Week 1 · Session 2
Forces on a 4×4: Load Transfer, Articulation, and Terrain
60 min lecture + 60 min lab

Learning Objectives
  • Derive lateral and longitudinal load transfer equations
  • Understand articulation and the Ramp Travel Index (RTI)
  • Recognise terrain types as forcing functions with distinct demands on suspension
  • Quantify the stability cost of lifting a 4×4
Lateral Load Transfer

When a vehicle corners, weight transfers from the inside wheels to the outside wheels. The amount depends on mass, lateral acceleration, and CG height relative to track width.

Lateral Load Transfer
Where m = mass (kg), ay = lateral acceleration (m/s²), hcg = CG height (m), t = track width (m).

This is why lifted 4×4s feel less stable in corners. A 50 mm lift on a Wrangler raises the CG by roughly 40 mm after accounting for tyre size. That directly increases load transfer — more weight to outside wheels, more body roll, less grip at inside wheels.

For a 2,200 kg Wrangler at 0.5g lateral, increasing CG height from 700 mm to 740 mm increases load transfer by about 5.7%. That’s the difference between confident and sketchy in a highway lane change.

Longitudinal Load Transfer

The same physics applies front-to-back. Under braking, weight shifts forward (nose dive). Under acceleration, weight shifts rearward (squat). On steep descents, the majority of weight can end up on the front axle.

This is particularly relevant for 4×4 use: hill climbing puts enormous load on the rear springs/shocks while unloading the front. The anti-squat and anti-dive geometric properties — determined by control arm angles — resist these transfers. When you lift a vehicle and change those arm angles, you change these properties too.

Build Connection

This is why adjustable control arms are on the geometry correction list for any lift over 2 inches. They’re not just about alignment — they restore the anti-squat and anti-dive behaviour the factory engineers designed in.

Articulation

Articulation is the suspension’s ability to allow one wheel to compress while the opposite droops — keeping all four tyres on the ground over uneven terrain. Measured using the Ramp Travel Index (RTI): drive one front wheel up a ramp until the opposite rear wheel lifts off.

Solid axle vehicles inherently articulate more than IFS, because the entire axle tilts as a rigid unit. IFS is limited by CV joint angles and independent travel range.

Sway bars are the biggest articulation limiter. They connect left and right suspension and resist differential movement. This is why sway bar disconnects (Wrangler’s electronic system, manual pin-pull) and Toyota’s KDSS exist. KDSS hydraulically decouples the sway bars off-road, then re-engages on-road — one of the most effective factory solutions for the articulation vs. stability trade-off.

Terrain as a Forcing Function

Desert driving is not a single condition. Each terrain type imposes different frequency and amplitude inputs on the suspension.

Terrain Frequency Amplitude Primary Demand
Washboard High (5–15 Hz) Low (10–30 mm) High-speed damper cycling, heat management
Dune crests Low (0.5–1 Hz) High (200–500 mm) Full travel, bump stop engagement
Rock shelves Single event Very high Impact absorption, compression damping
Soft sand Continuous Low Rolling resistance, weight, tyre pressure
Whoops Medium (1–5 Hz) Med-high (100–300 mm) Sustained damper performance, rebound control

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No single suspension setting is perfect for all of these. The art of desert setup is finding the best compromise for your dominant terrain — and having adjustment range to fine-tune when conditions change.

Desert Note

If you do one type of desert driving more than others, set up for that. A vehicle optimised for washboard cruising will handle dune crests adequately. A vehicle optimised for flat-out whoops will be stiff on washboard. Know your primary use case.

LAB
Load Transfer and Articulation Measurement

Duration: 60 minutes

  • Calculate load transfer for a specific vehicle at a given lateral acceleration and CG height.
  • Measure static articulation using a ramp or stacked blocks — document RTI.
  • If possible: drive over washboard at low speed and observe suspension movement.

ASSIGN
Lift and Load Transfer Analysis

For a Jeep Wrangler JL with a 3″ lift and 35″ tyres:

  • Calculate the change in lateral load transfer at 0.5g compared to stock. Use: stock CG ≈ 700 mm, lifted CG ≈ 776 mm, track ≈ 1,570 mm, stock weight ≈ 2,050 kg, lifted ≈ 2,200 kg.
  • Discuss implications for highway stability.
  • Explain why this vehicle might feel more planted in soft sand despite the higher CG.


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