pivoting frames and mounting campers
- Thread starter lehel1
- Start date
4 point / diamond mounting on this DAF truck:
Front pivot:
Link
Middle fixed mount:
Link
Note that the middle mount is on some kind of rubber bushing, but there is a large bolt going through the middle. I guess this allows for the twist caused by the rotation of the frame rails about an axis perpendicular to the truck centerline. Kind of like the tubes on the middle mount for a Unicat.
It's worth mentioning that I didn't see any trucks with a completely rigid mounting system. The Steyr was the most rigid mounting of the trucks I saw.
Front pivot:
Link
Middle fixed mount:
Link
Note that the middle mount is on some kind of rubber bushing, but there is a large bolt going through the middle. I guess this allows for the twist caused by the rotation of the frame rails about an axis perpendicular to the truck centerline. Kind of like the tubes on the middle mount for a Unicat.
It's worth mentioning that I didn't see any trucks with a completely rigid mounting system. The Steyr was the most rigid mounting of the trucks I saw.
i was reading a genuine mitsubishi workshop manual today 1994 book, it shows the genuine factory dump truck has a rigid full box chassis and not the C channel the std models have.
I am about to start building the new chassis for my 1991 canter 4x4, i was checking the engine model today it has a 4D33 engine which is 4.2 litre with an aftermarket turbo, from what i can work out someone has changed the engine to this model.
I am about to start building the new chassis for my 1991 canter 4x4, i was checking the engine model today it has a 4D33 engine which is 4.2 litre with an aftermarket turbo, from what i can work out someone has changed the engine to this model.
whatcharterboat
Supporting Sponsor, Overland Certified OC0018
Hey Alan, is it a "Superior Turbos" kit. From Toowoomba. They did alot of those for us back in the days before the FG649's. Had their own cast exhaust manifolds which were great and they went even better with an intercooler. Really good performers. Super reliable. No electronics. Not too thirsty. Perfect for the heavier trucks and buses.
gait
Explorer
I'm in the process of fabricating what is basically a three point mounting on a Canter.
At one point in the design process I made a simple meccano model to allow some observation of what may happen and aid discussion with Engineer.
A few observations came out of it. One was a decision to locate the pivot on the longitudinal line that the chassis probably twists about. I believe that is somewhere between the top and bottom surfaces of the chassis rails, probably central between the rails - the geometric centre of the the chassis ladder.
Reason is that 100mm above that (if rigidly coupled to the centre of twist), with a chassis twist of 5 degrees, there is a lateral movement of about 9mm. 10 degrees and its 18mm. etc.
While that movement is taken up by the pivot it simply seems better to not introduce lateral movement in the camper body, relative to the chassis, due to the pivot, if possible. Particularly if placing the spare tyres (heavy) on the rear surface of the body which is already subject to all sorts of movement when on corrugations. Esoterically, possibly the difference between a single pendulum and a double pendulum. The latter can become chaotic at some frequencies.
A second observation was that fixed vertical mounts bolted to the side of the chassis rail web would also exhibit fore and aft movement 100mm above the chassis rail. Hence the need for some flexibility in those mounts.
A third observation was that pivoting at a front mount ensures that all of the twist at the rear fixed mounts is transmitted to the front of the camper body. If the pivot is at the rear there may be less apparent lateral movement of the camper body relative to the cab. It seemed that there would be less chassis twist at the longitudinal middle than at the chassis extremities, regardless of whether its caused by a front or rear wheel lifting.
The use of an angle gauge in a static test of twisting the chassis confirmed that the chassis rails remained straight, they just twisted relative to each other.
Fourth observation from the meccano, which surprised me, was how torque resistant the ladder could be with minimal cross bracing. This led to some conclusions about the depth of the sub-frame and the addition of some cross bracing to that.
An observation from driving alongside a loaded refuse truck was that traveling over a mild, short, dip at 60 km/h, the middle (along its length) of the chassis moved downwards about 150mm. The chassis bent longitudinally. It occured to me that a 4 point mount would need to accommodate some vertical movement in the mounts. How much depends on the longitudinal stiffness of the chassis. Also that the three point mounts had to accommodate that bending as well as twisting. I guess 4 point is inherently more rigid than 3 points - as noted from observations of chairs and 3-legged stools on uneven ground.
Another principle (which I guess is what I'm really describing) is in the decision to mount tanks under camper body attached to the subframe. This will lower the centre of gravity of the camper body and, being closer to the c-o-g than the roof and solar panels, the weight of the tanks will hopefully compensate for the high roof of the camper body. Something about moments of inertia from a class long ago.
All just little things but may be useful.
At one point in the design process I made a simple meccano model to allow some observation of what may happen and aid discussion with Engineer.
A few observations came out of it. One was a decision to locate the pivot on the longitudinal line that the chassis probably twists about. I believe that is somewhere between the top and bottom surfaces of the chassis rails, probably central between the rails - the geometric centre of the the chassis ladder.
Reason is that 100mm above that (if rigidly coupled to the centre of twist), with a chassis twist of 5 degrees, there is a lateral movement of about 9mm. 10 degrees and its 18mm. etc.
While that movement is taken up by the pivot it simply seems better to not introduce lateral movement in the camper body, relative to the chassis, due to the pivot, if possible. Particularly if placing the spare tyres (heavy) on the rear surface of the body which is already subject to all sorts of movement when on corrugations. Esoterically, possibly the difference between a single pendulum and a double pendulum. The latter can become chaotic at some frequencies.
A second observation was that fixed vertical mounts bolted to the side of the chassis rail web would also exhibit fore and aft movement 100mm above the chassis rail. Hence the need for some flexibility in those mounts.
A third observation was that pivoting at a front mount ensures that all of the twist at the rear fixed mounts is transmitted to the front of the camper body. If the pivot is at the rear there may be less apparent lateral movement of the camper body relative to the cab. It seemed that there would be less chassis twist at the longitudinal middle than at the chassis extremities, regardless of whether its caused by a front or rear wheel lifting.
The use of an angle gauge in a static test of twisting the chassis confirmed that the chassis rails remained straight, they just twisted relative to each other.
Fourth observation from the meccano, which surprised me, was how torque resistant the ladder could be with minimal cross bracing. This led to some conclusions about the depth of the sub-frame and the addition of some cross bracing to that.
An observation from driving alongside a loaded refuse truck was that traveling over a mild, short, dip at 60 km/h, the middle (along its length) of the chassis moved downwards about 150mm. The chassis bent longitudinally. It occured to me that a 4 point mount would need to accommodate some vertical movement in the mounts. How much depends on the longitudinal stiffness of the chassis. Also that the three point mounts had to accommodate that bending as well as twisting. I guess 4 point is inherently more rigid than 3 points - as noted from observations of chairs and 3-legged stools on uneven ground.
Another principle (which I guess is what I'm really describing) is in the decision to mount tanks under camper body attached to the subframe. This will lower the centre of gravity of the camper body and, being closer to the c-o-g than the roof and solar panels, the weight of the tanks will hopefully compensate for the high roof of the camper body. Something about moments of inertia from a class long ago.
All just little things but may be useful.
gait,
Not sure if you saw this video, I had it posted in my camper build thread:
http://iandraz.com/post/175161730/
Might be helpful.
Personally, I don't feel the lateral motion due to twisting is significant enough to negatively affect the camper. The truck frame itself isn't 100% rigid laterally, so it will likely deflect some laterally as it twists. And the pivots should be on rubber bushings which give some more flex. The 9mm you calculated at 5 degrees isn't very much, and factoring these in it may be even less.
Regarding the 3 point design - I think the Hackney and Szulc truck problems mentioned in the forum seem to indicate the 3 point design does not sufficiently distribute the load, which leads to frame problems unless the frame is heavily reinforced (e.g. AATREC-FG). Although the 4 point (diamond) design does not 100% perfectly isolate the camper from the frame torsionally, the frame is not 100% rigid (otherwise it wouldn't be flexing!). So it really is more optimizing the interaction between the camper and the frame to reduce stresses, than completely isolating the box. In my opinion the 4 point design is a good compromise which both distributes the weight and frees the camper box from excessive deflection forces.
Not sure if you saw this video, I had it posted in my camper build thread:
http://iandraz.com/post/175161730/
Might be helpful.
Personally, I don't feel the lateral motion due to twisting is significant enough to negatively affect the camper. The truck frame itself isn't 100% rigid laterally, so it will likely deflect some laterally as it twists. And the pivots should be on rubber bushings which give some more flex. The 9mm you calculated at 5 degrees isn't very much, and factoring these in it may be even less.
Regarding the 3 point design - I think the Hackney and Szulc truck problems mentioned in the forum seem to indicate the 3 point design does not sufficiently distribute the load, which leads to frame problems unless the frame is heavily reinforced (e.g. AATREC-FG). Although the 4 point (diamond) design does not 100% perfectly isolate the camper from the frame torsionally, the frame is not 100% rigid (otherwise it wouldn't be flexing!). So it really is more optimizing the interaction between the camper and the frame to reduce stresses, than completely isolating the box. In my opinion the 4 point design is a good compromise which both distributes the weight and frees the camper box from excessive deflection forces.
dzzz
™
Looking at box attachment for the U500, I see a "disc spring" is at the front. Further back is the same attachment without the spring. This seems consistent with the advice here for those wanting a simpler attachment: Fixed at the back, with some 'give' near the cab. At least for the U500.
Mercedes hasn't engineered a pivot mount for the U500.
It's this approach or their four point quick release system that are official.
Mercedes hasn't engineered a pivot mount for the U500.
It's this approach or their four point quick release system that are official.
ntsqd
Heretic Car Camper
Commonly known as "Belleville Spring Washers" here in the States. For large deflections they're stacked big to big and little to little. For high spring force they're stacked to 'nest'. Can mix and match how they are stacked to achieve the desired results.
http://www.mcmaster.com/#disc-springs/=51ajau
http://www.mcmaster.com/#disc-springs/=51ajau
Commonly known as "Belleville Spring Washers" here in the States. For large deflections they're stacked big to big and little to little. For high spring force they're stacked to 'nest'. Can mix and match how they are stacked to achieve the desired results.
http://www.mcmaster.com/#disc-springs/=51ajau
Hmmm, wonder if I could use those on the existing U-bolts that hold the service body to the frame on my FG. There's probably enough room on the existing U-bolts for a short stack of those washers whereas the wouldn't be enough length for a regular spring.
dzzz
™
Should help keep the nut tight at least.
Looks like the mercedes implementation has one washer reversed, allowing just a little give. Even with that small possible movement they added a guide than is cut off on the right edge of the photo.