Overview
Performance suspension upgrades for 1973-1987 GM Square Body trucks transform these classic vehicles from their factory utility-focused setup into capable performers for street, strip, or off-road applications. The factory suspension, while adequate for work truck duties, leaves significant room for improvement in handling, ride quality, and performance applications.
Square Body trucks came with basic leaf spring rear suspension and independent front suspension (IFS) on 2WD models, or solid front axle on 4WD variants. Performance upgrades can dramatically improve cornering ability, reduce body roll, eliminate wheel hop, and provide better traction for drag racing or towing applications.
This comprehensive guide covers suspension modifications from mild street improvements to full race-ready setups, addressing both 2WD and 4WD configurations across all Square Body variants.
Common Issues
Factory Suspension Limitations
- Excessive body roll - Factory sway bars are undersized or non-existent
- Wheel hop under acceleration - Particularly problematic with V8 swaps or performance builds
- Poor cornering response - Soft springs and inadequate damping
- Inconsistent ride height - Sagging rear springs common on older trucks
- Limited adjustability - No provisions for corner balancing or fine-tuning
Age-Related Problems
- Worn bushings - Rubber components deteriorate after 35+ years
- Sagging leaf springs - Loss of arch and spring rate
- Blown shocks - Original equipment rarely survives decades of use
- Cracked spring hangers - Stress cracks in mounting points
- Worn ball joints - Particularly on 2WD IFS systems
Parts & Materials Needed
Basic Street Performance Setup
Front Suspension (2WD):
- Drop spindles: Belltech 2" (Part #2340) or DJM 2" (Part #2593)
- Performance shocks: Bilstein B6 (Part #24-186728) or QA1 Stocker Star (Part #SS501)
- Sway bar: Hellwig 1.25" (Part #7714) or Addco 1.125" (Part #704)
- Polyurethane bushings: Energy Suspension Master Set (Part #3.18105)
Rear Suspension:
- Lowering springs: Belltech 4" (Part #6403) or DJM 4" (Part #C2594-4)
- Performance shocks: Bilstein B6 (Part #24-186729)
- Sway bar: Hellwig 7/8" (Part #7672)
- Spring hangers: Classic Performance Products reinforced (Part #7584SH)
High-Performance/Racing Setup
Front Suspension:
- Coilover conversion: QA1 K-Member (Part #52315) or Ridetech StreetGrip (Part #11240298)
- Tubular control arms: Global West (Part #1042) or UMI Performance (Part #2043)
- Performance ball joints: Moog K6136 (upper) and K6024 (lower)
- Rack and pinion conversion: Flaming River (Part #FR1595)
Rear Suspension:
- 4-link conversion: QA1 (Part #52320) or Ridetech (Part #11169298)
- Coilovers: QA1 Pro-Coil (Part #PR502) or Viking Double Adjustable (Part #VCS502)
- Panhard bar: UMI Performance (Part #2090)
- Torque arm: Spohn Performance (Part #TA-7387)
Tools Required
- Floor jack and jack stands (minimum 3-ton capacity)
- Socket set (metric and standard)
- Torque wrench (ft-lb and in-lb)
- Spring compressor (coil spring applications)
- Ball joint separator
- Pickle fork set
- Cutting torch or sawzall (fabrication work)
- Welder (MIG recommended for modifications)
Step-by-Step Guide
Phase 1: Basic Street Performance (2WD)
Front Suspension Upgrade
-
Remove factory components
- Raise vehicle and secure on jack stands
- Remove wheels and brake components
- Disconnect tie rod ends and ball joints
- Remove spindles and springs -
Install drop spindles
- Torque specifications: Upper ball joint 125 ft-lb, lower ball joint 83 ft-lb
- Apply thread locker to all fasteners
- Verify brake line routing with lowered position -
Install performance shocks
- Mount upper shock through factory tower
- Torque: Upper mount 18 ft-lb, lower mount 65 ft-lb
- Ensure full compression/extension travel -
Add front sway bar
- Mount brackets to frame rails
- Torque: Frame bolts 35 ft-lb, end link nuts 25 ft-lb
- Use polyurethane bushings for improved response
Rear Suspension Upgrade
-
Remove factory leaf springs
- Support rear axle with jack
- Remove U-bolts and spring plates
- Remove front and rear spring hangers
- Note: Mark spring orientation for reference -
Install lowering springs
- Position new springs with proper arch orientation
- Install reinforced hangers if upgrading
- Torque: Front hanger 95 ft-lb, rear shackle 75 ft-lb -
Install performance shocks
- Mount lower shock to axle bracket
- Route upper mount through bed floor
- Torque: 65 ft-lb both ends
Phase 2: Advanced Performance Setup
Coilover Conversion (Front)
-
Modify K-member (if required)
- Some conversions require welding new mounting points
- Ensure proper alignment of mounting brackets
- Test fit before final welding -
Install coilover assemblies
- Set initial spring preload per manufacturer specs
- Typical starting point: 1" preload on 200 lb/in springs
- Adjust ride height through spring collars -
Tubular control arm installation
- Replace factory stamped arms with adjustable units
- Set initial length to factory specifications
- Fine-tune after alignment
4-Link Rear Conversion
-
Remove leaf spring setup completely
- Cut or unbolt all mounting points
- Ensure axle is properly supported during conversion -
Install 4-link brackets
- Weld frame brackets per kit specifications
- Critical: Verify axle centerline and pinion angle
- Use jig for proper positioning -
Mount coilovers and panhard bar
- Position coilovers for optimal geometry
- Install panhard bar for lateral location
- Set ride height and test suspension travel
Year/Model Differences
1973-1980 vs 1981-1987 Differences
Frame variations:
- 1973-1980: Heavier gauge frame rails, different shock mounting points
- 1981-1987: Lighter construction, updated suspension mounting locations
- Brake line routing: Earlier trucks require different brake line kits
Steering systems:
- 1973-1980: Recirculating ball steering standard
- 1981-1987: Some models available with power steering variations
- Rack conversion compatibility: Different kits required for each generation
Model-Specific Considerations
C10/C20 (2WD)
- Independent front suspension allows for easier coilover conversions
- Lower ride height achievable without major modifications
- Lighter front end weight benefits handling upgrades
K10/K20 (4WD)
- Solid front axle limits suspension options
- Higher center of gravity requires different spring rates
- CV joint angles must be considered with lift/drop modifications
K5 Blazer/Suburban
- Shorter wheelbase (Blazer) affects handling characteristics
- Heavier rear end (Suburban) requires stiffer spring rates
- Full convertible top (early Blazer) may require chassis reinforcement
Axle Ratio Considerations
- 10-bolt rear: Adequate for street performance, limit torque applications
- 12-bolt rear: Preferred for high-performance applications
- 14-bolt rear: Heavy-duty applications, requires specific suspension components
Tips & Best Practices
Installation Best Practices
- Always replace bushings when upgrading suspension components
- Use thread locker on all critical fasteners
- Check clearances throughout full suspension travel
- Perform alignment immediately after installation
- Break in gradually - avoid aggressive driving for first 500 miles
Spring Rate Selection
Street Performance:
- Front: 300-400 lb/in (depending on engine weight)
- Rear: 150-250 lb/in (based on intended use)
Drag Racing:
- Front: 150-200 lb/in (weight transfer optimization)
- Rear: 300-500 lb/in (anti-squat control)
Road Racing:
- Front: 400-600 lb/in (reduced body roll)
- Rear: 250-400 lb/in (balanced handling)
Shock Valving Guidelines
- Street: 70/30 compression/rebound split
- Track: 50/50 split for consistent lap times
- Drag: Soft compression, firm rebound (rear)
Budget Considerations
Phase 1 ($1,500-2,500):
- Drop spindles and springs
- Performance shocks
- Sway bars and bushings
Phase 2 ($3,500-5,500):
- Coilover conversion
- Tubular control arms
- Complete bushing replacement
Phase 3 ($6,000+):
- Full 4-link rear
- Adjustable coilovers
- Professional installation and tuning
Troubleshooting
Handling Issues
Excessive understeer:
- Increase front spring rate or decrease rear
- Add or increase front sway bar diameter
- Check front tire pressure (increase if low)
Excessive oversteer:
- Decrease front spring rate or increase rear
- Add rear sway bar or reduce front bar diameter
- Verify proper rear axle location
Harsh ride quality:
- Check shock valving (may be over-damped)
- Verify spring rates aren't excessive for application
- Inspect bushing condition and material
Installation Problems
Binding in suspension travel:
- Check control arm geometry and length
- Verify proper shock mounting angles
- Inspect for interference with brake lines or exhaust
Uneven ride height:
- Verify spring rates match side-to-side
- Check for sagging or damaged springs
- Ensure proper spring installation and orientation
Noise issues:
- Lubricate all grease fittings if equipped
- Check sway bar bushings for proper fit
- Verify all fasteners are properly torqued
Alignment Specifications
Street Performance:
- Caster: +3° to +5°
- Camber: -0.5° to -1.0°
- Toe: 1/8" total toe-in
Track/Autocross:
- Caster: +5° to +7°
- Camber: -1.5° to -2.5°
- Toe: 0° to 1/16" toe-out
Drag Racing:
- Caster: +2° to +4°
- Camber: 0° to -0.5°
- Toe: 1/8" to 1/4" toe-in
Remember that suspension modifications may affect other systems including brakes, steering, and drivetrain components. Always test thoroughly and consider professional installation for complex conversions.