Am I interpreting correctly that it has a fixed rear mount and a pivoting front mount just after the step down section of the frame?
pivoting frames and mounting campers
- Thread starter lehel1
- Start date
Am I interpreting correctly that it has a fixed rear mount and a pivoting front mount just after the step down section of the frame?
dhackney
Expedition Leader
Kerry,
I've got some photos of Carl & Mary's rig around, including details of the pivot, but it will be a while before I can dig them out and get them posted. We stopped at their place and looked at it in 2006.
Note that Whatcharterboat has indicated that if possible, it is best to pivot in the front rather than the rear.
Please also note that it is very important to spread the weight along the frame rails, even with a relatively lightweight camper box, lest you suffer frame bending and/or breakage, as more than one FG has recently. A pivot system that supports the weight solely on two widely separated longitudinal points can cause frame failure.
Doug
There are several aspects of Carl Hunter's subframe that are not optimal design.
I'd be cautious about using a single 1/2 inch bolt to act as the pivot at the front. I think some sort of greaseable "cylinder inside a cylinder" pivot would be stronger and more durable.
The rear mount perches sideways on the truck frame, concentrating the stress. It would be better to have a wider contact point.
The rear mount is held in place with a couple of bolts (two per side, I think). I'd prefer more bolts.
There are no "fishmouth" stress relievers in the subframe where it contacts the truck chassis. I'm not sure how you could add stress relievers to a subframe that sits across the truck chassis, rather than parallel to it.
The rear mount uses two gusseted steel "C" channel beams to support the camper. I think a square section tube would be stronger. I think they chose the "C" channel because it gives easy access to the bolts that extend down through the beams and truck chassis.
The bottom line is that, despite its theoretical shortcomings, this subframe worked sucessfully for two long overland trips (Asia and South America). The camper and truck chassis sustained no damage despite many miles of travel over primitive roads.
Chip Haven
I'd be cautious about using a single 1/2 inch bolt to act as the pivot at the front. I think some sort of greaseable "cylinder inside a cylinder" pivot would be stronger and more durable.
The rear mount perches sideways on the truck frame, concentrating the stress. It would be better to have a wider contact point.
The rear mount is held in place with a couple of bolts (two per side, I think). I'd prefer more bolts.
There are no "fishmouth" stress relievers in the subframe where it contacts the truck chassis. I'm not sure how you could add stress relievers to a subframe that sits across the truck chassis, rather than parallel to it.
The rear mount uses two gusseted steel "C" channel beams to support the camper. I think a square section tube would be stronger. I think they chose the "C" channel because it gives easy access to the bolts that extend down through the beams and truck chassis.
The bottom line is that, despite its theoretical shortcomings, this subframe worked sucessfully for two long overland trips (Asia and South America). The camper and truck chassis sustained no damage despite many miles of travel over primitive roads.
Chip Haven
ntsqd
Heretic Car Camper
If I understand correctly this is a frequent design in older tanker trucks. The sub-frame sits on an anti-chafing isolator of some sort (usually wood) for the full length of the frame. It is then clamped in place at several to many points along each frame rail with U-Bolts. In between the nut on each leg of the U-Bolt and the anchor plate is a very heavy rate coil spring. The nut is tightened to put pre-load on the spring, but not to the point that the coils are fully collapsed.
The coil springs allow the rails to twist while still maintaining the clamp load and do not fully transfer the torsional stress to the sub-frame. In a tanker that might be bad....
I've seen this system on dump trucks. I'm curious as to what the military version looks like. I'm probably going to do something like this with my service body.
jesusgatos
Explorer
Yeah, I can take some pictures for you.
That's almost exactly right. On the military trucks, they don't use U-bolts though. They use standard hardware (nuts & bolts) on the outside of the framerails. I'll post some pictures.
Thanks. I suspected it might not be u-bolts. I think the more we can see alternative systems, the more our collective creative juices can flow.
M.Bas
Adventurer
The ambulance box on my Volvo truck is bolted straigth onto the frame.
Not sure if that is possible because there is not much flex in the frame (don't even know how much flex the frame has) or the box is so rigid it eliminates all the flex.
All I know is that there are 4 bolt and 2 U-bolts and the box has a lot of metal framework on the inside.
Note too self: I should make time to take some pictures of it some day.
Not sure if that is possible because there is not much flex in the frame (don't even know how much flex the frame has) or the box is so rigid it eliminates all the flex.
All I know is that there are 4 bolt and 2 U-bolts and the box has a lot of metal framework on the inside.
Note too self: I should make time to take some pictures of it some day.
dzzz
™
By lateral do you mean parallel to the frame? On a traditional truck frame like the F700 the most neutral area is behind the cab. Not so with a cab over. This is an argument for the diamond shape being good for cabover. Both of these designs place the parallel pivots near the neutral point. With the driver getting to ride in the worst area.
I have an engineer designing a box with no metal frame above the pivot. Just composite. Can holy water be purchased?
By lateral do you mean parallel to the frame? On a traditional truck frame like the F700 the most neutral area is behind the cab. Not so with a cab over. This is an argument for the diamond shape being good for cabover. Both of these designs place the parallel pivots near the neutral point. With the driver getting to ride in the worst area.
I have an engineer designing a box with no metal frame above the pivot. Just composite. Can holy water be purchased?
I am running the pivot behind the cab "across" the frame to prevent the camper box from pivoting right behind the cab. That section of the frame is quite rigid because of all of the cab mounts, the bare frame twists pretty good from that point all the way to the rear, I figure the pivots should let the frame flex the way it wants to along those nice long rails rather than engineer a stress point into the middle of the frame. I'll post some pics of the build soon, I'll take some shots of the way the frame twists with some help from the forklift.
Byrdseye
Observer
My M2 currently has a very heavy steel flatbed mounted. The bed was built crazy heavy out of 1/4", 3/8", and 1/2" steel because the previous owner wanted as much weight as possible for a better empty ride. The gooseneck ball is also mounted directly to the bed and it wouldn't suprise me if the whole thing weighs 3000 lbs. Built this heavy, the frame under the bed can't flex at all. I'm sure that a living quarter box mounted on a frame like this would survive just fine...........But, not only does it add a ridiculous amount of weight, all of the frame flex occurs right between the bed and the cab. Traveling off road you can see the bed and the cab move independently as though the bed was on a three point mount. I worry that the concentrated stress could cause frame fatigue failure down the road. I've posted in the past about the virtues of a torsionaly stiff structure for suspension tuning etc. but I think that for this application I'm going to stick with what has worked for others and build a 4 point flexible mount with the stress spread along the frame rails as much as possible.
jesusgatos
Explorer
I had that problem with our Tacoma after we built the bedcage. I was never planning on leaving it like that, but the flex/stress where the bedcage ended (under the cab) was crazy. It went away after I finished building the rollcage though. I don't think it would be too difficult to build a box that would be strong enough to keep the frame from flexing (especially if you used a few strategically-placed structural interior walls), but I don't know what you'd do about that concentrated stress where the box ended. If I was to build something from scratch, or on bare framerails, I think I'd ditch the cab altogether and make one large motorhome-type enclosure. I don't see how that would be any more complicated than mating a box to a cab. Just look at all the work that's gone into the ecoroamer.
dzzz
™
Here's a very good article, but you need to register:
http://trailer-bodybuilders.com/mag/trucks_tips_truck_frames/index1.html
Supporting my earlier point:
"He said as wheelbases get longer, deflection becomes important. The rule of thumb is that with wheelbases under 160", design for strength; and with wheelbases over 160", design for deflection. For the same load, a 10% increase in wheelbase requires a 10% increase in RBM for strength and a 33% increase in moment of inertia for the same deflection."
other points from the article:
"Toner said the purpose of body mounts is to attach the body to the truck frame without doing more harm than good and retain the body in all horizontal directions.
“Harm can be done by concentrating stress and not allowing the frame to flex,” he said. “Rear mounts could be shear plates or some other rigid configuration that prevents movement. Front mounts should be flexible or placed to avoid stress concentrations and high moments.”
He said mounts can be combined to take advantage of the best features of more than one type. Spacer strips used between the body and truck frame perform multiple functions: cushioning member, sacrificial wear member, and stress-spreading member. Rigid mounts should be used at the rear and flexible mounts at the front. The front mount should not be at the front of the body.
Toner said there are three basic types of body mounts: flexible, rigid, and combination. Rigid mounts should be used at the rear, and flexible mounts at the front.
He said that even though U-bolt body mountings are popular, they are among the least effective mounting systems.
“Are they bad by themselves? No,” he said. “But they don't tend to stay tight.”
He said if they are used, proper frame spacers must be used. Some of the problems are loosening, not preventing forward movement of the body, and frame damage. He said frame flanges should never be notched for a body mount.
“The body should not be rigid at the front,” he said. “It is better to be mounted solidly at the rear and float at the front. Avoid high moment areas for the mounting brackets. Use existing frame holes when possible.”
http://trailer-bodybuilders.com/mag/trucks_tips_truck_frames/index1.html
Supporting my earlier point:
"He said as wheelbases get longer, deflection becomes important. The rule of thumb is that with wheelbases under 160", design for strength; and with wheelbases over 160", design for deflection. For the same load, a 10% increase in wheelbase requires a 10% increase in RBM for strength and a 33% increase in moment of inertia for the same deflection."
other points from the article:
"Toner said the purpose of body mounts is to attach the body to the truck frame without doing more harm than good and retain the body in all horizontal directions.
“Harm can be done by concentrating stress and not allowing the frame to flex,” he said. “Rear mounts could be shear plates or some other rigid configuration that prevents movement. Front mounts should be flexible or placed to avoid stress concentrations and high moments.”
He said mounts can be combined to take advantage of the best features of more than one type. Spacer strips used between the body and truck frame perform multiple functions: cushioning member, sacrificial wear member, and stress-spreading member. Rigid mounts should be used at the rear and flexible mounts at the front. The front mount should not be at the front of the body.
Toner said there are three basic types of body mounts: flexible, rigid, and combination. Rigid mounts should be used at the rear, and flexible mounts at the front.
He said that even though U-bolt body mountings are popular, they are among the least effective mounting systems.
“Are they bad by themselves? No,” he said. “But they don't tend to stay tight.”
He said if they are used, proper frame spacers must be used. Some of the problems are loosening, not preventing forward movement of the body, and frame damage. He said frame flanges should never be notched for a body mount.
“The body should not be rigid at the front,” he said. “It is better to be mounted solidly at the rear and float at the front. Avoid high moment areas for the mounting brackets. Use existing frame holes when possible.”
jesusgatos
Explorer
I should have mentioned that the wear-plates between the box and the frame on my M109 are tapered towards the front and rear (like leafsprings). I can see how that would be an important feature with the type of mounting system that they used on those vehicle.