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Using different roll bar holes

Motorsport Wheel Alignment Fundamentals

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Referring to Adjusting Anti Roll Bars module, my thinking is that using different linkage mounting points on LH and RH side will have an effect of the bar not balanced during right and left hand corners/bumps. I can't see how we can fine tune the bar this way.

*** EDIT ***

Please see my updated response to this post that you will find lower down after Gord's first post. I have left my original response here for clarity.

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Hairul, I understand the confusion on this! I'll try to explain with an example to be as clear as possible for everyone else.

Let's take the example of an ARB (anti-roll bar) that has 3 holes at each end for roll stiffness adjustment. If you have the bar set such that one end is set to the stiffest setting and the other end is in the middle setting, this will result in a total roll stiffness set to part-way between the stiffest setting (if both ends of the bar were set to full stiff) and the medium setting (if both ends of the bar were set to medium).

Remember that an ARB is a spring that only acts BETWEEN the two sides of the suspension. The only time the ARB provides a resistive force is when there is a difference in position between the two sides of the suspension.

For this reason, regardless of the settings you use on each side of the ARB, the resulting roll stiffness will always be symmetric for both left and right side turns. Does that help?

This is how I think of it. 3 holes on each side.

On LH side, I put it on the softest hole, which I believe, is farthest one from the bar. On RH side, the stiffest hole which is nearest to the bar. Not sure if my these are correct.

Now let's say that LH wheel goes over a bump. Since that side use the farthest hole, it will have more leverage and easier for that side to twist the bar.

When RH wheel rides over a bump, it doesn't have as much leverage, hence harder for it to twist the bar.

Attached Files

You are correct with which holes tend to make the adjustment stiffer and softer in general. A shorter lever arm equals less mechanical advantage, which will give you a higher effective ARB stiffness.

However, for the effect of running different adjustments on each end of the bar, both sides of the suspension experience the same stiffness regardless if the adjustment is different on each end. The ARB is simply a spring that acts between both sides of the suspension. The only thing the ARB is reacting against is the opposite side of the suspension.

In the situation, you describe where you have 3 adjustments on each end, if you put one side full hard and the other full soft, this will be roughly equivalent to putting both sides in the middle hole.

If the ARB was reacting against the chassis rather than the opposite side of the suspension, then your reasoning would be correct, but that's not where the reaction force is. Remember that the ARB mounts on the chassis are only there to support the ARB and allow it to rotate and twist as freely as possible. None of the torsion forces within the ARB are reacted through the chassis.

Have a look at how the "Blade" style in cockpit adjustable ARB's work. One side is tubular arm with the connecting link to the suspension at the end of it, with the other side being the blade that is rotated by a mechanical device to alter the angle of the blade relative to the force applied by the connecting link to the suspension. The more in line with the force axis that the blade is, the greater the stiffness of the bar.

Good example Stephen!

Hairul, I think you can see Stephen's example has the same reasoning that I was trying to explain. The end result is that regardless of where you change the stiffness in the bar both sides are affected in the same way.

Just a comment. It is important that there is no pre-load in the ARB, as if there is any present it will add to the total spring rate to one side and remove it from the other - you would be adding 'wedge'.

Hairul,

I would like to make a correction to the first response I gave to you on this topic. I was contacted by someone on this topic that challenged me on my previous response which I am grateful for. I will try to be clear in my response to this topic below!

Take the example of the ARB I have shown below. The red arrow indicates the adjustment hole to reduce the effective stiffness of the ARB (SOFT) and the blue arrow is the adjustment hole to increase the effective stiffness of the ARB (HARD).

Using the SOFT hole will mean less vertical load transfer at the tyre contact patch, using the HARD hole will increase it.

If you were to set the car such that one side used the SOFT setting and the other side used the HARD setting, you would get different roll stiffness effects from the ARB in left and right-hand corners. The effective behavior from a load transfer perspective of the ARB will not be symmetric.

The torque within the ARB will be equal and opposite at each end of the torsion bar (the middle section of the ARB), but the force reaction at the suspension attachment point (be it the lower control arm, or strut) will be different on one side to the other. This gives you a different vertical load at the contact patch due to the lateral load transfer between left and right corners.

While I was thinking about this, something else occurred to me as well: If you set one side the HARD and the other to SOFT, in the case where you have a pure compression situation where both wheels move up the same amount, because you have a different motion ratio between wheel and ARB on each side, this movement will induce a vertical load transfer and therefore a body roll. This is happening in the absence of any cornering force. This is something that will likely lead to some unexpected handing traits and poor response to bumps depending on which end of the car this was occurring on. There are multiple potential courses for the wheels compressing the same amount, things like wide dips in the road, or ride height changes due to downforce!

I apologise for the confusion and I hope this post is clear. πŸ™Œ

Tim

There is a critical difference between using a blade (or two) for adjusting the ARB - the leverage, and hence forces on the links, are the same, whereas using different length arms will mean a inversely proportional force applied through the linkages - there is not just the input forces at play, but the inter-reaction with the chassis mounting points.

I am not convinced, either way, how this may affect the balance left to right - perhaps one can try one way, and then the other, to see if there is a change in behaviour with the change?

When I did some more reading on this, I found some test-bench results (which I can try link here at some point) which tested this exact situation to prove the points. Their findings were that you did indeed get non-symmetric behavior. Agreed that it would make a nice on-track test, it may indeed be something we include in an upcoming suspension fundamentals course πŸ‘Š

Ah, thought that might be the case, but didn't know for sure.

It does open up an interesting possibility for some tracks - say there were primarily fast corners in one direction, with some tight corners in the other, one could use odd length arms to give different roll resistance, and balance, for each type corner.

You're right about it opening up some possibilities to take advantage of the non-symmetric behavior, there are definitely specific situations where this could be a benefit. I think if I had the choice of layout, a blade type would probably still be my preference though because of the symmetric motion ratio side to side.

If you were to set the car such that one side used the SOFT setting and the other side used the HARD setting, you would get different roll stiffness effects from the ARB in left and right-hand corners. The effective behavior from a load transfer perspective of the ARB will not be symmetric.

The torque within the ARB will be equal and opposite at each end of the torsion bar (the middle section of the ARB), but the force reaction at the suspension attachment point (be it the lower control arm, or strut) will be different on one side to the other. This gives you a different vertical load at the contact patch due to the lateral load transfer between left and right corners.

Are you saying that my assumption is correct or is there another whole new view point on this topic?

While I was thinking about this, something else occurred to me as well: If you set one side the HARD and the other to SOFT, in the case where you have a pure compression situation where both wheels move up the same amount, because you have a different motion ratio between wheel and ARB on each side, this movement will induce a vertical load transfer and therefore a body roll. This is happening in the absence of any cornering force. This is something that will likely lead to some unexpected handing traits and poor response to bumps depending on which end of the car this was occurring on. There are multiple potential courses for the wheels compressing the same amount, things like wide dips in the road, or ride height changes due to downforce!

From what I can understand, suspension on both sides travel the same amount vertically, but since the ARB rotates with different radius, it'll affect the other wheel with different distance? Seems like in car setup, every little bit of change might have a bigger unintended negative effect 🀯

Btw Tim can you post the link to the test bench result?

Are you saying that my assumption is correct or is there another whole new view point on this topic?

Hairul, I went back to read your initial posts on this thread and I think your initial assumption is the correct one!

From what I can understand, suspension on both sides travel the same amount vertically, but since the ARB rotates with different radius, it'll affect the other wheel with different distance? Seems like in car setup, every little bit of change might have a bigger unintended negative effect 🀯

Yes, exactly. This could cause the car to get upset even on an even 2 wheel bump! Always a lot to take into account in car setup! πŸ˜ƒ

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