This is in response to Wisby's question on forced articulation on EZZZZ's Homebuilt 3 link thread.
When we were testing the Hybrid prior to glazing and painting the bodywork, I tried every conceivable test , up hill,down hill, extreme offcamber up and down hill, driving into and out of deep ditches at an oblique angle etc in an effort to catch the suspension out and never once did it feel unstable. In fact the only time I tipped it over was in the workshop when I was mocking up the forced articulation driveshafts and lifted the right front wheel up about 5 feet with the floor crane to check articulation. I walked over to the work bench to answer my telephone when I heard a loud crack. I looked over to the hybrid just in time to watch it ever so gently lay over on its side. I had tack welded one of the forced articulation driveshaft flanges to a temporary makeshift shaft and the loud crack was the tack weld breaking thus negating the stabilising effect causing the fully compressed right front spring to unload.
As tested on the videos ,the truck was centrally sprung at the rear, with no side coils and it was easy to see the forced articulation doing what it was designed to do. However in order to have it certified for road registration The accredited engineer required that we fit side coils in the normal location, and although the truck is still extremely capable I believe it lost a little at the extreme limits . Nigel the hybrids owner told me that on one offroad jaunt one of the forced articulation driveshafts became detached. he picked it up and threw it in the back to refit when he got home. He reported that the truck wasn't anywhere near as capable or as easy to drive without the system working.
Wilsby, A year or so ago on Outerlimits we discussed a suspension consisting of interconnected centrally pivoted front and rear "A" frames, wishbones, One links, call them what you will, the interconnection was to be via an old diff carrier bolted to the chassis and a short driveshaft from each side gear to respective "A" frames. Did you ever give the idea any more thought?
bill.

Just because I love the vehicle so, to help with any searches, and so others know exactly what this thread refers to here is the link to the photos and video over on outerlimits. The photos are on the second page and videos on the third

http://www.outerlimits4x4.com/PHP_Modules/phpBB2/viewtopic.php?t=22705&postdays=0&postorder=asc&highlight=volvo&start=0&sid=a3a6dedce07add7c55ef607a9ba8d405

Bill, I remember the thread at outerlimits, but at the time I think I was considering it for my street legal 110, and decided against it because of weight and legality reasons, not counting the complications from actually pulling it off in real life.

I think it makes sense on a ligther rig, though, particularly if street legality is not a prerequisite. A plan to put Volvo portals (that I have) on an old RRC 2 door (that I have) is slowly taking shape. Bobbed frame and cut body behind the doors is part of the plan. I think forced articulation could be the ticket to get articualation without making it unstable off camber, which I dread. I don't think I got the part with diff carriers at the time, but may be doing so now. I have a Salisbury rear and a 4 pin P38 carrier lying around, so always a start, I guess.

On a slightly longer timeframe, I may consider building a prototype trialer as well.

Why wishbones and not your original design, btw?

ive got spare drivshafts (lots of them) and abit of room to fit the gear system needed for forced articulation.....

Bill - I think we need to chat :D

Bill, I remember the thread at outerlimits,
Why wishbones and not your original design, btw?


Why wishbones.? Well I have what is effectively a wishbone on the rear of my series 2 and after I fitted it several years ago I believe it transformed the trucks offroad performance. I don't personally subscribe to the notion that antisquat is an undesirable commodity, particularly on a swb.
Unlike so called 3 or 4 link systems a wishbone or what some call a 1 link combined with a Watts Linkage for lateral location ,always keeps the axle perpendicular to the frame no matter how much the suspension articulates, so no unwanted rear axle steer. And no matter how much the axle articulates the drive thrust is always centred through the single frame attachment point.
A wishbone is relatively simple to fabricate compared to multi links and only requires one bushing or ball joint, compared to 6, 7 or even 8 for multi links.
A wishbone locates portal axles positively in a simple design whereas multi links require a lot of vertical separation of the links at the axle to provide control. I also think that both front and rear wishbones can be more easily interconnected via the afore mentioned diff carrier than the complicated , heavy and space consuming set up I fitted to the Hybrid. There would be 4 less Universal joints and 3 less slip joints with their clearances (lash) that have to be taken up before the forced articulation becomes effective.
I would really love to have the time again to build such a system but nowadays family commitments mean that even minor jobs that would only take a day or so usually have to wait several months before I can get around to it.
Bill.

OK, I get it. What you call a wishbone is in fact how virtually all successful Prototype trialers attact their Volvos or Mogs to the chassis. Some end up with weird caster, but that doesn't seem to bother anyone.

What would you use between the carrier and the wishbone to take up axial and radial loads, and provide flex?

And how did you design the Watts linkage to accept more axle movement than stock vehicles?

I think it makes sense on a ligther rig, though, particularly if street legality is not a prerequisite.

Wilsby, AFAIK (bill can correct me) the rig was fully engineered (approved by the equivalent of the TÜV) in Australia. Our laws are almost as strict as the laws in sweden.

Wilsby, AFAIK (bill can correct me) the rig was fully engineered (approved by the equivalent of the TÜV) in Australia. Our laws are almost as strict as the laws in sweden.

Newer trucks, like my 2001 Defender, have EU-wide certification that do not allow much in terms of modification. If you stick to the letter, you can only fit equipment that was submitted for certification by the manufacturer. I managed to get re-registration with a winch bumper and full exo cage, but it seems that window has closed now. They also turned a blind eye to my lift and lack of muffler. And were very intrigued by the movement in Sam's holey bushes. But slapped me for a bad ball joint in the pitman arm, so what they do actually make sense.

You can also take an older vehicle an re-register as "Modified vehicle" or go all out and register as "Amateur built". There is a Swedish enthusiast organization with a network of accredited scrutineers that do pre-inspections before registration with SBP (MOT equivalent). They will look at weld quality, chassis design, brakes etc. So in this sense Sweden is quite liberal, but the treshold is high and you have to do your homework. Tobbjo is halfway through this with his Volvo portaled classic, and I think he can attest to the fact that this is not done in an afternoon.

I think a bobbed classic with Volvos is about as far as you can go without being upgraded to "Amateur built". The older the original vehicle is, the more you can do.

OK, I get it. What you call a wishbone is in fact how virtually all successful Prototype trialers attact their Volvos or Mogs to the chassis. Some end up with weird caster, but that doesn't seem to bother anyone.

What would you use between the carrier and the wishbone to take up axial and radial loads, and provide flex?

And how did you design the Watts linkage to accept more axle movement than stock vehicles?
This is going to be a bit long winded and maybe confusing but here goes.
The wishbone I made for the front my old 6x6 that worked very well used a metalastic bushing around 100 mm OD and around 50mm ID.(tip truck hendrickson beam bush or similar) sleeved the ID down to 1 inch with an aluminium slug and attached it to a crossmember with a 1 inch bolt. I figure for forced articulation,for the front end, if I mounted the same bush on a spindle in a similar way to a fullfloat hub on a spindle, I could run a universal jointed shaft(say a series 2 or 3 front axle shaft) through the spindle to the side gear in the forced artic diff carrier which would be bolted to the other side of the crossmember and the other end of the universally jointed shaft would be attached to the wishbone . I am beginning to confuse myself now!
The same arrangement would need to be made for the rear end.
As you know if you jack up both wheels of an axle and put the E brake on you can rotate a wheel in one direction and the other wheel will rotate in the opposite direction. The same thing will happen with forced articulation. cross country if the front axle articulates in one direction the front wishbone driven universally jointed shaft will twist the rear wishbone universally jointed shaft, wishbone and rear axle assembly in the opposite direction, or level out the vehicle body.
The castor variation during vertical suspension travel of a wishbone is similar to the normal RangeRover front radius arms assuming length from axle to chassis bush is the same.
The vertical travel limits of a Watts Linkage is determined by the distance between all 3 holes on the central rocker. the larger the distance the more travel.

As Ben mentioned, the hybrid was engineered and certified and to do so it had to pass a handling and braking stability test.
Bill

This is going to be a bit long winded and maybe confusing but here goes.
The wishbone I made for the front my old 6x6 that worked very well used a metalastic bushing around 100 mm OD and around 50mm ID.(tip truck hendrickson beam bush or similar) sleeved the ID down to 1 inch with an aluminium slug and attached it to a crossmember with a 1 inch bolt. I figure for forced articulation,for the front end, if I mounted the same bush on a spindle in a similar way to a fullfloat hub on a spindle, I could run a universal jointed shaft(say a series 2 or 3 front axle shaft) through the spindle to the side gear in the forced artic diff carrier which would be bolted to the other side of the crossmember and the other end of the universally jointed shaft would be attached to the wishbone . I am beginning to confuse myself now!
The same arrangement would need to be made for the rear end.
As you know if you jack up both wheels of an axle and put the E brake on you can rotate a wheel in one direction and the other wheel will rotate in the opposite direction. The same thing will happen with forced articulation. cross country if the front axle articulates in one direction the front wishbone driven universally jointed shaft will twist the rear wishbone universally jointed shaft, wishbone and rear axle assembly in the opposite direction, or level out the vehicle body.
The castor variation during vertical suspension travel of a wishbone is similar to the normal RangeRover front radius arms assuming length from axle to chassis bush is the same.
The vertical travel limits of a Watts Linkage is determined by the distance between all 3 holes on the central rocker. the larger the distance the more travel.

As Ben mentioned, the hybrid was engineered and certified and to do so it had to pass a handling and braking stability test.
Bill

OK, I think I follow. Maybe. Assuming the rubber bush thing, the UJ would be between the "spindle" and the carrier? If so, how does the carrier handle the radial movement of the UJ/axle when articulating? Carrier mounted on rubber? Now, if you meant that the UJ would in fact be on the wishbone side of the bushing, that would solve the potential problem with the carrier, but wouldn't the relatively weak circlips in the UJ break if the UJ would need to take a significant radial load, as when braking hard?

How are torque tubes attached to Mogs, btw?

... but wouldn't the relatively weak circlips in the UJ break if the UJ would need to take a significant radial load, as when braking hard?
...
The UJ's only take the articulation torque (similar to a driveshaft with slip joint).

Accel/braking loads are taken from the wishbone to its chassis mounting bush. Think of a front wheel hub, where the vert and horiz loads are taken by the bearings on the spindle and the cv joint only sees the torque.

OK, I think I follow. Maybe. Assuming the rubber bush thing, the UJ would be between the "spindle" and the carrier? If so, how does the carrier handle the radial movement of the UJ/axle when articulating? Carrier mounted on rubber? Now, if you meant that the UJ would in fact be on the wishbone side of the bushing, that would solve the potential problem with the carrier, but wouldn't the relatively weak circlips in the UJ break if the UJ would need to take a significant radial load, as when braking hard?

How are torque tubes attached to Mogs, btw?

The UJ would be on the wishbone side of the bushing and the shaft would be free to slide in and out of the side gear slightly so it would only be subject to torsional loading, not axial or radial loads. Mind you an appropiate sized UJ would be capable of coping with all those loads. The rear axle driveshafts of many Jaguars for example also double as the suspensions upper control arms, so thay have to cope with radial, axial and torsional loads.

Aside from the Aussie Military U1700's there aren't too many Mogs over here, but I assume the torquetubes are attached to the transmission in a similar way that the swing axles on an early VolksWagen were attached to the transaxle, ie the torque tube has a male hemisphere welded to the end of it that fits into a housing and clamped in position by a female hemisphere.
Bill.

I've been thinking of a different method of forced articulation, using sway bars.

If each sway bar is in 2 halves and the halves are joined using a hydraulic cylinder between arms on each bar. Excuse the sketch below showing roughly what a sway bar would look like from above.
|____| |____|

Or like this (ignore ... used for padding):
|____ .. ____|
....... | |

The hyd cylinders for the front and rear bars would be connected together so that when the front axle articulates one way, it's cylinder would extend (or retact) and the displaced fluid would force the cylinder on the rear bar to extend (or retract). Then the bars would generate forces to balance the articulation diagonally.

I was assuming the rubber bushing was mounted longitudinally, but that doesn't make sense, I guess. Now assuming it is mounted transversely? I think a clarification here would spare you a few stupid questions...

Now, wouldn't you be able to simlify this by using two LR spindles and hubs, and mount the bushing to the hub, and just run short driveshafts into the sidegears, and no UJ's? Or am I still not getting it?

Pic's would be nice...

On a sidenote, unless these Jaguars have swing axles, the UJ's would only take axial and torsional loads, and nothing radially, I think?

I've been thinking of a different method of forced articulation, using sway bars.

If each sway bar is in 2 halves and the halves are joined using a hydraulic cylinder between arms on each bar. Excuse the sketch below showing roughly what a sway bar would look like from above.
|____| |____|

Or like this (ignore ... used for padding):
|____ .. ____|
....... | |

The hyd cylinders for the front and rear bars would be connected together so that when the front axle articulates one way, it's cylinder would extend (or retact) and the displaced fluid would force the cylinder on the rear bar to extend (or retract). Then the bars would generate forces to balance the articulation diagonally.

That looks quite plausible to me John, and probably more elegant and reliable than say diagonally connected hydraulic struts at each wheel.
Bill.

I've been thinking of a different method of forced articulation, using sway bars.

If each sway bar is in 2 halves and the halves are joined using a hydraulic cylinder between arms on each bar. Excuse the sketch below showing roughly what a sway bar would look like from above.
|____| |____|

Or like this (ignore ... used for padding):
|____ .. ____|
....... | |

The hyd cylinders for the front and rear bars would be connected together so that when the front axle articulates one way, it's cylinder would extend (or retact) and the displaced fluid would force the cylinder on the rear bar to extend (or retract). Then the bars would generate forces to balance the articulation diagonally.

I think the sway bars would effectively become part of the suspension system then, but with no stabilizing effect on fore and aft trim. Similar to diagonally cross-linked air springs, to add yet another concept!

Hydraulic struts at each wheel crossed my mind, but that would negate the suspension of the truck. See comments above.

I was assuming the rubber bushing was mounted longitudinally, but that doesn't make sense, I guess. Now assuming it is mounted transversely? I think a clarification here would spare you a few stupid questions...

Now, wouldn't you be able to simlify this by using two LR spindles and hubs, and mount the bushing to the hub, and just run short driveshafts into the sidegears, and no UJ's? Or am I still not getting it?

Pic's would be nice...

On a sidenote, unless these Jaguars have swing axles, the UJ's would only take axial and torsional loads, and nothing radially, I think?

The axis of the bush would be longditudally, ie parallel with the chassis rails.
The landrover hubs and spindles in place of a compliant bushing would only permit suspension travel in roll, not vertical unless you "hinge" the wishbone off the hub, which may work, I will have to ponder that for a few minutes.

No, Jaguars don't have swing axles, and because the driveshaft is also the upper control arm it cannot have a slip joint either. The lower control arm and the driveshaft are more or less parallel, so within the cars suspension travel limits there is negligable camber change.
Bill.

Hydraulic struts at each wheel crossed my mind, but that would negate the suspension of the truck. See comments above.

Maybe a hydraulic accumulator/expansion chamber in the line would permit suspension.
Bill.

I guess it would, But it seems easier to go pneumatic.

The axis of the bush would be longditudally, ie parallel with the chassis rails.
The landrover hubs and spindles in place of a compliant bushing would only permit suspension travel in roll, not vertical unless you "hinge" the wishbone off the hub, which may work, I will have to ponder that for a few minutes.

No, Jaguars don't have swing axles, and because the driveshaft is also the upper control arm it cannot have a slip joint either. The lower control arm and the driveshaft are more or less parallel, so within the cars suspension travel limits there is negligable camber change.
Bill.

OK, so maye a stiff bushing would work as a robust hinge? I suspect an all metal contact system will be nice when new, but soon develop annoying play? Reminds me I need to tend to the rose joints on my Bilsteins. Click, click, click...

The bushings used for rear torque bars on C303's seem stiff enough.

OK, so maye a stiff bushing would work as a robust hinge? I suspect an all metal contact system will be nice when new, but soon develop annoying play? Reminds me I need to tend to the rose joints on my Bilsteins. Click, click, click...

The bushings used for rear torque bars on C303's seem stiff enough.

The hinge would only need to work in one plane, so even a Landy rear lower control arm bushing, or the 3.5 inch long series 3 front shackle chassis bushing would be ok.
Aside from the possible Bling factor ,I really cannot understand the use of Heim/rose joints on anything but high speed racing cars. I used rose joints on various suspension and dampers on my old 6x6 and they were forever wearing out. I ended up replacing most of them with greasable Landy tie rod ends, which are at least dust and mud resistant and self adjusting to a degree.
Bill.

Seems we're on the same page with this. I was thinking big bad bushing, because it will se a lot of load, and you want it to transfer the torsional load to the carrier system without too much flex. Series fronts sounds OK, but the coiler rear trailing arm bushings appear smallish.

Yes Christer I think the hub ,spindle,and hinged wishbone would be the most practical way of doing it. It eliminates the need for a two piece universally jointed shaft at the small expense of a couple of long lasting bushings at the hinged end. The hubs roller bearings may suffer brinelling damage as they wouldn't get to roll much, but these could be replaced with plain metal friction cones running in the stock bearing cups.These could even be preloaded to provide a degree of damping ?
I don't know if it could be made strong enough, but if it could, it may even eliminate the need for any lateral control arm such as Watts Linkage, Panhard rod etc?
Bill.

Now that we are dreaming, why not dream big?

If we consider two spindles/hubs and a carrier, what stops us from mounting this in an extremely cut down axle housing? I'm not sure there can be made room for such a contraption, but assuming it is possible, turning the pinion would tilt the entire vehicle for off camber driving.

Now that we are dreaming, why not dream big?

If we consider two spindles/hubs and a carrier, what stops us from mounting this in an extremely cut down axle housing? I'm not sure there can be made room for such a contraption, but assuming it is possible, turning the pinion would tilt the entire vehicle for off camber driving.

It's early and cold and my brain hasn't fully woken up, but space considerations aside it sounds interesting. Let's see. A strong, compact diff of about 3 :1 ratio. A double acting hydraulic ram acting on a lever attached to the pinion flange. Range of movement at lever around 120 degrees, half that either side, 60 degrees, divide by 3 = 20 degrees of leveling capability. Could probably get a wider range of movement at the lever, or use a higher ratio diff for more leveling capability. I don't know whether or not this would reduce the probability of roll over?That would depend on the height of the trucks roll centre ?

Or we could eliminate the ring and pinion and have a stronger hydraulic ram acting against a lever off the carrier itself.
Bill.

Roll center = wherever you mount the spindles, so as low as practically possible to have a positive effect?

Lever directly on the carrier is probably a good idea.

It's early and cold and my brain hasn't fully woken up, but space considerations aside it sounds interesting. Let's see. A strong, compact diff of about 3 :1 ratio. A double acting hydraulic ram acting on a lever attached to the pinion flange. Range of movement at lever around 120 degrees, half that either side, 60 degrees, divide by 3 = 20 degrees of leveling capability. Could probably get a wider range of movement at the lever, or use a higher ratio diff for more leveling capability. I don't know whether or not this would reduce the probability of roll over?That would depend on the height of the trucks roll centre ?

Or we could eliminate the ring and pinion and have a stronger hydraulic ram acting against a lever off the carrier itself.
Bill.

Roll center = wherever you mount the spindles, so as low as practically possible to have a positive effect?

Lever directly on the carrier is probably a good idea.

If the roll centre is indeed at the spindle then they should be mounted as high as practical to get it close to the centre of gravity. If the spindles were mounted low and you tilt the body on a side hill, the C.O.G would move down the slope, causing instability. Like yourself I am making this up as I type, so I may be wrong.


edit. I think you are correct and I am wrong. If the roll centre was well below the COG then the hydraulic levelling should have a more positive effect.Just don't push the control lever the wrong way. It's without the levelling system that it would be more desirable to keep the COG and Roll Centre close together.

Bill.

If the roll centre is indeed at the spindle then they should be mounted as high as practical to get it close to the centre of gravity. If the spindles were mounted low and you tilt the body on a side hill, the C.O.G would move down the slope, causing instability. Like yourself I am making this up as I type, so I may be wrong.


edit. I think you are correct and I am wrong. If the roll centre was well below the COG then the hydraulic levelling should have a more positive effect.Just don't push the control lever the wrong way. It's without the levelling system that it would be more desirable to keep the COG and Roll Centre close together.

Bill.

I must admit I'm new to the concept of roll center, and I haven't read a definition.

But bottom line is, if you're able to tilt the truck vs the axles, you want to move the COG uphill in an off camber situation.

I wonder if it is possible to design a chassis where roll center is at or above COG with people in the truck. If yes, forced articulation would be rendered redundant. :D

I must admit I'm new to the concept of roll center, and I haven't read a definition.

But bottom line is, if you're able to tilt the truck vs the axles, you want to move the COG uphill in an off camber situation.

I wonder if it is possible to design a chassis where roll center is at or above COG with people in the truck. If yes, forced articulation would be rendered redundant. :D

It can and has been done, but apparently if you build the roll centre too high it is not as stable as one may think.An extreme example I can think of would be a childs swing. If you put a wheel on each corner and replaced the chains with bars and placed the swing on a sideslope with a person on the seat, what happens ? The roll centre is at the very top of the swing. the COG represented by the person on the swing who is several feet or about 1 metre below swings down the sideslope.
If the slope is steep enough the person may swing past the lower wheels and the whole contraption falls over.
bill.

I must admit I'm new to the concept of roll center, and I haven't read a definition.

But bottom line is, if you're able to tilt the truck vs the axles, you want to move the COG uphill in an off camber situation.

I wonder if it is possible to design a chassis where roll center is at or above COG with people in the truck. If yes, forced articulation would be rendered redundant. :D
During articulation, the axle swings (relative to the chassis) about a roll axis. There are 2 points that do not move relative to the chassis during articulation. The roll axis passes through both of these points.

Take the rear suspension of a rangie. One of these points is the ball joint on top of the diff (at the apex of the A-frame). The 2nd point is where the projected line along the lower trailing arms converge. Because the arms are parallel, this point is at infinity. Then the roll axis passes through the ball joint and is parallel to the trailing arms (looking from the side).

With a panhard, one of the points is where the panhard crosses the centerline.

The point where the roll axis crosses a vertical plane though the axle is the roll center for that axle.

A line that connects the front and rear roll centers, is the roll axis for the chassis. Body roll will be reduced if this roll axis is closer to the COG of the sprung weight. If it is too low, the body flops (resisted by roll stiffness of suspension) in off camber situations and moves the COG further down hill (less stable).

With a high roll axis, the axle and wheels shift well to the side of the vehicle centreline during articulation (or vise versa).

This is a disadvantage on a high speed climb. The axle can't articulate as well as they would at low speed because it is opposed by the inertia forces from accelerating the sprung masses from side to side.

John, how do you work out the roll centre of a leaf sprung vehicle?

I have noticed that come lightly sprung (leaf sprung) vehicles can have a fair bit of sideways axle movement (due to springs and bushes deforming) during travel, so would this mean that the roll-centre changes slightly as the vehicle articulates?

John, how do you work out the roll centre of a leaf sprung vehicle?

I have noticed that come lightly sprung (leaf sprung) vehicles can have a fair bit of sideways axle movement (due to springs and bushes deforming) during travel, so would this mean that the roll-centre changes slightly as the vehicle articulates?

I've pondered that too, general consensus is the roll-centre is axle height and yes they do migrate as a vehicle rolls for almost all suspension types. The 3 link top ball is the easiest one to visualize.

But the actual roll centre of a leaf sprung vehicle would change a lot depending on the loading, I suspect the springs are far from linear in compression and extension as well.

I've been hinting for a copy of Milliken for my birthday, it seems a bit obscure though.

I suspect the springs are far from linear in compression and extension as well.


If you have springs that are "single rate" springs (all leaves near the same thickness and all set to the same arch), then they should be linear (or very close to, over the range of normal road suspension travel. However with articulation offroad when you get to the point where the whole pack is trying to twist along its length, then some nonlinearities could occur. Dual-rate springs are of course non-linear.

John, how do you work out the roll centre of a leaf sprung vehicle?

I have noticed that come lightly sprung (leaf sprung) vehicles can have a fair bit of sideways axle movement (due to springs and bushes deforming) during travel, so would this mean that the roll-centre changes slightly as the vehicle articulates?
In the side view:

Draw a line connecting the front and rear spring eye/shackel pins at the respective chassis mounts.

Now draw a vertical line through center of the axle.

The roll center is the point where these two lines cross.

The roll center would not change significantly during articulation (for normal leaf sprung suspension) as the points for the 1st line drawn above, are fixed laterally.