I mean, wouldn't the bindless action be effective when you're turning only one direction? I dunno, maybe I'm just missing something.

 

Yes and no. Personally, I think a combo panhard rod is pointless because if everything else is working as it should there's no way a panhard rod would bind. But to it working on one turn and not another, no it wouldn't make a difference. It would work both ways just the same.

 

Think side view. A panhard bar binds slightly as the pinion angle changes relative to the chassis, so the combo arrangement is at least theoretically better. If nothing else, it removes all of the torsion from the PHB, which increases the compressive strength (right turn loading on the PHB is compression). Whether or not you can feel the difference is another matter . . .

Norm

 

I think I'm not understanding the function of the panhard bar correctly. Okay, the rear seems to be a mostly symmetrical design right? Stresses should be similar on either side of the suspension when making either left or right turns. Now what advantage would the sperical side (let's say this side is mounted on the passenger's side) have when making either turns. Hmm, maybe I am confused about the purpose of the panhard. Thanks for the replies to help me figure this out.

 

The magnitudes of the stresses are similar in left vs right turns, but their effect on the PHB is not. The PHB goes into compression in a right turn but is in tension for a left turn. Compression is the more severe condition, since the PHB will buckle (collapse) at a lower load than whatever tension load would pull it apart. It's just the nature of the beast, structurally speaking.

The combination of compression with torsion is more severe than tension plus torsion because the torsional distortion of the PHB reduces its stability and hence its buckling strength.

Mostly in normal driving, buckling issues with a PHB are academic even with the cheapie OE PHB, hence my earlier use of the term 'theoretical'. Until you clip a curb in a sideways slide with the left rear wheel, that is. In competition, where the loads are considerably higher and the overall consequences of a PHB buckling failure more severe, you do have to consider it.

Structurally there's no difference which end of the PHB gets the spherical. Just having one there at all makes it impossible for the PHB to have any torsion developed in it (a second spherical would also remove the small amount of lateral bending that remains with the combo arrangement). But from a transmitted noise standpoint with the combo PHB I'd put my spherical on the axle end so that the poly would isolate at least some of the eventual rod end noise.

Norm

 

I thought of that, which is why I commented "if everything else is working as it should". Do you think the pinion angle can change drastically enough to bind the panhard rod if the torque arm is doing it's job? Especially if that bad boy is aftermarket or at least if the clamshell bushing is polyurethane. I did not consider the compression/tension effect of left/right turn loading, but like you said, it's pretty academic. Of course, I don't consider flexing the bushings slightly to be "binding". I consider it to be when the bushes flex to the point of metal-to-metal contact as binding. That's a good explanation NP.

 

Thanks for the great explanation. If I were to have spherical on both ends of the PHB the noise would be very noticeable in normal driving conditions? I hear spherical ends are pretty loud, but for a weekend showcar I should be ok.

 

Perhaps we're looking at this from slightly different perspectives, and it's easy to let the water get muddied a bit by the magnitude of the forces and moments involved. Bind exists when there is anything but tension or compression loads developed in a link (i.e. the LCA's and the PHB) or vertical shear and bending in a beam (the TA) as the chassis rises, falls, and rolls with respect to the axle. That's only possible with an all ball-jointed LCA and PHB configuration and a friction-free sliding connection at the front of the TA.

As soon as the bushings for any link can support any load other than tension or compression along the axis of the link, you have bind as a function of the 3-D arc that the ends of the link move through and the stiffnesses of the bushings. Consequently, any cylindrical bushing will develop bind. It's true that, given soft enough bushing material, the forces and moments that are developed in reaction to bushing flex are small. That's the compromise that GM and most other mfrs take, and that this solution for minimizing bind also helps to control NVH and minimizes periodic maintenance requirements makes it a good solution for the normal driving of most people.

As soon as all the bushing compliance is all used up (essentially the metal to metal scenario) or if there's very little to begin with (any hard cylindrical bushing replacement) the amount of bind goes way up. In other words, bind doesn't magically start here, it just gets bigger. Ten times OE is in the ballpark for poly/poly LCA's if that's any help. Maybe it's not quite so bad for the PHB since it's longer and is also more or less a channel-section.

To put some numbers on the pinion angle change, it's a function of TA length and suspension movement from design ride height. Given a 48" TA, each inch of rear suspension motion represents about 1.2º of pinion angle change. That's if the TA is completely rigid and its front bushing is not deflecting at all; any flex in these components exaggerates the above-calculated pinion angle change.

And thanks

soulbounder - As I understand it, they rattle as clearances open up. The looser they get, the louder they become.

Norm