To the Point: Torque Specifications

In simple terms, torque is a twisting force that is likely to cause an item to rotate.Torque
is calculated as force multiplied by length with the length being the distance between the
rotational axis and the point where the force is applied. Torque is measured most commonly
as Newton meter (Nm), pound-inch (lb-in), or pound-foot (lb-ft) units. The last two are common
in the U.S. but are often referred to as inch pounds and foot pounds. Newton meters are
standard on most bicycle components. As it relates to bicycle mechanics, torque is most
often related to component fasteners. These fasteners are threaded, which converts the torque
into a linear force used to hold components in place or to provide a clamping force to hold two
components together. A common example of this is the interface between a stem and handlebar.

Torque is calculated by multiplying the amount of force (F) by the length (L) of the lever.

As a human powered vehicle, the weight of a bicycle is a major contributor to performance. This
requires engineers to push the limits of each material and design they choose. The engineer must
factor in material properties, part shape, riding conditions, product life, and more. The torque
specification provided with a bicycle or component makes sure the forces applied to these parts
are within the intended limits. If the torque is too low, a component is likely to slip or fall off. Conversely,
if the torque is too high, the component or fasteners may be over-stressed and fail. In either case,
significant injury could result. While torque specifications are important with all type of materials,
the growing number of carbon fiber components has led to an increasing focus on torque and the use
of torque wrenches. While carbon fiber allows for more optimal design and provides a far higher strength
to weight ratio compared to steel and aluminum, it is also more susceptible to crushing and cracking
when improperly set up. Simply put, the margin for error is much smaller. For this reason, using a
torque wrench has become essential.

The general principle of a torque wrench is that as torque is applied to a fastener, the
torque wrench uses a calibrated mechanism to display the torque applied or otherwise
indicate when a specified torque has been reached. The three most common types of
torque wrenches are beam type, click type, and digital. A beam type torque wrench
uses two parallel rods, one being the wrench handle, and the second having a torque
display scale. The handle rod is designed to bend as torque is applied to the fastener.
The torque display scale remains unbent allowing the relative angle between each rod
to indicate the torque. These are the least expensive and simple type of torque wrench,
but rely on the user to constantly look at the torque scale. This may be challenging in
some bicycle applications where wrench positioning is limited.

A click type torque wrench is the most common type offered in the cycling industry.
This type of torque wrench uses a calibrated clutch mechanism and connected pivoting
head to indicate when a preset torque has been reached. When the torque setting is
reached, the head displaces slightly and makes a clicking noise as a result. Click type
torque wrenches are available in fixed setting, specific to a single torque value, and
adjustable setting, which can be set to a range of torque values. Pedro’s Demi Torque
Wrench and Pro Torque Wrench are click type wrenches adjustable from 3 Nm to 15 Nm
and 6 Nm to 30 Nm respectively. We chose to offer this type of torque wrench because
we felt it was the best blend of accuracy, ease of use, ease of calibration, and value. A
digital torque wrench uses a strain gauge attached to a torsion rod to measure torque and
convert it into the common torque units using a processor. The digital torque wrenches work
on similar principles as a beam type torque wrench, but are more advanced, displaying the
torque value digitally as the fastener is tightened. In addition, many digital torque wrenches
sounds a digital alarm when a preset torque value is reached. These types are the most
expensive.

The technique for using each type of torque wrench varies, but the general principles are the
same. First, read any technical documents provided to determine torque specification and
thread treatment indicated by the manufacturer. Second, set your torque wrench to the specified
torque and double check the torque units. There is a huge difference between 10 inch pounds
and 10 foot pounds! A handy trick, though a bit finicky, is that Google search will do unit
conversions for you. Try it out by entering “5 newton meters in pound inches” and see what
you get. Worst case, you’ll find many free conversion tools. Third, apply treatment, such as
grease or threadlock, to the fastener threads if required by the manufacturer. Fourth, while
holding the torque wrench by its handle, begin tightening the fastener. When the correct torque
is reached, as indicated by the torque scale on a beam type, or by the click/alert on click or
digital types, stop tightening the fastener. Do not continue to tighten the fastener. This is
especially important on click/digital types as continuing beyond the set torque can damage
the torque wrench.

These four steps will cover the basics of using a torque wrench. When tightening components
using multiple fasteners, as commonly found on stems, it is very important to tighten them
using an alternating method, tightening half a turn or less before moving back to the other
fastener, and repeating this until both are tightened to the specified torque. This is important
because when one fastener is tightened, it loosens the others. Using a torque wrench, each
fastener should be tightened to the torque specification, and then re-checked for torque after
the other fasteners are tightened, repeating for each fastener until all meet the specified torque.
Just as important, this alternating method should be using when removing torque specific
hardware as well because when one bolt is loosened, the other is tightened which can lead
to thread damage. If there are four or more fasteners, they should be tightened in a crossing
pattern by moving to the fastener opposite to the one just tightened whenever possible. This is
the same technique used for tightening lug nuts on a car wheel ensuring equal tension on all
fasteners.

As a calibrated device, all torque wrenches should be used with care and stored in a protective case. For
adjustable click type torque wrenches, the wrench should be set to its lowest torque setting or approximately
20% of the maximum torque. For example, the Pedro’s Demi Torque should be set to 3 Nm for storage as this
is 20% of the maximum torque of 15 Nm. If the wrench has be unused for a long period of time, set the wrench
to 50% of the maximum torque and operate the click mechanism five to ten times before using the wrench on
a fastener. This is also suggested before the first use of a new torque wrench.

Calibration is most important for click type torque wrenches. The exact process varies from wrench to
wrench but the process typically involves adjusting preload on a calibrated spring inside the wrench.
The wrench is then checked against a device with a known and calibrated torque. We suggest having
Pedro’s torque wrenches be calibrated every three to six months with heavy (daily) usage. The occasional
user could likely wait longer if the wrench is treated well and hasn’t been dropped or abused. We also
strongly recommend wrench owners work with a professional calibration service to ensure the proper
equipment and techniques are employed. We have had good luck with ESSCO Lab located in
Massachusetts but any similar calibration service should be able to provide calibration or at very
least determine if the wrench is calibrated within tolerance.

Most mechanics are guilty of not using a torque wrench at least some of the time. However,
bicycles and riding itself have changed considerably in a very short time. Considering the
variables of fastener material, multiple fastener clamps, component material, wrench size,
thread treatment, component design, etc., there is simply no way to get a torque specification
correct by feel. Many mechanics still disagree, arguing that their years of experience have
given them the "feel" needed to properly torque hardware, but test after test has shown this
is rarely true.