Trailer Building Indepent Arm Building - Official Guide

[dieselcruiserhead]

The bushing never gave up the ghost, they come back true when the trailer is unloaded.

Also I know it is not correct but I have a lot of time into the airbag mounts and so for those reasons unfortunately the airbag will stay where it is. It would also take a fairly major redesign to move the airbags lower. I know that is not as ideal as the big right on the axle tube and as close to the wheel as possible but unfortunately that is how its going to be. I believe the upgrades I've listed particularly with the delrin bushings will solve the problem to the point where I won't need to move the air bags. If I'm wrong (again) then its my loss...

Thanks guys,
Andre

 

[ntsqd]

Is it the inner or the outer bushings that distort the most?

If it is the inner bushings then I would suggest moving that assembly further inboard.

For the single tube design I had not considered that someone might want to use a single long bolt. I would use a piece of bar stock (CRS for it's surface finish) that was drilled and tapped on one end for a keeper bolt & washer and tabbed on the other end to bolt to the mounting ears and provide anti-rotation function.

 

[Ca. Bowhunter]

I went with a dual long tube design and just had it apart to check the Mcmaster Carr bushings and my 3/4" axle's for wear. I am glad to report after 8200 miles of on and offroad activities it shows only minor polishing on the axle and bushing surfaces. When I saw the soft style mounts I was a bit envious thinking those would probably help my trailer have a better ride but with this one finished I might go that way with the new project so keep this tread going until I get the suspension started.


Randy

 

[R_Lefebvre]

For the single tube design I had not considered that someone might want to use a single long bolt. I would use a piece of bar stock (CRS for it's surface finish) that was drilled and tapped on one end for a keeper bolt & washer and tabbed on the other end to bolt to the mounting ears and provide anti-rotation function.

I probably used the term "bolt" too loosely. Yes, it would only make sense to use a rod for that length. But, it still just doesn't seem right to me. If you had short bushings in the ends, you'd have a long length of unsupported rod in the middle. If you tried to put a full length bushing, it would be hard to push in, and there would be a lot of stiction on the rod. You're also supporting all that load on 2 tabs instead of 4. Just doesn't seem right to me. <shrug>

 

[MotoDave]

lower pivot point- the air bag is not positioned close enough to the tire
giving too much leverage.

I think you're thinking the same thing I am. Not to move the airbag down toward the axle, but outward toward th frame rail.

If you look at the forces acting on the arm, you have the axle pushing upward at the very end, the air bag pushing down on the opposite side, and the reaction forces of the two bushings. Since the airbag and axle forces are on opposite sides of the arm, they're basically applying a big twisting force to the arms, with only the bushings to resist.

I know there are space constraints that limit where you can mount the airbag, but I think if you can move it outward towards the frame rail, it will reduce the twisting forces on the arm. It probably should ideally be located on a line between the center of the 2 bushings and the tire contact patch, but I doubt physically that's possible.

Here's a quick SolidWorks model I made while thinking out the problem. The forces acting on the arm are in yellow, and the reaction forces on the arm are in green. Keep in mind the forces seen by the bushings are the reverse of this, so I'd expect the inner bushing to be deflecting downward, and the outer bushing upward.

 

[UK4X4]

"I think you're thinking the same thing I am. Not to move the airbag down toward the axle, but outward toward the frame rail."

Correct it would reduce the distance between the center of the wheel
and the closest pivot point - the air bag

Less distance = less forces produced

 

[maXTERRA]

Another cool and informative thread! Subscribing....
Really got me thinking about a new trailer now.

 

[R_Lefebvre]

I keep thinking, IF I were to do an independent suspension, why not do transverse arms instead of longitudinal. It's easier to get the forces lined up with the spring that way.

 

[UK4X4]

I did look at that and in a wide trailer would work well.

in a 45" wide trailer your arms would be rather short for the height of the bags
unless you staggered -or crossed the legs.

Not saying it can't be done....just more work

Also the common trailing arm slides up and over obstacles, cross mounted all you would hear would be a large
"thunk" and come to a standstill

 

[ntsqd]

If I'm understanding correctly, traverse arms would put the pivot pins/mounts/etc. effectively in torsion. Could be done, but seems like extra effort for little gain. Maybe I'm missing something? Wouldn't be the first time, nor is it likely to be the last.

I don't see a problem with using a single pivot per trailing arm. Obviously the mounting needs the design & structure to handle the loads, but it is very doable.

 

[R_Lefebvre]

UK4x4, that's probably a good point, about running into things. Of course, is it really any different than a trailer with a solid axle? Same thing, if it hits something like a stump, you stop.

Also, the arms could be long if you did them like an old I-beam front suspension, with the left arm pivot point being on the right side of the trailer.

NTSQD: I'm not quite following you, but yeah, probably little gain. Just kind of a pie in the sky thing if you decided you needed the perfect suspension setup with excellent geometry, which... we don't.

Now, another idea though, what about a twist beam rear suspension. Just go look at any pre-2005 water cooled VW for an example. The beam is further up out of the way.

 

[dirty Bakers]

I am thinking maybe a sort of suspension subframe for maintanance and strength reasons if adding independent or any mods for that matter to a military trailer. Remeber they were going for less weight and low costs.
I work in the heavy equipment industry and when it comes to shock loads Brass bushings are used with very little room for movement for the hardened pin. This set up is used on a skid steer where the bucket pivots. Skid steers arent treated nice and rarley do we find well maintaned greasable joints. If you were to use the brass bushing and hardened pin (I'm thinking 1" pin) set up and kept them greased and added a little width to your swing arms it should last a lifetime with no flex, just up and down motion which is all a trailer needs.

 

[Dmarchand]

Heim or uniball joints in application rather than the delrin or other bushing. These include zerks for greasing.

 

[ntsqd]

Rob, what I'm saying is that lateral arm(s) will try to twist when an obstacle is encountered by the tire. That twist puts some complex loadings on the mounts. Versus a trailing arm's simple tensile and shear loadings.

Most of the SRE's that have a grease zerk in them have the zerk hole drilled in just about the worst possible spot for strength. SRE's of quality won't have a zerk option.

 

[Trailpsycho]

Heim or uniball joints in application rather than the delrin or other bushing. These include zerks for greasing.

How thick is that angle? Those look nice, but I am not sure about the piece of angle that you used to anchor your heims from. I would add gussets to "box" in the angle.... I dont think its much better than the c-channel that the original poster had in his setup. Otherwise the arms look great.