Sprung Idler Test Vehicle (SITV)

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The Sprung Idler Test Vehicle (SITV) was a specialised experimental platform developed under the auspices of the Military Vehicles and Engineering Establishment (MVEE) at Chertsey.

The Sprung Idler Test Vehicle (SITV) #

In the early 1970’s the Military Vehicle Engineering Establishment (MVEE) started research work into active suspension systems at about the same time it was working on the hydrogas suspension system for the Challenger Main Battle Tank and built the Suspension Research Vehicle (SRV) based on a CVR(T).

Initial work made use of Citroen gas springs and an early electronic controller. Lucas supported the project with a different active controller but the overall system was too complex to be adopted.

In 1980, MVEE looked again, this time with Lotus, also using a CVR(T). The trials Scorpion made use of a digital controller which provided much improved reliability and performance than the MVEE and Lucas units trialled a decade or so earlier.

MVEE merged with the Royal Armament Research and Development Establishment (RARDE) in 1984

Other projects in conjunction with Horstman developed a greater understanding of utilising hydrogas suspension in smaller armoured vehicles resulting an an unusual vehicle called the Sprung Idler Test vehicle (SITV) that was a CVR(T) Striker in a reverse configuration with rear mounted engine and drive sprocket.

The SITV served as a testbed for advanced suspension systems in tracked vehicles, emphasising improvements in ride quality, pitch dynamics, and crew comfort during high-speed off-road operations.

It incorporated hydrogas suspension units, which allowed for detailed evaluation of dynamic responses, including whole-body vibration (WBV) and pitch behaviour under acceleration, braking, and terrain irregularities.

The vehicle was not intended for operational deployment but contributed to broader research on controllable suspensions and tracked mobility enhancements.

Development of the SITV stemmed from ongoing work at MVEE and later RARDE on hydrogas and active suspension concepts, building on computer modelling and scale models to validate real-world performance.

It drew from prior experiments with hydrogas springs, including those applied to main battle tanks (MBTs), and focused on variables such as temperature effects, damper stiffness, and idler compensation to mitigate pitch oscillations.

Aspect	Details
Suspension System	Hydrogas units with polytropic index ≈1.66 (at 0.001–1.0 Hz frequencies), increasing to 1.69 under gas chamber pressure; damper rates 120–220 kN (m s⁻¹)⁻¹; limiting forces 50 kN (bump), -40 kN (rebound); spring and damper characteristics evaluated across temperature ranges.
Pitch Damping	For a six-wheel-per-side setup (1.0 m wheel spacing): C_p = 32.26 C_w (where C_w is individual wheel damping in kN (m s⁻¹)⁻¹); critical pitch damping C_pc ≈ 5,520 × 10³ kN m (rad s⁻¹)⁻¹, based on pitch inertia ≈439,256 kg m² and natural frequency ≈1.0 Hz.
Ride Height & Displacement	Suspension travel 350–450 mm (bump/rebound); effective spring force/displacement loops tested at 0.8 Hz (-50 mm to +350 mm); wheel loads up to 200 kN; ride height variations e.g., +59 mm at 50°C, -100 mm at -30°C (without tracks).
Additional Features	Compensating idlers for pitch control; compliance with WBV standards (BS 6841:1987, ISO 2631-1:1997, ISO 2631-5:2004); variants of hydrogas units including external telescopic, in-arm, and twin-cylinder configurations.

The platform was instrumental in refining suspension designs for lighter armoured vehicles, addressing challenges like vibration-induced crew fatigue and component wear.

Tests included the following

Ride Performance: Pitch responses to braking and acceleration using sinusoidal inputs (0.001–2.0 Hz, amplitudes ±175–200 mm); assessments of spring/damper interactions and terrain-induced WBV via absorbed power metrics.

Pitch Response: Manoeuvres on turns with idler adjustments (e.g., as implied in dynamic modelling figures); multi-wheel evaluations on specialised test courses, validated against quarter-car and full-vehicle simulations.

Vibration and Shock: Compliance testing to ISO and BS standards for crew WBV; extreme temperature trials to quantify damping near critical levels (e.g., C_w ≈158 kN (m s⁻¹)⁻¹)

Notes: Explanations of Key Terms and Acronyms #

Hydrogas units: Advanced hydropneumatic suspension systems that utilise hydraulic fluid and compressed gas (typically nitrogen) to provide springing and damping. These units are employed in armoured vehicles such as the Challenger 2 tank to enhance mobility, reliability, and performance under heavy loads, up to 75 tonnes when equipped with additional armour and systems.
Polytropic index: A thermodynamic parameter (denoted as n) in the equation PV^n = constant, describing the compression and expansion behaviour of gas in systems like hydrogas suspensions. In the context of vehicle shock absorbers, it characterises the gas pressure-volume relationship during piston movement.
Hz: Hertz, the unit of frequency, representing cycles per second.
kN: Kilonewton, a unit of force equivalent to 1,000 newtons.
m s⁻¹: Metres per second, a unit of velocity.
Damper rates: The damping coefficients, measured in kN (m s⁻¹)⁻¹, which quantify the resistance provided by the damper to motion.
Limiting forces: The maximum forces the suspension can withstand; ‘bump’ refers to compression (positive force), and ‘rebound’ to extension (negative force).
Pitch damping: The damping of rotational motion around the vehicle’s lateral axis (pitch), which helps control oscillations and maintain stability over uneven terrain in tracked vehicles.
C_p: Pitch damping coefficient for the vehicle.
C_w: Individual wheel damping coefficient.
C_pc: Critical pitch damping, the minimum damping required to prevent oscillatory motion without overshoot.
rad s⁻¹: Radians per second, a unit of angular velocity.
kg m²: Kilogram metre squared, the unit for moment of inertia.
Natural frequency: The inherent frequency at which the suspension system oscillates when disturbed, typically around 1.0 Hz for vehicle ride comfort.
Ride height: The vertical distance from the ground to the vehicle’s chassis or hull, which can vary with temperature and load.
Suspension travel: The total range of vertical movement available in the suspension, divided into bump (compression) and rebound (extension).
mm: Millimetres, a unit of length.
Spring force/displacement loops: Hysteresis loops plotting force against displacement, used to evaluate the energy absorption and stiffness characteristics of the suspension.
Wheel loads: The vertical forces acting on each wheel or roadwheel, up to 200 kN in heavy vehicles.
Compensating idlers: Adjustable idler wheels in tracked vehicles that maintain track tension and assist in pitch control during suspension articulation.
WBV: Whole Body Vibration, referring to mechanical vibrations transmitted to the human body, which can affect operator health and comfort.
BS 6841:1987: British Standard for the measurement and evaluation of human exposure to whole-body mechanical vibration and repeated shock.
ISO 2631-1:1997: International Standard for mechanical vibration and shock, providing general requirements for evaluating human exposure to whole-body vibration.
ISO 2631-5:2004: Part 5 of the ISO 2631 series, specifying methods for evaluating vibration containing multiple shocks.
External telescopic, in-arm, and twin-cylinder configurations: Variants of hydrogas units; ‘external telescopic’ resembles conventional shock absorbers mounted outside the arm, ‘in-arm’ integrates the unit within the suspension arm for compactness, and ‘twin-cylinder’ uses two parallel cylinders for enhanced capacity.