Nuts and bolts are part of almost every toy building set - either in form of easy-to-grap wooden toys or in form of small brass parts. Also on a roadbike, bolts and nuts are everywhere - but did you know that a normal roadbike has around 120 to 130 bolted connections with completely different functions?
The bolts at the bottle cage are typical fixation bolts, with the nuts on the wheels' axes you can fine-tune and fix the bearing play; end stop bolts at front and rear derailleur limit the pivot range of the bicycle gearing, but except from this only have to hold themselves. The one commonality they have is that the way they are assembled decides if they can accomplish their task.
The wheels' spokes and nipples are just as well a bolt-and-nut-system - and a good example of how forces develop in bolts and what influence they have. (...)
This resilience of bolts is also taken advantage of with handlebar and stem, for example. In order to fix the handlebar firmly, the stem bolts' resilience has to be high enough to build up sufficient friction between handlebar and clamping area. If the bolt is overtightened, however, the stem constricts the handlebar and dents it. Other problems: Too much force applied can cause bolt heads to be pulled of or threads being torn out of the stem. (...)
Such failures are caused because you cannot directly determine the bolt's resilience. Therefore, the torque is used as controlling authority. This is the force a bolt is tightened with multiplied with the moment arm. (...)
(...) Unfortunately, most torque wrenches are quite expensive - but, considering the possibility of a broken part or a material failure, they are worth the money.
Author: Dirk Zedler