Safe wheel nut thread engagement

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Phil Bate
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Safe wheel nut thread engagement

Post by Phil Bate » Sun Oct 18, 2015 1:32 am

After removing my spacers to fit aftermarket wheels, I found it less than straightforward to establish if my wheel studs still had a safe amount of thread left after trimming them. Whilst they would have been fine had I been refitting the standard wheels, the aftermarket ones had a thicker hub face, and I ended up having to replace the studs. Here is what I established in case you find yourself in a similar situation:

There are varying opinions and some interesting mathematics to be discovered if you look into it, but as a rule of thumb for similar material studs and nuts, a thread engagement length of 1 x the thread diameter is recommended to ensure the thread will not strip before the stud shears.

The best way to measure thread engagement length is to count the number of turns from "bite 'til tight" and multiply this by the thread pitch (for metric threads). The easiest way to do this is to torque the wheel nut correctly, and then remove it while counting the number of full turns until it disengages from the stud. Multiply this figure by the thread pitch to obtain the engaged length.

For our 12mm studs of 1.5mm pitch, this equates to 8 full turns as a minimum engagement for steel nuts: (12x1)/1.5 = 8.

As long as you can count at least 8 turns until the nut disengages, you have at least 12mm of engagement which is safe. Anything less and you should fit longer studs.

For softer wheel nuts (ie. aluminium 'racing' nuts) the engagement length should be increased to 1.5x the thread diameter, which in our case would be 18mm. This would require 12 turns to achieve (18/1.5 = 12).
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