Using upper stock frame rails for subframe

[Kevin Means]

Hi. Kevin here and this is my first post. I've been reading LOTS of posts over the last few months and found them to be very helpful, but now it's time to start asking questions. We purchased a 1998 Stewart & Stevenson M1083 a few months ago for the purpose of turning it into an off-road RV. We've been RVers for decades, but we'd really like to go places that stock RVs can't go.

I understand that we'll have to build a subframe to avoid breaking the habitat, because we're planning to build a habitat with "foam" core walls. (We haven't decided exactly which type yet) The M1083 has upper and lower frame rails that are bolted together with bolts and Huck bolts. I removed the 20 Huck bolts in the upper rails that attached the factory bed to those rails (boy, that was fun.) I was initially planning to put the subframe on top of the upper rails, but as I looked at it, I started wondering if I could just use the upper set of frame rails as the subframe. That would help keep the overall height down, which is something I'm concerned about.

We're planning on installing a pass-through, so all of the spare tire equipment and air intake stuff will be relocated. While studying subframe designs, I've found that the majority of builders rigidly attach the rear of the subframe to the rear of the chassis' frame rails, and then they use springs to allow the forward part of the subframe to move independently of the frame rails. It seems to me that that could cause the pass-through boot to move around a lot. Is there a reason that we couldn't reverse the mounts? That is - rigidly attach the forward part of the subframe to the frame rails and spring-mount the rails toward the rear?

Kev

 

[Ronmar]

Hi. Kevin here and this is my first post. I've been reading LOTS of posts over the last few months and found them to be very helpful, but now it's time to start asking questions. We purchased a 1998 Stewart & Stevenson M1083 a few months ago for the purpose of turning it into an off-road RV. We've been RVers for decades, but we'd really like to go places that stock RVs can't go.

I understand that we'll have to build a subframe to avoid breaking the habitat, because we're planning to build a habitat with "foam" core walls. (We haven't decided exactly which type yet) The M1083 has upper and lower frame rails that are bolted together with bolts and Huck bolts. I removed the 20 Huck bolts in the upper rails that attached the factory bed to those rails (boy, that was fun.) I was initially planning to put the subframe on top of the upper rails, but as I looked at it, I started wondering if I could just use the upper set of frame rails as the subframe. That would help keep the overall height down, which is something I'm concerned about.

We're planning on installing a pass-through, so all of the spare tire equipment and air intake stuff will be relocated. While studying subframe designs, I've found that the majority of builders rigidly attach the rear of the subframe to the rear of the chassis' frame rails, and then they use springs to allow the forward part of the subframe to move independently of the frame rails. It seems to me that that could cause the pass-through boot to move around a lot. Is there a reason that we couldn't reverse the mounts? That is - rigidly attach the forward part of the subframe to the frame rails and spring-mount the rails toward the rear?

Kev

I see no reason not to attach at the other end(front) and float the rear. either way is basically a 3 point attachment, with the front two points at each forward subframe end and the 3rd point shifts from rail to rail depending upon which direction the frame twists as one rail will push up on the sub frame as the other frame rail twists down and away.

I am in the process of doing exactly what you describe, turning the upper frame rail into the floor structure(floor structure built, will start on the walls as the weather improves). There are a few considerations that you must address and I don't have time right this minute to dive in, but I can describe my build considerations later.

 

[Keith Knight]

this how the built the 1079 model. Frame then another frame on top which puts that frame level with the transmission the habitat sub-frame on top. I’ll refer to them as bottom, middle and top frame. My fear is without the middle frame that it
would allow the bottom frame to flex accessibly potentially causing bottom frame damage.
Also note the top frame and middle frame have a 1/4” gap and all the weight is on the rubber in that caged area. Not sure why they mount the springs to the front and not the rear. I would assume after testing and research that it is the best choice. I’ve seen it all across the industry. “Tried and true”

 

[Keith Knight]

Oh and why you see bolts on the middle frame attaching to the bottom frame is because mine started as a M1078 then purchased an older M1079 and swapped the middle and top frame.
Which had all the mounting points.

 

[Ronmar]

I think the methodology of attaching at the rear of the frame is that the frame then acts like a spring allowing the whole box more overall movement instead of imparting more torsion thru the box or tank when heavily loaded.

but I see that as having drawbacks as well, as it stores energy which will enhance rebound and rocking instead of dampening it(they call them frame slappers for a reason). it will also add to frame flex as the mass of the box rocks back and forth...

With referencing the front of the box to the middle of the chassis or at the back end for that matter, with captured spring the box structure must be strong enough to withstand the forces that will be applied to it. IE: with the box bolted down to two front mount points, jack it up at either the left or right frame point at the opposite end of the box. The structure must be built to easily withstand this without too much deformation. That can be an issue with pure composite boxes. The panels themselves are fairly rigid, but the issues then becomes force concentration at the seams where the panels are bonded. these boxes don't like twist, so a low torsion subframe is usually recommended.

I am building an all steel rib and stressed skin box(like a ships hull), to better deal with these forces and also to try and maintain axle balance. With a bare cab and chassis, I need to add around 6000# to maintain a weight balance between front and rear axle... That will put me around 20K, which is still on the light side of what these trucks are capable of(4K less than a M1079 with 5000LB of payload).

If you look at the original Styer 12M trucks that these were based on, they don't use a secondary frame, just the main, very substantial, frame rail... here is an example.

 

[serpico760]

View attachment 941047
this how the built the 1079 model. Frame then another frame on top which puts that frame level with the transmission the habitat sub-frame on top. I’ll refer to them as bottom, middle and top frame. My fear is without the middle frame that it
would allow the bottom frame to flex accessibly potentially causing bottom frame damage.
Also note the top frame and middle frame have a 1/4” gap and all the weight is on the rubber in that caged area. Not sure why they mount the springs to the front and not the rear. I would assume after testing and research that it is the best choice. I’ve seen it all across the industry. “Tried and true”

To that I would say the m1088 only has the lower frame not an upper frame. Think the only reason for the second frame for the versions with the bed or a box is to put it above the transmission and also to reduce any flex that would make its way to the bed. If I were going to do this again I would try to start with an M1088 semi then there's no bed to take off no second frame to deal with, and you get a bigger gas tank I think it's like 78 gallons.

 

[Ronmar]

To that I would say the m1088 only has the lower frame not an upper frame. Think the only reason for the second frame for the versions with the bed or a box is to put it above the transmission and also to reduce any flex that would make its way to the bed. If I were going to do this again I would try to start with an M1088 semi then there's no bed to take off no second frame to deal with, and you get a bigger gas tank I think it's like 78 gallons.

it may also have been a commonality thing. They made the M1078 pickup, M1079 box truck, and the M1080 cab and chassis, with the M1080 being easily adapted to create a special purpose vehicle(rocket launcher, gun or AA missle vehicle) with the built in load distributing upper frame.

These things are Grossly overbuilt!

 

[serpico760]

it may also have been a commonality thing. They made the M1078 pickup, M1079 box truck, and the M1080 cab and chassis, with the M1080 being easily adapted to create a special purpose vehicle(rocket launcher, gun or AA missle vehicle) with the built in load distributing upper frame.

These things are Grossly overbuilt!

Yep I believe you are correct. But the second frame section you can mount anything and it's above the transmission it's also above the wheels. With my m1083 doing a flex test in the desert the wheels never go above the top of the second frame. So anything you mount to the second frame can be flush with the top of it and the tires are very unlikely to rub it.

 

[B-Dog]

Before you make any decisions, I think it would be a good idea to remove the upper rails and then go drive it offroad to see how much the frame flexes. Your spring system will have to accommodate the movement if you want to limit the the flex transferred into the habitat. My hunch is that without the second rails, your frame is going to twist more than you want, like a Unimog. You could add plate the main rails to stiffen them up but like Serpico said above, at some point your tires are going to rub.

To that I would say the m1088 only has the lower frame not an upper frame.

FYI, That particular frame has additional plates on the inside and the outside plates are longer than the plates on the M1083.

 

[serpico760]

Nice image yeah I see that they supplemented the lack of a second frame rail with the plates. Even more reasons why the semi would be the best platform option for building a camper.

 

[Ronmar]

I think even without the upper frame, main frame twist is not going to be too bad. In the end frame twist is a factor of the weight applied, a habitat weight is probably going to be less than what the truck was built(over built) for. And even if it does twist, so what, that is why we use the captured springs between the chassis and habitat.

The habitat restraint springs do require some thought, and that also relates to how much torsion the habitat box design can withstand without buckling the walls or tearing apart the seams. Springs are simple proven, robust and inexpensive, but they have a few drawbacks in this type application.
1. They increase in force the more they are compressed. Now if you are trying to support something, this is a good thing, but if you are trying to allow something to move, but yet still restrain something AND limit force transfer, this is not a desirable trait. As the frame twist increases, so does the force applied to the habitat by the springs.
2. They have limited travel. Take the M1079 springs for example, as I have looked those up. ~1000#/inch, with ~1.7” of total travel, so 1700# of force at full travel?
3. In order to provide initial restraint, the springs have to be pre-loaded, further reducing their overall travel.
4. Springs store energy, and in this case it is in opposition to the frame twisting and storing energy. When the twist is released, these two stored energies tend to accelerate toward one another, back to the term “frame slapper”…

Thats why I am not going to use them. I am going to use a constant tension hydraulic restraint Instead. I can set an initial restraint tension similar to what a spring system will deliver, but I can have several inches of travel at that same tension without increasing the force applied from the frame twist to the habitat box. Because it is hydraulic, I can also set the force required to return as the frame untwists, to provide some rebound dampening…

 

[aw113sgte]

I don't see any issue using the stock frame rails for subframe as long as you build from the top of that rail. If you do, there is adequate clearance for you tires.
I am using a spring mount everywhere, no rigid mounting. It's been working great. I did try rigidly mounting the shipping container to the rails but way too much force was being transmitted (enough to bend the front ribs of the container).
Regardless of how you mount the box, you will have to have a compliant section for the pass though, the cab twists in relation to the frame rails. Also the cab air suspension causes more movement.

 

[aw113sgte]

This page goes into my methods a bit https://www.steelsoldiers.com/threads/from-fire-truck-to-mtv.216741/page-6