Physics of anti-roll bars

[sosidge]

Can somebody with a bigger brain and more education than me clarify the physics behind our bent wire anti-roll bars.

I'm pretty certain that the straight centre section of the bar is stiffer if of greater diameter and/or shorter length.

But the bit that has me confused is the effect of the mounting position on the arm and the length of the "lever" part of the bar. Is it cancelled out by the mountings being identical on both sides of the car? Or is the effect cumulative on both sides?

[Garry Driffill]

A lighter front sway bar increases front traction off-power, But has less on-power steering.

A heavier front sway bar decreases off-power front traction making the front of the car more predictable when entering a turn but gives more on-power steering.

Going acrross to the rear of the car..

A lighter rear sway bar will increase rear traction but decreases on-power steering.

A heavier rear sway bar increases stability in the middle of a turn and increases on-power steering, Heavier sway bars also make the car more stable on high speed, high traction tracks like astro or carpet..

[sosidge]

Originally Posted by Garry Driffill

A lighter front sway bar increases front traction off-power, But has less on-power steering.

A heavier front sway bar decreases off-power front traction making the front of the car more predictable when entering a turn but gives more on-power steering.

Going acrross to the rear of the car..

A lighter rear sway bar will increase rear traction but decreases on-power steering.

A heavier rear sway bar increases stability in the middle of a turn and increases on-power steering, Heavier sway bars also make the car more stable on high speed, high traction tracks like astro or carpet..

That doesn't explain the physics though... just the effect. I understand the effect, but want to understand the physics too.

[Garry Driffill]

Wouldnt it work as in further out with a kink gives a smoother the transfare?

[glypo]

Vehicle dynamics (suspension) interests me very little. I did however read a book once which had comprehensive physical analysis of roll-bars.

It was either:

Pacejka, Hans B. 2005. Tyre and Vehicle Dynamics. 2nd Ed. Butterworth-Heinemann.

or:

Genta, G. 1997. Motor Vehicle Dynamics. World Scientific Publishing.

[simoncrabb]

My very simplistic view of roll bars in RC is they increase the springing when one side is under compression, but not when both are under compression.

Our wire roll bars are like a torsion bar I guess?

[sosidge]

Well thanks for the answers so far. I have been trying to find a clear reason on t'internet why a longer lever arm results in a softer ARB rate. Much is said about the greater leverage of the longer lever, but I still believe the added leverage is cancelled out by the equal size of the lever on the opposite side of the car, my schoolboy physics implies it would have to be.

However the wire ARB levers are not perfect - they flex a lot, and act more like springs. I think this may be the reason for the change in ARB rate, the long lever acts like a softer spring and absorbs a lot of the energy before it can be transferred to the other side of the car, the effect is duplicated on the other side.

I did do an empirical test on the B4 and yes, the bar is stiffer if you shorten the lever length. Wish I knew where the physics was actually happening but I suppose I shall have to either take another degree or just accept the results!

[DCM]

the longer arm, has a lower ratio of torsional twist to the upstroke of the suspension arm.

eg.

Short link, 15mm of suspension travel = 47' of torsional twist (stiff action)
long link, 15mm of suspension tavel = 30' torsions twist (soft action)

Running an ARB can allow for slightly softer springing and damping on your suspension, to cope with a rough track, with the ARB controlling pitch into and out of a corner.

[sosidge]

Originally Posted by DCM

the longer arm, has a lower ratio of torsional twist to the upstroke of the suspension arm.

eg.

Short link, 15mm of suspension travel = 47' of torsional twist (stiff action)
long link, 15mm of suspension tavel = 30' torsions twist (soft action)

Running an ARB can allow for slightly softer springing and damping on your suspension, to cope with a rough track, with the ARB controlling pitch into and out of a corner.

If we were to assume that the bar was perfect (ie no actual twist, just complete transmission of the forces), 47 degrees or 30 degrees would not matter as the opposite side of the cars suspension would need be lifted by an identical amount, as the linkages are identical on both sides of the car.

If we assume that the levers are perfect but the centre section twists at a uniform rate (say, 50%), the lift on the opposite side would also be constant (certainly as you approach zero the relationship would be linear). An input of 20deg from a short link that lifted the other side 10deg would be effectively the same as a long link, deflecting 10 degrees, and lifting the other side 5deg.

But obviously the levers are not perfect, they flex, and this takes up more of the input energy before it gets transferred to the centre section of the bar, and more again as it is transferred back to the opposite side. The longer the lever, the greater the energy loss. And the softer the effect of the bar.

I'm fairly certain that it is the "spring" nature of the lever rather than the leverage itself that causes the bar to be stiffer with a shorter lever. But in all honesty it is not that relevant now as after 5 minutes with the car I have seen the lever effect in action. I probably shouldn't have bothered posting the thread in the first place!

[Alfonzo]

Surely the length of the lever arm dictates the amount of torsional twist or 'spring' that gets loaded up within the ARB for any given suspension travel. It's the gearing if you like.

Slow one knows what he's talking about. Your comment about the longer lever giving a softer result through its own 'springiness' might be true, given the scaling issues with model car design, although not at all how an ARB is designed to work. It would effectively be the same as modestly increasing the spring rate.

[SlowOne]

Sosidge, you are confusing two things - what you see on your car and the Laws of Mechanics and Physics. I hope this helps...

In a 'proper' ARB, the arms don't bend. In a model car ARB, we use piano wire and, as you say, the arm does bend. Ignore this! Now go through DCM's good explanation and you will see the reason for a softer action from a longer lever.

The wishbone moves a fixed distance. If the lever is a long way from the centre section, it will move teh lever a small amount, and thus twist the centre section a little. Move the lever closer to the centre section and for the same movement of the wishbone, you will twist the centre section a lot. More twist tries harder to lift the other wishbone, thus making the front end 'stiffer'

By the same token, make the bar thicker and you will get more lift on the opposite wishbone, again making the suspension 'sitffer' at that end.

Always remember that an ARB is transferring weight across the chassis, and taking grip away from the end the ARB is attached to. Your first port of call should always be the roll centres, in order to change the roll-rate of one end or the other. ARBs come into their own when one can no longer get the desired result from this approach (most cars have limits as to where the suspension pick-up points can be placed) or when the car needs to have suspension capable of absorbing bumps, yet remain stiff in roll.

HTH