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. 

 

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.

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. 

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

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. 

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.

In the meantime, here are some preliminary recommendations we feel worth sharing:

 

  • Dry/standard lug nuts: NOT RECOMMENDED for track or racing use on aluminum wheels without steel inserts.

    • The friction between a dry/standard lug nut and an aluminum wheel conical seat is immense and can almost never generate the extraordinary preload that super high strength (beyond grade 12.9) studs like ARP and MSI are capable of.  There will be variations from wheel to wheel in terms of paint and coatings that can affect friction, but you've all seen how beat up a lug nut seat gets.  There's almost always evidence of galling and marred material.

  • Lubricant coated (MSI lug nuts) or externally applied lubricated nuts (most importantly applied to the aluminum wheel lug nut seat, or the lug nut seat itself): Torque to 90+ lbs-ft with ARP or MSI studs (12mm).

    • There is some information out that states to torque MSI lug nuts on MSI studs to 70 lbs-ft (12mm). DO NOT torque to this value, especially if endurance racing and/or if the lugs nuts are well used!

      • Dry lubricant coatings (MSI lug nuts), lose their effectiveness after 10+/- torque cycles, which means they slowly degrade to only slightly better than a dry lug nut in terms of friction. It is recommended to add some additional lubrication on the wheel lug nut seat after this usage and treat it as a dry/standard lug nut.

    • Standard lug nuts: apply a small amount of lubricant to the wheel where the lug nuts seat, and/or on the lug nut itself.

      • Lubricating the lug nut seat is FAR more important and effective than lubricating the threads in a wheel interface application!

    • Typical lubricants to use: regular grade anti-seize works great. Dry lubes, like moly or graphite spray work just OK.  They need to be reapplied more often (~5 cycles). 

  • New wheels: it is MANDATORY to re-check torque settings after 30-60 minutes.

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

  • Racing/Endurance Racing

    • We recommend checking torque during pit stops, or at least every few hours. Yes, we want you to check torque when “hot” during pit stops, contrary to what dogma has led many to believe.  Over the course of a race/endurance race, many factors can contribute to self-loosening of lug nuts. The implications of losing clamping force are far greater than torquing when “hot.” Lower torque 10-15% if you feel there’s lots of heat input at the stud-hub interface and check.

  • It should go without saying: Make sure your torque wrenches are calibrated!  ​​

 

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

12.11.2019