Spring Mounted Flatbed?'s

[RES]

I know the solid vs. spring vs. 3 point debate has been pretty well discussed.

I have a 1998 dodge 2500 long bed that I am wanting to put an aluminum flatbed on. The flatbed will be mounted with a
2011 Northern Lite that I will be modifying/improving (water damaged, so making it more what I want) There will be boxes mounted under the
flatbed for storage and boxes built onto the camper where the cutouts for a typical bed are. I am guessing I will be around 2200 - 2500 lbs for
the camper loaded on the flatbed.

On to the question. I have been leaning toward wanting to do a spring mount for the flatbed to eliminate some stress on the bed and camper.
I was thinking of fabricating mounts that would be welded to the flatbed and then bolt to the existing 6 mounting points that the stock bed fits on. One question is regarding whether to have spring mounts at all six locations or whether to hard mount the front or back two? My initial thought was to hard mount the front two and have
springs at the other four. I have been reading of people doing the opposite and solid mounting the back and spring mounting the front. I am not
an engineer, but it seems like the back of the frame would be more prone to movement. Anyone care to elaborate or explain? Another question is if the mounting to these 6 locations, do I need contact at the frame rails for support or would the 6 mounts be enough? I understand I need to have bracing at the mounts to stop side to side movement and forward to back. I would also be interested in any suggestions for springs to use based on the above mentioned weight?

Can anyone share an example of a 2500 or 3500 with a spring mount? I have asked around my region quite a bit but haven't found anyone with the confidence to help with this. I have some welders and fabricators that can help, but I will need to decide on the design. Any other suggestions or things to take into consideration? It's a used flatbed I have already purchased, so if pictures of the underside of the flatbed or the truck mounts would help I can take them and post.

Once I get this part figured out I will start a build thread on the project. i have really appreciated everyone's sharing and knowledge on the site and hope to give a little back on the camper build. I have rebuilt a few campers at this point, but the spring mount is new territory.

Thanks for any help!

 

[Tire_Marx]

I am not an engineer

I am a mechanical engineering technologist, so I can hopefully shed some light on the matter.

One question is regarding whether to have spring mounts at all six locations or whether to hard mount the front or back two? My initial thought was to hard mount the front two and have
springs at the other four.

Spring mounting is a great idea, although it may not be required based on many things, including expected usage of the rig, frame stiffness from factory (old long beds are not known to be overly stiff, but I cant comment on a dodge of that era) and suspension (and tire) setup. I would hard mount at the back if you did do a spring mount system. See below for my why.

I have been reading of people doing the opposite and solid mounting the back and spring mounting the front. I am not
an engineer, but it seems like the back of the frame would be more prone to movement.

Ok, so the back of the frame may look like it will move more, but as a system, it will actually be the middle section that will move more. The reason? Bending. Specifically torsional bending.
I can easily confuse you on the topic, but that wont help you. The easiest way to visualize this is to take something that is somewhat flexible. A debit card will do, or an envelope. Picture the card as your trucks frame. Put two fingers on each edge for where the front axle is, and two at the back. (You can use the corners of the card) Now hold one end steady, and twist the other. This is what happens to a frame as the suspension articulates.

The card still has (well, will roughly still have) a straight line along where the axles are, both front and back. But notice the middle has twist/flex in it. This is an idea as to how a truck frame flexes.
Now we get to why I am suggesting you hard mount the back. At the rear of the truck, you are not likely to see frame twist, meaning you are more likely to have a good connection that wont cause grief.
Hard mounting all 6 points could cause issues by not allowing the frame to flex, which could possibly lead to cracking something. But possibly not.

I have some welders and fabricators that can help, but I will need to decide on the design. Any other suggestions or things to take into consideration? It's a used flatbed I have already purchased, so if pictures of the underside of the flatbed or the truck mounts would help I can take them and post.

Was the flatbed designed around this truck or a different truck?
I would also contemplate using bushings (think truck body mount bushings, typically made of polyurethane) for mounting. This is what pickup trucks use from factory.

Hope this helps, or at least helps you think of a new series of questions to start fleshing out your design options.

 

[RES]

Tire Marx

Thanks, that was a helpful explanation. I can now see how there would be more stress on the front points. It seems like the back wouldn't be as stressed as long as they were somewhat near the axle.

I did discuss the option of doing urethane body mounts with one fabricator that does some very high end old power wagons. I just wasn't sure there was enough movement. Might still consider this.

The truck/camper is most likely going to get a basic Carli suspension, just for better performance and as minimal a lift as I can. We do a lot of forest service and fire roads in Idaho, Wyoming, and Montana, as well as off the beaten path in the southwest deserts. Nothing to extreme, but also like to get away from the typical paths. What the future holds for travel who knows, but it could be much longer trips. This will be used mostly as a camper (wait overland rig!), with some lumber, construction, firewood hauling at times.

The flatbed was not designed for this specific truck, it is the size I wanted, as well as the right price and figured I would be making custom mounts anyway. I will try and grab some pics and maybe make a quick sketchup mock-up of what I initially had in mind. Not looking for anyone to commit to telling me if it is right or wrong, but would be interested in opinions and feedback.

I would still be interested in anyone sharing a few pictures of any done on a 2500/250 or 3500/350. I have seen a number of ones done on larger trucks, but think they are overkill for this size.

Thanks

 

[damon1272]

The doges of that era mount the bed directly to the frame. The frame is boxed until partially after the cab so flexible from the middle on back as tire max demonstrated. Two ways to build a vehicle flexible or solid. Both work but in-between does not, it end up tearing everything apart.

Since what you are looking at is not all that rigid I personally would look to bushing mount so there is some flexibility across the bed as an easy way to get some flex but not too much. 3 mount and a spring setup may be better but may not be worth the effort over the long haul. Irrespective of how you mount I would recommend mounting another 1.5" back from the cab than the factory bed to give you more room for movement and less chance for the bed to contact the frame.

 

[javajoe79]

This is a well documented subject on this forum. I would just do the standard solid rear mount with sprung mounts forward of that. Just downsize a bit for your application maybe?

My setup will be two subframes actually. One subframe for the camper to mount to in the back, mounted solid in the back with sprung mounts forward. One subframe in the front with the fuel, water and batteries hanging from it as well as a large storage box on top, Solid forward mount and sprung rear mount.

 

[javajoe79]

Here is my forward subframe.

 

[gringoinloscabos]

This is a well documented subject on this forum. I would just do the standard solid rear mount with sprung mounts forward of that. Just downsize a bit for your application maybe?

My setup will be two subframes actually. One subframe for the camper to mount to in the back, mounted solid in the back with sprung mounts forward. One subframe in the front with the fuel, water and batteries hanging from it as well as a large storage box on top, Solid forward mount and sprung rear mount.

I hope you do not mind me troubling you for some assistance... I want to make sure my mounting system is best on my vehicle... it is a flatbed mounted to an F-550. A camper weighing 5,500 lbs sits atop the flatbed. 1.) Do you think this system is best for that? 2.) do you have a great photo of the front spring mounts in this particular application? and 3.) What material do you think is best as a buffer between the subframe and frame?

 

[gringoinloscabos]

Here is my forward subframe.

Is there a missing photo or link here?

 

[billiebob]

Spring mount sure, definitely anchor at the front. Every semi with a sleeper today does this, "Air Ride Cabs". I would only consider air ride springs. Look into modern semi truck designs. They typically run a million miles in 5 years.

 

[1stDeuce]

Here's the thing with the spring mount idea... For the bed to live, it will need to be stiff enough to resist any flexing due to the truck frame pulling away from one corner, plus whatever force the spring exerts on it trying to pull it back down to the frame. You may find that achieving this amount of stiffness in a flatbed is quite difficult. Additionally, you will also find that the bed will "slap" the frame on rougher roads. Neither of these is ideal IMO.

My $0.02 as a mechanical engineer, and as someone who built a flatbed for my 02 GMC and then bolted it solid to the bed mounts with no issues, is that the frames of newer trucks don't flex enough to be a concern in the first place. Your Dodge had a pretty stiff frame from the factory, so I believe it fits into this category.

Even the factory bed contributes some stiffness to the whole system, so if you eliminate that, and 3 point mount a new bed with a higher CG and a LOT more weight, you're actually adding quite a bit of stress to the factory frame. If you still think that is a good plan, I would strongly advise you to anchor it at the front of the bed, not the rear. The ends of the frame are moving in opposite twist directions as the frame flexes. The point where the frame is most stable is the MIDDLE, which sees the average twist, or pretty much zero twisting. Mounting to the rear of a flexy frame will have the bed and camper tossing from side to side as the rear of the frame moves up and down with flex, and this is NOT good for lots of reasons. (This mistake was made early on by Earthroamer, and it cost them a lot of money...)

I do believe that for isolation, using poly mounts between the frame and bed is a good idea. I rode in my truck camper down the highway once, and it was eye opening just how rough a ride it is compared to riding in the isolated cab of the truck. This was with the camper sitting on a rubber bed mat, which I also recommend. No wonder campers come apart over time! I'd use ~1/2" thick isolation pads to mount the flatbed right at the OE box mount locations. Just be sure to place pads both above, and below the frame, and to use large washers below, and locking nuts on the bolts.

Good luck!!

 

[javajoe79]

Is there a missing photo or link here?

Yes it's a facebook link so they change occasionally. Here it is again.

Also this thread is a great resource.....

pivoting frames and mounting campers

hey doug here you go ; ) look forward to seeing you at the expo cheers lehel and laura

www.expeditionportal.com

 

[Motafinga]

I hope you do not mind me troubling you for some assistance... I want to make sure my mounting system is best on my vehicle... it is a flatbed mounted to an F-550. A camper weighing 5,500 lbs sits atop the flatbed. 1.) Do you think this system is best for that? 2.) do you have a great photo of the front spring mounts in this particular application? and 3.) What material do you think is best as a buffer between the subframe and frame?

I just designed and completed a spring / rail on rail subframe for my 97 F250 which runs between a Tool bed and Alaskan camper. There is a build thread in my sig line and the subframe is near the end. Without a doubt a trucks frame flexes the most behind the cab (middle point) not the very rear btw. I hard mounted at the rear and put the springs forward of that. I used HDPE plastic as "wear-strips" between the sub and truck frames. I also use "guide plates" to keep the subframe centered side to side, those are also lined with HDPE and there's no metal on metal contact between the articulating points. We just did a 3k mile shakedown trip through all sorts of rough terrain and it's working great, no frame slap and the trucks handling just fine. Our rig is a pop up fwiw

 

[richxd87]

@Motafinga what springs/spring rate did you use?

 

[Motafinga]

@Motafinga what springs/spring rate did you use?

I use color coded Die springs from McMaster Carr, I used the red https://www.mcmaster.com/9584k76(medium/heavy) rate in the rear and blue https://www.mcmaster.com/9573k73 (lighter) towards the front. I am going experiment with a gold https://www.mcmaster.com/9588k448 (stiffer) in place of the red on my next run. From the research I've done you want more stiffness towards the rear where it is rigidly mounted and progressively lighter towards the front.
The links give specs on rates, dimensions etc...

 

[warrengreen]

I just designed and completed a spring / rail on rail subframe for my 97 F250 which runs between a Tool bed and Alaskan camper. There is a build thread in my sig line and the subframe is near the end. Without a doubt a trucks frame flexes the most behind the cab (middle point) not the very rear btw. I hard mounted at the rear and put the springs forward of that. I used HDPE plastic as "wear-strips" between the sub and truck frames. I also use "guide plates" to keep the subframe centered side to side, those are also lined with HDPE and there's no metal on metal contact between the articulating points. We just did a 3k mile shakedown trip through all sorts of rough terrain and it's working great, no frame slap and the trucks handling just fine. Our rig is a pop up fwiw

Thanks so much for sharing this!