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Let's Talk Torque!

Let's Talk Torque!

Bike torque wrench
It's important to know about torque as most bike parts require specific torque settings for many installations. So let's talk some torque.
First, the definition: torque is force applied in a rotating or twisting motion. Think of a bottle cap. The effort (force) it takes to break the cap loose is torque ("break-free torque"), and when you put the cap back on, at the moment you stop turning the cap, the force required in that moment is the torque value to which the cap has been secured ("security torque").
There are many torque units, because they can be the result of a combination of any number of force units combined with any number of lever-length units.
Think of tightening a nut or bolt with a wrench that is one foot (twelve inches) long. If applying a force of 10 pounds to the end of the foot-long wrench, the security torque achieved would be 10 foot pounds (10 pounds x 1 foot). Because "foot pounds" takes a lot of space to write, and because it is conventional to express measurement units as abbreviations or as symbols, "10 foot pounds" usually appears as "10 ft-lb." Interestingly, it's not "10 ft-lbs" because "lb" is an abbreviation for the Latin word "libra" (the singular Latin word for "pound of weight"), the plural of which is "librae" (so both abbreviate identically to "lb"). Fortunately, when we verbalize these torque values, we don't have to say "1 foot-libra, or 10 foot-librae).
Enough of the grammar lesson. In the above example, with the wrench length and the force remaining the same, if you express the wrench length in inches, then the torque value becomes "120 in-lb" (1 ft. x 12 in. x 10 lb. = 120 in-lb).
Inch pounds are by far the most common primary units utilized on mechanical torque wrenches used in the USA (in the torque ranges typically found on bicycle parts). However, bicycle-component manufacturers often provide torque specifications in Newton Meters (abbreviated "Nm," or sometimes "N*m").
Many mechanical torque wrenches have two scales (in-lb and Nm), but the Nm scale is secondary, which results in all the Nm values being in decimal form instead of in whole units (for example, 120 in-lb appears as 13.56 Nm on the metric scale of a tool that's primary scale is non-metric).
A new category of torque wrench is becoming common, known as "torque limiting" torque wrenches. These may be adjustable, or set to a fixed value, but they give way once the value has been reached, making overshooting the intended torque impossible. This category is usually marked in Nm units, and this style of tools are made available in the range of torques typically found on stem bolts. It has also become conventional for manufacturers to silk screen Nm torque values onto components. Digital torque wrenches exist for many (but not all) applications, and these can be effortlessly toggled between the common torque units.
Consequently, today's mechanic must be familiar with both inch-pound and Newton-meter units, and be able to convert between them. The math is simple: in-lb x .113 = Nm, and Nm x 8.85 = in-lb; alternately in-lb / 8.85 = Nm and Nm / .113 = in-lb. Following are links for two online converters (the same website has the capacity to convert between almost any two known torque units):
A few "rogue" torque units may be encountered that are primarily vestiges of the past. For example, in the metric realm, kgf-cm (kilograms of force applied to centimeters of lever length) sometimes appears. Also, while "foot-pounds" were used earlier in this article, it's rare that this unit appears any more as a bicycle-component torque spec. Foot-pounds are more commonly used on cars and other heavier machinery.
Thanks to John Barnett for some great torque talk. John started the Barnett's Bicycle Institute which is the premier training facility for all qualified bicycle mechanics.
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