Torsion-Free Sub-Frame
- Thread starter Bajaroad
- Start date
dhackney
Expedition Leader
Has anyone thought of/tried using adjustable shocks on the sub-frame? You could tighten it up for use on smoother maintained roads, then soften them up for use on unmaintained surfaces. This could help control body roll on more heavily used roads where traveling with traffic would mean higher safety needs, while still providing off-road flexibility with a simple adjustment. I'm sure someone could be creative enough to run a remote adjuster inside the cab.
The 50/50 shocks for our 3 point pivot frame are not adjustable from the cab, but you can alter their characteristics by varying the charge pressure. They made a significant difference in our system. Recommended.
ntsqd
Heretic Car Camper
BajaRoad is using the air springs to do this. Shocks really aren't designed to support loads, only to damp motion. To support moving loads a spring of some sort is needed.Has anyone thought of/tried using adjustable shocks on the sub-frame? You could tighten it up for use on smoother maintained roads, then soften them up for use on unmaintained surfaces. This could help control body roll on more heavily used roads where traveling with traffic would mean higher safety needs, while still providing off-road flexibility with a simple adjustment. I'm sure someone could be creative enough to run a remote adjuster inside the cab.
Hi all
Very interesting thread!
I have driven Unimog 416, and know how much they flex!
I have another Q though:
We (my girlfriend and I) have just bought a Volvo TGB 1314, six-wheel drive army truck. We are scrapping the original ambulance-box and are going to build a camper-box for it in Nida-core.
The frame should be stiff, so no 3-point sub-frame should be necesary. This is supported by the fact that Volvo made a van version which were bolted directly to the frame, and I mean directly!
There's no rubber insulation of any kind between the frame and the cab!!!
I know this must be a great source for cab noise, vibrations ect. All other framed vehicles I have had and worked on have some sort of rubber mounts (I have, amongst other vehicles, a Toyota HJ60)
Our plan is to join the cab and the camper rigidly. I know this is un-conventional, but the original volvo van is in one piece, as mentioned above, so it should be doo-able.
My question is:
What bodymounts would give us the best insulation between the frame and the cab/camper?
I hope I'm not high-jacking, I think it relates perfectly to the discussion on airbags, motormounts etc.
Very interesting thread!
I have driven Unimog 416, and know how much they flex!
I have another Q though:
We (my girlfriend and I) have just bought a Volvo TGB 1314, six-wheel drive army truck. We are scrapping the original ambulance-box and are going to build a camper-box for it in Nida-core.
The frame should be stiff, so no 3-point sub-frame should be necesary. This is supported by the fact that Volvo made a van version which were bolted directly to the frame, and I mean directly!
There's no rubber insulation of any kind between the frame and the cab!!!
I know this must be a great source for cab noise, vibrations ect. All other framed vehicles I have had and worked on have some sort of rubber mounts (I have, amongst other vehicles, a Toyota HJ60)
Our plan is to join the cab and the camper rigidly. I know this is un-conventional, but the original volvo van is in one piece, as mentioned above, so it should be doo-able.
My question is:
What bodymounts would give us the best insulation between the frame and the cab/camper?
I hope I'm not high-jacking, I think it relates perfectly to the discussion on airbags, motormounts etc.
dzzz
™
I'm reading this older threading deciding on a camper subframe.
Looking at the video above what I see is the camper moving from momentum in addition to frame movement. What I don't like about the Unicat 3/4 point is that the camper is always "busy", even when the surface change is well withing the range of the suspension.
The racing guys talk about a flexible frame having unpredictable loading and unloading. With the Unicat subframe I see the camper storing energy which affects the downforce on the wheels in unpredictable ways.
Our aussie members who build for tour operators probably couldn't use a suspension like Unicat due to the large movement of the cabin.
So, am I crazy?
ntsqd
Heretic Car Camper
Road racers started out not liking a flexible chassis because it is an un-tunable variable. In their world if the foundation isn't stiff then nothing that you do with springs, dampers, sway bars, or tire pressures makes much sense. Down-force considerations just elevated the threshold for what was considered "stiff."
Any time that you have a mass able to move you will have inertia. If you can control the rate that it can change how it moves, then it isn't a bad thing. I recall reading somewhere that if the natural frequency of the suspension is tuned to 3 Hz (?) that it will duplicate a human's normal motion when walking. That is essentially undetectable and is comfortable. Witness the slow frequency 'bounce' that older Cadillac's displayed after a bump. Un-nerving maybe to a performance driver, but not uncomfortable.
Any time that you have a mass able to move you will have inertia. If you can control the rate that it can change how it moves, then it isn't a bad thing. I recall reading somewhere that if the natural frequency of the suspension is tuned to 3 Hz (?) that it will duplicate a human's normal motion when walking. That is essentially undetectable and is comfortable. Witness the slow frequency 'bounce' that older Cadillac's displayed after a bump. Un-nerving maybe to a performance driver, but not uncomfortable.
dzzz
™
The way I'm looking at it is that a more dampened subframe response would cause more suspension movement. I don't see the point of that much rocking motion when the suspension is not at a limit.
Take the other end or design - a fixed rear mount with spring-loaded Ubolts at the front. Isn't the point of the springs to only move to prevent damage to the frame or camper?
Regarding the FG with the camper on air bags: A comment by that owner was that the truck settled down a lot when shocks were added to the subframe airbags.
Take the other end or design - a fixed rear mount with spring-loaded Ubolts at the front. Isn't the point of the springs to only move to prevent damage to the frame or camper?
Regarding the FG with the camper on air bags: A comment by that owner was that the truck settled down a lot when shocks were added to the subframe airbags.
ntsqd
Heretic Car Camper
Need to separate out what drives the box's motion. Is it free to move (within limits) regardless of what the chassis is doing, or is it only free to move in a stress-relieving direction when the frame twists or bends? The latter is desirable, the former should be avoided.
The box will follow the angle or tilt of the frame at the point where the cross-chassis pair of mounts are placed. On the Unicat that Mr. Myers drove to the 2nd Annual Central Coast Meet-n-Greet that was forward of the rear axle, though I don't recall the distance.
The box will follow the angle or tilt of the frame at the point where the cross-chassis pair of mounts are placed. On the Unicat that Mr. Myers drove to the 2nd Annual Central Coast Meet-n-Greet that was forward of the rear axle, though I don't recall the distance.
dzzz
™
My concern is that without dampening the inertia of the movement of the camper is unnecessarily exaggerated. The first goal of the mount is that nothing breaks. If that goal can be met with one of several designs, what should the second goal be? Maybe cost, or maybe the comfort and control of the vehicle.
I'm liking the way Rob Gray did his truck because those mounts apparently have more dampening and also have vibration control. The 4 point (or 2 x 3 pt) rely on the undesigned dampening characteristics of the subframe and floor.
The term "torsion free" is the problem, perhaps. The term "torsion management" removes the implication of perfection.
I'm liking the way Rob Gray did his truck because those mounts apparently have more dampening and also have vibration control. The 4 point (or 2 x 3 pt) rely on the undesigned dampening characteristics of the subframe and floor.
The term "torsion free" is the problem, perhaps. The term "torsion management" removes the implication of perfection.
ntsqd
Heretic Car Camper
If the box isn't free to move unless the twist in the frame drives it to move, then up to the point that the box's inertia twists the frame, that excludes any box inertia induced relative motion between the box and the frame. Spring loaded mounts can't do this, but the pivot type mounts do.
If the box's inertia is high enough to twist the frame then the mounting method doesn't matter, it will twist the frame once the travel limit is hit.
With the triangle or diamond mounting arrangement, the two mounts that are across the chassis centerline from each other are tasked with controlling side to side inertial movement (box rotation about an axis parallel to the chassis centerline). If properly designed they will only allow rotational motion about an axis that is perpendicular to the chassis centerline. Any rotational motion about a axis parallel to the chassis centerline should only happen at the aft and/or fore mounts. Any chassis axis rotational motion of the box is that directly of the frame at the point of the transverse mounts.
If the box's inertia is high enough to twist the frame then the mounting method doesn't matter, it will twist the frame once the travel limit is hit.
With the triangle or diamond mounting arrangement, the two mounts that are across the chassis centerline from each other are tasked with controlling side to side inertial movement (box rotation about an axis parallel to the chassis centerline). If properly designed they will only allow rotational motion about an axis that is perpendicular to the chassis centerline. Any rotational motion about a axis parallel to the chassis centerline should only happen at the aft and/or fore mounts. Any chassis axis rotational motion of the box is that directly of the frame at the point of the transverse mounts.
I might be splitting hairs, but I don't think the "box" moves at all. It's rigidly attached to the frame either somewhere in the center of the box like unimog or at one end like Rob Gray and Carl.
so the box can't move relative to some part of the frame.
The frame is twisting and the Cab is moving relative to the box.
And the frame twists way before the suspension limits are reached because frame twisting is part of the susupension system and helps keep the wheels in contact with the ground.
Twist is a function of any large, heavy piece of material whether it's wood, steel or concrete. Look at pictures, video, etc of great lakes barges, ocean going ships and bridges. You can't make a ship rigid enough that it won't twist when opposite ends are supported by different phase waves. It has to twist or it will break.
so the box can't move relative to some part of the frame.
The frame is twisting and the Cab is moving relative to the box.
And the frame twists way before the suspension limits are reached because frame twisting is part of the susupension system and helps keep the wheels in contact with the ground.
Twist is a function of any large, heavy piece of material whether it's wood, steel or concrete. Look at pictures, video, etc of great lakes barges, ocean going ships and bridges. You can't make a ship rigid enough that it won't twist when opposite ends are supported by different phase waves. It has to twist or it will break.
dzzz
™
You mean doesn't move relative to the frame. That's what ntsqd is saying too. But the frame doesn't have a single state of torque. Looking at videos of other trucks with other companies campers. The bodies are not as active as unicat.
Does a car perform better with springs and shocks or just springs?
What's going on in a moving truck is a lot more that one torque on the frame. A theoretical completely ridged box/frame combination would load the suspension more. Does this truck perform better or worse that the 4 point version?
I found it interesting to compare the Oka video posted recently with the Unicat videos. The Oka appears to have a totally rigid mounting:
My thought is that for fast rally-type driving over rough terrain, the rigid mounting may be better, to reduce the possibility of harmonic oscillations building up and causing damage.
But for slower driving, the flexible mounting may be better, since it improves traction (more articulation for better weight distribution) and reduces the stresses at the mounting points and in the suspension.
Off road I usually drive pretty slow, especially with all the gear I would be carrying in my camper. So I like the flexible mounting.
For those really concerned about dynamic motions, one idea is a hybrid design - a diamond or 3 point design using shocks at the pivot mounts. This would behave like a rigid mounting in the dynamic case, but a flexible mounting in the static case.
My thought is that for fast rally-type driving over rough terrain, the rigid mounting may be better, to reduce the possibility of harmonic oscillations building up and causing damage.
But for slower driving, the flexible mounting may be better, since it improves traction (more articulation for better weight distribution) and reduces the stresses at the mounting points and in the suspension.
Off road I usually drive pretty slow, especially with all the gear I would be carrying in my camper. So I like the flexible mounting.
For those really concerned about dynamic motions, one idea is a hybrid design - a diamond or 3 point design using shocks at the pivot mounts. This would behave like a rigid mounting in the dynamic case, but a flexible mounting in the static case.
ntsqd
Heretic Car Camper
The frame does have one state of torque at any given moment. That torque produces a displacement that varies along the length of the frame, but is a direct function of the torque.
No doubt there, but are the dampers there for the sake of the structure and it's contents, or for the occupants?
I'll not argue for or against adding dampers, just ask what the goal(s) is/are and if they're worth that added complexity. Don't assume that the intended flex points are without internal damping, it is there as friction. Whether it is significant to the situation is another story.
I found it interesting to compare the Oka video posted recently with the Unicat videos. The Oka appears to have a totally rigid mounting:
My thought is that for fast rally-type driving over rough terrain, the rigid mounting may be better, to reduce the possibility of harmonic oscillations building up and causing damage.
But for slower driving, the flexible mounting may be better, since it improves traction (more articulation for better weight distribution) and reduces the stresses at the mounting points and in the suspension.
Off road I usually drive pretty slow, especially with all the gear I would be carrying in my camper. So I like the flexible mounting.
For those really concerned about dynamic motions, one idea is a hybrid design - a diamond or 3 point design using shocks at the pivot mounts. This would behave like a rigid mounting in the dynamic case, but a flexible mounting in the static case.
The OKA has a rigid chassis.