A Jacobs ladder is a triangulated dual-link rear suspension locator used on sprint cars and midgets to center the live rear axle laterally while setting the rear roll center height.
Unlike a single Panhard bar that swings in an arc, a Jacobs ladder bolts to the right side of the frame rail and links via two angled straps to a vertical pin on the rear-end birdcage or axle center section. Where those straps intersect geometrically defines the rear roll center height, typically adjusted by moving the chassis pin up or down through pre-drilled mounting holes. Raising the Jacobs ladder raises the rear roll center and plants harder initial rear traction on corner entry, though going too high can make the car skate or hike on a dry-slick surface; lowering it calms the rear down and adds side-bite consistency.
Moving the Jacobs ladder one hole on the frame changes rear roll center height immediately without touching torsion bar stops or stagger.
Read the full post:
Panhard bar adjustment and J-bar setup: how height and angle change your car