How Many Mounts Needed on Spring Mounted Flatbed?

[brp]

I have read through many threads on this, and other, websites about flexible flatbed design.

Doug has been a big help too.

Now I am about to start bolting mounts to the frame of my cab/chassis F450, with 9' frame rails going out behind the cab. I am going with the Aussie/ozzie style, see picture of an M35's mounts to give a rough idea.

The trouble is there are only 4 good points on the frame to attached the angle iron for the mounts. These spots are essentially at the corners and in my picture below you can see the 2x6 chunks standing on end to indicate those spots. There are no clearance issues and pre-existing holes in the frame for these locations.

Where there is a 2x6 chunk on it's side, I could put a mount but only on the passenger side, the driver side has the emergency brake mechanism there.

Where you see the tape measure laying across, I could add a cross member and a single spring that is centered lengthwise, just over the diff.

There is no room to place mounts above the leaf springs, there is possible room within the forward mount for the leaf spring. The drawback here is I am looking at using a 6" or 8" spring and the 6 might not even clear there.

Other options would be to just use the four corners but with 2 springs at each corner, or try to add mounts that are tight to the inside of the frame.

Any thoughts or advice would be great. I would like to have more than 4 mounting points, there will be a camper going on the flatbed and I would feel better about a tall load having more points.

 

[pappawheely]

Why such long springs? You should be able to match the spring rate and travel with a shorter spring.

 

[Buliwyf]

Aren't there factory mounting brackets out there?
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I wouldn't drill or weld the top of the frame. Just the sides. Remove rivets in the frame and use G8 bolts In place of rivets if they're too close to drills or welds.

 

[brp]

Why such long springs? You should be able to match the spring rate and travel with a shorter spring.

I have not been able to find anything definitive on spring rates and deflection % as it pertains to flatbeds. I just got one spring from an M35, pictured above, it is actually one spring within another, and is 7 3/8" long. The springs I have looked at have max deflections of 50%, and then you need to tighten it down some initially to hold the bed static, just keep it in place, give an inch for that. That leaves you with about 2.5" of room for deflection, which seems like a good amount to me.

I would love to hear anything beyond what I know, it is just a thoughtful guess.

As a side note, I was working to remove a welded on landscaping/commercial style hitch from the truck and was able to twist the frame rail a surprising amount with just my body weight and a 3 foot lever perpendicular to the rail.

 

[brp]

Aren't there factory mounting brackets out there?
-
I wouldn't drill or weld the top of the frame. Just the sides. Remove rivets in the frame and use G8 bolts In place of rivets if they're too close to drills or welds.

I am trying to not alter the chassis in anyway. I will not be welding on it, I might enlarge existing holes. Yes, grade 8

 

[pappawheely]

I am planning to build a flatbed for my truck and have the same concerns you do. I was planning to tie into the front spring hangers and use a pivot at the rear. If you notice, some trucks have bolt locations to hold the bed down and also pads that just sit on the frame. The more mounting locations you use, the less the frame can move. Fixturing a part when machining or locating a bed both deal with degrees of freedom, (possible movement) Three points of contact are all you need to locate an object on a flat plane.

Spring calculator: http://www.calculatoredge.com/mech/hspring.htm

 

[brp]

Thanks Papawheely.

I forgot to mention in response to Buliwylf that I did find that Eastern Surplus sells the mounting brackets for the M35, $47/each. I bought a length of angle iron (3"x5"x4' and 1/4" thick) for about 20$ that I will be using to make mounts. I'll gusset it with plate as well.

Anyway, both options are fine, the M35 would be more costly, but could be less work if all your mounts are really straight forward.

Most of my brackets will need an odd size hole, a notch, etc., so I am just going to use the angle iron, but both options have their place.

 

[brp]

I took some more measurements and I think my best option may to be to make a mount that goes into the forward mount of the rear leaf spring.
Since this is too shallow to use the long springs I would like to use I will cut the spring and put a section of spring on top of the angle iron coming off the flatbed.
I should be able to fit about 1.5" of spring on top of the mount and almost all of the remaining approx. 5.5" inch of spring below the mount.

I imagine this would give similar spring rate/performance. It is not as important because it is near the center (front/back) so these springs will not move as much as those front to rear.

I attached a photo that Doug sent me of such an example. In my case the lower spring would be about 5.5" and the upper about 1.5"
Thoughts on that?

How about on having 6 points, I am sure some would advocate for 8 or more?

Thanks