A torque wrench is a tool designed to apply a specific amount "tightness" or torque to a bolt or nut. Stereotypically torque wrenches are designed in the shape of a socket wrench with an internal mechanism that can be externally adjusted for different torque settings. So why do we use torque wrenches on mountain bikes? Just like with cars or airplanes, certain components of the bike need to tightened enough so they don't come loose. At the same time, if a particular component is overtightened, it might damage the part. Having a torque wrench is a great way to quantify exactly how tight bolts are on the torque sensitive areas of your bike.
Handlebars and Stem - With lightweight materials like carbon and titanium all over mountain bikes now a days, using a torque wrench has almost become mandatory so that you don't damage those extremely lightweight parts. Once you get in a good habit of using a torque wrench, you will want to use it on just about everything on your bike. One of the most important torque sensitive areas on your mountain bike is the cockpit. With carbon handlebars especially, it's very important not to over tighten the stem clamping bolts that hold the bars in place. Over tightening carbon handlebars can cause them to crack or possibly break. With the Deity Skywire Carbon handlebars for example, Deity recommends a maximum torque range of 4-4.5 Nm. Keep in mind this torque range would be different for different handlebars and manufacturers.
Suspension Linkage - Another highly torque sensitive area on your full suspension mountain bike is all of the suspension linkage hardware on the frame. It's important that each bolt and linkage axle on the frame is torqued properly that way the suspension system isn't binding or causing premature bearing wear. If some of the linkage hardware is overtightened, the bearings or bushing may be preloaded too much. Even if the suspension system is running smoothly, you may be killing those bearings. With a torque wrench you won't have that problem.
Derailleur - Some threads and bolts you have to be more careful with than others. Where your derailleur mounts up to the derailleur hanger is one of those spots. Sram is nice enough to put the torque spec right on the outside of the bolt head. On all of the Eagle 12 speed derailleurs, Sram recommends 10-12 Nm.
Crankset - Another torque sensitive component on your mountain bike is the crankset. Both threaded bottom brackets and crank arms recommend a much higher toque than most other components on your bike. With Sram's new DUB crankset, Sram recommends that the cranks be tightened to 54 Nm. This is where having a higher torque range torque wrench like the Park Tool TW-6.2 comes in handy. While you might not use is as often, they are still handy to have in the tool box.
Here at the shop we are big fans of Park Tool and all of their products. We also sell torque wrenches from brands like Pedros, Ritchey, and Effeto Mariposa Giustaforza, but you can't go wrong with any the different options from Park Tool. Here are some of our favorites that our head mechanic Liam uses every day.
The Park Tool TW-6.2 is a clicker style torque wrench with a 10-60 Nm range (88-530 Inch Pounds) that uses a 3/8 inch drive socket head. This torque wrench is designed for higher torque applications like threaded bottom brackets, cranksets, center lock brake rotors, etc. The TW-6.2 is both left and right hand thread compatible and includes its own case with Nm to inch pounds conversions.
The Park Tool TW-5.2 is the younger brother to the TW-6.2 above and offers a 2-14 Nm range (18-124 Inch Pounds) with a 3/8 inch drive socket head. Between the two, the TW-5.2 torque wrench is the one that will get used most of the time on components like stem and handlebar hardware, brake caliper bolts, brake lever mounting hardware, etc. The TW-5.2 is also left and right hand thread compatible and includes its own case.
The Part Tool ATD-1.2 is an adjustable torque driver that is designed as a quick and easy way to prevent over torquing different fasteners. The ATD-1.2 offers a 4-6 Nm range in .5 Nm increments. This torque driver comes with 3mm, 4mm, 5mm, and T25 bits that store conveniently in the handle. The magnetic sockets securely holds each bit in place when in use. The ATD-1.2 is convenient, easy to use, and works best with seat post clamps, chain guide mounting hardware, and brake lever mounting hardware.