Week 2 · Session 4
Springs, Damping, and Travel for Solid Axle Rigs
60 min lecture + 60 min lab

Learning Objectives
  • Select spring types and rates for solid axle 4×4 applications
  • Understand the shock technology hierarchy from twin-tube to bypass
  • Grasp travel budgets, droop, and bump stop design
  • Recognise shock heat as the primary desert performance killer
Coil Springs for Solid Axles

Coil springs are standard on modern solid axle 4×4s. Rate selection is based on load and target static deflection:

Spring Rate from Load
Where Fstatic = static load (N), δdesign = target static deflection (m). Off-road target: 100–130 mm.

Progressive vs. linear: Progressive starts soft and gets stiffer — compliance over corrugations, resistance over big hits. The most versatile desert choice.

A “2-inch lift spring” is simply longer free length and/or higher rate. Trade-off: more clearance but less droop unless shock and linkage accommodate the new range.

The Shock Technology Hierarchy

Shocks don’t hold weight — springs do. Shocks control the rate of motion by forcing oil through orifices and past valving shims.

Type Heat Capacity Stage
Twin-tube Poor — fades quickly Not for desert
Monotube Adequate Stage 1 minimum
Remote reservoir Good (+30–50% oil) Stage 2 standard
Internal bypass Good + position-sensitive Stage 2–3
External bypass Excellent + zone-adjustable Stage 3

Bore size: 2.0″ is entry performance, 2.5″ is the sweet spot, 3.0″+ is for race trucks. A 2.5″ bore has ~56% more oil than 2.0″.

Understanding Shock Heat

A 2.0″ bore shock through whoops at 80 km/h can exceed 120°C within minutes. Viscosity drops, damping disappears. Solution: oil volume (bigger bore, reservoir), synthetic fluid, proper nitrogen charge. Your shocks matter more than your springs in the desert.

Travel Budget and Bump Stops

Total wheel travel = bump + droop. Touring 4×4: 200–250 mm. Competition: 350+. Shock length dictates the usable range; springs just position ride height within it.

Hydraulic bump stops (King, Fox) provide progressive resistance at end-of-travel instead of a jarring hit. They transform desert capability — you can use the last 2–3 inches of travel safely. The single biggest quality-of-life upgrade for desert driving. Stage 2 essential.

Limit straps prevent over-extension that can stretch brake lines, damage ABS wiring, or overextend shocks. Essential for long-travel builds.

Complete Solid Axle Springs & Damping Map

Part Function Stage
Coil springs (linear) Hold weight, set ride height 1
Progressive / dual-rate Soft initial rate, stiffer for hits 1+
Leaf springs Support weight AND locate axle 1
Monotube shocks Desert minimum — floating piston, better cooling 1
Remote reservoir +30–50% oil volume, desert standard 2
Internal bypass Position-sensitive damping 2–3
External bypass Zone-by-zone adjustable 3
Rubber bump stops Basic end-of-travel protection 1
Hydraulic bump stops Progressive end-of-travel. Transforms capability. 2+
Limit straps Prevent over-extension damage 2+

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LAB
Spring and Shock Measurement

  • Measure installed spring length and free length. Calculate approximate rate from known vehicle weight.
  • On stands: measure full droop and full bump. Calculate total travel.
  • Compare stock shock to Dobinsons MRR: measure dimensions, hand-cycle at different speeds.

ASSIGN
Spring Rate and Shock Selection

A Patrol Y61 owner wants a 2″ lift for touring with 300 kg constant load. Stock rear rate ~30 N/mm.

  • Calculate required spring rate to maintain ride frequency at loaded weight.
  • Recommend a Dobinsons spring and justify.
  • Specify what shock to pair — explain bore size, reservoir type, thermal capacity considerations.


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