torque specification on headset

Question

I was wondering why there is a torque specification on the steer tube end of the stem for mountainbikes.

The torque specification that goes to the handlebar is reasonable, after all handlebars are often made of lightweight materials and depending on the construction and the stem pressure distribution I might run in danger of crushing/cutting it. But the steer tube on mountainbike forks is usually very very solid aluminium, there is no way I could crush it. So why do some stems have torque specifications on the steer tube end?

Answer 1

We all easily understand the problems of under tightening a screw or bolt - if is too loose, things move, it ends badly and we know why. Most of us also know that too much torque can damage the components, and in extreme cases strip the thread of the fastener. What many people do not understand is that bolts stretch when torqued. At the correct torque the bolt is elastic or springy, and is this elasticity that holds things together under vibration.

An over torqued fastener can be stretched too far and loses elasticity long before it breaks or threads strip - at this point, the bolt may not continue to hold the surfaces tight under vibration, and can work loose. As the bolt has been stretched beyond its elastic ability, it will not return to its original size and essentially stuffed - however you cannot see this visually. Here in lies a big problem - it came lose, things come lose because they were too lose, so people assume it needs to be put on, just tighter. Unfortunately, that over tightened bolt (which is now stuffed) will only hold if it is over tightened even more.

On cheap bikes with lots of steel (very forgiving material) and excess weight, its never a real issue - you can give a gorilla a power bar and not do much damage. The use of things like titanium and aluminium rather than steel for fasteners, along with the desire for weight savings means these components are designed with fine tolerances - hence its easy to over do it and the correct torque becomes critical.

Essentially if you bike is not a chain store special, over torquing will damage something - hopefully just the bolt that is easily and cheaply replaced. If you torque a bolt up to specification and it does not hold reliably, replace the bolt - no matter how tempting, do not over tighten it.

Answer 2

The stem cap torque specification is for safety and bearing tolerance. The cap provides a preload on the headset bearing. This preset is the point where there is minimal movement between the steerer tube and the headtube. At the same time the bearing is loose enough to allow free turning of the fork. This is why the top cap is adjusted prior to the bolts that clamp the stem to the steerer. If the preload is too light the bearing will move resulting in premature failure and possible frame damage. Too much preload will also cause early failure by overloading the bearing. The stem to steerer clamp bolts require an amount of torque sufficient to prevent movement but not so tight that the bolts are stretched to the point of being damaged. Since most stems are aluminum it is easy to overtighten the steel bolts to the point that the internal aluminum threads can damaged.