Rollcenter Control and Suspension type

[De Ranged]

Something I've had in the back of my mind for a bit now, I've brought it up now as it may have implications to do with Bills thread on retained coils... I'm curious to see if I'm right lol

Now please bear in mind I'm looking at this from a low to med speed point of view (typical 4wd'n speeds)

How a Panhard suspension systems rollcenter height performs compared to a triangulated system with no panhard

I know this may sound daft but.... I have come to the conclusion at low speed the panhard systems is not as is commonly thought

"Half way between a virtual line between the two mounting points of the panhard bar, so effectively half way between axle mount and chassis mount..."

Here's my thoughts on it

I'll start first with a comment that I think is why most miss what I'm getting at "speed"... most of the calculations for forces in automotive handling are worked out for vehicles with motion and inertial mass.... now we are trying to apply these to our sport but where we are going alot slower, alot of the time we will have minimal inertia so a minor change of direction wont have the same resistance or possibly no resistance at all

(Sorry to all those who know science speak, I know I'm not describing this right lol please bear with me )

Right let me describe something so this makes sense, I'll start with the triangulated system, lets say the rear "A" frame setup of a coil sprung landy .... the rollcenter is the ball at the end of the "A" arm, now forcing the body to pivot on this is the "A" frame so this locks the body into this relationship... this can't change due to speed, orientation to a side slope (yes I know it changes in relation to the body CoG due to extension and compression of the suspension but both front and rear do this so I'm ignoring it)

Now on a panhard system what is to resist and force the relationship so that rollcenter height stays there

Most books and people I've talked too say... that it is the sideways motion of the body as the panhard bar swings under compression or extension, as the bar swings in an arc that creates a sideways motion between the chassis and the axle

In my opinion this amounts to little to no resistance at our speeds, also the amount of sideways movement is not that great compared to the weight that is in the body and supposedly pivoting on it

Even if it did have some resistance lets look at what is resisting it... imagine the truck with the front panhard bar under compression, this wants to force the front of the truck to one side so what resists this, remember there is little force from inertial change so the only real resistance is the wheels in contact with the ground at the back of the truck and a long lever (length of the truck) working against these... remember its only a few mm or sideways movement

The way I see it... it is the axle pivot for the panhard is the real rollcenter height, I can't see any resistance to the body's movement coming from the chassis pivot at low speed

I haven't put this to any of the trials guys over here yet..... but curious to see if anybody out there that has gone from a panhard to triangulated suspension and noted an improvement in roll resistance, but with similar RC heights?

Comments ?

[Soren Frimodt]

I'm no suspension guru, by any Means, but from looking at the workings of different suspension systems that I have come across IRL I'd say the the position of the Panhard definitely has something to do with RC. The taler it sits the more "stiff" the vehicle seems to be when on a sideways tilt. When people do high steering arm mounts and subsequently move the Panhard upwards to avoid bumpsteer, you easily see this effect.

[De Ranged]

I'm not saying it doesn't.... just that instead of the RC height being half way between chassis and axle mounts on the panhard as is in all the automotive books I think due to our slow speed I see the body pivoting on the axle mount of the panhard which makes this lower

This is why I was hopeing to find some people who have gone from say radius arm front to a triangulated link setup to see what they noticed... or if I'm being daft, challenge my logic

[Soren Frimodt]

Hehe then we are saying the same thing are we not? that it pivots on the axle end of the panhard, and that the higher this sits the less of a lean your body will get. However this is only good to a certain extend, because when it finally overcomes this point it'll move drastically faster it seems, so instead of a smooth lean you get a sudden agressive one. The Norwegian Super triallers have been fighting this problem for ages, having a hard time finding the ideal placement of the Panhard. Sorry I only have this to add seeing as I haven't ever met anyone who converted to three link.

[De Ranged]

It also brings up an interesting issue..... if I am right and it does pivot on the axle panhard mount then that brings up the issue you've mentioned that instead of tipping evenly from the center it tips with a pivot off to one side... this means that tipping to one side will have more resistance than the other

Yrs ago in the trials scene here there was a move to short panhards mounted to the center of the diff (still alot of trucks running this) I have play'd on trucks with this setup and from an anecdotal point of view I felt they flexed free'er and since the panhard was mounted in the middle of the diff at low to med speed there was minimal bump steer, alot less than I expected, unfortunately I wasn't able to push them on sidelings to see handling changes... but it makes sense it should be more predictable and even side to side

.... now I'm not even remotely suggesting this for a road going truck, it would take very carefull consideration if you were to do this!

Oh and 3 link is still panhard based, the ones I was thinking about is triangulated 4 link or "A" frame styles

[bill van snorkle]

I'm not saying it doesn't.... just that instead of the RC height being half way between chassis and axle mounts on the panhard as is in all the automotive books I think due to our slow speed I see the body pivoting on the axle mount of the panhard which makes this lower

I think you have a point there. When doing front axle articulation cycling on a hybrid I was building with a 3 link plus panhard rod some years ago, I discovered that with the R/H/Side tyre at full stuff, the inner sidewall would foul heavily on the upper coil spring mounting, whereas there was adequate clearance when the L/H/Side wheel was at full stuff. Legalities prevented the fitting of rims with less back spacing, so I tried a Watts Link which gave equal left to right roll geometry, and surprisingly didn't cause massive bump steer on whoops.

[Bush65]

Roll centre depends upon the suspension geometry and has nothing to do with speed or time.

The statements about roll centre location are not strictly correct.

It is not correct to say that the roll centre for A frame suspension is the ball joint. If the ball joint happens to be on the transverse vertical plane through the centre of the wheels, then, by coincident, the roll centre just happens to be where the ball joint is. Admittedly this is a likely scenario, but it it doesn't make the statement correct.

The roll centre for suspension with a pan hard bar is not at the centre of the pan hard bar, unless the centre of the pan hard bar happens to be on the transverse vertical plane through the centre of the wheels. However if you are looking at a front view, and the roll axis happens to be horizontal, then the roll centre will be directly inline with the centre of the pan hard bar, but on the transverse vertical plane through the centre of the wheels.

The transverse vertical plane through the centre of the wheels is used for the roll centre definition because the front and rear roll centre heights affects the transverse load shift distribution to the tyres during roll. This is important because it affects the traction and determines under/over steer.

The suspension roll axis passes through the roll centre, and its slope determines bump/roll steer.

As for the point where the pan hard bar is attached to the axle, that is not the roll axis, it is the instant centre for the pan hard bar.

Consider if it was (which it isn't), and consider if the pan hard bar was horizontal with the ends inline with the springs and the left end attached to the axle.

Now consider what would happen when the chassis/body rolls to the right.

The right end of the pan hard bar will force the chassis/body to move down in an arc. There will be a large vertical downward and small horizontal to left displacement at that point in the chassis.

Now if we look at a point on the chassis at the perch for the left spring. This point will also swing in an arc about the roll centre. There will be a large displacement to the right and a small displacement vertically downward.

How can you reconcile these displacements?

Now consider the loads on the springs. The right spring will compress a large amount and the left a small amount. This is a significant nett increase in the total load on the pair of springs.

Where will that extra load come from?

Unless you have a good explanation to those questions, it should be obvious that the roll centre can not be at the end of the pan hard bar.

[Bush65]

BTW, the SAE definition of roll centre is the point where you can apply a load on the spring mass and not cause it to roll, or something to that effect.

[Daan]

the roll centre is only there at right height while driving in a straight line. The moment it does start leaning in corners, it starts moving around, on a panhard rod especially. A wattslinkage is better for this, as the roll centre is an actual pivot axis.

Daan

[De Ranged]

Sorry for the slow reply been working away and when i'm home my partner wants her time with me lol

Thanks for the replies

Bill, that sideways motion is the nature of the beast the panhard is a link and as it moves (swings) it will have a sideways motion as well as an up and down motion

John, agree with everything up till the example.... I get the impression you are considering the pivot point of the suspension roll center, I'm just considering the height, the pivot point will still be a virtual point half way between the panhard axle and chassis mounts (as seen from the front) .... this does bring up the issue of an off center pivot point in relation to the weight carrying springs, those early trials trucks here in NZ but this is something to go over someother time lol

The way I see it, using your example as the body rolls right the chassis mount will move down, and slightly left .... the left spring will expand as much as the right spring compresses this will force the body to pivot on the chassis pivot as much as the panhard pivots on the axle mount

Now the definition I was given for Roll center height is the height the body rotates at on the suspension

LOL it clicked Thank you John.... the virtual pivot point on the panhard because the springs carry the weight, the point is half way between the springs.... as the body rolls right this will lower and as it rolls left it will raise

This fits with what your saying Dan

Alright then what about in the case where the panhard isn't mounted centrally like in the case of the early trials trucks here, where they mounted the right side to the chassis rail and the left side to the center of the axle (viewed front on) then in this case axle mount is roll center height ?

[CR88]

Roll Centre, this is a very difficult topic even for people with vehicle dynamics back ground.

There two ways of identify the RC location, the geometrical and force definition. The SAE definition is inline with the force definition, and where the vehicle dynamics guys are more familiar. The geometrical is more easy to obtain in independent suspension like double-wishbone or SLA (short long arm as the Americans use to name it).

The RC location is important for two important reasons:

1. define the Roll Axis;
2. define the arm between this axis and vehicle CG.

There are other characteristics to be taken in account in a suspension definition and the roll centre is only one of multiple points. The RC is only defined by one axle, and depending on the vehicle this should be lower or higher, depending on the other(s) axle(s) RC location.

The RC can be in a static position of the vehicle defined by physical point or not, but in the most of the times isn't common to find the RC specific joint.

The Panhard link isn't the best arm configuration for a suspension, but sometimes is the only one possible. This specially makes sense in front axle where the available space is limited with engine other components such as propeller shafts.

Other solutions of configurations are more interesting like the 4-link, link the rear suspension of the RRC, Defender & Discos. The A-frame is a special case of 4-link where the two top arms connect in the same point. Ideally, the front should have a similar solution, w or wo A-frame or two link instead. But isn,t easy.

The suspension is tricky to be define, and in the most of cases, isn't the ideal one but the better possible solution.

[De Ranged]

I understand the triangulation used to locate the RC on any of the triangulated suspension types (be it triangulated beam axles, or independent suspension) but a panhard system I've always taken the mid point of the panhard bar with an adjustment due to the distance off the vertical side axis of the axle, and that adjustment is back to the instant center of the top and bottom links

But due to its offroad handling quirks I've never bothered dealing with its dynamics till now lol

There is another reason for the panhard front and that is steering linkage bump steer.... so long as the panhard matches the draglink on a X over steering system this is eliminated (this isn't bump steer due to high RC that is still there)

Legally here (NZ) we have to comply with a performance handling test (I could say something about the lack of standards for this test... lol) it is that tough that I have had to fit panhard bars to leaf sprung vehicles to eliminate some of the bump steer

And as we arn't allowed hydro or electric steering, our laws dictate steering must be mechanically linked so the panhard style suspension is pretty much a requirement on long travel beam axle suspension

Been thinking on my post above

I'm not so certain that the pivot point (as seen from the front) would be set by the mass of the body on the springs.... to my original way of thinking, the pivot point will be vertically in line with the CoG of the body.... as the body will want to balance on the springs on this axis

But what happens when you have something like these early trials trucks I mentioned.... I'm now certain I'm wrong about the last part of my post, the axle pivot of the panhard will not be the pivot as the RC height will still change due to the up and down swing of the panhard

With the short panhard and the center of it now being off to the side of the body CoG and it is the center of the rod that dictates height.... does that mean there is now a lever between the pivot point and the mass of the body ?