Torque converter

What it's for

Tightening torque is the figure printed next to every bolt that matters: car wheels, the cylinder head, bicycle components, assembling a heavy piece of furniture. The trouble is that the figure arrives in different units depending on who wrote the manual, and the torque wrench in your hand shows only one. Here you move from one to the other.

The conversions, and the one that does damage

1 kgf·m = 9.81 N·m (older continental manuals talk in kilogram-metres) and 1 lbf·ft = 1.356 N·m (British and American manuals). But the real trap is another one, and it's inside the anglosphere system: lbf·ft and lbf·in are not the same thing, they differ by twelve times. Bicycle components and small fastenings are nearly always in inch-pounds, and reading them as foot-pounds means tightening twelve times harder than you should.

Why tightening «properly» doesn't mean tightening hard

A bolt holds because it's stretched like a spring, and the torque is there to bring it to exactly that stretch. Under it, vibration works it loose; over it, the metal yields and from then on it doesn't hold at all, even though it feels very tight. That's why the values are so precise: car wheels are generally between 110 and 130 N·m, while the stem of a carbon bicycle is at 5-6 N·m, twenty times less, and giving it an extra squeeze «to be safe» cracks it.

Torque and power are not the same thing

Torque is the twisting force, power is torque multiplied by revolutions: an engine can have plenty of torque and little power (a tractor) or the opposite (a racing engine that only pulls high up). Spec sheets always show both, and they get confused constantly. For power units there is Power converter.

Nearby tools

For force units there is Force converter, for tyre pressure Pressure converter and for lengths in inches Length converter.