Anti-roll bars or not, that is the question
Posted: 26 Jul 2011 10:27
Note: This topic has been split off this trip report.
In my opinion anti-roll bars aren't nearly as important to the handling abilities of vehicles with solid axles. Here is my theory:
The most important geometrical parameter that determines the grip available from a tyre is how close the camber angle is to optimal. With solid axles, the camber angles of the wheels are fixed by the axle geometry. The only (slight) change that occurs under hard cornering is due to deflection of the tyre itself, and that can not be helped by the anti-roll bar in any case. The body of a solid axled vehicle without anti-roll bars will exhibit a much greater roll angle than that of a vehicle with the anti-roll bars. This will mainly contribute to discomfort of the occupants, but will also move the center of gravity towards the outside of the corner (slightly) and therefore lead to a lower allowable lateral acceleration (and therefore cornering speed around a fixed radius corner). For roll angles below 15° or so I would imagine that this is still quite a small effect.
With independent suspension, things are very different. The camber angles of the wheels are not controlled directly. Instead, the wheel motion is controlled relative to the vehicle chassis. Now if the whole vehicle has adopted 15° of roll, the wheel camber angles all go for a ball of chalk. In addition, the vehicle will also suffer from the same lateral displacement of the center of gravity relative to the wheels on the outside of the bend as with the solid axles. There is only so much that can be done in terms of suspension geometry design to control the wheel camber angles at extreme roll angles, so it becomes very important to limit those roll angles by fitting beefy anti-roll bars. Normally one would find that disconnecting the anti-roll bars on an independant suspension leads to much greater gains in flex (percentage wise, based on original values) than on solid axles.
There is another subtlety to anti-roll bars that is relevant to both independantly suspended and rigid axled vehicles: The anti-roll bars are one of the most important items that are used to control whether a vehicle will understeer or oversteer. This is because they affect only the roll stiffness of the suspension and not the vertical stiffness and can be stiffened a great deal without affecting ride comfort. Normally vehicles are set up such that the roll stiffness is higher in the front than in the rear (actually it is a little more complicated than that and also involves the height of the front and rear roll centres and the mass distribution, but let's not go there right now ; let's assume the roll centre heights are identical front and rear and the mass distribution is 50% front and 50% rear). The higher front roll stiffness will tend to induce understeer at the limit due to the fact that the front wheel on the outside of the corner has the highest vertical load and loses grip first (tyres exhibit non-linear friction behaviour).
Understeer is deemed to be easier for less experienced drivers to control, because natural reactions (backing off the throttle) lead to a return to control and not further destabilisation.
If one removes the rear anti-roll bar permanently (leaving the front one on), the handling balance could only change to become more prone to understeer. If one removes only the front anti-roll bar, the handling balance could move away from understeer and the vehicle could become prone to oversteer. If both are removed, the change to the handling balance is not easy to predict without more information about the suspension parameters. Now Jules' GQ came from the factory with only a rear anti-roll bar as far as I know, and the GU is so similar that I reasoned I would not induce wild oversteer on the limit on my GU by removing all anti-roll bars.
So, to recap:
Anti-roll bars basically have the following functions:
* Limiting roll angles to keep the occupants happy: People feel increasingly uncomfortable and unsafe as the roll angle increases, even if the car is nowhere near rolling. In fact, people seem to have a disproportionate fear of roll angles.
* Maintaining the angle of the tyre relative to the road to maximise grip in independent suspensions. This role is irrelevant with beam axles.
* Tuning on-the-limit handling balance to induce oversteer or understeer as the designer wishes.
The optimal solution might be to fit disconnects to front and rear anti-roll bars and always connect them for on-road travel, but that adds complexity and is a fiddle every time you need flex. Manual disconnects are normally reasonably easy to reach on the front where the wheels can be steered for access, but can be a real pain at the rear (no pun intended). Some power actuated/automatic systems have been developed (some Y61s, and Y60s I think, had this option for the rear anti-roll bar in certain markets), but invariably these are quite complicated and difficult to retrofit to vehicles that did not have them originally.
Russ, I have actually done the unspeakable and completely removed the front and rear anti-roll bars. I have been running like this for thousands of kilometers in all kinds of different conditions, including town driving, open road tar driving, dirt road driving, caravan towing on dirt and tar, etc. and have no complaints.Russ Kellermann wrote:Gerrit, although i have not seen your GU set up yet, am i right in assuming that you have installed sway bar disconnects which you use to disable the bars on trails only ?
I have different thoughts on this issue.Russ Kellermann wrote:I only say this as i have heard horror stories of rear sway bar removal on tarmac, is this not correct?
In my opinion anti-roll bars aren't nearly as important to the handling abilities of vehicles with solid axles. Here is my theory:
The most important geometrical parameter that determines the grip available from a tyre is how close the camber angle is to optimal. With solid axles, the camber angles of the wheels are fixed by the axle geometry. The only (slight) change that occurs under hard cornering is due to deflection of the tyre itself, and that can not be helped by the anti-roll bar in any case. The body of a solid axled vehicle without anti-roll bars will exhibit a much greater roll angle than that of a vehicle with the anti-roll bars. This will mainly contribute to discomfort of the occupants, but will also move the center of gravity towards the outside of the corner (slightly) and therefore lead to a lower allowable lateral acceleration (and therefore cornering speed around a fixed radius corner). For roll angles below 15° or so I would imagine that this is still quite a small effect.
With independent suspension, things are very different. The camber angles of the wheels are not controlled directly. Instead, the wheel motion is controlled relative to the vehicle chassis. Now if the whole vehicle has adopted 15° of roll, the wheel camber angles all go for a ball of chalk. In addition, the vehicle will also suffer from the same lateral displacement of the center of gravity relative to the wheels on the outside of the bend as with the solid axles. There is only so much that can be done in terms of suspension geometry design to control the wheel camber angles at extreme roll angles, so it becomes very important to limit those roll angles by fitting beefy anti-roll bars. Normally one would find that disconnecting the anti-roll bars on an independant suspension leads to much greater gains in flex (percentage wise, based on original values) than on solid axles.
There is another subtlety to anti-roll bars that is relevant to both independantly suspended and rigid axled vehicles: The anti-roll bars are one of the most important items that are used to control whether a vehicle will understeer or oversteer. This is because they affect only the roll stiffness of the suspension and not the vertical stiffness and can be stiffened a great deal without affecting ride comfort. Normally vehicles are set up such that the roll stiffness is higher in the front than in the rear (actually it is a little more complicated than that and also involves the height of the front and rear roll centres and the mass distribution, but let's not go there right now ; let's assume the roll centre heights are identical front and rear and the mass distribution is 50% front and 50% rear). The higher front roll stiffness will tend to induce understeer at the limit due to the fact that the front wheel on the outside of the corner has the highest vertical load and loses grip first (tyres exhibit non-linear friction behaviour).
Understeer is deemed to be easier for less experienced drivers to control, because natural reactions (backing off the throttle) lead to a return to control and not further destabilisation.
If one removes the rear anti-roll bar permanently (leaving the front one on), the handling balance could only change to become more prone to understeer. If one removes only the front anti-roll bar, the handling balance could move away from understeer and the vehicle could become prone to oversteer. If both are removed, the change to the handling balance is not easy to predict without more information about the suspension parameters. Now Jules' GQ came from the factory with only a rear anti-roll bar as far as I know, and the GU is so similar that I reasoned I would not induce wild oversteer on the limit on my GU by removing all anti-roll bars.
So, to recap:
Anti-roll bars basically have the following functions:
* Limiting roll angles to keep the occupants happy: People feel increasingly uncomfortable and unsafe as the roll angle increases, even if the car is nowhere near rolling. In fact, people seem to have a disproportionate fear of roll angles.
* Maintaining the angle of the tyre relative to the road to maximise grip in independent suspensions. This role is irrelevant with beam axles.
* Tuning on-the-limit handling balance to induce oversteer or understeer as the designer wishes.
The optimal solution might be to fit disconnects to front and rear anti-roll bars and always connect them for on-road travel, but that adds complexity and is a fiddle every time you need flex. Manual disconnects are normally reasonably easy to reach on the front where the wheels can be steered for access, but can be a real pain at the rear (no pun intended). Some power actuated/automatic systems have been developed (some Y61s, and Y60s I think, had this option for the rear anti-roll bar in certain markets), but invariably these are quite complicated and difficult to retrofit to vehicles that did not have them originally.
)