Understanding torque
Force times lever, expressed in three notations.
What torque is physically, the N·m / lbf·ft / kgf·m units, and why torque wrenches come in three flavours that don't quite agree.
Torque = force × lever arm.
A force applied at a distance from the axis of rotation produces torque (also called moment of force). The lever arm is the perpendicular distance from the axis to the line of force. SI unit: newton-metre (N·m). Same physical dimensions as energy (joule = N·m), but conceptually different — torque is a vector quantity related to rotation.
τ = F × r (perpendicular)
The four units to know.
N·m (SI, used in automotive specs worldwide). lbf·ft (US automotive, shop manuals). lbf·in (smaller fasteners). kgf·m (older European specifications, slowly disappearing). Conversion factors: 1 lbf·ft = 1.356 N·m; 1 kgf·m = 9.807 N·m; 1 lbf·in = 0.113 N·m. Memorise the rough ratios — most people only ever convert between two specific pairs based on their industry.
A worked conversion.
A bike's bottom bracket needs to be torqued to 35 N·m. Imperial spec: 35 / 1.356 ≈ 25.8 lbf·ft, or 35 / 0.113 ≈ 310 lbf·in. The bike shop's torque wrench reads in lbf·in for low-range fasteners, so 310 lbf·in is the working number. Convert once, work in the spec your tool reads.
Bicycle BB torque
35 N·m → lbf·ft → lbf·in
One value, three notations across the metric/imperial divide.
35 / 1.356 = 25.8 lbf·ft ; 35 / 0.113 = 310 lbf·in
= 35 N·m = 26 lbf·ft = 310 lbf·in
Torque wrench accuracy.
A consumer torque wrench is accurate to ±4 % at the upper 80 % of its range and significantly worse at the lower end. Reading 10 N·m on a tool calibrated for 50-250 N·m is unreliable — pick a wrench whose range centres on your target. For aerospace and racing applications, calibration matters more (±1 % wrenches exist, ~10× the cost). For everyday automotive and bicycle work, a consumer wrench is fine provided you use it within its sweet spot.
Engine torque is different from fastener torque.
The "200 lb-ft of torque" on a car spec sheet is the engine's rotational output. Same units as fastener torque, very different application. Engine torque varies with RPM; the peak number on the spec sheet is the maximum of that curve. Tools and machinery (impact drivers, drills) report a maximum torque output too. Same physical units; different real-world meanings.