Frame flex and type and kind of spring?

[gait]

I had my springs made at Marsh Springs, Carole Park, Brisbane. They commented on how easily they could be stressed and permanently deformed.
http://www.marshsprings.com.au/

I went from 3 point to "6" when I rebuilt the sub-frame. Lighter. Easier. The pivot was a bit complex. Hard to access etc.

Important bit ..... the 6 mounts are on the last 2m of chassis. At the front of the box (4m) I retained the lighter springs between sub-frame and chassis so that there is movement. Supporting the cantilever from the 6 mounts is one of of thinking of it.

I occasionally here a "clunk" which my imagination says is at least one of 6 mounts reseating itself after the small movement provided for. A little movement over narrow chassis rails and short distance (2m) can provide quite a large angle. I have some polyurethane in the way.

 

[steve66]

ATW mounts are about $300 a pop, hence the quest to make my own. But does make it easy.(edit:that price was for the whole mount, never thought to ask them how much just for the springs)

Marsh springs was one that never replied to me, maybe I should have perservered!

Gait, do you have a source for the polyurethane please?

Living in a smaller/remoter location has meant my project is mostly 'built by google'.

cheers steve

 

[Flameffects]

Lots of choices!

I used McMaster Carr for my springs. The springs I used are called die springs. They are red in color...the color is a coding and is important. I went through three sets of springs to get it right. Below is what I used.

High-Strength Red Chrome-Silicon Steel Die Spring for 1.5" Hole Diameter, 4" Overall Length
I have three sets of four. Hope this helps.

 

[SkiFreak]

They are red in color...the color is a coding and is important.

Maybe I need to get my eyes tested, as those springs don't look red to me.

 

[trackadda]

Maybe I need to get my eyes tested, as those springs don't look red to me.

my guess is that blue was first attempt, every thing still looks nice and shiny. thank you every one for your replies, gives me a bit more to work on. i messaged atw re their springs but no reply

 

[trackadda]

I used McMaster Carr for my springs. The springs I used are called die springs. They are red in color...the color is a coding and is important. I went through three sets of springs to get it right. Below is what I used.
View attachment 400728
High-Strength Red Chrome-Silicon Steel Die Spring for 1.5" Hole Diameter, 4" Overall Length
I have three sets of four. Hope this helps.

https://www.mcmaster.com/#die-springs/=17nkbh0
Good list of spring sizes ect here, thanks flameffects

 

[1aquaholic]

Similar thought process as those above. I built the spring mount to capture side and fore/aft movement.

Sent from my SM-G935T using Tapatalk

 

[trackadda]

Hi Tracka,

Thanks for opening an old wound for me


I reckon I've stressed/pontificated more about the subframe mounts than any other aspect of my build. The ex aircraft engineer part of my brain says this is a critical aspect of the design. But once you read everything available and look at a few in the flesh it seems that anything goes.


I ruled out 3 (or 4) point systems long ago, all the pro builders in Oz for these trucks use spring mounts, I see Gait has just converted his from pivot to springs but not sure why. I had spoken with Skifreak but didn't get to Industrial Springs before they closed down.


There doesn't seem to be any consensus on spring rate, some just chuck in whatever fits, ATW use the same springs for all builds, some guys use old truck valve springs, some of the photos you see of home builds appear to use very light springs HOWEVER I've never yet seen a report of anyone breaking a spring so eventually figured it can't be much of an issue in real life.


Decided in the end I wanted springs that were 40mm O.D, 40mm compression, 20kg/mm, length about 150mm. Just sounded about right!


Now the fun bit. Last year I contacted 8 spring makers in OZ, out of the 6 that responded 2 said can't be done, one wasn't interested, everyone said that 20kg/mm was a very big load on such a spring. Would not be a 'happy' spring they said. There was about 300% difference in prices as well, Hmmmm, had me worried again. put it back in the too hard tray.


Last month I had to decide as it's holding me up. Ended up going with Boynes Springs from Perth, they should arrive this week, 8 custom springs for $407 delivered. Their design sheet gives me about 30 mm compression before the spring enters the highly stressed zone. I consoled myself by saying I can always swap them out if need be, but doubt I will need too.


Am planning on 4 mounts each side but may end up with one as solid, depends on what my engineer says when he sees it. ATW use all springs, some guys have the rear most mount as solid, SLR have their solid mount above the rear axle, so again anything goes it seems!


Ahh, I feel better now Good Luck!


Cheers Steve

sorry to start your nightmare again steve, did you get your springs from BOYNES and if you did how do they look, you may have even fitted them by now. do they give a producy number to the custom made springs so that i could cheat off you if they look ok. my box will be a pop top 4.6mtr x 2.1 mtr with styro max panels so i cant see the all up weight being much over 1000kg sitting on the springs. at this stage looking at mounting water tanks and batteries off chassis?

 

[steve66]

Hi Trackadda,

Yep, they turned up last Thurs, takes a while for the Aust Post camel to walk this far , they appear well made and exact size. They painted them black for free as well.

If you PM me your email I will send you the design sheet, has all of the load numbers etc and they can match up the quote number if you decide to use them. Bear in mind that my springs are only 40mm OD, most guys I've seen use 50mm, so means mine are more highly stressed but Boynes will be able to make any size you want. I wanted to limit my overall mount width to 50mm to save space. Skifreak sent me a pic of a slim mount off a RFS truck that I'm basing my mounts on.

I'm hanging all tanks, boxes and spare off the subframe. 1040 amps of Lithium batts plus inverters is another 200kg, I estimate I will be between 2000-2500kg depending on water/fuel load at the time. There doesn't appear to be any hard and fast numbers for working out the spring rate. I figured 6.4T overall was plenty (40mm x 20kg x 8 mounts). My Thought process was I need the load to stay in one plane and the chassis to flex under it. Lots of the structural strength for the camper is in the bond between the composite floor and the subframe so I didn't want a huge spring rate that would be trying to tear it apart. I got Boynes to do a design at 15kg/mm but that only gave me another couple of mm travel outside the highly stressed zone so stayed with the 20kg option. I'm prepared to make the rear pair solid mounts if need be.

Am using 16mm HT bolts which are rated for 13t each.

Cheers Steve

 

[glennm01]

Resurrecting this thread to hopefully get some insight on how to choose the right springs for my subframe. McMaster-Carr has tons of options, as someone indicated above:

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

As has been stated, I realize that there is no one-size-fits-all solution to this problem, so my question concerns how to actually go about determining the right spring specs? Hypothetically, let's say you've got a 5k pound camper box, four spring mounts on each frame rail, each with a single spring:

1. How much spring travel is recommended/typical for a Fuso camper?
2. Given #1, along with our hypothetical setup, any thoughts on what McMaster options above might be good?

 

[SkiFreak]

1. How much spring travel is recommended/typical for a Fuso camper?

How long is a piece of string?

I am no engineer, but I can tell you that just knowing the overall weight does not give you all of the answers.
That is because there is a significant difference between static weight and dynamic weight (liquids), as well as where that weight is located on the camper.
As an example... if you are going around a corner and hit a bump (or pothole), the fluids will move and create different forces.

To reflect here a little... the primary reason for implementing a kinematic mount system, whichever form that may take, is to reduce the stress on the chassis that twisting can cause.
To that end, anything that achieves this can only be seen as beneficial. That said, and I have raised this issue previously, that too much spring travel can be just as bad, if not worse, than no travel at all.

Over the years I have had lengthy discussions about this "maximum movement" with numerous people (that are smarter than me).
So, to answer your first question, I believe that a maximum separation of 30mm should be the design goal between the chassis and subframe.
Don't ask me how you calculate the correct spring rate to achieve this design goal, because as I have stated above, this calculation will be specific to your build and will be dictated by the types of weight and their position.

I have been working with steel in the general engineering field since being a teenager and my own approach to this problem was to take a "best guess" as a starting point.
My logic was that if I guessed incorrectly, it would not be that complicated to change the springs to get the desired result.
I should add... getting the spring rate right can only really be done at the completion of the build, because only then do you really know what forces you are dealing with.

 

[glennm01]

But this is a hypothetical scenario, so the camper is effectively done, and hypothetically weighs 5000 pounds. I agree with using a "best guess" approach as a starting point and working from there, so I guess I what I'm really asking is where/how to start the best guessing.

On that McMaster-Carr page, it looks like a little over an inch or so of compression at max load is about all one can get from springs in the length typically used for this sort of application (I'm assuming in the 4" ballpark), which sits well with your 30mm max separation number. How then does one determine what sort of max load and/or spring rate would be advisable for our hypothetical box?

 

[SkiFreak]

How then does one determine what sort of max load and/or spring rate would be advisable for our hypothetical box?

I'm sorry, but there is no "simple" answer to your question.
It's probably the reason why mechanical engineers get paid well, because unlike most people, they like doing complex math, which is what is required here.

If you want to try and work it out yourself, look up the formulae for a "class 2 lever".
Your fulcrum is the opposite frame rail (or mount) where the subframe will pivot, the input force is a proportion of the camper's weight, divided by the number of spring mounts on one side and the output force (the weight of the camper between the pivot and the outside edge) will allow you to determine the spring rate required.
Note that the weight is being pivoted about 350mm off the center line of the camper, not on the outside edge of the box. There is also friction to take into account, if you wanted to be more accurate.
Something else I have not mentioned is that the Fuso also has a stepped chassis, which adds even more complexity into the mix, as you have more than one pivot plane.

The simplest way to get a close approximation would be to mount the camper onto the truck without any springs in the mounts. Then, using a hoist, with a crane scale attached, lift the side of the camper and see how much force is required to separate the subframe from the chassis. Divide that number by the number of spring mounts on that side and use that figure as your output force in your lever calculation.
You would also need to know the distance from the pivot to the spring mount and the distance from the pivot to the outside edge of the camper.
Remember, that this would only give you an indication for static loads, which is not what you have in a moving truck.

Other than that... take a guess.

 

[glennm01]

I get it -- it's a complex problem, especially if a dead nuts answer is the goal. But I'm thinking one isn't. There's likely an acceptable ballpark, within which one will be fine, and I'm just looking for info on how to get to said ballpark. There are, after all, thousands of self-built spring-mounted campers out there doing just fine, no? How did they arrive at their spring specs once the weight of the box was known? Sure, some may have made a wild guess, but I'm betting few if any hired a team of physicists to work out the precise math. So what did the rest of them do?

 

[SkiFreak]

So what did the rest of them do?

From my observations, the majority of people that do custom builds don't really understand much about kinematic mounting systems, but accept that their build would be better off with this kind of setup.
As I have described, to work out the correct spring rate for the mounting springs can be an involved process, but as I also mentioned, any system that gives you some disconnect between the chassis and subframe should be beneficial.
It should also be noted that most people do not actually do their entire self build themselves; they outsource quite a bit to others. I would also guess that a significant amount of DIY builders just copied an existing setup, opposed to actually working out what the spring rate should be for their specific build.

I believe that by using the information I described you can actually make a pretty good "educated guess" regarding the appropriate spring rate.