When I have removed crank bolts later, they have felt much looser than the original torque settings from installation.
Hi mickeg! Joint relaxation (creep), bolt stretch and materials compression are all factors in making bolts loosen at lower torque than they tighten (to).
Since my crank arm pullers always indicated that the cranks were on really tight, having bolts that were less than initial tightness has not bothered me at all as long as the bolts were tight enough to not fall off of the bike.
<nods> Torque to the correct value in one continuous motion of possible, then...stop. Don't go back for a second go. A few applications require repeat torquing to torquing an intermediary value (cylinder-head bolts come to mind) or to a torque-angle (crankshaft pulley bolts), or if a torque-to-yield fastener is used, but these are all rare in bicycles.
One thing to keep in mind when torquing fasteners coated with threadlock compound (in its liquid state) is the difference between "dry torque" and "wet torque". See:
http://www.thorncycles.co.uk/forums/index.php?topic=6805.msg42269#msg42269 In bicycles, assume "dry" unless instructed to use "wet". A handy table of dry torque values for metric fasteners I often use is here:
http://www.engineershandbook.com/Tables/torque1.htm For comparison, take a look at dry vs wet values and variations caused by bolt coatings:
http://raskcycle.com/techtip/webdoc14.html Though this last link is for SAE fasteners, the difference in values and rues of thumb hold true regardless. In general, there will be more torque variation with a dry fastener than a lubed one.
Most other bolts like water bottle cages, fenders, etc., do not really need a high torque setting as long as they are tight enough to stay on. I often grease these, the viscous fluid helps keep them from vibrating off if then get loose.
All true, but there's another factor in play here as well: If one uses dry torque on a lubed fastener, it will be tightened more.
I have noticed that bolts on Cane Creek or Tektro brake levers that have a polymer body appear to loosen over time, I find that I am tightening these a few times months after a new installation, same with shoe cleat bolts. I suspect that the plastic shoe sole material and the plastic in the brake lever body deforms a little, reducing bolt tension.
Yep; materials compression.
Pre-applied threadlocker (the dry-crusty stuff already on the threads of new bolts sometimes included with components) adds some complications. It is not just "dry", but sometimes is robust enough to clog the threads and so cause a momentary rise in tightening torque that is artificial.
Loosening torque can sometimes be greater than tightening torque due to galvanic corrosion between the metals in a joint, or grease/anti-seize drying-out a bit or if threadlocking compound is used or if washers of various sorts are used.
For all these reasons and more, it would not be a good idea to use loosening torque as a measure of tightening torque. Tightening torque is just that.
An interesting article on torque in automotive applications is here:
http://www.popularmechanics.com/cars/how-to/repair/torque-wrench-101-how-to-get-the-right-amount-of-force-2Best,
Dan. (...who is a "nut" about correct torque values and the things that affect it)
