When you tighten a bolt, you want it to stay tightly done up until the next time you come to undo it, right? We've all heard horror stories about brake calipers falling off, and it's doubtless too easy to think, 'It'll never happen to me'. It can happen, but it won't if you've got the right tools and know how to use them. Equally, you don't want to tighten up a bolt so much that you can't get it undone again, nor do you want to damage the thread. Even more importantly, you don't want to place stresses on parts of your motorcycle where they shouldn't exist. All of which means, you need a torque wrench.
A torque wrench allows you to accurately dial in the tension to which you fasten your motorcycle's bolts. The old, beam-type wrenches were pretty unsophisticated, as they simply measured the bend of the wrench handle, under strain, in order to estimate the torque. Modern wrenches, have an internal 'ball-and-spring' clutch mechanism.
You dial in the required amount of torque, which sets the preload of the internal spring holding the ball in its socket. The spring holds the ball in place, under tension, until the force you push through the wrench counteracts this tension. When the force handle reaches the required level, the preload force of the spring is overcome and the ball pops out of its socket. Job done. The owner's manual of your motorcycle contains torque settings for all of your motorcycle's important bolts. Refer to it religiously, and take care when dialing the measurements into your torque wrench. On most motorcycles, the rear-wheel bolt is the one that needs to be done up tightest. Unless you ride a tractor, you'll never need to tighten any bolt to more than about 130Nm.
Of course, most bolts require far less torque than that. And in the same way you wouldn't use a sledgehammer to crack a nut, you don't want a meter-long wrench for tightening small, delicate bolts. Hence, it's good idea – for the sake of convenience and torque accuracy, one small and one large.
As you probably expect, torque wrenches vary in price a great deal. Unless you're a professional mechanic or doing lots of work on your motorcycle, you really don't need a top of the line torque wrench. Of course, some folk just like the feel and finesse of top-notch tools, and there's nothing wrong with that. But it doesn't matter how much you spend on a torque wrench, it's only as good as its accuracy level.
No torque wrench can be 100 percent accurate, because other variables come into play, such as the friction between the bolt and its mating surface. Ideally, your wrench should be accurate to within a few percent. If it gets damaged through overuse or ill treatment, it can become de-calibrated and will need to be sent to a specialist with the necessary machinery to get it back in pristine order. So, treat your torque wrench with care.