Power Trains and Transmissions
Core concepts
Trace power from engine to clutch or torque converter, transmission, differential, final drives, and wheels. Know manual gear transmissions, power shift, hydrostatic drives, and continuously variable transmissions (CVT), plus the clutch, damper, and PTO clutch packs. Understand differential action, final-drive ratio, and how tire size affects ground speed and torque. For four-wheel drive and MFWD, know engagement methods and lead/lag ratio between axles.
Traps
Questions often assume a differential divides torque equally at all times; it divides torque equally but allows speed difference. Another trap is mixing up power shift (clutch packs shift under load) with hydrostatic (variable displacement pump and motor). Tire size mismatch across an axle or between axles causes driveline wind-up and premature wear; do not assume identical nominal sizes match rolling circumference.
Rule of thumb
Gear ratio = driven teeth / driving teeth. Overall ratio multiplies each stage, and output speed = input speed / overall ratio. For MFWD, front-to-rear lead is typically a small percentage; mismatched tires change that lead and stress the driveline.
Worked example
Engine 2200 rpm, transmission ratio 4:1, final drive 6:1. Overall ratio = 24:1, so axle speed = 2200/24 = 91.7 rpm. With a 1.6 m rolling radius, ground speed = 2 x pi x 1.6 x 91.7 / 60 ≈ 15.4 m/s (about 55 km/h), confirming how gearing and tire radius set speed.