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What is torque?
Torque is the concept of applying a twisting force around an axis. Torque is defined as the value obtained when you multiply of a force and the perpendicular distance from the axis.
We can demonstrate it with a bolt and nut in the above diagram. Torque is simply the application of a force some distance from the axis to make that twisting action happen. As shown in above diagram, if I double the length of the wrench, I can get the same torque at the bolt by halving the force.
Torque is measured in units that relate this lever arm length and the force applied at a particular distance. In the imperial scale, generally, torque is measured in foot pounds and in the metric scale, it’s measured in newton meters.
This is an important concept for not just bolts and nuts and other fasteners on your motorcycle, but also concepts like acceleration of a motorcycle. Torque is what does the accelerating of your bike, but we can talk about that some other time. To start with, we just need to understand the relationship between this lever arm length and the force that you apply at the end of it.
Want to watch a video rather than read about it? Check it out…
Why is torque important?
Well, we need to go back a few steps.
Bolts are used to clamp parts together. The size and numbers of bolts used in the design of the parts depend on the expected stress the joint will see in service. The engineers have to determine through calculation, testing or simulation what sort of forces might act on the joint. If they are the bolts that hold your cylinder head onto the cylinders, those bolts have to withstand the pressure acting on the surfaces due to the controlled explosions happening in the cylinder.
Bolted joints work by clamping the parts together very tightly. Once they’re clamped together, friction between the parts makes them difficult to slide apart.
Bolts work in “tension”, which means they act like a spring. If we bolt two parts together, the bolt will stretch like a spring as the nut is tightened. As the bolt is stretched, an equal and opposite clamping force acts on the parts.
On the small bolts found on your motorcycle, it’s not difficult to “overtighten” and even break bolts. If I pull on a spring, I can pull it so far that eventually it will deform, and not return to its original shape. Bolts are no different.
When I tighten a nut, or a bolt that assembled into a threaded hole, I am stretching the bolt. If I continue to tighten, the bolt stretches and stretches and eventually goes past its “elastic limit”. It won’t spring back to its original shape. In some specific applications, bolts are tightened into this range. Often the service manual will instruct you to replace bolts that have been tightened in this manner. This is more commonly seen in heavy duty applications, like trucking and mining equipment.
Once the bolt passes the elastic limit it will break soon after if it continues to be tightened.
If the bolt is too loose, the joint will be loose, and it will move when it is used. This is a bad outcome as you can imagine.
In practice, there’ll be a recommendation in your bike’s service manual that specifies the torque to use on that bolt in that particular assembly and that’s determined by the engineer through the design and testing of that assembly.
To tighten the bolt to the specified torque, you need a tool called a torque wrench. There are a variety of torque wrenches available, so we’ll just focus on the commonly available “click-type” wrench found in retail automotive shops, or from online tool vendors. Similar to your first wrenches, I would suggest a 3/8″ drive torque wrench should be fine for motorcycle work, and will be compatible with the socket wrench set you might buy.
Most modern click-type torque wrenches are adjustable for a range of torque values. You can adjust the scale for the torque you wish to achieve at the socket end. Once you reach that value, it will give you a loud audible click to tell you that you’ve reached the desired torque.
Using a torque wrench
If you are reusing a bolt, clean old dirt, grease and paint off the threads. A bench grinder with a wire wheel is ideal for this. Alternatively, clamp the bolt head in a vice with the threads pointing up, and use an angle grinder with a wire wheel. At worst, use some kerosene, or WD-40 to loosen deposits, and brush off any deposits with a wire hand brush.
Obtain the correct torque value from the service manual. If the units differ between your wrench and the service manual, use the Norbar app to convert the units.
Check the instructions in the service manual. Is the bolt supposed to be dry, or lubricated? Lubrication will allow the bolt to stretch further before reaching the torque wrench setting.
Install the bolt and tighten it down by hand or with another wrench until it is “snug”.
Select the correct-sized socket to suit the bolt or nut and install it onto your torque wrench.
Loosen the small locking nut at the bottom of the wrench handle.
Inspect the scale on the side of the wrench to determine where your desired torque value is located.
Twist the handle and screw the graduated edge of the collar up to your desired value.
Hand-tighten the small locking nut at the bottom of the wrench handle.
Use the torque wrench to tighten the bolt to the desired torque. Apply smooth movement to the wrench until you hear a loud “click”. Stop pulling on the wrench immediately. You have reached the desired torque.
Loosen the small locking nut at the bottom of the wrench handle.
Unscrew the handle and re-set the at the lowest value.
Care and storage of torque wrenches
Click-type wrenches contain a spring in the handle to operate the mechanism. When you turn the handle to increase the setting of the wrench, you are putting a pre-load on the spring. It’s good practice to loosen the adjusting mechanism off as mentioned above. This reduces the chance the spring will take a set while under tension as it sits in your toolbox for six months.
A torque wrench is generally considered a precision instrument, so don’t drop them, or using them in everyday repairs as a socket wrench. If you are working in a quality-approved organization, these would be calibrated on a regular schedule to ensure they are accurate. While researching some details for this article, I found a couple of useful articles showing how to calibrate a torque wrench in your own shop. They’re listed in the Resources section below. This might not pass muster if you were repairing a Boeing 787, but would be adequate for most of our custom bike repairs.
Having torque wrenches roll around loose in your tool box is probably not going to help them. Most wrenches are supplied in a plastic case. I keep mine in the case supplied. Worst case, just keep them somewhere where they’ll avoid knocks.
Click here to download a free 18-page eBook all about torque:
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Torque units converter app – iPhone and Android (Norbar)
Proper Torque Wrench Use and Maintenance (Snap On)
Calibrate a Torque Wrench
How-to calibrate a Torque Wrench