Last year, the For Dummies series celebrated its 25th anniversary. What started with a book on DOS programming morphed into an iconic brand with more than 2,700 titles and 200 million books in print. The books are written under the premise that the reader knows nothing about a particular subject and needs to be educated on the basics in a way that is not condescending or judgmental.
Another common feature of the For Dummies series is small, easily digestible sections that allow the reader to either start from the beginning or skip right to the part that is the most important. In that spirit, following is my version of Wheel Torque for Dummies.
Proper torque guarantees nothing:
Torque wrenches and pricey torque control devices are not magic wands that guarantee the wheels will stay on the vehicle. Their sole purpose is to measure and deliver precise amounts of twisting force to the fasteners. Clamping force is the physical property that holds the wheels on the end of the axle, and it cannot be measured in the field. The only way to approximate the correct clamping force is to control the torque, but the correct torque can still result in the wrong clamping force.
Clean mating surfaces are a key ingredient to clamping force:
Think of clamping force as a recipe and torque as the temperature of the oven. One of the key ingredients to clamping force is a clean mating surface. A mating surface is any place where metal touches metal so the areas between dual wheels and between the wheel and the hub/drum must be clean. If the mating surfaces are not clean, then the foreign material will eventually work itself free and result in the loss of clamping force. The vehicle may leave the shop with the correct torque, but the clamping force will start dropping as soon as the wheels settle into place under the weight of the vehicle. The recipe still fails with the wrong ingredients at the correct temperature.
Component inspection is important:
On all commercial wheel and rim systems, the condition of the components plays a huge role in clamping force. If the parts are worn, broken, or damaged, the clamping force will be reduced at the correct torque. Besides the obvious condition of the rims or wheels, the fasteners must also be inspected. Flange nuts with flanges that do not spin, stud-piloted cap nuts with worn ball seats, and demountable rim nuts and studs with worn threads will all result in the loss of clamping force at the correct torque.
Snug in a star pattern to seat the wheels/rims:
Another key to success when installing commercial wheels or rims is to snug the fasteners in a star pattern starting at 12 o’clock to seat the components on the hub/drum. Hub-piloted wheels also have piloted drums; failing to seat the wheels and the drum on the hub will result in loose wheels.
While most stud-piloted wheels are used on inboard drums, the uneven distribution of torque and clamping force can be prevented when the inner and outer cap nuts are tightened in a star pattern. On demountable rim systems, snugging the nuts in a star pattern helps to ensure they are properly aligned and not wobbling as they rotate.
Set the torque for the middle of the range:
Most torque specifications are given as a range, like 450-500 lbs.-ft. Even the most accurate torque wrenches have some degree of variation on the plus or minus side. If the wrench has a +/- of 5%, then setting the wrench to 475 lbs.-ft. with a 450-500 lbs.-ft. range puts the minus 5% at 451 lbs.-ft. and plus 5% at 499 lbs.-ft., so the working torque stays within the specification.
Don’t ignore the torque check:
To quote one of my favorite movie lines, “Screws fall out all the time; the world is an imperfect place.”
Stud fatigue is difficult to identify in the field even with the help of thread gauges. There’s no way to tell if the studs, nuts or components will deliver the proper clamping force at the recommended torque. Checking the torque after the first 50-100 miles of service is the best way to determine loss in bolt tension, which obviously results in the loss of clamping force.