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Torque and Clamping Force

(Your business phone line rings…)

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“Hello, I’m Attorney John Smith, and I am representing Mrs. Jones who came to your shop last week. It appears that your technician left the lug nuts on her vehicle loose and she has been in an accident that totaled her car and caused bodily injury. We are accepting her case and will be filing a lawsuit.”

As you stand there pondering all your years of hard work and the sacrifices you made to build your business, it all flashes before you as you face the stark reality of having to not only go through a legal proceeding, but also face the negative ramifications of such an incident on social media and possibly the news media, as well as rebuilding trust in the community. All because industry processes weren’t followed, and a 50-cent piece of metal wasn’t properly secured. Do I have your attention now?

We can’t prevent losses like this from ever happening, but we can be proactive in putting processes in place that greatly diminish the possibility. Many people think we must tighten lug nuts until they can’t turn anymore. Nothing could be further from the truth. We measure torque, but the goal is to achieve proper clamping force. In addition to torqueing lug nuts to spec, there are other factors that need to be considered. TIA’s certified training is one good source of information. Downloadable articles can be posted in a conspicuous place and used in safety meetings.

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R.I.S.T. and Other Safeguards

TIA has developed the R.I.S.T. procedure, which stands for Remove debris, Inspect the lugs and mating surfaces, Snug the lug nuts and Torque to spec. Overlooking the first two steps could negate the benefits of the latter two. Taking the time to clean the mating surfaces of any visible rust, grease and corrosion will help prevent vibration caused by uneven surfaces and joint settling that can occur when the mating surfaces have rust or debris that falls out over time. This can lead to decreased clamping force. To do the job properly, inspecting the mating surfaces and lug holes goes beyond a quick glance.  

Photo-1-Powdercoated-Centerbore
Photo 1

Recently, I was asked to install a set of wheels that were powder-coated black by a local company (see Photo 1). The painter didn’t mask the mounting pad, centerbore or the lug seat surfaces. The centerbore of the wheel would not fit over the hub on the rotor due to the thickness of the coating. I had to sand the paint off to get the wheel to fit back on the hub. The other issue was that the mounting pad and lug nut seats of the wheel were now thick with paint, which compresses over time. I was not able to remove the paint on this area, so I torqued the lugs to spec and informed the owner that he will need to sand these surfaces and check the torque periodically.  

Photo-2-Tinnerman
Photo 2

Another commonly overlooked item are Tinnerman clips (see Photo 2). If the vehicle has aftermarket wheels, be sure to look at the back of the wheel to see if the mounting pad is smooth. If it is and there are Tinnerman clips over the factory lug studs, immediately remove these clips. Leaving them on will cause a vibration problem, at the very least.

Next, if you are installing aftermarket wheels that are lug centric, you must rotate the assembly while running the lugs down by hand in a star pattern. This causes the lugs to center the wheel. If you let the wheel hang on the lug studs and run the lugs on with an impact gun, this can lead to a false torque reading. As the vehicle is driven, it will experience a vibration and the wheel can loosen up.

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Another aspect of the “snug process” is to use torque sticks that are rated at 60% of the final torque value. Most techs use pneumatic impact guns, which can have a wide range of torque depending on the amount of air pressure and volume. Using a torque stick to reach the final torque value is not always accurate. Think about how many drains on air from other tools or equipment are running at the same time that causes variations with the impact gun. For this reason, always secure the lugs with a calibrated clicker or digital torque wrench. These wrenches must be calibrated periodically or replaced. If yours have been in the shop for more than a year, it is time to have them calibrated and verified.

So, where do you find the correct torque value? One source is the TIA TPMS Relearn Guide that includes lug nut torque (in the far-right column) for any vehicle that came with TPMS dating back to the early 2000s. If you are working on a trailer or older vehicle where data like this is harder to find, you can use the accepted guide produced by the SEMA Wheel and Tire Council that shows torque values for different-sized lug studs. 

Photo-3-Porsche-Centerlock
Photo 3

Lastly, center-lock wheels (see Photo 3) are unique in that you need to use a special tool to release a spline lock in the center of the hub that allows you to unscrew the lock ring. For installation, the surface of the wheel hub needs to be coated with anti-seize lubricant. In this example using a ¾-in. torque wrench or the torque multiplier tool from Porsche, aim for the mid-400 ft.-lbs., depending on the model. Then, back off a quarter turn and finally retorque to spec. It is critical here that the spline lock completely pops back out and is flush with the outer edge of the hub (see Photo 4). If not, the wheel could come loose.

Photo-4-Porsche-GT3-Center-Lock_spline-lock
Photo 4

These are just some of the variables that need to be managed in your dealership to secure the wheels and ensure safe and reliable wheel service for your customers.

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