Interview with graduate engineer Dirk Zedler, bicycle expert
Screwing based on your feeling can crush parts
BIKE: Many amateur mechanics tighten screws on the bike purely by feeling. How dangerous is that?
DIRK ZEDLER: From the mechanical engineering point of view, bicycles are absolutely lightweight. In fact, weight is saved on every detail. Tightening screws by feeling cannot only damage the thread itself. Carbon components are actually at risk of being crushed. As a result, previously damaged parts can fail in operation.
Is aluminium just as sensitive to excessive torque values as carbon?
Crushing is not the big issue, and if the aluminium frame is affected by a dent, this is neither a fracture origin. The clamping points on the handlebars can be critical if neckings occur. In this case, the expert speaks of notch effect, and sooner or later fatigue fracture can occur.
Is there only the problem of too tight, or can too low torque values also cause problems?
In the event of an impact, handlebars can twist abruptly in the stem or stems can twist in grooves on fork steerer tubes. But cranks or pedals can also come completely loose and suddenly fall off if they are not tightened properly. Bolts of seat posts are also particularly critical. For this reason, the urgently recommend that you stay within the ranges of torque. And keep in mind: The bolted connections should be checked for the first time with the torque key after about 10-15 hours of operation and then at regular intervals and depending on the hardness of the use about every 50 hours. More details are indicated in the operating instructions.
A part is still loose in spite of the proper torque value. What can I do?
Tolerances in manufacturing can hardly be avoided, so the first thing to check is whether the parts are matching. Grease should be absolutely avoided on clamps where one partner is made of the composite material carbon. A thin layer of friction-promoting carbon assembly paste applied evenly on both sides creates magic results here, then it works within the specified torque values.
What shall I do if a component has been over-tightened?
This is not an easy question and definitely not a question that can be answered in general. Once over-tightened and then loosened immediately is typically not the end of the world. With critical carbon, creaking noises often announce during tightening already that the pressure was too high. You can also hear them when loosening. If this is the case, it is definitely time for replacement. I also recommend that you contact an expert.
The interview was held by Stefan Frey
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