This article will discuss the basics of torque and torque wrench use. See also related article on Basic Thread Concepts. This article includes a table of various torque recommendations. The Bicycle Specific Torque Values is available as an Adobe Acrobat PDF file.

Threaded fasteners (nuts and bolts) are used to hold many components to the bike. As a fastener is tightened, the fastener actually flexes and stretches, much like a rubber band. This stretching is not permanent, but it gives the joint force to hold together, called "preload", or tension. Each fastener is designed for a certain range of tension. Too much tightening will deform the threads or the parts. Too little preload will mean the fastener will loosen with use. This can damage components, such as a crank ridden with a loose mounting bolt. Loose bolts and nuts are also generally the source of various creaking on the bike.

Tension in the fastener depends largely upon the amount of torque (tightening) and the size of the thread. Generally, engineers will specify a thread size large enough to handle the anticipated stresses. For example, the M5 bolt of a water bottle cage bolt would not be a good choice for holding a crank. Even if the bolt were as tight as possible, it would not provide enough force to hold the arm secure to the spindle. The crank-to-spindle interface receives quite a lot of stress, making larger threads (M8, M12, M14) a better choice. The amount of pressure applied by a thread can be substantial in order to hold the joint secure. For example, a fully tightened crank bolt can provide over 14,000 Newton force (3,000 pounds) as it holds the arm in place.

It is commonly believed that bolts and nuts often come loose for no apparent reason. However, the common cause for threaded fasteners loosening is simply lack of tension during initial assembly. Vibration, stress, use, or abuse cannot typically overcome the amount of clamping force in a properly sized and secured threaded fastener. As a simple rule of thumb, any fastener should be tightened as tight as possible without failure of the thread or the component parts. This means the weakest part of the joint determines the limits of tension, and hence, torque.

Torque Measurements

Torque for mechanics is simply a twisting or turning motion around the axis of the thread. This resistance can be correlated to, but is not a direct measurement of, fastener tension. Generally, the higher the rotational resistance, the greater tension in the threaded fastener. In other words, the more effort it takes to tighten a bolt, the tighter it is.

Torque is measured as a unit of force acting on a rotating lever of some set length. In the USA, the common unit used to measure torque is the inch-pound (abbreviated in-lb.). This is a force of one pound acting at the end of a lever (wrench) only one inch long. Another torque unit used in the USA is the foot-pound (abbreviated ft-lb.), which is the force in pounds along a one-foot long lever. It is possible to convert between the two units by multiplying or dividing by twelve. Because it can become confusing, it is best to stick to one designation. The units given on the torque table here will be the in-lb.

A more universally accepted torque measurement is the Newton-meter (abbreviated Nm). One Newton-meter is a force of one Newton on a meter long lever. Another option sometimes used is the Kilogram-centimeter (abbreviated kgf-cm), which is a kilogram of force acting on a lever one centimeter long. It is possible to convert between the various systems.

in-lb = ft-lb. × 12   EXAMPLE: 5.5 ft-lb × 12 = 66 in-lb
in-lb = Nm × 8.851   EXAMPLE: 9 Nm × 8.851 = 79.7 in-lb
in-lb = kgf-cm × 0.87   EXAMPLE: 300 kgf-cm × 0.87 = 261 in-lb

Torque Wrench Types

Torque wrenches are simply tools for measuring resistance to rotation. There is a correlation between the tension in the bolt and the effort it takes to turn it. Any tool, even a torque wrench, should be used with common sense. A cross-threaded bolt will not properly tighten even with a torque wrench. The mechanic must be aware of the purpose of torque, and what torque and fastener preload doing to the component joint. It is also important to consider thread preparation, which is discussed in detail at the end of this article.

Torque wrenches available to general industrial work, including bicycle work, are typically accurate to plus or minus four percent. In other words, a torque wrench set for 100 in-lbs might tighten to 96 in-lbs, or 104 in-lbs. There are basically three types of torque wrenches, the beam type, the click type and the dial type. The click type and dial type wrenches have more moving parts than the beam, and are susceptible to wear and will require re-calibration. The bicycle has relatively light torque specifications (20-inch pound) to relatively heavy torque specifications (600 inch-pound). There is no single torque wrench accurate for this wide range. Consequently, Park Tool offers two different wrenches.

Beam Type

Park Tool TW-1, and TW-2 torque wrenches are beam-type. The beam design is relatively simple, and is accurate for both left-hand and right-hand threading. The socket head holds two steel beams, a primary beam and an indicator or pointer beam. The primary beam deflects as the handle is pulled. The separate pointer beam remains un-deflected, and the primary beam below flexes and moves with the handle. The reading is taken at the end of the pointer, at the reading plate on the primary beam. The handle is moved until the desired reading is attained. These wrenches rarely require re-calibration. If the pointer needle is not pointing to zero when the tool is at rest, it is simply bent back until it does align. Fatigue in the steel is not an issue.

Beam Type Torque Wrench Calibration

It is possible for any torque wrench to come out of calibration. The beam type wrenches use a simple principle of deflection that allows the user to re-calibrate the wrench. Inspect the pointer when the wrench is at rest. If it is pointing to zero on the scale, the wrench is calibrated. If the pointer is off to either side, the pointer beam can be bent back so it again points at zero. It is easiest to use a lever between the two beams and pry small amounts, checking the scale and the pointer often.

Re-calibration of the beam type wrench.

Bicycle Torque Specifications

Below is a table of torque equivalents and formulas for conversions follow the torque table. The table is also available as a PDF file HERE.

Formulas for converting other torque designations into Netwon meter (Nm) and inch pounds (in-lb.):

Nm = in-lb x 0.113

Nm= ft-lb x 1.356

Nm= kg-cm x 0.0981

in-lb = ft-lb × 12

in-lb = Nm × 8.851

in-lb = kgf-cm x 0.87

 

Torque Equivalencies