Wheel Stud Torque & Friction

We wanted to share some initial observations seen in an experiment we're conducting to determine what lug nut installation torque is the "correct" torque to generate the desired stud clamping force ("preload") for securing the wheels on your car. Click here for pictures and videos of broken stud incidents.

 

How much preload a stud generates is the single most important factor in how a stud/bolt operates, regardless of a press-in or thread-in type.  It is fundamental in how much a bolted joint can resist externally applied forces; in the case of cars, lateral & longitudinal accelerations from turning, braking, engine torque, and discontinuities/bumps in the road surface, etc.

 

This doesn't change the fact that thread-in studs are inferior in that they have twice as many stress concentrated areas to fail; one of them in a peak stress zone inside/flush with the hub, where many have experienced them failing either on their own car, or someone else's (Figure 1).  The frequency of thread-in studs vs press-in studs failing is proof enough and why our press-in stud hubs are a real solution.

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Figure 1: Typical Thread-In Stud Failure and Worn Lug Nuts

What we've learned so far in this experiment: too much friction is detrimental. This may be of no surprise to some of you. Friction is the single most important factor for what determines the preload achieved on any torqued stud/bolt. It is also why stud preload can vary by thousands of pounds with minimal changes in friction.  

 

But, friction is also paramount in why a bolted joint functions. Without friction, a nut would fly off as soon as you let off after tightening. Friction is also why the torque tightening method to achieve a desired preload is actually not accurate in many cases, with error ranging in the +/- 15-20% range. More critical fasteners are usually called to be angle tightened, and more accurate than that, by measuring bolt elongation, i.e. - how much a bolt stretches (Figures 2a & 2b).  Anyone who has built an engine is familiar with those techniques.

Where excessive friction comes into play is especially applicable to aluminum wheels that do not have steel inserts (what most of us use).  Aluminum is a terrible material when used in a friction/sliding/bearing interface; like when a lug nut is sliding on the wheel surface while being tightened. Dry lubricant coatings, like what MSI lug nuts have, do help, but aren't infinite in longevity.  As we remove and install them multiple times, the coating wears off and loses lubricity. Case in point, Figure 1 shows yellow lubricated MSI lug nuts with almost all of the coating worn away. 

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Figure 2a: Bench Test Setup wth Load Cell Measuring Clamp Force

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Figure 2b: Bolt Elongation Measurements

We found that after approximately 10 +/- torque cycles, the effectiveness of the lubricant coating is almost negligible and only marginally better than using a standard lug nut. When this happens, aluminum on the wheel can start to "gall" (weld/fuse itself to the opposing interface) and friction sky rockets. This large increase in friction on the nut-wheel interface is detrimental to developing preload. Galling examples in Figures 3 and 4 below observed during experimentation.

 

At this point, not only is the resultant stud preload several thousand pounds below the ideal capacity of ARP/MSI studs, it barely changes regardless of increasing tightening torque. In other words, all the extra torque put into tightening a lug nut passed a certain value is dissipated by the friction of the lug nut on the aluminum wheel and NOT into tensioning the stud. 

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Figures 3 & 4:  Galling of aluminum from wheel onto lug nut

This testing is still ongoing. We plan to test effects of elevated temperatures, impact gun tightening, as well as how the adhesive race teams use to hold lug nuts in wheels for quick wheel changes affects friction. We will update accordingly as our knowledge and testing evolves.

GENERAL RECOMMENDATIONS AND SUGGESTIONS

  • When using USED standard 1-piece lug nuts on USED wheels (apprx. 5-10 tightening cycles):
     

    • MUST BE inspected frequently on aluminum wheels that don't have steel seat inserts to see if galling occurs (see figures 3 & 4).
       

    • The increased friction between a lug nut and an aluminum wheel nut seat when galling occurs will prevent sufficient preload/clamping force, which greatly increases the chances of fatigue failure.
       

    • Extra high strength studs like ARP and MSI (class 12.9) are capable of over 15,000 lbs of clamping force/preload PER stud (12mm). It is very difficult to achieve this preload on an aluminum wheel with the typical tapered/cone nut seat interface; galling or no galling. Extra clamping capacity is the main advantage of moving to stronger fasteners!
       

    • If galling starts occurring, it must be cleaned off thoroughly OR you can apply a SMALL amount of anti-seize lubricant to reduce friction.
       

    • IF there's excessive galling, a very small amount of standard grade anti-seize lubricant works well. Be sure no grit or dirt gets caught in the lubricant during wheel changes as this will affect lubricity and will need to be cleaned off. Clean and re-apply after approximately ~10 wheel changes or when you notice insufficient lug nut rotation while tightening.

 

  • Dry lubricant coated MSI lug nuts and wheel studs:
     

    • There are torque recommendations for MSI lug nuts on MSI studs to 70-75 lbs-ft for 12mm, 85-90 lbs-ft for 14mm. We HIGHLY recommend NOT torquing to these values. Dry lubricant coatings (yellow or black MSI lug nuts) lose their effectiveness after two tightening cycles which makes torquing to these values HIGH RISK for insufficient preload (short cycle fatigue failure). 
       

    • Independent torque-tension testing we’ve had done at an OE level test facility shows that 70-75 lbs-ft with a 12mm increases the chances of fatigue failure significantly after the first to second tightening cycle.
       

  • New wheels, rotors, spacers, studs, hubs, and/or lug nuts: it is MANDATORY to check lug nut torque after 30-60 minutes run time.
     

    • Phenomena known as “preload relaxation” and "embedment loss" can and WILL OCCUR, where fresh mating surface imperfections/roughness can deform/flatten enough to cause a loss of clamping force.  All it takes is 0.002-0.005 inches (i.e.-thickness of a sheet of paper) of permanent deformation to make a stud lose all meaningful preload.
       

  • All wheels: REMOVE ALL PAINT ON MATING SURFACES
     

    • Paint finishes under the extreme pressure of the lug nuts will break down and extrude from under the lug nut seat causing an unexpected loss of preload. If painting wheels, mask off the lug nut seats, rotor mount face and center-bore! If your wheels are new and have paint finish in the lug nut seats, IT MUST BE REMOVED until a raw aluminum finish.
       

  • Racing/Endurance Racing
     

    • Over the course of a race/endurance race, many factors can contribute to self-loosening of lug nuts (thermal expansion, vibration, insufficient starting preload, etc.) The implications of losing clamping force are far greater than torquing when “hot.” If you are concerned with potential preload loss or excessive heat if hot, lower torque 10-15% and check torque.

 

Hopefully this all makes sense and if you want to further the discussion, please feel free to contact us!