Mustang II A Arms
Mustang II A-arms—the upper and lower wishbone control arms that govern steering axis inclination, roll center dynamics, and dynamic wheel camber on classic Mustang II independent front suspension systems—are the mechanical heartbeat of custom hot rod chassis engineering. While installing tubular A-arms provides structural rigidity and aesthetic appeal, achieving exceptional road manners, razor-sharp steering turn-in, and zero tire wear requires mastering the delicate geometric alignment relationships of the Mustang II system. Navigating upper cross-shaft shim packs, dialing in optimal dynamic camber curves, engineering anti-dive braking geometry, and correcting the infamous 'bump steer' defect inherent in modified rack-and-pinion front ends ensures your custom build drives like a modern sports car.
Suspension Geometry: Camber Curves and Anti-Dive Angles
The fundamental geometry of the Mustang II double-wishbone suspension relies on unequal-length A-arms: a short upper A-arm paired with a long lower A-arm. In vehicle dynamics, this unequal-length arrangement is intentionally engineered to generate negative camber gain during cornering. When a car enters a hard turn and body roll causes the outside suspension to compress (jounce), the shorter upper arm swings through a tighter radial arc than the lower arm, pulling the top of the tire inward.
This dynamic camber gain counteracts body lean, keeping the tire tread footprint flat against the road for maximum cornering grip. Furthermore, the angle at which the upper A-arm cross-shaft mounts to the frame crossmember dictates 'anti-dive' geometry. By tilting the rear of the upper control arm cross-shaft downward slightly relative to the lower arm (typically 3 to 5 degrees of anti-dive rake), braking torque produces an upward mechanical vector that naturally resists front-end nose dive under hard emergency braking.
The wheel alignment geometry table below details optimal street rod specifications for Mustang II A-arms.
| Alignment Parameter | Factory 1970s Ford Specs | Modern Street Rod / Cruiser | Pro-Touring / Track Handling | Handling Impact |
|---|---|---|---|---|
| Caster Angle | +0.5° to +1.5° Positive | +3.0° to +4.5° Positive | +5.0° to +6.5° Positive | High-speed highway tracking & self-centering |
| Camber Angle | +0.5° Positive (Tire tilts out) | -0.5° Negative (Tire tilts in) | -1.0° to -1.5° Negative | Maximizes tire contact patch in aggressive turns |
| Total Toe-In | 1/8 to 1/4 Inch Toe-In | 1/16 to 1/8 Inch Toe-In | Zero to 1/16 Inch Toe-In | Prevents tire feathering; ensures crisp turn-in |
| Anti-Dive Angle | 0 to 2 Degrees | 3 to 4 Degrees rake | 4 to 5 Degrees rake | Prevents front bumper dipping hard during stops |
| Scrub Radius | Positive scrub (1.5 in) | Near-zero scrub (+0.5 in) | Slight negative or zero scrub | Reduces steering kickback on bumpy roads |
Modern radial tires demand negative camber (-0.5 deg) and generous positive caster (+3.5 deg); setting a hot rod to 1970s bias-ply factory specs will cause unstable highway wander.
Bump Steer Elimination and Rack-and-Pinion Positioning
The single most notorious handling defect encountered on custom Mustang II front suspensions is 'bump steer.' Bump steer occurs when a vehicle's front suspension moves up and down over road bumps and the front tires autonomously steer left or right without any driver input on the steering wheel. This terrifying phenomenon is caused by a geometric mismatch between the arc of the A-arms and the arc of the steering tie rods.
To achieve zero bump steer with Mustang II A-arms, the steering rack-and-pinion inner tie rod pivot point must align along an imaginary vector drawn from the upper A-arm inner pivot through the lower A-arm inner pivot. Furthermore, the tie rod must run parallel to the lower control arm at ride height. If a custom crossmember mounts the steering rack too low or too high—or if builders install an aftermarket rack with incorrect inner pivot width—the tie rod pushes or pulls the steering knuckle over every bump. Correcting bump steer requires adjustable outer tie rod end kits with precision aluminum spacer shims.
The troubleshooting guide below diagnoses common handling and alignment issues with Mustang II A-arm suspensions.
| Handling Symptom | Underlying Geometric Cause | Diagnostic Measurement | Mechanical / Alignment Remedy |
|---|---|---|---|
| Car Darts Over Bumps (Bump Steer) | Tie rod not parallel to lower A-arm | Toe changes as suspension cycles | Install adjustable bump-steer tie rod spacers |
| Highway Wandering / Floating | Insufficient positive caster (< +2 deg) | Check alignment caster readout | Add shims to rear upper bolt / remove from front |
| Tire Outer Edge Wear | Excessive positive camber or toe-in | Tire tread depth gauge across ribs | Adjust upper shims to achieve -0.5 deg camber |
| Harsh Bottoming Out | Shock absorber too long / coil spring too soft | Measure shock travel at curb height | Install stiffer rate springs or adjustable coilovers |
| Loud Creaking Over Speed Bumps | Dry unlubricated polyurethane bushings | Inspect grease fittings on pivots | Inject synthetic waterproof grease via Zerk fittings |
Always perform final alignment checks with the vehicle at true curb ride height with full fluids and ballast weight placed in the driver's seat.
How to Align Mustang II A-Arms Using Shims in 4 Steps
Follow this alignment procedure to dial in caster and camber on a Mustang II crossmember.
Establish Exact Curb Ride Height and Level Floor
Place the vehicle on level ground or alignment turnplates with normal tire pressures and fuel tank filled.
Adjust Upper Cross-Shaft Shims to Set Positive Caster
Add alignment shims behind the rear cross-shaft bolt (or remove from the front) to tilt the upper ball joint back to +3.5 to +4 degrees.
Equalize Shims on Both Bolts to Dial In Negative Camber
Add equal shims to both front and rear cross-shaft bolts to push the upper arm out, or remove shims to achieve -0.5 degrees camber.
Adjust Steering Tie Rod Sleeves to Lock Toe-In
Loosen rack-and-pinion tie rod jam nuts; rotate the adjuster sleeves to achieve an exact 1/16-inch total front toe-in.
Frequently Asked Questions (7 Questions Answered)
Q1: What is the proper caster setting for Mustang II A-arms?
For modern radial tires, set caster to +3.0 to +4.5 degrees positive; this ensures high-speed straight-line stability and steering wheel return.
Q2: What causes bump steer on a Mustang II front suspension?
Bump steer occurs when the tie rods are not parallel to the lower A-arms or the rack pivots do not align with the control arm pivot vectors.
Q3: How do you adjust camber on a Mustang II suspension?
Camber is adjusted by adding or removing horseshoe-style alignment shims between the upper A-arm cross-shaft and the frame mounting brackets.
Q4: What is anti-dive geometry on Mustang II control arms?
Anti-dive tilts the rear of the upper control arm cross-shaft downward, producing a mechanical force that resists front-end dipping during braking.
Q5: Can I use dropped spindles with Mustang II A-arms?
Yes. 2-inch drop spindles are the most popular way to lower a Mustang II front end because they lower the car without sacrificing suspension travel.
Q6: What size alignment shims are used on Mustang II upper arms?
Standard 1/2-inch slot horseshoe alignment shims (available in 1/16", 1/8", and 1/32" thicknesses) are used behind the cross-shaft bolts.
Q7: Why does my hot rod dart to one side when I hit a pothole?
This is bump steer; your steering tie rod is travelling on a different arc than the A-arms, forcibly turning the wheel as the suspension compresses.
Final Thoughts & Key Takeaways
In conclusion, understanding mustang ii a arms provides essential clarity, practical strategies, and actionable advice. By incorporating these foundational insights, adhering to verified safety guidelines, and following structured best practices, you ensure reliable, long-term outcomes while preventing common mistakes. Stay informed, consult certified professionals when needed, and maintain consistent quality care.