A 4 bar suspension is a common rear suspension design used in off-road vehicles, hot rods, and custom builds. It provides precise control of the axle's movement, improving traction, stability, and ride quality. Understanding the individual parts that make up a 4 bar setup is essential whether you're building from scratch, upgrading an existing system, or troubleshooting a problem. This guide covers the core components, how they work, and what to look for when selecting parts.
Shop on Amazon
Browse the latest options and prices.
TRQ Front Steering & Suspension 12 Piece Kit with Ball Joints Tie Rods Drag Links Sway Bar End Links
What Is a 4 Bar Suspension?
A 4 bar suspension uses four rigid links—two upper and two lower—to locate the rear axle. Unlike leaf springs or triangulated setups, the four bars control the axle's fore-aft and lateral movement while allowing vertical travel. The links attach to the chassis and axle via brackets and bushings or rod ends. This design offers high tunability and is popular in rock crawlers, desert trucks, and street rods.
Key Components of a 4 Bar Suspension
Upper and Lower Control Arms
Control arms are the primary links. The upper arms are typically shorter and angled to control pinion angle, while the lower arms are longer and carry most of the braking and acceleration forces. Arms come in various materials:
- Steel: Durable and affordable, but heavy. Good for most builds.
- Aluminum: Lightweight, reduces unsprung mass, but more expensive and less forgiving in extreme abuse.
- Chromoly: Strong and light, often used in race applications.
Arm ends also vary: rubber bushings offer comfort, polyurethane bushings add stiffness, and heim joints (rod ends) provide maximum articulation but transmit noise.
Brackets and Mounts
Brackets attach the control arms to the frame and axle. They must be strong and precisely located. Frame-side brackets often come as weld-on or bolt-on kits. Axle brackets may also include perches for coil springs or coilovers. Proper bracket geometry is critical—incorrect placement can cause bind, poor travel, or driveline vibration.
Bushings and Joints
Bushings absorb vibration and allow pivoting. Common types:
- Rubber: Best for street use, quiet, and isolates noise.
- Polyurethane: Firmer, less deflection, but can squeak.
- Heim joints: Spherical bearings for full articulation, require maintenance.
Joint choice affects ride quality and maintenance intervals. For a daily driver, stick with rubber or poly. For off-road or track use, heim joints are common.
Coil Spring or Coilover Hardware
Most 4 bar setups use coil springs or coilovers mounted between the axle and frame. Coilover shocks combine spring and damper in one unit. Parts include spring perches, adjusters, and shock mounts. Proper spring rate and shock valving are essential for the intended use.
How 4 Bar Suspension Parts Work Together
The four bars create a parallelogram-like motion, keeping the axle square to the chassis during travel. The upper arms control the axle's rotation (pinion angle), while the lower arms handle forward/backward forces. Together, they prevent axle wrap under acceleration and reduce squat or lift. The arrangement can be parallel or triangulated. Parallel links are simpler but may require a track bar or Watts link for lateral location if used without a triangulated setup.
Instant center, antisquat, and roll axis are all determined by the lengths and angles of the arms. Changing one component affects the whole system. That's why aftermarket parts are often sold as matched kits.
Common Upgrades and Considerations
Many enthusiasts upgrade from factory leaf springs to a 4 bar for better articulation and ride. Common aftermarket parts include:
- Adjustable control arms: Allow tuning of pinion angle and wheelbase.
- High-clearance brackets: Improve ground clearance for off-road use.
- Polyurethane or rod-end bushings: Reduce deflection for more precise handling.
- Coilover conversion kits: Replace leaf springs for progressive damping.
When upgrading, consider the vehicle's primary use. A rock crawler needs maximum flex, so heim joints and flexible arms. A street rod needs comfort and stability, so rubber bushings and well-damped coilovers.
How to Choose the Right 4 Bar Suspension Parts
Start by determining your goals: off-road performance, street driving, or drag racing? Then measure your chassis width, axle type, and available space. Key factors:
- Link length and angle: Most kits come with pre-measured arms. Adjustable arms are worth the extra cost for fine-tuning.
- Bushing type: Match to your NVH tolerance and performance needs.
- Material: Steel is cheaper, aluminum saves weight.
- Brand reputation: Established companies provide better geometry engineering and support.
Avoid bargain kits that use thin-wall tubing or poor-quality hardware. Inspect all parts for weld quality and hardware grades.
Installation Tips and Maintenance
Installation requires basic fabrication skills: welding brackets to the frame and axle, setting pinion angle, and aligning the driveline. Always follow the kit instructions for torque specs and link lengths. After installation, check clearance at full compression and droop. Grease all fittings and torque bolts after initial settling.
Maintenance involves inspecting bushings for wear, tightening rod ends, and checking shock mounts. Off-road use demands more frequent checks—especially after hard runs.
Final Recommendation
A 4 bar suspension offers excellent control and tunability, but the parts must match your vehicle and use case. For a first-time builder, a complete bolt-on kit from a reputable manufacturer is the safest choice. If you're experienced, mix and match components from suppliers known for quality. Prioritize bushing type and arm adjustability—they make the biggest difference in ride and performance. Invest in proper bracketry and hardware, and don't skip the setup process. With the right parts, a 4 bar suspension can transform your vehicle's handling both on and off the road.