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4WD Suspension Technology
MICHAEL HAYES REVEALS AND EXPLAINS ALL ABOUT 4WD SUSPENSION TECHNOLOGY.
Big Lifts And Caster Correction
If you own a coil-sprung 4WD, you probably understand the versatility of the radius arm suspension platform and just how well they can handle most off-road conditions. When you lift a radius arm vehicle, the arms are fixed to the diff and the rear pivots around the chassis bushes. This added lift causes the diff to rotate and effectively reduces the caster.
So what is caster? When a vehicle's front suspension is aligned, caster is adjusted to achieve the self-centring action of steering. Improper caster settings will often cause vague steering response and the lack of self-centring. This, combined with larger or greater offset tyres, will amplify any slack or loose suspension or steering joint play.
Positive caster
is when the steering knuckle pivot points of the diff are angled such that a line drawn through them intersects the surface slightly ahead of the contact patch of the tyre. This makes is easier to drive and improves directional stability. An example of positive caster is the front forks on a bike that tilt forward from the body.
A 4WD with negative caster will tend to be easier to steer but will lack directional stability and handling, like a wheel on a shopping trolley. The vehicle will also be easily affected by any track surface variation such as small irregularities or bumps.
HOW DO I FIX THE CASTER?
Caster Bushes
Caster correction bushes replace the original front radius arm bushings and have off-centre pins that put the caster angle back within vehicle specifications.
PROS: Cheap
CONS: Reduced bush life, reduction in suspension travel
Caster Plates
Caster correction plates are a bolt-on adapter plate that provides an alternate mounting hole position for the front of the radius arm. Lower angle plates can require welding, larger plate correction angles of 6in plus lift are generally fully bolt on.
PROS: Cheap, DIY
CONS: Added strain to the pivot pin
Drop Boxes
The 'Drop Box' is a fabricated alternate rear mount for the radius arm. This mount lowers the rear mount of the arms to counteract the same amount of lift, essentially returning the suspension to its original geometry.
PROS: Cheap, DIY, drive-ability
CONS: Reduction in ground clearance
Dropped Radius Arms
When performance matters most, dropped radius arms accurately correct caster and also address the pin or bush mounting geometry to allow for maximum wheel travel and greater handling. As well as correcting caster, added design features such as Superior Engineering's SuperFlex mounting technology can also be incorporated to dramatically increase wheel travel in radius arm type vehicles.
PROS: DIY, driveability, strength, clearance, articulation
CONS: Cost
Hybrid Drop Box / Long Arms
Superior Engineering has developed a combination Hybrid Long-arm Drop box style radius arms. The extended arc of the longer flatter arm allows the suspension to properly soak up the bumps, and the new drop box that changes the mount orientation to a radial rear bush. Combine this with SuperFlex mounting and wheel travel is massive while still retaining excellent manners.
PROS: DIY, driveability, ride, strength, clearance, articulation
CONS: Cost