Week 6 · Session 12
Wheels, Tyres, and the Contact Patch
60 min lecture + 60 min lab

Learning Objectives
  • Understand how wheel and tyre selection affects suspension performance
  • Grasp tyre pressure as the cheapest and most impactful suspension adjustment
  • Quantify alignment angles and their effects on tyre wear and handling
  • Map the universal parts that apply to every 4×4 desert build
Wheels and Tyres — The Universal Parts

Part Function Key Consideration
Wheels (width/offset) Stance, clearance, scrub radius Lightweight forged = less unsprung mass
Beadlock wheels Clamp tyre bead mechanically Essential below 15 psi; Stage 2–3
Wheel spacers Push wheels out for clearance Temporary; adds bearing load. Prefer correct offset.
Tyres (size/construction) Only contact with terrain Bias vs. radial; ply rating; flotation vs. puncture resistance
TPMS Real-time pressure monitoring Essential for desert — catching slow leaks, monitoring heat
CTIS Air up/down from driver’s seat Ultimate convenience; expensive

← Scroll →

Tyre Pressure as a Suspension Tool

Airing down from 35 psi to 18 psi effectively softens the first stage of your suspension. The tyre sidewall becomes a compliant element that absorbs small impacts before the spring and damper see them.

In soft sand, lower pressure increases the contact patch for flotation. This is the cheapest and most impactful “suspension upgrade” you can make in the desert. A $50 ARB deflator and a good compressor deliver more capability per dollar than almost any bolt-on mod.

Beadlock wheels mechanically clamp the tyre bead to prevent it unseating at very low pressures (below 12–15 psi). For serious dune work where you’re running 8–12 psi, beadlocks are essential. For touring at 18–22 psi, standard wheels are fine.

Alignment: Camber, Caster, Toe

Camber (γ) — tyre lean. Negative = top leans inward. Positive = top leans outward. Lift without UCAs creates excessive negative camber → inner edge wear.

Caster (δ) — steering axis tilt. More caster = better self-centering and high-speed stability. Lift reduces caster → steering feels vague.

Toe (τ) — direction the wheels point relative to centreline. Toe-in = stability. Toe-out = turn-in response. Toe change during suspension travel (bump steer) makes the vehicle dart over bumps.

Pull Radius from Misalignment
Where v = speed, g = gravity, α = effective misalignment angle. Smaller radius = harder pull.
Critical

Use a 4WD specialist for alignment, not a tyre shop. Every suspension change requires realignment. Budget for it as part of every build.

Brakes and Skid Protection

Bigger tyres increase rotational inertia and braking distance. Desert heat adds thermal stress. Slotted rotors and high-temp pads are the minimum upgrade for any vehicle running larger tyres off-road.

Skid plates protect engine, transfer case, fuel tank, and differentials from impact. Aluminium for weight-conscious builds, steel for maximum protection. Essential for desert crests and rough terrain.

Extended differential breathers — factory breathers clog with dust. Routing them high prevents sand and water ingestion. One of the cheapest and highest-value mods on any 4×4.

LAB
Alignment and Tyre Pressure

  • Measure toe using the string method.
  • If alignment machine available: measure camber, caster, toe on a lifted vehicle.
  • Air down a tyre in stages (35 → 25 → 18 → 12 psi) and observe sidewall bulge and contact patch change.

ASSIGN
Misalignment Diagnosis

A vehicle pulls right with inner-edge wear on the left front. Diagnose the most likely alignment fault. Calculate the pull radius at 100 km/h. What corrections would you make?


← All 16 Chapters