Vehicle Body, Wheels, Tyres and Workshop Practice
Vehicle Body, Wheels, Tyres and Workshop Practice
This domain covers body construction, wheel and tyre specifications, and safe workshop procedures. The exam tests monocoque versus ladder frame, tyre size decoding, wheel balance, torque tightening and basic tool safety.
Core rules
- Monocoque bodies use stressed skin panels; ladder-frame chassis use separate rails and cross-members. Monocoque is lighter and stiffer for cars; ladder frames suit heavy loads.
- Tyre size 185/65 R15 means width 185 mm, aspect ratio 65% (sidewall height = 0.65 × 185 mm), radial construction, rim diameter 15 inches.
- Wheels must be balanced to prevent vibration; wheel nuts are tightened in a star sequence to the specified torque.
- Workshop practice: use jack stands, not only a jack; use the correct spanner size; disconnect the battery before electrical work; wear eye protection.
Worked example
For tyre size 185/65 R15, sidewall height = 185 × 0.65 = 120.25 mm. Rim diameter = 15 × 25.4 = 381 mm. Overall tyre diameter = 381 + 2(120.25) = 621.5 mm. Reasoning: the aspect ratio is a percentage of section width, and the rim diameter must be converted from inches to millimetres before adding two sidewalls.
Traps
- Reading 65 as 65 mm rather than 65% of width.
- Mixing inch and millimetre units in diameter calculations.
- Tightening wheel nuts in a circle rather than a star pattern.
- Working under a vehicle supported only by a hydraulic jack.