Vehicle Controls, Handling, Dimensions and Stability
What the exam tests
This domain covers control use, steering technique, gearing and retarders, vehicle dimensions, mass effects, centre of gravity, off-tracking and rollover risk. Questions often ask what a driver should do before a hazard, not during it.
Core concepts
- Pre-select a low enough gear before a descent; use the retarder or engine brake to hold speed and save the foundation brakes for stopping.
- Brake and slow down before a curve, then maintain a smooth throttle through it. Hard braking mid-curve transfers weight and reduces roll stability.
- Centre of gravity: high, off-centre or shifting loads increase rollover risk. Liquid loads surge; hanging meat or livestock moves the effective CoG.
- Off-tracking: the rear wheels of a long vehicle cut inside the path of the front wheels. In a turn, check the trailer's swept path and use mirrors through the whole movement.
- Rearward amplification: in a multi-trailer combination, the last trailer can swing wider than the lead unit during a rapid lane change.
- Know your height, width, length, axle loads and turning circle. Measure clearance, not estimate it.
- ABS helps steering control during hard braking but does not shorten stopping distance on all surfaces.
Worked example
A tanker enters a decreasing-radius curve too fast. Reasoning: braking hard in the curve adds forward weight transfer and reduces the outer tyre's grip, while liquid surge pushes the CoG outward. The correct action is to brake before the curve, select a safe gear, and use gentle steering and steady throttle. If already too fast, straighten the steering slightly while braking to regain grip rather than adding more lock.
Traps
- Braking late and hard inside a curve.
- Using the retarder as a parking brake or on a slippery surface without checking traction.
- Forgetting trailer cut-in when turning near kerbs or poles.
- Estimating bridge or canopy height instead of checking the posted clearance.