Captive Spring Mounting Questions?

So I've been looking into the black hole of subframes and mounting methods. I think theres a good chance I'll go with a captive spring system as my chassis will be extremely rigid and I don't plan on rock crawling with my rig. (Torsion free subframe is not in the budget at the moment so I'd appreciate if there's no attempted convincing in the replies.) That being said, I have some questions.

I noticed most if not all of the Captive Spring Systems were rigid mounted in the rear and the spring mounts were the in front. Why is that the best option? I figured the best option would be to rigid mount the habitat toward the front where the frame flex is minimal and then do the spring mounts rearward to accommodate the flexing frame?

What do people use for the springs? Curious of spring rates and material and can't seem to find much information.
 

simple

Adventurer
I cant give you any design guidelines. The box on my f450 has springs front and rear. Tomorrow I can measure length diameter and wire size to give you a basis for comparison.
 

simple

Adventurer
There are 4 spring mounts total. These are on a 2006 f450 single cab with 12' chassis. The box is 2500-3000lbs estimated weight.

spring1.jpg
spring2.jpg
spring3.jpg
spring4.jpg







spring5.jpg
 
@simple thank you for the pictures thats a great start, looks like the bolt is 3/4 inch, spring OD is about 1.5, and the length was probably something around 3" uncompressed.

The one question I have about that is that it doesnt look like the spring would allow for much travel, based on how close the coils already are to eachother. what are your thoughts?
 

simple

Adventurer
Yea. Not much room left for compression. Maybe 3/8 to 1/2". I guess every time you see a box truck, take a look underneath and see how it is set up. This setup might be more about constant clamp pressure on the joint versus room for articulation. I didn't look closely but maybe there is a rubber isolator strip between the box and the frame. If so, fastening the two together by torqueing U-bolts would squish the rubber and not stay tight.

The overall diameter is about 1.63 when I set the caliper on it. It just looks funny because the caliper is held closer to the camera.
 
Last edited:
Yea. Not much room left for compression. Maybe 3/8 to 1/2". I guess every time you see a box truck, take a look underneath and see how it is set up. This setup might be more about constant clamp pressure on the joint versus room for articulation. I didn't look closely but maybe there is a rubber isolator strip between the box and the frame. If so, fastening the two together by torqueing U-bolts would squish the rubber and not stay tight.

The overall diameter is about 1.63 when I set the caliper on it. It just looks funny because the caliper is held closer to the camera.
I've looked at a handful of box trucks and a lot of them are just really long U Bolts that clamp the two frames together.
 

natevolk

New member
A solid mount on one end with the other sprung is cheap and easy. Generally the rear of the frame flexes more so to keep things small, they solid the rear and spring the front. It kind of doesn't really matter though. I did 4x each side on a 20' box. You can see the design on my build playlist towards the beginning of the build series:

 
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mog

Kodiak Wrangler
The one question I have about that is that it doesn't look like the spring would allow for much travel, based on how close the coils already are to each other. what are your thoughts?
I took the below photos at the 2018 Abenteuer Allrad show in Bad Kissingen of one of the 4Wheel24 trucks. As you can see fixed in the rear and the springs get progressively longer travel as they move forward. BTW- @terra-exp in Ft. Myers, FL is a supporting member here on the portal and is associated with 4Wheel24 in Germany.

Click to enlarge

DSCN3043.JPG DSCN3044.JPG DSCN3045.JPG
 

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