Hi kenshireen,

I recommend using a torque wrench. As dsbrantjr wrote, "40 Nm is a surprisingly large amount" and it's true. I use SRAM cassettes on all my family's bikes and they use 40 Nm. I would never have tightened the aluminum SRAM lockring as tight as I should if I was doing it "by hand" and I think I have a fairly good "feel" for this kind of work.

When purchasing a torque wrench, I recommend going "old school" and buying the old "beam" type because they don't go out of calibration as easily as the "click" type. Park Tool's TW-1 and TW-2 are good examples. Plus, they are usually less expensive. If you purchase a click type, be wary of the cheap ones because they may not be calibrated correctly. And be extra wary of inexpensive automotive click-type torque wrenches because some only work in one direction. Bicycles have a number of prominent parts that use reverse threads (drive-side bottom bracket end caps and pedals) so your daughter should have a torque wrench that works in both directions.

Torque is much more important with modern bikes than old bikes. I care for two carbon-frame bikes in my family and you MUST be very mindful of torque or you can damage the frames, carbon handlebars and aerobars. I think your daughter is probably showing wisdom by recognizing the benefit and, in some cases, need for a torque wrench.

The range of torque used in bikes is quite wide. Some components may require only 2-5 Nm of torque like the bottle cage bolts attached to a carbon frame. They must be tight enough so the parts don't rattle apart on a bumpy road---but if they are tightened too much, they may pull the metal sleeves out of the carbon. Other components may require over 500 Nm of torque (like a bottom bracket end cap). It is impossible for one torque wrench to handle such a wide range accurately. So most bike mechanics have two---a small low-range torque wrench for small torques and a larger high-range torque wrench for big torques.

Finally, there is one more thing your daughter may need to know. One way to achieve a good grip at a lower torque is to use friction compound at the joint. It is available at most any bike shop (I use Park Tool SAC-2). It is used for most joints that include carbon. But it is just as good with metal joints and is becoming popular with softer metals like aluminum. It allows you to get a good joint that won't move or squeak at a lower torque, thereby not over-tightening and breaking bike parts.

Kind regards, RoadLight