Prado: Re-valving Foam Cell Shocks for Heavy Loads

Toyota Prado 120 Series
1

This document details the process of selecting, re-valving, and testing long-travel hydraulic foam cell shocks for the rear of a Toyota Prado, focusing on fade resistance and performance under heavy loads. It outlines the limitations of smaller twin-tube and monotube shocks, the advantages of larger bore hydraulic shocks, and the steps involved in customising the valving for optimal performance.

Problem

Standard 2" twin-tube shocks are prone to fading quickly under heavy loads due to their small oil volume. Monotubes offer a bigger bore but still have limited oil volume, leading to overheating. Furthermore, the factory valving in some larger bore shocks (like Koni 90-5404) is too soft for lifted vehicles or those carrying heavy loads, resulting in poor handling characteristics such as body roll and a “pogo stick” effect.

Solution

The solution involves installing and re-valving Ironman 4x4 Foam Cell Pro shocks (specifically the 45682FE model) to improve fade resistance and handling. The process includes:

  1. Selecting appropriate shocks with sufficient oil volume and travel.
  2. Collaborating with the manufacturer (Ironman 4x4) to develop custom valving based on specific needs and coil rates.
  3. Disassembling the shocks, replacing the valve stacks, and reassembling them.
  4. Testing the re-valved shocks under demanding conditions to ensure optimal performance and fade resistance.

SOLUTION: Install and re-valve Ironman 4x4 Foam Cell Pro shocks (45682FE) with custom valving.

Relevant Parts & Specs

  • Ironman 4x4 Foam Cell Pro Shocks: Part number 45682FE
    • Open Length: 618mm
    • Closed Length: 383mm
    • Oil Volume: Approximately 680cc
  • Koni 90 series RAID shock: Part number 90-5404
    • Open length: 587mm
  • Bilstein B46-1478S
  • Possible Alternative (but not directly compatible): Ironman 4x4 45683FE (80/105 series front shocks)
    • Open Length: 615mm
    • Closed Length: 370mm
  • Coil Rate Recommendation: ~280lb/in (for heavy rear)

Disassembly and Re-Valving Process

  1. Disassembly:

    • Use the Ironman disassembly tool to remove the top dust seal.
    • Remove the oil seal/rod guide/pressure tube assembly.
    • Drain the oil and remove the foot valve using a cold chisel.
    • Hammer off the rod guide.
    • Disassemble the foot valve and piston head.

  2. Valve Stack Replacement:

    • Replace the valve shims on the foot valve and piston head with the new custom-designed valve stacks.
    • Re-torque the shaft nut and foot valve nut, ensuring the foot valve is functioning correctly.

  3. Reassembly:

    • Refill the pressure tube and overflow chamber with oil.
    • Cycle the piston head to bleed air from the system, topping up the rod guide bleed holes as needed.
    • Reassemble the top works using the assembly tool, forcing the rod guide back into the pressure tube.

Tools Required

  • Ironman Disassembly Tool
  • Cold Chisel
  • Hammer
  • Deadblow Hammer
  • Basic hand tools

Performance Testing

The re-valved Ironman shocks were tested under extreme conditions (45°C in the shade, 60°C sand temperatures) and demonstrated excellent fade resistance. The shock temperatures remained within a manageable range (around 50-60°C) and did not exhibit any signs of fading during hard driving.

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