Learn about Torque
Our friends over at pinkbike.com interviewed Pedro’s about the importance of torque specifications and use of torque wrenches for their “To the Point” technical series. Take a look below or view the original story here: To the Point: Torque Specifications
What is torque? How is it measured?
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.
Why is knowing the proper torque spec important for bicycle components? What happens if you over-tighten a bolt?
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.
How does a torque wrench work? Are there different types?
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.
What is the proper technique for using a torque wrench?
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.
What about storage? Does a torque wrench need to be at a certain setting before putting it away?
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.
How often does a torque wrench need to be calibrated? How is this done?
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.
Occasionally, you’ll hear someone say “I don’t need to use a torque wrench – I can just tell when it’s tight enough.” Any thoughts on this statement?
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.