Rail on rail spring sizing

[Ramdough]

All,

I am doing a rail on rail designed subframe and need advice on the spring selection.

I can easily estimate displacements, but I am not sure what if any rule of thumb applies to the spring constant? Is there a target spring force for a given camper weight or some other way to determine a good spring constant?

Thanks in advance!


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[Ramdough]

[mention]Joaquin Suave [/mention]

[mention]DiploStrat [/mention]

[mention]Joe917 [/mention]

[mention]IdaSHO [/mention]

Tagging in a few that may know….


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[javajoe79]

Every time this comes up the answer is something like, trial and error will get you the spring that works best. I'm using these https://www.mcmaster.com/96485K386/ So far they allow similar separation as no spring but keep the subframe from flopping around. I have minimal trail time with them though

 

[RoamIt]

I'll probably be doing the rail on rail/spring subframe as well. At Expo East, I met a guy who used this:
https://www.acxesspring.com/spring-calculator.html To choose springs.

I'm not there yet, but perhaps this may help.

 

[Ramdough]

Thanks guys for the comments…..

Maybe we can learn together.… so far, I heard one guy that guessed that he wanted the sum of the max spring forces to equal the weight of the box at full compression.

That to me sounds like just a guess as I see no reason to pick that number. You are not going to hang upside down. When you hit bumps, I could see wanting enough force to not open gaps too much.

To me, it sounds like you want to flex the truck with no springs and see what kind of deflection you get, then choose spring with more travel.

The force you want at that deflection is my concern.


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[RoamIt]

My plan was to measure how much flex there is in the frame by raising various wheels, to get an idea of how much travel I may be dealing with. This would help with sizing the the springs.
Next step would be getting a weight for the fully loaded habitat.

Then knowing these parameters, use a spring calculator, like the one I linked, to determine the spring rate.

But then again, I've never done this before, so this was the best plan I could come up with

 

[rruff]

I can easily estimate displacements, but I am not sure what if any rule of thumb applies to the spring constant? Is there a target spring force for a given camper weight or some other way to determine a good spring constant?

I haven't gone down this road, and I'm not planning on it, but... it's not going to be simple!

The spring system is designed to reduce twisting stress in the subframe+habitat (assuming these are hard mounted) and the main frame as well. So you want it to not be too high/stiff... but enough so that the subframe+habitat doesn't flop around. Every application will be different. Semi educated trial and error is probably the best approach.

 

[Ramdough]

Yeah….. that is the part I am trying to get close on the first try, then maybe adjust preload from there by tightening the nuts.

I do plan to have a control rod to prevent slewing, but I will also have some side plates that are there just in case the control rod fails.


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[Ramdough]

Found a good source (I think) for reasonably priced springs. Still need to guess what load rating to start with.

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