RENNtech Suspension & Engine Components Suspension Upgrade – 4 Link Rear (Heims) | R129 SL 320/500/600RENNtech rear suspension upgrade with heim joints utilizes four control arms (two upper and two lower) to connect the rear axle to the vehicle's frame, offering enhanced articulation, adjustability, and control for off road vehicles. Heim joints (also known as rod ends) are spherical bearings that allow for movement in multiple directions, providing increased suspension flex and articulation compared to traditional rubber bushings.