[4ePikanini]

I think this is well worth the investment to eliminate possible stability issues on corrugated gravel

I'm sure it will add stability and extra safety to the ride.

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http://www.ultraracing.com.my/Bars.asp?ID=3795

I don't know if they can ship to SA or if there is an equivalent in SA

[Gerrit Loubser]

I am not convinced that adding roll stiffness at the rear will improve matters...

[4ePikanini]

I am not convinced that adding roll stiffness at the rear will improve matters...

could you elaborate? I was under the impression that was the main cause for the instability.

[Gerrit Loubser]

Marius, I don't claim to know the exact root cause of the Fortuner instability issue and I doubt whether anyone on this forum or 4x4commie (or whatever other forum) really does know the cause. I know that many people have jumped to the conclusion that it must be the lack of a rear anti-roll bar, as this is an obvious and easily observable thing, but vehicle systems are complicated and subtle things can make a large difference.

As far as I know, the Fortuner is known to experience a loss of grip at the rear axle in some cases on dirt roads. This then leads to oversteer and could also lead to a total loss of control and a vehicle roll-over (probably due to the driver overcontrolling the vehicle once un-anticipated oversteer occurs).

Anti-roll bars are used for the following reasons:

A) To limit body roll angle under conditions of lateral acceleration. Body roll seems to be a particularly disconcerting sensation to humans and creates an impression of iminent roll-over, even though this might not nearly be the case physically.

B) To tune the vehicle's handling balance. In a four wheeled vehicle with two axles, the load transfer that occurs during lateral acceleration is typically not identical at each axle, but is related to the roll stiffness at each axle; the axle with relatively higher roll stiffness would experience greater load transfer, all else being equal. Because tyres are non-linear, the increase in available lateral force on the more highly loaded tyre will never be as much as the loss of available lateral force on the less highly loaded tyre, so the nett result is that the total lateral force available from an axle with higher roll stiffness is less than that of the axle with lower roll stiffness. This is one way of biasing the handling towards oversteer or understeer. If one wishes to tune the handling balance towards oversteer, the rear roll stiffness would be increased relative to the front and if one wants to tune the handling towards understeer, the opposite would apply.

C) To tune out any tendencies for the vehicle to experience a continuous low frequency side-to-side swaying motion on its suspension by increasing the roll stiffness.

D) To control the angle of the tyre with respect to the road on axles with independent suspension. In the case of an independent suspension system, the suspension linkage is attached to the sprung mass (vehicle structure/body) and as this rolls on the suspension, the whole linkage is tilted, so the wheel attached to the linkage would also tilt. The maximum lateral force that a tyre can generate reduces as the tyre is tilted away from being perpendicular to the road, so the anti-roll bar is an important item on an independently suspended axle. This concern does not apply in the case of a beam axle.



Now in the case of the Fortuner, it seems that the vehicle can be tail happy under certain conditions (i.e. prone to oversteer), so I would not want to pursue a course of action where the roll stiffness of the rear (beam) axle suspension is increased by the addition of an anti-roll bar while the front axle is left standard. This would tend to make the car even more prone to oversteer. If one wanted to mess around with the roll stiffness, a stiffer front anti-roll bar would be a better starting point, although I think that there is more to the Fortuner issue than mere roll stiffness tuning (e.g. suspension bush compliance issues).

By the way, I think that one of the reasons for the Gen 1 / Gen 2 Pajeros' tidy dirt road handling might well be the rather high front suspension roll stiffness (the very same thing that limits flex on these vehicles...).

[RoelfleRoux]

Gerrit,

Thanks for another superbly explained technical issue.

I would like to ask your opinion on a few issues though:
1)I came to a personal conclusion that the shock absorbers were woefully inadequate. That the tail would unexpectedly step out because dampening failed on a pair of overheating shocks. Does the fact that ridiculously low tyre pressures often seems to help, support my shock-theory?
2)If corrugations are taken perfectly parallel to the acle, then the anti-roll bar will not have an effect, but if the corrugations are not parallel, or if the road is slightely rutted as well, then the wheels start to perambulate on their own. Is it possible that an anti-roll bar will be helpfull under those conditions to stabilise the car?
3)The angle of the panhard rod also makes me wonder (or is it wander, like a Fortuner tail). I was behind a Fortuner the other day at idle speed over speed bumps. The lateral movement as the suspension is depressed at the rear is scary. The passengers literally sway from side to side. Could that also impact on rear instability?

All this said, I fully agree with you that suspensions are complicated and interactive systems and seldome can a single factor be a cause for something like this.

Cheers
Roelf

[Gerrit Loubser]

1)I came to a personal conclusion that the shock absorbers were woefully inadequate. That the tail would unexpectedly step out because dampening failed on a pair of overheating shocks. Does the fact that ridiculously low tyre pressures often seems to help, support my shock-theory?

This might very well be the a contributor. Many have reported an improvement when fitting better dampers, but not all have found this to completely solve the issue, if I recall correctly.

2)If corrugations are taken perfectly parallel to the acle, then the anti-roll bar will not have an effect, but if the corrugations are not parallel, or if the road is slightely rutted as well, then the wheels start to perambulate on their own. Is it possible that an anti-roll bar will be helpfull under those conditions to stabilise the car?

An engineering definition of handling stability would be something like the ability of a vehicle to react in accordance with the driver's steering inputs in a predictable and not in a divergent manner (e.g. the vehicle would not attempt to keep reducing the corner radius until it spins) and would return to running in the straight ahead direction when the steering s neutralised. This means that the anti-roll bar can play a role by tuning the handling between a bias towards oversteer and a bias towards understeer as described in my previous post. Adding an anti-roll bar could improve or reduce vehicle stability in this sense, depending on the initial vehicle design.

If one considers a vehicle traversing terrain where the severe cross-axling occurs, then the ability of the vehicle body to remain at an even keel will be maximised if the roll stiffnesses at both ends of the vehicle are as low as possible and are matched. This is the way that a Land Rover Defender typically works (especially derivatives without anti-roll bars). Adding roll stiffness either to the front or the rear of a vehicle such as a Defender Hardtop would probably lead to more violent motion of the body on the suspension as the body is forced to follow the motion of the axle with greater roll stiffness more closely.

3)The angle of the panhard rod also makes me wonder (or is it wander, like a Fortuner tail). I was behind a Fortuner the other day at idle speed over speed bumps. The lateral movement as the suspension is depressed at the rear is scary. The passengers literally sway from side to side. Could that also impact on rear instability?

Normally Panhard rods are arranged so that they are as close as possible to horizontal in the mid-ride position. Designers also attempt to make the Panhard rod as long as possible. Both of these things help to limit the lateral motion of the chassis relative to the axle as the suspension is deflected. I have not noticed this about the Fortuner, but if the Panhard rod is at such an angle that it induces visible lateral motion over a speed bump, it definitely is not good.

[4ePajero]

I said it on the other forum, and will repeat it here.

I am not even half as clued up on these things as Gerrit is, so my opinion has to be read as such (ramblings of an amateur).

The fact that only some examples of the same vehicle model exhibit these tendencies brought me to the following conclusions:

• if is not an inherent design flaw (if it was, most, if not all examples would act similarly)
• it can barely be assembly tolerance slackness, since these vehicles are built to tight tolerances on "jigs"
• one should look for components where there is the possibility of high manufacturing tolerance.

I think these vehicles suffer from "rear-end steering", due to the axle alignment going out of geometry under certain conditions.
Rear-end steering is a terrible flaw, since

• it cannot be "instinctively" corrected by the average driver
• it gets out of control very quickly, due to the whole alignment geometry

(Test the above by pushing a shopping trolley "in reverse" and at speed. It now steers with its rear wheels. Try to make smooth turns and slalom turns!)

After considering the above, I still suspect that the rubber bushes in the control arms are to blame.
Under certain conditions, the rear axle geometry is thrown out of kilter, causing the vehicle to steer with its rear wheels.
My theory is that the bushes are to soft (designed as such to give a softer ride and less chassis feed-back).
When the rear axle starts to steer the vehicle, the wheel(s) start "slewing" / bouncing, which releases the tension on the bushes, which "corrects" the problem, which now causes the vehicle to either steer straight or in the opposite direction.
This vicious cycle is close to what drivers describe (violent lateral movements, which progressively become worse).

Why will better shocks and tyres improve the problem?
Most probably because it dampens the movement of the components that cause the problem.

Will the 3rd paty add-on anti-roll bar help?
Most probably it will (possibly limiting changes to the axle geometry).
I am sure it won't do much harm.

MHO
(use it or lose it)

[JonnyD]

4e, a friend of mine has exactly the same theory (rear wheel steering).

The peculiar thing to me is that some vehicles has no problem and others have.

[Jaco Vosloo]

I drive a 2007 D4D 4x4 Fortuner and must say that I have not experience any instability with or without towing. This December I swerved at 80km/h for a goat in Moz while towing a heavy offroad trailer... nothing. Even towing on a gravel road at 80-90km/h I do not feel any instability. The Fortuner is stock.
Question?
Opposed to the Hilux the 4x4 Fortuner is permanant 4x4. Could the fact that it is permanant 4x4 conribute to the roll over when one loses the rear end and (as 4e says) instictively tries to correct?
Lastly how many people that have opinions about this issue have actualy driven a Fortuner.
I think that allot of people use to drive a platkat like a 3 series BM and now owns a Fortuner and still want to apply the same driving style as in the platkar...

[4ePikanini]

this is a pajero forum so I think I may stir a bit

This is pretty impressive for a 1993 design suspension on a 4x4 SUV
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And then the famous moose test
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