Torque is a turning force: a force applied at a distance from a pivot. The SI unit is the newton-metre, N·m: a force of one newton applied one metre from a bolt produces one newton-metre of torque. Imperial tools and older manuals use pound-force feet, lbf·ft, often written ft-lb, and for small fasteners pound-force inches, lbf·in. Some older Japanese and European manuals use kilogram-force metres, kgf·m. One lbf·ft is about 1.356 N·m, and one kgf·m is about 9.807 N·m.
Where you meet it
Most people meet torque when they use a torque wrench, on car wheels, bicycle components or engine work, and when reading car specifications, where engine torque is quoted in N·m or lb-ft. Bicycle parts are an area where precision really matters. Carbon handlebars, seatposts and stems are often specified at around 4 to 6 N·m, and over-tightening can crack them. Many bike torque wrenches are marked in N·m, but some US-made tools use inch-pounds.
Torque isn't energy
Torque and energy share the same dimensions, a force multiplied by a distance, which is why both can be written in newton-metres. But they're different quantities. Torque is always written N·m, and energy is written in joules, even though one joule equals one newton-metre of work. A torque spec in "joules" or an energy figure in "newton-metres" is usually a sign something has been mixed up.
Worked conversions
A 5 N·m torque spec for a carbon seatpost clamp is about 44 lbf·in, the figure you'd set on an inch-pound wrench. A car wheel nut specified at 110 N·m is about 81 lbf·ft. An engine producing 350 N·m of torque delivers about 258 lb-ft. When converting for a torque wrench, round to the wrench's scale divisions rather than over-precision, and never round upwards on delicate parts.
Engine torque and power together
Car specifications usually quote both torque and power, and they describe different things. Torque is the turning force the engine produces, which you feel as pull when accelerating from low speed. Power is torque multiplied by engine speed, which determines how fast the car can go and how quickly it accelerates at higher speeds. Diesel engines and electric motors produce high torque at low engine speeds, which is why they feel strong when pulling away or towing. Petrol engines often produce their peak torque higher in the rev range.
Using a torque wrench well
Torque wrenches are precise but easy to misuse. Click-type wrenches should be stored at their lowest setting so the spring doesn't lose calibration, and they should be pulled smoothly until they click rather than jerked. Settings below about 20% of a wrench's range are less accurate, so a large car-wheel wrench isn't suitable for small bicycle bolts.
Checking your answer
One lbf·ft is about 1.36 N·m, and one N·m is about 0.74 lbf·ft. Small bicycle bolts are usually single figures of N·m; car wheel nuts around 80 to 140 N·m; large truck wheel nuts several hundred. If a conversion puts a seatpost bolt at 50 N·m, an inch-pound figure has probably been taken as foot-pounds.
One more worked example
A motorcycle manual specifies an axle nut at 65 lbf·ft. On a torque wrench marked in newton-metres, set about 88 N·m. If the wrench is marked in kgf·m, as some older tools are, set about 9.0 kgf·m. Always confirm whether a figure is for a dry or lubricated thread, because lubrication changes how much clamping force a given torque produces.
Torque angle
Some modern fasteners, such as cylinder head bolts, are tightened to a torque and then turned through an extra angle, for example 90 degrees. The angle step stretches the bolt to a precise clamping force more reliably than torque alone. Follow the manufacturer's sequence exactly, and replace single-use stretch bolts rather than reusing them.