Three Link or Indy?

[GeoTracker90]

Well so far it looks like the voting has been pretty close, but with the three link system getting the slight edge on the independent system. That's OK with me because I've wanted to give that style a try. So I think I'll go ahead and begin with making the links for the three link. I have some 1-1/2" OD tube that I'll use as well as all four of the poly bushings that I need for the axle end of the links. I just bought the threaded inserts for the rod ends, and still need to buy the rod ends at this point.

As I begin construction on the links I'll begin a seperate construction thread on the three link. I will also try to document my sources as well as the cost of the parts that go into this style of suspension. I'll probably list the raw parts with out any shipping/handling because that will vary depending on where you are in the country.

I guess I had better finalize some of the dimensions of the trailer as well as the link design. Wish me luck!

Mike

 

[dieselcruiserhead]

I'm going with independent. I'm doing a three link with coils on the front of my mini truck pretty shortly so I know that style well and don't think it would be too difficult either. But I'm psyched to try the independent arms and see how they turn out... I'll post progress in my thread...

 

[ntsqd]

Any chance of some dimensions for those air springs? What would be handy to know:
Max/Min bladder OD, Max/Ride/Min length, OD of end plates, Stud spacing & size

 

[Cruiser]

I have a late 60's early 70's era trailer that has a simple coil spring link suspension.. Would you like me to post pics of how they did it back then??
Steve

 

[fj40fax]

I have a late 60's early 70's era trailer that has a simple coil spring link suspension.. Would you like me to post pics of how they did it back then??
Steve

Yes, please.

 

[ntsqd]

This is my preliminary 3 link design. I have some details to work out with regard to the air springs, but the 'bones' of it are there. Note that I removed one section of the 2" socket receiver tube in the middle of the frame for clarity. For a sense of scale, the tires are true 33-10.50 and the outer dimensions of the rectangular part of the frame is 44" wide by 60" long. I suspect that the tongue is a bit short, but I've not yet done the Shear-Moment Diagram as I have no idea what the loading is. At the current ride height, shown in the side view, there is 3" of compression travel. I'm not convinced that this is enough, more work to be done there.

 

[GeoTracker90]

Any chance of some dimensions for those air springs? What would be handy to know:
Max/Min bladder OD, Max/Ride/Min length, OD of end plates, Stud spacing & size

I did a search for "Firestone Air Spring #6781" and came up with the following from the Truckspring.com website:

• End plates have new environmentally friendly metal coating with superior corrosion resistance.
• 4-layer reinforcement air spring construction provides years of worry free service.
• Braided wire girdle hoop is stronger and increases the burst pressure.
• Each Firestone air spring is pressure tested before leaving the factory.
• Air helper spring delivers exceptional ride.
• Rolled crimp design for durability.

Inflation up to 100. PSI
Compressed Height: 2.80"
Extended Height: 8.30"
Ride Height: 5.50" - 6.50"
Maximum Diameter: 6.60"
Firestone Bellows Number: 267C1.5
Air Inlet: 1/4" NPT
Capacity: 2400 lbs.

 

[GeoTracker90]

This is my preliminary 3 link design. I have some details to work out with regard to the air springs, but the 'bones' of it are there. Note that I removed one section of the 2" socket receiver tube in the middle of the frame for clarity. For a sense of scale, the tires are true 33-10.50 and the outer dimensions of the rectangular part of the frame is 44" wide by 60" long. I suspect that the tongue is a bit short, but I've not yet done the Shear-Moment Diagram as I have no idea what the loading is. At the current ride height, shown in the side view, there is 3" of compression travel. I'm not convinced that this is enough, more work to be done there.

Very nice. I like your design and how it incorporates a triangulated upper link. What is the length of the lower links center to center? From the looks of things I imagine that the air springs will mount directly above the axle and under the cross member/frame side rails.

Keep up the great work!

Mike

 

[ntsqd]

I've updated the model and now both link lengths are 32"
What I'm running into that I do not like is how much fore/aft movement happens at the air spring. Using the dims that you found I'm seeing almost an inch of change of position of the axle relative to bottom of the air spring through the suspension cycle (Bump to Droop). That seems like a lot to ask the air spring to absorb. I fear that some articulated mounts might be required.

Current ride height is such that for level tow the effective tow ball height would have to be ~26" AGL.

 

[Ireland]

Thats a lot of really useful information you are posting, a very interesting project

 

[GeoTracker90]

I've updated the model and now both link lengths are 32"
What I'm running into that I do not like is how much fore/aft movement happens at the air spring. Using the dims that you found I'm seeing almost an inch of change of position of the axle relative to bottom of the air spring through the suspension cycle (Bump to Droop). That seems like a lot to ask the air spring to absorb. I fear that some articulated mounts might be required.

Current ride height is such that for level tow the effective tow ball height would have to be ~26" AGL.

I'm not even close to being an expert in suspension geometries, but I wonder if the movement might be mitigated with links of unequal lengths. Also changing the mounting position of the links could play into this as well.

Wish I knew a little more about my modeling program. Wish I had more time to learn more about my modeling program.

Mike

 

[UK4X4]

"I had thought about fabricating the trailing arms from components, but the more I think about it the more I believe that it would be better to order a 3,500 lb axle tube and using a jig weld up the trailing arm pieces. The two trailing arms could then be cut loose from each other. "

Thats what I did..still have to oval a mounting hole to straighten up the wheels though.

The arc movement of the spring is not going to go away, to minimise you'd need the mount at the same level as the center of the air spring at ride height., that should then give you 1/2" movement at it lowest and highest positions

issue with that though is you have to add tall mounts...

which ever way you build you will have spring twist-movement, so its not a major issue

 

[ntsqd]

It's pretty much a fact of life that with radius arms that the axle will travel in an arc, resulting in some lateral translation. I was just hoping that with linkage that long that the total fore/aft change would be less.

The attached proposal (based on the famous Watts Link) has other issues (axle rotation), but for a known travel distance it can be fine tuned to have a nearly linear travel path.

 

[Cruiser]

Sorry it took so long, but everytime I wanted to take pics it was either raining or snowing,, or both.. These were at night a week ago,, It's a light duty trailer and rated for 1K #'s.. But I have seen ones slightly bigger and took some heavy weight. This tows nice with three bikes on it, close to 800#'s.. The trailing arms go out at an angle from the main center bar, and you can see the centering link in the back.. If you do one you need to have this bar close to level, so the travel will not push it side to side.. This would break it like a lifted Jeep without a trackbar bracket.. Just breaks off the axle..

 

[Backwoods Rambler]

It's pretty much a fact of life that with radius arms that the axle will travel in an arc, resulting in some lateral translation. I was just hoping that with linkage that long that the total fore/aft change would be less.

The attached proposal (based on the famous Watts Link) has other issues (axle rotation), but for a known travel distance it can be fine tuned to have a nearly linear travel path.

I don't see a major drawback to this design with a trailer axle application.

However, I also don't see the 1" of movement you're concerned about in the original design as a problem. Just start building the darn thing :gunt: