Question on roll centre/ roll axis ?

[bill van snorkle]

I have, for no particular practical reason, been trying to work out the actual roll centre height and roll axis of WildFing. With my unusual front/rear suspension design which depends much on closely spaced short travel dislocating springs, cross axle see saw effect, double rear spring shackles, no rear panhard rod etc, I don't think the standard means of calculating the roll centre/axis necessarily apply.

I have been playing around with my hi lift jack, placed exactly mid wheelbase on the rocksliders. If I jack either side of the truck over to about 30 degrees before suspension travel ceases, the front/rear roll centres appear equal because the for/aft attitude is level. So with the truck sitting level again, and with a piece of chalk I draw a vertical line down the lateral centre of the vehicle, and place a vertical stick on a stand in line with that chalk line, does the intersecting point of the chalk line and the vertical stick indicate my roll centre height ? There you go Serg (aka Uninformed), this ones right up your street.

[uninformed]

at that particular point, maybe??? Im not 100% on your link set up Bill. Is the front 3 link +panhard? If so this is easy to figure the front roll centre height and the front axle roll axis angle. What about your rear links? Once you figure there rear roll centre height out, you can draw a line through it and through the front roll centre height. This can give you the roll centre line of the vehicle (Vehicle roll axis)

As you ar probably way more aware then me, these things are fine on paper, but what happens real world is more important. Your understanding and experience is that of about 5 of my life times.

Are you trying to figure it out so you have a base line if you change to a one link or change the rear?

regardless, Im going to try this tomorrow and see if my rear is higher than the front, given I know there is that difference in the roll centre heights.

[bill van snorkle]

The way I figure it Serg, calculating RC height by measuring height of Panhard rod at centre of vehicle, around 62cm from ground level, only works with constant rate springs, and while both springs are in contact with their upper and lower mountings. My short, stiff inboard mounted front springs that dislocate after 7 degrees of roll may upset that calculation formula.

Although the rear suspension is basically leaf springs with One Link, the formula for calculating RC height should be as for conventional leafs, ie half distance between spring base and upper shackle pin.That would be about 65cm above ground level, but once again I feel would only apply up to the point of dislocation. I have twin bumps stops both sides, positioned on the springs, for/aft of the axle to progressively increase the leaf springs stiffness as they compress.

Yes, my present front end is 3 link +Panhard.

WB measurement wasn't a typo ! Just bad maths from measuring between the hi lift jack and front /rear wheel centres and adding the two values together, Actual WB is 2.36 mtrs.

No particular reason for wanting to know this stuff. Just another mental distraction.

[uninformed]

The panhard is only part of the equation. It is the starting point, but a line must be drawn including your link geometry, where this passes over the axle centre line is the roll centre height. Bush65 mentioned to me he feels it changes for a panhard type suspension depending on which way the axle is articulating. In my book, it shows it being the chassis centre line intersecting the panhard as the starting point, regardless if the panhard is equal either side of centre or not……..

Does your front 3 link have any triangulation in the links when viewed from above?

[bill van snorkle]

The panhard is only part of the equation. It is the starting point, but a line must be drawn including your link geometry, where this passes over the axle centre line is the roll centre height. Bush65 mentioned to me he feels it changes for a panhard type suspension depending on which way the axle is articulating. In my book, it shows it being the chassis centre line intersecting the panhard as the starting point, regardless if the panhard is equal either side of centre or not……..

Does your front 3 link have any triangulation in the links when viewed from above?

John (bush65) I feel is correct when he states that roll geometry changes left to right. When I was building Nigel Gamblins Hybrid on 38's all those years ago, when cycling the front suspension through its articulation range, the right tyre wall would stuff into the upper spring mounting, whereas when I articulated it the other way there was significantly more clearance between the left tyre wall and the spring mounting. That is why I went to a watts link on that one.

When viewed from above, all my 3 links are quite parallel, plus or minus a degree or three, due to me being corrupted by LandRovers traditional build tolerances

[uninformed]

ok. Am I correct in saying you have 2 lowers and one upper? If so, your lowers will determine your axle roll axis angle, AND, when combined with the panhard starting point, your front roll centre height. Using your panhard starting point (which ever YOU feel is correct, ie half way etc) plot a line running parallel to the lowers when viewed from the side (i.e. the same angle front to back) the point (height) that this new line intersects a vertical line drawn through your axle centre is your front roll centre height. The angled line is your axle roll axis.

Are your rear leaves retained each end?

[discomark10]

Why not run your measurements through the 3 link calculator from pirate?

[Bush65]

Taking the problem of finding roll axis back to basics, at any particular time there exists 2 points that don't move when the axle articulates relative to the chassis. The roll axis passes through those 2 points. The roll centre is the point where the roll axis passes through a transverse vertical plane at the axle centreline.

Now as the suspension moves the location of the fixed points can changed, i.e. they aren't in the same spot forever, only at some instant in time.

Take the stock rear coil sprung suspension for an example. Now you both know this.

The pivot centre of the ball joint on the axle end of the 'A' frame is one of those fixed points - it doesn't move when the axle articulates/rolls a small amount.

The second fixed point is at the intersection of the trailing arms. Because the trailing arms are parallel they intersect at infinity. So the roll axis is on a line through the ball to the intersection of the trailing arms at infinity, so it must be parallel with the trailing arms.

As the suspension moves those points can change.

When a panhard rod is used, the conventional wisdom is to assume one of the fixed points is midway along the panhard, or where the panhard intersects a longitudinal vertical plane on the centreline of the vehicle. Serg and I had a brief discussion about the validity of this. I think it may not be strictly correct, but because of the many assumptions, such as centre of gravity location, that we make affecting other aspects of suspension behavior, any error is not going to cause enough concern in practice.

[bill van snorkle]

Thanks John, Serg, well explained.

Unfortunately the very sudden arrival of hot dry weather in Melbourne outer regions has given me a much more urgent distraction, in that grass growth on my land has exploded within the space of only a couple of days. So all LandRover related work goes on the back burner for 2 weeks while I get busy with the slasher and make this place fire safe.

One suspension related issue that just became apparent this evening after doing a spot of clearancing on my 3 link ball joints, is that at maximum front articulation left and right, the tie rod ends of my steering drag link now reach the absolute limits of their angular movement. Cranking the draglink would only limit articulation on the opposite plane.

[Bush65]

Bill, what I had in my mind, and should have stated in my last post is the suggestion that you examine your suspension, as it articulates, with the aim of finding 2 points that can't (at that point in time, but may change with suspension travel). That is why I pointed out what you already understand, the obvious example (the ball joint) and a not so obvious example (the intersection of the parallel links at infinity).