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# Anti-Roll Bar Motion Ratio and Lever Arm Length

### Tech Articles

Discussion and questions related to the course Suspension Tuning & Optimization

Ignoring packaging, and using a spline-bar style anti-roll bar, what factors influence anti-roll bar motion ratio selection? What about the lever arm length?

I want to gauge the degree to which anti-roll bar motion ratio and lever arm length influence performance in order to evaluate those factors compared to cost and packaging when choosing a compromise.

How much to you want to control roll with ARB vs. Springs? The age old question. I have one race car designed with no anti-roll bars, and one with an ARB at only one end.

What you need to determine is the wheel rate due to springs, and the wheel rate due to ARB at whatever your desired roll angle is. Generally you need enough ARB range to cover the spread in your available spring inventory (so if you have springs in 50 lbs increments), then an ARB range with arms/holes allowing fine tuning of +/- 50 lbs would be good.

Yes, but that can be accomplished with a variety of anti-roll bar motion ratios.

My question here is just about the motion ratio of the anti-roll bar itself (not about a setup or how to determine a setup for a car). Motion ratio effects damper function, but influences spring function much less (at a given wheel rate). Is the same true for anti-roll bars; does anti-roll bar motion ratio primarily influence just the size and shape of the anti-roll bar? Or do things like the metallurgy and length of the torsion spring cause a greater or lesser anti-roll bar motion ratio to be preferable?

Hey Sam,

It sounds like you may actually be looking for information on "Anti Roll-Bar Stiffness" and the relative influence of Motion Ratio, material, and torsion bar dimensions?

If that is the case, we have the calculations for ARB Stiffness down the right side on the 'Suspension Formula Cheat Sheet' as part of the Suspension Tuning and Optimisation Course. Which will provide some insight into the relative contribution of the materials and dimensions you use. I've also attached an ARB Stiffness Calculator Spreadsheet which uses these same calculations and should allow you to quickly test different torsion bar diameters, wall thicknesses, lever arm lengths and materials to help you understand the contribution to stiffness each of these parameters has on the ARB in isolation.

Once you have an understanding of that, the motion/installation ratio becomes a multiplier which can raise or lower the effective stiffness depending on where you connect the ARB to the suspension.

In simple terms, you can seperate out the Torsional Section Stiffness, Lever Arm Length & Stiffness and Motion Ratio/Connection Point to balance packaging and performance.

For example if you can connect the ARB to the suspension directly to the upright/damper you might expect close to a 1:1 Motion Ratio which means you're effective stiffness of the ARB would be as designed in the calculations we provided. If you connect somewhere along the arm or on a rocker at a Motion Ratio of 0.5:1 you will have halved the effective stiffness of the ARB meaning you may have to drastically increase the torsional section stiffness or shorten the lever arm length to achieve your target effective stiffness.

Hopefully that helps, let me know how you go with the calculations.

We usually reply within 12hrs (often sooner)

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