Camper mounting help

[bee]

I plan on using a spring system like this for my Dodge 2500. I think 1 to 2 inches of travel will be sufficient for my build. Does anyone know what strength springs I should be using? Does it go by payload? I will be around 2000lbs

These mounts will not be locating the the camper front to back or side to side, just controlling vertical motion. Anyone have some insight into this setup.

 

[Alastair D(Aus)]

Bee,
I am planning to use the same method to attach a sub-frame to my Isuzu NPS 300. The subframe will hold storage bozes and tanks and also provide mounting rails for a custom built slide on that will be held down by sprung turnbuckles. This way I will be distributing the weight over the chassis frame and providing 2 levels at which flex can be accomodated. I will also have 1/2" rubber (conveyor belt) between metal at both levels to even out the junction and also reduce noise.

I consider the springs are there to hold the camper & frame down against minor bumping and general road movement, not push against the flex. If in the rest position each spring mount (4 each side in my case) exerts 100kg force then that is a total of 800kg holding down what I estimate will be a 1200 to 1400 Kg camper. Simple physics suggests that if I go over a broad hump in the road than on the downside the camper will be left 'up in the air' and will then come down with a thump. So I should increase the spring load so each has 200kg and hence a total of 1600kg which should restrain the camper for 'slow' rises and falls. This level of tension should not spoil the flexibility when going over uneven ground.

Front to rear and side movement are taken care of by simple brackets and wear pads etc.

The above is my logic and I am happy if you or anyone else can point out a flaw.

cheers
alastair

 

[Scoutman]

I think I saw where someone used valve springs for what you're talking about. I forget what thread that was in though...

 

[snowcat]

Use valve springs

Hi All, I've used V-8 valve springs for mounting several beds over the years. I've mounted several large water tanks and many flatbeds using valve springs with a grade-8 bolt up the middle. Never had a failure. I build off-road wildland fire trucks and water tenders for use in the American West- I used valve springs to help mount a twelve foot bed with 850 gallon water tank on my 2004 U500 with no problems. My 2500 gallon water tender is also mounted for flex using this method.

 

[SChandler]

Snowcat, any advice on how to choose a spring rate for the load or do you always use the same spring rate on all of the bodies, regardless of load?

Thanks.

 

[bee]

damn I thought I had notifications turned on, didn't realize anyone had responded to the post. I do not think valve springs will offer enough travel, though I have heard of people using them. Mcmaster.com sells compression die springs in various lengths and spring rates. My gut feeling is that very little clamping force is needed. Pretty sure I remember reading that fast guns or something like that use 300 in/lb springs, but they also have to help locate the camper.

 

[Scoutman]

I found the thread I was thinking of....
http://www.expeditionportal.com/forum/threads/9502-Alaskan-Camper-Build-Up/page60

 

[westyss]

damn I thought I had notifications turned on, didn't realize anyone had responded to the post. I do not think valve springs will offer enough travel, though I have heard of people using them. Mcmaster.com sells compression die springs in various lengths and spring rates. My gut feeling is that very little clamping force is needed. Pretty sure I remember reading that fast guns or something like that use 300 in/lb springs, but they also have to help locate the camper.

Lee Springs have a huge selection of springs, before selecting one you have to have an idea of the travel needed and how much resistance, it can be a bit daunting to start but some main points to look for are the length of the spring at rest and at full compress, that will give you the travel available, after that you can choose wire diameter, overall spring diameter, pounds of resistance etc etc...........if you cant find the spring you need at lee Spring it doesnt exist.

 

[bee]

Alastair. If I am following what you are saying then you could hang your truck upside down and the springs wouldn't allow the camper to separate from the truck frame. Isn't that excessive?

Scoutman hadn't seen that thread before thanks for the link. Gonna take awhile to sift through all those pages.

Westyss once I figure out what spring rate I need lees should be helpful, thanks.

Honestly I drive around all the time with stuff just loose in the bed of my truck. Haven't had anything fly out over a bump yet. I am guessing that it has to do with the difference in weight between the back of your truck and the camper. So If the back of your truck weighs more then the camper box, then theoretically no springs would be needed just a method of locating it to the truck.

So my theory is that if the back of my dodge weighs 2500lbs and the camper weighs 3000lbs then I would only need a total of 500 lbs in spring force to keep the camper and truck together over any size bump. Any thoughts? Its been a while since physics class.

 

[Scoutman]

Start around page 60.

 

[Alastair D(Aus)]

Alastair. If I am following what you are saying then you could hang your truck upside down and the springs wouldn't allow the camper to separate from the truck frame. Isn't that excessive?

Scoutman hadn't seen that thread before thanks for the link. Gonna take awhile to sift through all those pages.

Westyss once I figure out what spring rate I need lees should be helpful, thanks.

Honestly I drive around all the time with stuff just loose in the bed of my truck. Haven't had anything fly out over a bump yet. I am guessing that it has to do with the difference in weight between the back of your truck and the camper. So If the back of your truck weighs more then the camper box, then theoretically no springs would be needed just a method of locating it to the truck.

So my theory is that if the back of my dodge weighs 2500lbs and the camper weighs 3000lbs then I would only need a total of 500 lbs in spring force to keep the camper and truck together over any size bump. Any thoughts? Its been a while since physics class.

Bee,
I am not planning any aerobatics just trying to work out where between two extremes I should load the mounts. Anyway I have made the decision and ordered the springs. I am having 5 mounts each side with a springs that are 85mm long and have 32mm compression at ~15kg/mm rating. In the rest position they will be compressed by 2mm leaving 30mm travel for the flexing whilst on the move, before they hit the limits. So this equates to 10 x 2 x 15 = 300kg (660lb) force restraining the camper initially over the full length - about half the weight of what I expect the camper to be. This feels about right and I have plenty of travel left if I feel they need more tension. If I find there is not enough movement of the subframe when I test flex the truck when finished then I will just swap the springs for lighter units. Fingers crossed.

Thanks for your reply and it looks like we are thinking in the same ballpark.

Incidently when I visited the spring works to talk about and order my springs the guy showed me some springs that they regularly supply to a company that build water tankers. They are 100mm (4") and have a rating of 30kg/mm. He said they had been making them the same for years so it seemed they were happy. Mine are half the rating and which given the relative loads may still be too stiff. Physics & guesswork !

cheers

 

[bee]

I asked xp camper how they deal with chassis flex and basically they do not isolate the camper. The camper accepts the stresses without failure. I mention this because his campers are sandwich composite construction making them relatively sensitive to frame flex. What I am taking from this is that properly constructed campers being mounted on 3/4 or 1 ton chassis do not need any special considerations.

On another note I noticed that the frame on my 95 dodge 3/4 ton is thicker then on my friends 1/2 ton, making it stiffer. Now this may be a reason to use a heavier duty truck then what you really need.
On the other hand heavier duty trucks have heavy duty leave springs that will transfer more forces to the frame possibly negating most benefits.
Well I am just thinking out loud hear. I can't really draw any conclusions except that frame flex may vary greatly from one truck to another.

There should be more talk on camper mounting that is not related to 3 or 4 point systems on unimogs. Those systems have advantages but also major disadvantages. A spring mount system is cheaper, lighter, lower profile, produces less stress concentrations on the frame. Yet I haven't seen one thread where someone has tried to optimize the system. 3 point mounts on the other hand have a million threads.
It would be nice to come up with a system of measuring the effectiveness of a spring mount system so that someone could tailor there spring rates to their actual camper. For example How do I know if my springs are to soft? What should I look for?

 

[Alastair D(Aus)]

spring mounts & ratings

Bee,
As a part of the workup for my design I read as much as I could about mountings and found it quite confusing with the large variation of approaches. Eventually I came to the following conclusions
1. for campers mounted on the back of ~1 ton vehicles, the camper is quite rigid and the amount of vehicle flex is relatively small hence the majority of people quite successfully get away with using turnbuckles with or without some spring in them. It is only those who intentionally or by accident push the limit who have problems.
2. those using big trucks with complex chassis systems like the Unimog need to use 3-4 point systems and they need to be carefully designed to match the vehicle dynamics.
3. those of us using small trucks with a GVM ~6000Kg, eg Mitsu/Fuso Canter and Isuzu NPS300 are probably best to go for the spring approach as the majority of flex takes place in the region just behind the cab where in the earlier version both vehicles have a curved chassis section. The Fuso has 4.5mm chassis thickness whereas the Isuzu uses 6mm. The later model Isuzu has straightened the chassis which simplifies things somewhat. Having talked with a number of people with experience with these vehicles there is little concensus on spring rates and some professional builders use surprisingly light mounts with apparently no problem. I have recently talked with a professional who has spent the last 30 years or so building truck frames and bodies and he suggested that I not bother with spring mounts at the rear, use fish plates instead and have spring mounts from half way forwards to cope with chassis flex. He said this is what he has used for years on trucks of the size of my NPS300 and had no problems. He also felt my springs were too heavy and I should back off to half the rating.

I have talked with another person who contributes to this forum (Canter4x4) and he is building up a camper on a Fuso chassis. I have gained a number of ideas from him and although we have differences our basic approach is similar. When I have completed the subframe I plan to do some tests to try and guage how my mounts perform. I will also ask Canter4x4 to document something similar so we can try and learn along the way. Obviously it will only be once the full camper is completed that the real loads will tell the story.

cheers

 

[bee]

I look forward to seeing how your build goes. We should probably start a thread dedicated to spring mount systems where we explain our approach, and experiences. I bet once information starts accumulating on this mount system it will become more common.

 

[graynomad]

It's been a while, any conclusions here?