't (most) a/r bars solid?

At equal bar stiffness the solid one would be much heavier.

[RPD Motorsport]

Alex I've sent you an email with regards to roll bar calcs. If you send me youe email I will send a tutorial explainging how to calculate twist angles as a .pdf.

SD

[Belatti]

A nice practice you can do after doing and installing your own ARB is to put a scale and a jack under one wheel with disconnected damper/spring and replace the damper/spring of the other wheel with something rigid enough.

With the car lifted, start jacking up and read the scale every 1cm.

You will measure not only ARB wheel rate but also know your motion ratio.

[JohnD]

Pheonix mentioned David Gould, Mike Pilbeam and Fred Puhn. I would add Allan Staniforth, in whose book "Competition car suspension" (Haynes) appears Staniforth's formula:
Stiffness (Q)=(10^4 x Track^2 x K [fractional lever arm ratio]^2 x Diameter^$) / (R[effective arm length]^2 x L[half length of bar])

AND Gould's, but he takes a twelve page appendix to discuss it!

I really recommened you buy this book. The subject of ARBs is one that desrves and needs study - as you are doing.

John

[phoenix]

Originally Posted by JohnD

Pheonix mentioned David Gould, Mike Pilbeam and Fred Puhn. I would add Allan Staniforth, in whose book "Competition car suspension" (Haynes) appears Staniforth's formula:
Stiffness (Q)=(10^4 x Track^2 x K [fractional lever arm ratio]^2 x Diameter^$) / (R[effective arm length]^2 x L[half length of bar])

AND Gould's, but he takes a twelve page appendix to discuss it!

I really recommened you buy this book. The subject of ARBs is one that desrves and needs study - as you are doing.

John

Just to say that Allan Staniforth attributes his ARB formula to Mike Pilbeam in 'The race and Rally Car Source Book' and a different formula to David Gould in "Competition Car Suspension"

DIAMETER^$ IN THE ABOVE SHOULD BE DIAMETER^4 - an easy to make typo, John.

[chevy34]

Alex I have a Excel spreed sheet that does Hollow and solid bars and you can compare 2 bars on the same screen Flick me your e-mail and I will forward it too you

[Andrei]

Has any one of you experience with calculating an ARB with blades?

I have such an ARB on my work bench done by a proffesional company in Germany, but I would like to understand how have they calculated everything - something the company does not want to tell.

Thank you!

[RKTuning]

Originally Posted by chevy34

Alex I have a Excel spreed sheet that does Hollow and solid bars and you can compare 2 bars on the same screen Flick me your e-mail and I will forward it too you

Hi Could you please send me the spreed sheet to please!
having trouble working a solid bar into a tubular type
Thanks Ron
ron@rktuning.com

[Joaquin Topiso]

Originally Posted by Belatti

A nice practice you can do after doing and installing your own ARB is to put a scale and a jack under one wheel with disconnected damper/spring and replace the damper/spring of the other wheel with something rigid enough.

With the car lifted, start jacking up and read the scale every 1cm.

You will measure not only ARB wheel rate but also know your motion ratio.

If I've understood your test, I don't think it would work:
If you don't somehow clamp the car to the ground, part of the reading on the scale will be due to the weight of the car, which you are lifting with the jack.
Also, the displacement will mesure the combination of wheel travel and body lift. You would need to measure wheel travel with a ruler fixed to the car, not to the ground.

[Goran Malmberg]

What sort of car do you have, and do you have downforce data?
Goran