LCA and Pivot Shaft confusion

[BMosely]

I need some help on how the pivot shaft acts once installed in the k frame. Assuming rubber bushings, the pivot shaft is pressed into the LCA which then becomes fixed, no rotation. The back side is the torsion bar pocket which once installed is fixed and the T bar twists as needed for the suspension to work. The question is on the pivot shaft side inserted into the k frame. Is this greased on assembly and the pivot shaft rotates inside the k frame? That is what makes sense to me but that is not what I am seeing in the assembly details. If the shaft is a hard fit inside the k frame, then what allows the LCA to rotate as needed for suspension travel?

 

[threewood]

Not greased in the k frame. Secret is to not torque it down until it is on its own weight and at ride height. The rubber bushing is what gives it limited up/down. If you torque the nut with the suspension hanging, it will rip the bushing.

 

[Don Frelier]

The bushing twists.

 

[69Bee]

The arm pivots on the fixed points (shaft, bushing, torsion bar) that are are on the arm. The bolt to adjust the suspension height pushes on the arm lever that rotates when uninstalled, but when installed, the arm rotates around the fixed parts. The bushing does NOT rotate, but rather the arm does.

 

[dadsbee]

Some translation.. the rubber twists! And as Threewood points out DO NOT tighten the pivot shaft nut until the vehicle is set at ride height and the vehicle weight is on everything at time of tightening. Then the rubber bushing flexes back and forth through the arc of suspension travel.

 

[BMosely]

Seems like a pretty good argument for the poly bushing/greasable shaft. It just seems like it should be free to rotate and have the T bar do it’s job.

 

[1 Wild R/T]

Seems like a pretty good argument for the poly bushing/greasable shaft. It just seems like it should be free to rotate and have the T bar do it’s job.

You do what you feel comfortable with... But realize engineers designed it to work with rubber bushings.... Poly may allow it to twist slightly easier but trust me the forces involved won't notice the difference...

However the rubber doesn't allow fore/aft movement at the inboard end of the control arm, poly does allow about 1/2" of movement.... This movement happens under hard braking....

If Mopar used bushings like GM where there was a tremendous amount of rubber allowing allot of flex I would see the value in changing to poly but Mopar has very little rubber & therefore very little flex...

 

[Nate S]

You do what you feel comfortable with... But realize engineers designed it to work with rubber bushings.... Poly may allow it to twist slightly easier but trust me the forces involved won't notice the difference...

However the rubber doesn't allow fore/aft movement at the inboard end of the control arm, poly does allow about 1/2" of movement.... This movement happens under hard braking....

If Mopar used bushings like GM where there was a tremendous amount of rubber allowing allot of flex I would see the value in changing to poly but Mopar has very little rubber & therefore very little flex...

The braking comment is incorrect. The strut rod is in between and is the fulcrum. When braking, or even just drag from spinning the wheel the hub is pushed rearward, the strut is fixed in length and won’t let the arm move but what torque is generated pushes the inner end of the control arm forward. (Also bends the strut rod).

 

[1 Wild R/T]

The braking comment is incorrect. The strut rod is in between and is the fulcrum. When braking, or even just drag from spinning the wheel the hub is pushed rearward, the strut is fixed in length and won’t let the arm move but what torque is generated pushes the inner end of the control arm forward. (Also bends the strut rod).

Well your right the movement if forward... You are wrong that it doesn't move... The inboard end of the control arm tries to join the K member, The bushing dampens it but the arm does move..

 

[Nate S]

Well your right the movement if forward... You are wrong that it doesn't move... The inboard end of the control arm tries to join the K member, The bushing dampens it but the arm does move..

Oh sure, it’ll squish the rubber but it doesn’t go far (1/16” or so). Doesn’t pull out under any normal circumstances though.

 

[1 Wild R/T]

Oh sure, it’ll squish the rubber but it doesn’t go far (1/16” or so). Doesn’t pull out under any normal circumstances though.

Hang a Go-Pro on it and watch it.... Closer to 1/2" on the car I did it to.....

 

[Nate S]

Hang a Go-Pro on it and watch it.... Closer to 1/2" on the car I did it to.....

Did it with an endoscope, same idea. Flange on the pivot shaft bottoms out pretty quickly.

 

[gkent]

But realize engineers designed it to work with rubber bushings

Because back when this suspension system was developed "poly" was in its infancy and the price was prohibitive. Even today poly is more expensive than rubber.

All that aside, unless you're building a track car you really don't want the suspension freed up any more than it is.

 

[gkent]

(Also bends the strut rod).

I seriously doubt that !! The strut may be fixed in length BUT the rubber insulators on the K-member allow minor for and aft movement . This is necessary to minimize the fore and aft movement of the LCA as it moves in its arc. So actually, at both ends of suspension travel a) the LCA will move slightly forward and b) the strut will be pulled slightly rearward.

In a nutshell: you have the ends of two fixed points moving in an arc BUT because the "fixed" points are in rubber mounts, they give a little to minimize the arc.

 

[Nate S]

I seriously doubt that !! The strut may be fixed in length BUT the rubber insulators on the K-member allow minor for and aft movement . This is necessary to minimize the fore and aft movement of the LCA as it moves in its arc. So actually, at both ends of suspension travel a) the LCA will move slightly forward and b) the strut will be pulled slightly rearward.

In a nutshell: you have the ends of two fixed points moving in an arc BUT because the "fixed" points are in rubber mounts, they give a little to minimize the arc.

I agree with what you’re saying. The level of bending I’m referring to is quite small, probably not worth mentioning.