Physics of anti-roll bars

[Lee]

Steady on Fabs, anyone would think you knew what you were talking about

I do agree with what you are saying though, people forget that the roll bar absorbs energy, it is not an equal output from the input of the loaded side.

[sosidge]

Yes, I agree with the energy analysis, and also an analysis based on cosines of a difference in position of the input and output of the bars. I think I have mentioned the energy analysis myself already. It's the lever length analysis that I can't agree with.

Doesn't really matter ultimately - I have done an empirical test and seen the results. I have seen a few explanations based on sound science. My mind is at rest.

I only asked the topic in the first place because the .047thou bar (~1.2mm) on the B4 behaves in a much softer way than the 1.2mm bar on the Yokomo, and the 1.2mm bar on the Tamiya.

[Fabs]

I THINK I know what I'm talking about but odds are I'm just talking rubbish

[Lee]

What are the visual differences? a 1.2mm bar with 2 bends/kinks will always have more flex than a 4 kink bar as im sure you know as there will be less flex due to there being shorter lengths. (if that makes sense)

[Fabs]

Originally Posted by sosidge

Yes, I agree with the energy analysis, and also an analysis based on cosines of a difference in position of the input and output of the bars. I think I have mentioned the energy analysis myself already. It's the lever length analysis that I can't agree with.

Doesn't really matter ultimately - I have done an empirical test and seen the results. I have seen a few explanations based on sound science. My mind is at rest.

I only asked the topic in the first place because the .047thou bar (~1.2mm) on the B4 behaves in a much softer way than the 1.2mm bar on the Yokomo, and the 1.2mm bar on the Tamiya.

Two different roll bars design may be different for a same diameter for multiple reasons : Pick up points on the arms is an obvious one, but also the length of the central section of the ARB has a big influence as the twist rate / unit length is mainly what we're interested in. And ultimately the length of the ARB arms as we've established that they do indeed bend in our case.

Also don't forget the angle between the ARB arm and the link between it and the wishbone has an influence too.

[sosidge]

Originally Posted by Lee

What are the visual differences? a 1.2mm bar with 2 bends/kinks will always have more flex than a 4 kink bar as im sure you know as there will be less flex due to there being shorter lengths. (if that makes sense)

The bar on the B4 has a much longer centre section and the lever arms are somewhat longer too. All the bars have only 2 kinks.

So I think there are two things making it softer - the long levers (absorbing energy), and the long centre (behaving like a softer torsion spring).

[blue_pinky]

Originally Posted by sosidge

So I think there are two things making it softer - the long levers (absorbing energy), and the long centre (behaving like a softer torsion spring).

Spot on!!! A* Simple as that!

[Duncs]

Originally Posted by Fabs

Two different roll bars design may be different for a same diameter for multiple reasons : Pick up points on the arms is an obvious one, but also the length of the central section of the ARB has a big influence as the twist rate / unit length is mainly what we're interested in. And ultimately the length of the ARB arms as we've established that they do indeed bend in our case.

Also don't forget the angle between the ARB arm and the link between it and the wishbone has an influence too.

And the material the ARB is made from, of course. Two different grades of steel will behave differently, Young's Modulus IIRC.

[SlowOne]

Great - we all got there in the end! Keep in mind that the basics 'Rules' of ARBs apply no matter what shape or size they are - roll will increase if the material is thinner or more ductile, and if the lever points are further away from the pivot points. The length of the centre section will have an impact, but since that is fixed for any given car, it can be ignored when making tuning changes. Good luck you rock and (anti) rollers!