Effect of roll centre height on stability verses ability.

[ToyRoverlander]

From everything I've read over the years it's good to aim for a horizontal vehicle roll axis. For some reason I always thought it had to be pointing slightly downwards towards the front as you say, but no idea why that would be.

The difference between our vehicles is that mine is a softtop with a basic rollcage. It has all 4 batteries (split 24V system) between the chassis rails in front of the fuel tank. The engine/gearbox hangs quite low in the chassis. The whole suspension, springs axles wheels and stuff is very heavy. The result is a very low CoG.

His vehicle has the same drivetrain, axles, suspension (only difference are my longer rear springs) but it is a 4door stationwagon with roller drawers and such in the back. Not only does it weigh about 200kg more than mine, his CoG must be a lot higher with the heavy tropical roof, sidepanels and full rear door. Batteries, 2 of them or located next to the chassis rails and there's one underneath a seat.

I would say that our rollcentre heights are very close to each other but his has a much heavier setup, thus a higher CoG. When offroad, in offcamber situations, all that weight results in lots of body lean giving an uncomfortable feeling. Mine feels like it lets the suspension work harder and it keeps the body more level, giving a feeling of better stability.

We have the same front springs (rear 3leaf parabolic springs), only difference is in the rear springs (his is on parabolic 4leaf, mine sits on 63" 3leaf + overload chevy springs). I'm sure that the higher weight of his vehicle, combined with a higer CoG resulst in all this.

I'm not sure if it actually needs fixing on his vehicle, I think it would be something you have to learn to adjust to. It's the result of a lifted top heavy offroad vehicle. It would be very hard to do anything about it while sticking with leaf springs.

On a leaf sprung vehicle the roll centre can be found by drawing a line between the chassis mount of the leaf spring and the chassis mount of the shackle, where it intersects the axle vertical line that's the roll centre.

Leaf springs with a larger free camber will either be softer to obtain the same ride height or stiffer to raise the vehicle. When it raises the vehicle it also raises the CoG, and the roll centre together the same amount. So that doesn't do much.

More cambered springs with raised spring mounts does indeed raise the roll centre, as long as the body stays at the same height it would work. Only problem is, there's is no way you can raise the mounts, only way to do it is by outboarding the springs and mounting them next to the chassis rail. Makes it difficult up front as the tires will hit the springs when turning.

Not sure what you mean with shackles on top of the springs, you mean as in upside down shackles? The chassis mount lower than the leaf mount? That lowers the roll centre, so exactly opposite of what you want to achieve.

Going to a linked suspension would be best.... but that's a helluva lot of work.

[bill van snorkle]

As can be seen by the dimensions, W/fing's wheel track is significantly wider than most vehicles that form wheel ruts, so generally , it is only one front wheel at a time, when attempting to climb out of them. The way I'm seeing it is that for the body/chassis to shift laterally relative to the front tyres, verses the tyres shifting relative to the chassis is the same thing, and in certain circumstances, the resistance to that occurring is greater than the resistance to just ploughing straight ahead. The comment that it's difficult to design a very low roll centre may apply to a leaf SOA suspension, but LandRover managed to achieve it easily enough on their coil sprung vehicles, unfortunately IMO at the expense of having to locate the steering trackrod in a vulnerable position behind the axle.

Unless I am prepared to do the same, I would not be able to make adjustable height Panhard mountings on W/Fing to prove/disprove my theory, although it would be do able on the RRC, but not very practical as it would introduce a bump steer geometry conflict with the draglink.

[De Ranged]

Right I'm back on a pc (joys of working away)

This may get a bit messy there is a few things to reply to in the above posts and my head is in cotton wool lol got the flu

Bill you made a mistake in your drawing.... I am guessing you pivoted the axle at the rollcenter height in the center of the axle this will bias the result, remember the front axle is pivoted on a panhard with pivots at the axle and the chassis, now if your front suspension was an A frame top link your drawing would be right.... the length of the panhard is the important dimension for this as it is the arc the axle swings in

Toy your comment about vehicle front and rear rollcenters and wanting the rear to be higher is due to understeer/oversteer characteristics by having the front lower the front axle suspension will move and track better than the rear (similar to front and rear swaybars) having the front track and the back lift will mean the rear will break traction and drift meaning the driver will be oversteering, this will make the vehicle feel "forgiving"

And your comment about CoG to Rollcenter is spot on, keeping or getting the CoG makes more difference than lifting the RC as the RC is basicly body roll.... in most trucks this is what tips them, they get to a point the body moves and this tips the high mass outside the triangle from CoG to tyre contact patch.... If they could get the CoG lower then the triangle is flatter and takes more to tip it and when the body flops there is less high mass to act like a lever

Now back to what I was hinting at and what I think the real reason for your observations Bill

Dan

I think the roll resistance in the rear suspension has an effect on climbing out of ruts. As it allows the suspension to walk up and out of the rutted rather than pushing straight on.

When I sliding a-framed the back of out trialler we noticed a huge improvement in turning out of ruts and wheel placement in general, the front suspension was unchanged so the improvement had to come from the back.

Now when you get to the rut the front wheel pushes up, as it cycles up there is some sideways motion from the high rollcenter lets look at this a little bit closer how is this being resisted by the body.... we arn't hurtling along at speed so there is no inertia, so this only leaves the rear axle and suspension to resist it on a 2.2m long lever

Lets simplify things first lets remove all slop from suspension tyre sidewalls etc yes I know this is totally unrealistic but it will show something

now if we were to turn the truck on the rear axle like a wheel barrow (thanks jamie a bloody good way to look at it lol) it will rotate on the center of the diff due to the rear diff.... Does Wildfing have a welded rear diff?

Now add in all the other movements suspension slop, tyre sidewall flex, chassis flex

Now if we consider rear RC as the front suspension tries to articulate and twist the body it is forcing against the rear rollcenter the higher the rear RC the longer the lever it has to resist the body roll from the front suspension, so this should push down harder on the wheel that is trying to rise out of the rut ?

I was struggling to sort it in my head last week and I'm not much better now but it fits with what Dans found and your mate with the Zuk..... I just feel like I'm missing something lol it hasn't quite clicked.... even tho I don't quite have it right I'd suggest looking at making the rear RC height adjustable

[bill van snorkle]

To answer your question Deranged. No, W/fing has selectable difflocks but for comparison purposes they were left open. Due to the rear axle location via One Link and dislocating leaf springs, there is no rear axle steer as it articulates or with body roll, Whereas RRC's do rear steer a little, so that could be one of the reasons for the difference.

Like I stated earlier, I am not exactly disappointed with W/Fings abilities. I was just trying to determine the reason why a very stock RRC on small wheels and sagged original springs, plus stiff front antiroll bar, performs so much better than its specification suggests that it should. Problem is that I just could not live with the vulnerable low hanging trackrod, so to relocate it up front would require that the panhard rod also be lifted up to clear it, and was interested to determine if that would diminish the vehicles talent for stepping up over obstacles.

[De Ranged]

I didn't think it would be diff lock anyway for the amount of twisting motion the tyre sidewall flex would have countered it, but it could have been part of the resistance.... as I've said I think its the rear roll center height that is creating the resistance here, I'm just not convinced I have the forces correctly worked out as to "how"

I would suggest raising the panhard, and I'd be game enough to put a box on it that it won't change this effect lol

On a practical note even if I'm wrong, packaging concerns are going to limit the lift you can get and thus the change in effect

If I am right this brings up the issue of where the compromise is between good handling characteristics (oversteer/understeer) and improving this ability to climb obstacles

But I'll wait till after you've proven me right or wrong lol

[bill van snorkle]

If I am right this brings up the issue of where the compromise is between good handling characteristics (oversteer/understeer) and improving this ability to climb obstacles

But I'll wait till after you've proven me right or wrong lol

Might have to wait a while before I can prove right or wrongness Lol. Have finally made the decision to swap my portal axles over to my RRC, but that is not going to happen til summer at the earliest.