Wheel bolt grade

Eric eric_audi.ql at mindspring.com
Tue May 11 08:13:51 EDT 2004

>Sounds interesting Eric. But if the bolts are threaded
>all the way to the head, the risk is that they will
>snap somewhere because of stress risers.

Well first off I might have answered my own question.  Based on the required torque for our wheel bolts it falls in line for the torquing of a grade 8.8 metric bolt for that diameter.

As for the stress risers would you not have this on any threaded part regardless of thread length?  The shear point (assuming the bolt is not properly torqued) is also threaded from the factory.  In that sense the factory bolt is also a fully threaded bolt just coming from the other direction.  Also four M14x1.5 bolts offer 0.173 in^2 each of usable cross section after you remove the threaded area.  If grade 8.8 have a tensile strength of 116 ksi then the bolt will not break until over 20,000 lbs of tension is put on it.  Now like you said there are stress concentrations in any threaded part.  This would would reduce this number.  There is also a prestress put in the bolt when you torque it.  I did not calculate that since I am too lazy this morning (exercise for the reader).  But remember there are 4 bolts and a pilot to carry the shear load of the car and bumps hit in the road.


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