panhard bar

A panhard bar prevents the rear axle from moving side-to-side, which makes the handling more predictable. Reports that the Lighting rear axle can move laterally an inch or more under hard cornering (especially with longer shackles) has been confirmed with video. Yet another Lightning owner installed Slide-A-Link traction bars, which are very tight to the outside of the frame rails, and experienced no rubbing, indicating that the axle did not move at all on his truck (see my leaf spring tech page for leaf bind as a possible explanation for this difference).

At least one suspension guru has stated that a Hotchkiss drive does not need an axle location device. I suspect that this is not a declaration that one would not be effective, but rather pointing out that one of the engineering advantages for a passenger car to the Hotchkiss drive is that one is not required for normal use.

There are two common devices used to control lateral displacement -- the Watts link and the panhard bar (also called a "track bar"). The Watts link is complicated, but keeps the axle totally centered at all suspension movements. The only kit that I know of that will fit our trucks is a weld-on kit from Totally Polished. They did not respond to my e-mails.

A panhard rod has the advantages of lighter weight and less complexity. A panhard should be as long as possible to minimize lateral displacement as the bar swings through its arc. The bar should also be level for the same reason.

The bar should also be as low as possible. An axle location device determines the rear roll center, which we want as low as possible (the generally accepted rule of thumb is a roll center at about 1" in the front and 12" in the rear). With a diff-mounted Watts Link, the roll center will be lowered to the center of the axle, about 14". But with a panhard, the roll center can be lowered even below the axle.

But deflection in the leafs can raise the effective roll center back up a bit. There is nothing that can be done about this. The deflection in the leafs themselves can't be fixed. And leaf spring bushings, at least the front bushings, need to remain free to deflect to avoid suspension bind, so urethane or solid bushings are out of the question.

The homemade setup below was fabricated from standard issue circle track parts. It corrects the bar length issue, but does not lower the roll center.

The panhard to the right was custom designed and fabricated by Paul Terry for his own truck. Although much more complex and heavy, the arc described by the longer bar means that the axle is forced to move less side-to-side as the axle travels up and down. This may or may not be a big deal, but hats off to Paul for making one on his own! I stole many of this design features for my new design.

[Update: April 2008] Finally, someone has made a proper Lightning panhard.

Craig Bearb built this from some traction bars that he scrapped. It's long and low. Nicely done.

Both RUSlow (Stan Martin; see his forum at nloc.net) and Develop Mental Racing (Garrett Gunther; see his forum at nhtoc.com) make panhard kits for Lightnings. I have the installed the RUSlow panhard shown below. Installation photos are here.

Because of the need for a simple bolt-in system, they each have rods that should optimally be longer and lower. These over-the-diff panhards actually raise the rear roll center to about 21-22". Nonetheless, a Ruslow or DMR panhard is a noticeable improvement over stock, even with the Hotchkis leafs.

I recently bought a used axle mount from a Nextel Cup team to get below the axle, and far enough out to clear the shocks. I'm shooting for a 12" roll center, but the bar will be adjustable from 10.5" to 14.5" on both the frame and axle ends.

The 2X2X.25" mild steel tubing for the frame mount downtube is also shown. The brace on the axle mount is for a quick change racing diff, so I will have to cut that off and make a new tube to weld to the axle. The device is designed to bolt onto a 3" axle, so I will have to cut off the bottom and contour it to be welding onto our 3.25" tubes. Note the trick adjustment mechanism and the machined-in misalignment washer. It's a great piece -- beefy as hell, but surprisingly lightweight.