Axial Loads Applied To Johnny Joints

Hello All,

I am doing some front suspension design for a project, and I am planning to use 2 johnny joints for the pivot bushings on the lower control arm. Here is a sample of the layout:

This pictures the bushings and ball joint cup of the front lower drivers side arm from the top. After some consideration, I believe that johnny joints should be up to the axial loading that will be experienced by the trailing bushing (on the strut rod). My question is this:

Has anyone used a johnny joint in axial loading before? How was your experience?

Here is an example of a johnny joint for those following along:

I can't answer your question, I'd assume the manufacturer Currie would be the only ones that could.

I just want to know how big your control arms are. Those things are HUGE!!

This 7 pound lump of steel is the smallest one they make. It has 1 inch thread. 

Trent,

I'm using a "2inch" joint from Rock Jock. It has a 3/4-16 thread and a ~2.1" OD:

In reply to Shavarsh :

Since they don't even publish max radial load, let alone max lateral load, they're going to be your only option here.

I'm assuming you're wanting to go this route due to factory radius rod mount configuration? Have you considered modifying the mount, or making a mount adapter, that would allow you to run the joint conventionally?

In reply to Driven5 :

Yes, I have considered (and still might) cut the area out and adapt to run either a flex joint in the conventional configuration, or cut out enough room to mount a regular bushing in the same axis as the other bushing. At least then I could use standard bushings instead of anything spherical. However, I don't want to change the rotation point if I can avoid it. I'd like to keep the axis parallel to the car to avoid introducing additional caster gain/loss. Also, with the pinto frame rails it should be fairly easy to weld in a flex joint cup in the stock position:

Are they Johnny joints because of the ends of the inner part, so you don't need spacers as compared to regular sperical joints? And is that the main advantage or is there another? I would turn the front one 90*....but I see why you want to do it that way. I think there should be enough  movement in the joint to work, but will it hold up in that orientation?

Trent said:

I can't answer your question, I'd assume the manufacturer Currie would be the only ones that could.

I just want to know how big your control arms are. Those things are HUGE!!

This 7 pound lump of steel is the smallest one they make. It has 1 inch thread. 

I have 3/4 thread units in my RX-7, and also the front suspension of Colin.

I would not load them axially, they're held together with a very small looking circlip.

MiniDave said:

Are they Johnny joints because of the ends of the inner part, so you don't need spacers as compared to regular sperical joints? And is that the main advantage or is there another? I would turn the front one 90*....but I see why you want to do it that way. I think there should be enough  movement in the joint to work, but will it hold up in that orientation?

They are nice because unlike rod ends, they are bushings, but they are still "spherical".  Like preloaded, spherical poly bushings.

They last a lot longer than rod ends or "rebuildable, adjustable" UMI joints that use hard plastic.  And they are great for NVH reasons.

In reply to Pete. (l33t FS) :

Good consideration Pete. I am thinking they will only see significant force in the compression direction. So if I orient the circlip toward the ball joint, it should see very little load. This is good brainstorming, let me know if you see a flaw with the logic.

Edit:

I may machine my own cups anyway, so I could integrate the rear washer into the cup.

You know, G-Force Engineering used to make a polyball kind of joint for RX-7s that attached in place of the tension rod bushing (RX-7 front suspensions look like a strut converted Falcon/Mustang) that allegedly had no resistance to movement and was fairly tough.  Of course, he went out of business like twenty years ago.  It was kind of like a four piece arrangement.

Practically speaking, there's nothing particularly bad about that tension rod arrangement.  Let me do a little digging.

Well,

After doing a bit more disassembly and measuring, the ball OD in this joint is 1.125" and the ID of the bushing half is .95". This means there is a supported frontal area of ~.29 square inches. Trent, do you know the same dimensions for the joint you bought? I'll have to see if I can find some dimensions for the large versions.

In reply to Shavarsh :

I can measure it tomorrow 

This one is significantly larger.

OD of the ball is 1.625"

ID of the bushing is 1.15"

BTW. The retaining ring is this guy right here Snap ring on McMaster and has a thrust load rating of 21,500 pounds!

This was the smallest joint Currie offered 5 years ago when I was designing some Fiat stuff. 

Have you looked into the Specialty Products "Sealed Flex joints"?

https://www.spcalignment.com/component/content/article?id=291

Clearly what they are doing is offering OEM joints like Toyota, BMW and Audi use, but they give dimensions, forged rod end recievers and weld in sleeves that work with them. They also have neat ball joints that assemble like the Johnny Joints in weldable sleeves

Johnny Joints, incidentally, are a complete mofo to assemble without the special tool.  I built control arms using the weld in sleeves, which meant I had to assemble them after the fact.  I couldn't imagine trying to do it in-car.

The rubber bushings are heavily preloaded, and then you also have to get the clip in.

Thanks Trent, super useful information! That joint provides about 1sqin of frontal area (when viewed along its axis). After playing around with the stock rubber bushings some more, it is clear that they offer far more load supporting area for axial impacts. They also do not impede motion as much as I had remembered. I may just continue to run the rubber for now.... Decisions decisions

Another method would be to use a full sphere with a stud welded on, and cast my own poly bushings. This would give the full area of the cross section support on the back side. Again, I'd have to run the math on how much deflection would actually be reduced vs. the stock setup that has a softer bushing but more supported area.

In reply to Shavarsh :

What about a 'lower' (load bearing not follower) ball joint? Would still require some sort of adapter though.

Edit: Nevermind, probably not enough threads to provide reasonable range of adjustment.

In reply to Driven5 :

Thats a great idea, and definitely would work, but there is not much NVH attenuation in a ball joint.