excellent question -Panhard bars are some times called track bars and when we are discussing them here we assume left hand turn round track.
Panhard bars, are designed to locate a solid axle side to side, or laterally. The panhard bar locates the rear roll center.
We have discusses rear roll center in previous posts but a typical race car has front and rear centers, which are determined by suspension geometry. An imaginary line called the roll axis connects the front and rear roll centers. A turning car experiences a outward inertial ( some call it centrifugal, wrongly) force at its center of gravity, which tries to roll the chassis around the roll axis.
As the myth goes “centrifugal force also transfers weight from the inside tires to the outside tires”. Actually we have Inertial force “torqing “ the frame toward the outside when entering a turn.
All else being equal, raising the rear roll center increases the rear roll stiffness and thus the percentage of the inertial force ( think transferred weight if you like ) that goes to the outside rear tire. That loosens the car. Over steer. Lowering the rear roll center has the opposite effect.
All panhard bars swing in arcs, which means that the roll center moves up and down with suspension travel, including chassis roll. If the panhard bar is mounted to the right side of the chassis then the rear roll center will become lower as the chassis rolls to the right in a left-hand turn. That loosen the rear end. Right-side chassis mounts are the most common on pavement, while left-side mounts are more common on dirt. With a left-side chassis mount, the roll center will rise as the chassis rolls to the right, tightening the car up.
Panhard bars can also push and pull the rear end sideways as the suspension deflects. If you run a short Pbar you get a lot of deflection. Longer Pbar minimizes this deflection. Lateral rear axle movement generally causes rear end rear roll steer. If the rear’s skewing increases the right-side wheelbase vs. the left, the car will have roll-over steer.
Just like a short Pbar inclined bars further increase the amount of axle movement if their inclination increases as the chassis rolls. For example, a short panhard bar connected on the left and inclined downward toward its attachment at the rear axle will pull the rear axle toward the car’s left side quite a bit as the car rolls right. This is common on dirt race cars. The panhard bar controls the amount of lateral axle motion, and the geometry of the other suspension links determines how much the rear axle roll steers. Raising the panhard bar where it connects to the chassis will increase the amount that it pulls the axle to the left as the car rolls. With most dirt suspensions, this adjustment will increase roll over steer, loosening the chassis up in the turns. The opposite is also true.
Inclined panhard bars also produce vertical forces that act on the car’s chassis and the rear axle at the bar’s attachment points.When a panhard bar is level, the rear tires’ entire lateral grip is transferred into the chassis at the bar’s end horizontally. But if the bar is inclined upward to the left as in dirt car inertial the forces transmitted into the chassis will have two components – one horizontal and another vertical.
On dirt cars with left frame mount of the panhard bar, this causes chassis roll because it pushes up at the car’s left rear. There will also be an equal but opposite force pushing straight down on the axle where the bar connects to it. These opposing vertical forces increase as the inclination of the panhard bar increases, especially with larger angles.
If the bar is connected on the right and inclined upward to the right, it will pull the chassis down on the right side, but it will also unload the axle with a vertical force up, where it connects to the axle.
With inclined panhard bars – and all bars incline at least a little as they swing through their arcs – the point where the bar connects to the rear axle is important. If it’s at the center, its vertical force will be equally divided between the two rear tires. If it’s to the right, proportionately more of the vertical force will go to the right rear tire. The J-bars commonly used on dirt race cars are generally mounted to the right of the axle’s center line and inclined upward toward their left chassis mount. That sends more downward vertical load to the right rear as the car turns. J-bars provide drive shaft clearance.
The amount of room you have will determine if you mount the panhard bar ahead of or behind the rear axle. Depending on how you mount it the axle to rotate about its centerline, moving the bar’s mounting point up and down under acceleration/deceleration. Do not forget your rear roll center will move due to the axle twisting under acceleration. It may lower if the bar is moved from the front to the rear. You have to take the time to find out where the rear RC migrates when you go from static to dynamic...this is where the chassis software comes in handy. As with the other variables affecting the rear roll center height, moving the roll center up will increase rear roll stiffness and thus loosens the car up.
I never was a fan of offset mounting. Non parallel non planar mounts of the Pbar as viewed over head. I like things square to the frame and 90 degree vectors and the like.
Summary - Panhard bars should be parallel to the ground when the car is at static ride height. Otherwise the axle will be forced to move diagonally. A Panhard bar will cause lateral movement y nature but you want to minimize this. One way is to use a longer the bar because you have less displacement. Never preload it. Attach the bar to the chassis on the right side for left hand turns. Your rear roll center is the point crosses the centerline of the car.
