Stiffening An Expedition Camper Frame

[PathNotTaken]

Trying to get some opinion on stiffening the frame of an expedition camper, which I get the impression many on this forum think is a bad idea. For reference, this is for a relatively short wheelbase E350. The plan is to have the cab rigidly mounted to the habitat, with springs of varying rate rearward to allow for frame flex. With that said, the frame will obviously still transmit some load to the subframe. While I'm confident the subframe will handle this, I'm wondering what the downsides are of just directly reinforcing the frame.

The rear of the frame will already be reinforced by the beefy rear bumper that will hold the tow hitch. What is the downside of bolting another cross member, potentially more towards the front to relieve stress from the cab-habitat interface? My understanding is that this would be effectively no different than the stiffness added by the subframe, but stronger and better placed. Thoughts?

 

[geoffff]

I've also wondered about this. This thread talks about E350 frames, and flex: https://forum.expeditionportal.com/...posite-camper-box-on-4x4-e350-chassis.224393/

 

[PathNotTaken]

I've also wondered about this. This thread talks about E350 frames, and flex: https://forum.expeditionportal.com/...posite-camper-box-on-4x4-e350-chassis.224393/

Hey Geoff, thanks for the response. Yep, I’ve been all over that thread but a lot of the discussion wasn’t super helpful. However, seeing the 4in vertical displacement on the frame is concrete proof that resisting this flex is crucial. Per @Victorian recommendation and after looking at @CalgaryMcLean build in person and chatting about his design, I still think rigid front and spring rearward is the best play. At the bare minimum my rear will be reinforce via the hitch, the question is whether or not to add an additional brace.

 

[PathNotTaken]

Interesting enough I was reading the ford body builder guide today and came across the fact that the stock body mounts allow for 3/4in of movement, which is quite a lot for no spring.

 

[IdaSHO]

Altering/stiffening an otherwise flexy frame is almost always a bad idea.
If this is an open-C channel frame, much of its strength and ability to NOT break comes from its flex.

Short of tearing the entire vehicle down to bare frame and stiffening the entire frame, you are likely to cause more problems than anything.

 

[Victorian]

Interesting enough I was reading the ford body builder guide today and came across the fact that the stock body mounts allow for 3/4in of movement, which is quite a lot for no spring.

Keep in mind that Ford does not put campers in consideration in their guides. Commercial bodies can and will flex. That's not a big deal as long as you don't have an expensive interior or windows/doors that would be sensitive to twisting.

 

[rruff]

Trying to get some opinion on stiffening the frame of an expedition camper, which I get the impression many on this forum think is a bad idea.

I assume you are talking about your van chassis, not the subframe or the camper itself?

The issue is that we tend to put a lot of twisting forces on the rigs when offroad, and stiffening just the part that the camper mounts to will cause a stress concentration at the point where that stiffness ends. There might be a good way to transition gradually that would help, but it will still increase stress on the forward part of the frame that hasn't been stiffened. Maybe a worry, maybe not?

The rear of the frame will already be reinforced by the beefy rear bumper that will hold the tow hitch. What is the downside of bolting another cross member, potentially more towards the front to relieve stress from the cab-habitat interface? My understanding is that this would be effectively no different than the stiffness added by the subframe, but stronger and better placed. Thoughts?

Cross beams don't add very much stiffness, as the side rails are still able to rotate about the neutral axis. The subframe will usually be composed of fully boxed tubes, including the sides, and will be much stiffer in torsion than the chassis.

I asked someone with experience in adding flatbeds and campers to Tundras (which are also C channel aft of the engine). He said every one he's done or heard about has simply bolted the (usually stiff) flatbed to the frame. As far as I know, no frames have failed because of this... but it didn't seem ideal to me.

I used Energy Suspension generic poly mounts, with the 4 front ones widely spaced and the 2 rear ones on the centerline, so they act like a pivot. So far it works fine, but I've only had it on there for 6 weeks or so. I thought it might be floppy, but it isn't in the slightest. The only issue I've noticed is that there is a bit of fore-aft bob occasionally on pavement where there are regular undulations, but I don't know if the mounts are causing that.

 

[joeblack5]

Just showing some experience with flexing on an e350 chassis.

Copied from my unimog 404 conversion thread at expedition portal.

We used to have a 4*4 Quigley camper based on a 1994 E350 ambulance raised roof ..7.3 IDI. WVO.
After an engine failure due to a broken serpentine and overheating resulting in a head gasket failure we decided to go to anE350 schoolbus 7.3 powerstroke and transfer the 4*4 system.

The back was 4" low and we used 6" air springs to raise that up and improve the ride at the same time. The winch and bumper I had built years ago for the ambulance was also transferred to the bus.

The busbody is pretty stiff. There is not much frame flex,

Johan

 

[rruff]

The busbody is pretty stiff. There is not much frame flex

Ambulances are another vehicle that is usually built like a tank. Granted these vehicles don't see a lot of offroading, so not a good case to go by. There thousands of stiff flatbeds and service bodies that are hard-pointed to the frame, but they aren't offroading much either.

I asked a Tiger owner about their subframe, and it's stiff square steel welded to the top of the chassis. I get the feeling that if the transition is done well, this would not be an issue.

 

[Alloy]

Cross beams don't add very much stiffness, as the side rails are still able to rotate about the neutral axis.

Not really.....adding cross members towards the ends will reduce torsion. If that's no possible plates in the middle on the top&bottom between the rails is next. Which one is a balance between material availability, weight, cost, ease of fabrication and being serviceable.

 

[PathNotTaken]

Really appreciate all of the informative information here. I think bus bodies/ambulances are out of the equation as a resource since they're built very different and for much different use cases than a camper.

@rruff The energy suspension mounts you used, but with increasing spring travel rearward is what I'm leaning towards, however they don't fit the holes already in the frame which is separate problem. May just have to fab a larger collar.

 

[PathNotTaken]

Keep in mind that Ford does not put campers in consideration in their guides. Commercial bodies can and will flex. That's not a big deal as long as you don't have an expensive interior or windows/doors that would be sensitive to twisting.

Absolutely a good point. If anyone you've built a box for on the e350 wants to chat about their mounting design I'd love to

 

[PathNotTaken]

An E CAW (cutaway) is my current dream camper so I very interested in all this.

Any particular reason you're thinking springs not pivot like Calgary ?

It was my dream too! Until I found one for sale and impulsively bought it, so now I have to build the thing! I'll start a build thread soon.

To answer your question the reason for spring, theres quite a few:

1.) There is a lot of sentiment on the forum regarding the complexities of pivot vs spring mounted. I think it's easier to DIY a reliable spring system than it is pivot. Pivot I think is more likely to be engineered poorly and fail.

2.) The way I see it, frame flex is only a major concern in heavy articulation situations. I offroad my vehicles heavily to get to the places I want to go, but even then the majority of that time is spent on bad roads with lots of ruts/washboard, not fully articulated. A spring system with properly rated springs is really acting different only in this event of flex: most of the time in regular driving the subframe/frame is acting as ford intended using factory mounting positions, where as a pivot is putting concentrated load on a small section of the frame in a way unintended by the manufacture, all of the time.

3.) Mine will be a pop-top and be aggressively small. The exterior width will be no wider than the cab, length minimized, and height too. IMO the biggest limitation for where I go is physical size. A Unimog-type vehicle may be a beast "offroad" but "off-roading" anywhere I've ever been is traveling on bad roads, not just across baren land. A large vehicle simply doesn't fit and defeats the intended purpose. All this to say a pivot point comes with increase height of the vehicle.

4.) @SkiFreak has mentioned it a few times, but I agree that it's better to have less movement than too much. a Pivot really allows things to move as much as they want. This makes good on road driving characteristic harder to achieve, although the way everlanders did theirs is pretty neat.

Currently I am designing my subframe and have most everything figured out other than the mounts and springs I plan to use. Tonight I plan to jack tires till I get a wheel to lift and see how much frame flex I am getting as is. (mine is still 2wd not 4wd converted, although I would think this wouldn't matter much? My rear bumper will eventually reduce this some, as will the subframe.But I think this will be a good starting point. Maybe I can test it with and without the factory rear bumper cross member in place to see how many inches it reduces the flex.

 

[WanderingBison]

It was my dream too! Until I found one for sale and impulsively bought it, so now I have to build the thing! I'll start a build thread soon.

To answer your question the reason for spring, theres quite a few:

1.) There is a lot of sentiment on the forum regarding the complexities of pivot vs spring mounted. I think it's easier to DIY a reliable spring system than it is pivot. Pivot I think is more likely to be engineered poorly and fail.

2.) The way I see it, frame flex is only a major concern in heavy articulation situations. I offroad my vehicles heavily to get to the places I want to go, but even than the majority of that time is spent on bad roads with lots of ruts/washboard, not fully articulated. A spring system with properly rated springs is really acting different only in this event of flex: most of the time in regular driving the subframe/frame is acting as ford intended using factory mounting positions, where as a pivot is putting concentrated load on a small section of the frame in a way unintended by the manufacture

3.) Mine will be a pop-top and be aggressively small. The exterior width will be no wider than the cab, length minimized, and height too. IMO the biggest limitation for where I go is physical size. A Unimog-type vehicle may be a beast "offroad" but "off-roading" anywhere I've ever been is traveling on bad roads, not just across baren land. A large vehicle simply doesn't fit and defeats the intended purpose. All this to say a pivot point comes with increase height of the vehicle.

4.) @SkiFreak has mentioned it a few times, but I agree that it's better to have less movement than too much. a Pivot really allows things to move as much as they want, and puts stress on other.

Currently I am designing my subframe and have most everything figured out other than the mounts and springs I plan to use. Tonight I plan to jack tires till I get a wheel to lift and see how much frame flex I am getting as is. (mine is still 2wd not 4wd converted, although I would think this wouldn't matter much? My rear bumper will eventually reduce this some, as will the subframe.But I think this will be a good starting point. Maybe I can test it with and without the factory rear bumper cross member in place to see how many inches it reduces the flex.

Well, well - can’t wait to see your build thread … and build! I really like a lot of your design considerations


Sent from my iPad using Tapatalk

 

[rruff]

There is a lot of sentiment on the forum regarding the complexities of pivot vs spring mounted. I think it's easier to DIY a reliable spring system than it is pivot. Pivot I think is more likely to be engineered poorly and fail.

A spring system with properly rated springs is really acting different only in this event of flex: most of the time in regular driving the subframe/frame is acting as ford intended using factory mounting positions, where as a pivot is putting concentrated load on a small section of the frame in a way unintended by the manufacture

If you are referring to that massive "pivoting frames" thread, that made my head spin, too... People like to over complicate it seems.

You can probably use all the mounting points on your frame and still have a "pivoting system". My frame has 8 and I used them all. The rear ones just need to be on the centerline so they can tilt.

I think you will be able to make it lower with springs, but if you are planning to forego wheelweels, that will define your minimum height I think, and you wouldn't gain anything.