Steering, Brakes, Wheels and Tyres
What is tested
Steering, Brakes, Wheels and Tyres tests steering geometry and operation, brake function and bleeding, wheel fitting, tyre size and inflation, and safe ballasting.
Core rules
- Ackermann steering makes the inner front wheel turn a tighter angle than the outer wheel, so all wheels roll without excessive scrub. Toe-in, camber, caster and kingpin inclination affect stability and tyre wear.
- Hydrostatic steering uses a pump, orbitrol valve and steering cylinder; loss of flow causes heavy or lost steering, so check oil and relief pressure.
- Brakes may be dry or wet, mechanical or hydraulic. Bleed from the wheel farthest from the master cylinder, keep the reservoir full, and use the specified fluid.
- Tyre size such as 16.9-28 means section width in inches and rim diameter in inches; it is not the overall diameter. Use the tyre data book for load and inflation.
- Wheel nuts must be tightened in a cross pattern to the specified torque; recheck after the first few hours.
Worked example
A brake pedal feels spongy and travels too far. The likely cause is air in the hydraulic circuit. The reasoning is that air compresses while brake fluid does not, so pedal movement is lost. Bleed the farthest wheel first, then the next farthest, maintaining the reservoir level and closing the bleed screw before releasing the pedal. Recheck for leaks and pedal firmness.
Traps
Mixing tyre sizes on the same axle, which affects rolling circumference and can damage the differential; over-inflating for field work; and using the wrong brake fluid. Another trap is fitting a wheel without cleaning the mating faces, causing nuts to loosen.