Choosing the right bolt-material, and the art of tightening.
Suprisingly many high-end (not ultra top-end) -brands, use steel bolts as standard, in their components !
-Sometimes because the strenght is needed (calculated or tested) !
-Or because of final Price of product.
Here, the four materials I always use :
-Stainless Steel weight 7.80 gr/cm3 (Derailleur hanger attachment, and Places I can't reach)
-Titanium 4.51 gr/cm3 (Strenght
Needed)
-Aluminium 2.70 gr/cm3 (Strenght
Wanted)
-Nylon 1.14 gr/cm3 (bottle cages and bell)
Carbon Bolts are intriguing. Carbon weighing 3.51 gr/cm3. Bolts also contain resin, -so, a lower gr/cm3.
Threads are carbon fibres, hard packed in circular arrangement, to obtain strenght... Very complicated, and frequently discussed.
And still -expensive, -discussed durability, and only available in special heads.
Not for Me, -Yet.
<< 5 Nm !!! Important Tighten to 40 Nm... max 2.5Nm ???
We all know this wrench, -and its a fine Tool !
It stops wrenching, when a certain torque (Nm) is reached.
In other Words, -When (eg. 6Nm) is reached, the job is done !..... But is the bolt applying force to the part-assembling, Or is there a great resistance(friction) in the threading ??
Like : -Corrosion, -Wear, -Fabrication precision, -Different materials contact, -Performance lubrication, -Thread lock glue.. Please find more factors, that make this precision Tool, a guide-line for how much 5Nm is, in Your hands, -and 15, -and the 40Nm required for cassette lockring.
The Torque Wrench, is calibrated with Standard Internationale Specified Bolts and nuts... We never see/use these Reference screws !
But We are humans, and so We learn of our experiences.. A trained mechanic, can tighten bolts very close to specs. of Part, and NOT to specs. of BOLT.. Hope it makes sense ?
Always check if parts are in correct contact after assembling..- Don't trust The Torque Wrench ! (But it's good to learn from)
.) 
