• Although the design is slightly different the principle is the same from the 405 beam and the 205 beam in the picture-

    The tube in the centre is stationary. A pair of bearings poke out around 4" from where you can see the mounting points going inside the trailing arm. This allows the hub to rotate on a point about 12" forward of the hub.
    The torsion bars are the two bars you can see travelling parallel to the tube, one end is mounted into the trailing arm and the other is mounted into the stationary casting at the end of the tube. As the axle moves it twists the bars. The length and the thickness change the stiffness of the bar, not in a simple straight graph like 10mm gives 100lb/ft and 20mm gives 200lb/ft - it curves upwards to a point where a 2mm change in thickness can give you an extra 60-70lb/ft.
    On the 205 the ARB sits inside the tube but on the 405 it sits outside and in front. The 205 is merely pulled into place and bolted into the ends of the bars. The 405 bars are put in place then clamped. A better design imo as they're less likely to seize

    The bonuses of this system is a wide flat floor and fast change of ride height or spring poundage on a race car. Downsides being that you can't change camber or castor without machining your own stub axles. The rear suspension is pretty good anyway as Peugeot have proven in their rally history with the 205 and 306

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