More on audi wheel bolts/studs/torque

[EricNMN at aol.com]

An Engineering textbook perspective:

The reason torque is applied is to apply load to the bolt (preload) during a non-loaded condition.  This results in a stronger connection when loads are applied.  Torquing also makes the threaded connection less likely to back off on its own (vibration, etc).

The recommended preload for re-used joints is 75% of the "Proof Strength".  Proof Strength is the maximum load a bolt can encounter without acquiring a permanent set.

A (perhaps) useful equation:  T=KFd

T - Torque   (applied at the wheel nut/lug)
K - "Torque Factor" which is dependent upon the bolt condition
F - Preload Force
d - diameter of bolt

Torque Factor is .12 with Bowman anti-sieze, .18 if lubricated, .20 if zinc plated and .30 for nonplated black finish.

A few more words of wisdom: "...for heavy-hex structural bolts, the turn-of-the-nut specification states that the nut should be turned a minimum of 180 degrees from snug tight... this is also about the correct rotation for the wheel nuts of a passenger car."  Also stated: "Above all, do not rely too much on wrench torque.  It is NOT a good indicator of preload.  Actual bolt elongation should be used whenever possible"

What does this mean?

Our true intent is to apply force on the bolt.  The easies way to measure the amount of force is the torque applied to the nut.

Larger diameter bolts need more torque to have the same holding power.

Bolts of different strengths (they can vary tremendously at your hardware store - but are fairly uniform for wheel nuts/bolts/studs) require different torque values.

Bolts with lubricant or anti-sieze should receive LESS torque to achieve the same amount of preload.

If you apply antisieze or lubricant and apply the amount of torque appropriate for a bolt with nonplated black finish, you could damage your bolts, nuts, threaded hole or studs.

If you apply too much force on your bolts, they will stretch permanently.  They may not fit as well next time.  Or, you might not be able to get them out.

In a perfect world, we'd like to measure bolt elongation.  But, nut torque is our best option.

Eric Schumacher
96 A4 V6 Q