Wheels are manufactured using steel or aluminum, they are also made to be as strong as they can be using the least amount of material possible. That means they are as thin as they can make them keeping the strength.
The most common wheel/hub configuration today is the Hub Piloted wheel end. This means there are “pads” on the hub the wheel fits against to center the wheel to the hub. The pad to wheel centering works well as long as the wheel was installed correctly.
Years ago wheels used a nut that had a “ball” shape that would center the wheel in a “cup” that was machined into each stud mounting hole, this took care of centering the wheel to the hub. The clamping force of this kind of nut left a bit to be desired, it worked well enough, but there was room for improvement. The nut made contact with the wheel even during tightening of the nut, causing wear to the wheel and potential loosening of the nut. Enter the two piece flange nut we use today.
The two piece flange nut looks like one piece when tightened against a wheel but when off the wheel stud it is obvious there is two pieces. The two pieces cannot be separated but the flange of the nut can spin on the nut. This configuration allows the flange to press against the wheel while the nut can be turned to tighten without spinning on the wheel causing damage.
I started working on trucks in 1980, we had a one inch drive impact that we changed tires with. Right or wrong, I was taught to use the big gun to remove and reinstall the lug nuts, and “get them tight”. I did just that, spin off the lugnuts, do the repairs as needed then place the wheel over the studs, hand start the nuts, gently tighten the bottom and the top nuts, then work my way around the rest of the nuts. I’m not sure just how much torque a one inch impact will put out but it’s a lot. There was no specific torque taught to me nor was there a torque pattern that I knew about. I did this for years, then came the hub piloted wheels and some training, I now know there was a better way all those years.
You have to look very hard to find the old style ball and cup wheels today, hub pilot is the standard now. With that comes a different way of tightening the wheels after they are installed onto a truck. If you read a service manual, there are parts that require tightening to a certain torque and in a specific pattern, such as cylinder heads and crankshaft main cap bolts, some even specify loosening those same bolts in reverse order. Wheels also have a torque spec and pattern. I was taught to bring torque up to spec slowly or in increments, if we were going to 175 ft lbs. of torque, we started at 50, then 100, then 150, then topped out at 175. All of this was done in the pattern the manufacturer specified, once 175 was reached we repeated the pattern once more.
I recently had our front wheels off and did just that, following the pattern specified by Freightliner, I started at 100 ft lbs, then 200 ft lbs, then 300 ft lbs, then to the 450ft lbs required. I didn’t want to keep looking at my phone for the pattern so I numbered the ends of the studs with a paint pen, this made following the pattern easy. Following a torquing pattern assures you that the wheel is mated to the hub properly. Failure to follow the pattern could cause the wheel to not seat properly against the hub and could result in a loosening of the nuts and/or a wheel failure. The flange nuts used on today’s truck wheels distribute the clamping force over a larger area of the wheel creating a better wheel/hub mating, lessening the chances of failure. Over torquing can cause thread failure on the stud or in the nut or even stress fractures in the stud that may not show up until it breaks off. This is one more reason to check lug nuts during your pre and post trips, especially after a wheel end service.
Failures still happen but I believe this is caused by improper mounting of the wheel to the hub. Any debris, rust flakes, any kind of dirt between the wheel and hub will be crushed and eventually work its way out from in between the wheel and hub, causing a gap and lessening of clamping force and a possible failure. Following proper cleaning and mounting procedures will minimize this possibility.
Read the owner's manual for torque spec’s and patterns and follow the recommendations. Clean the mating surfaces of both the wheel and the hub. I believe not doing this can result in vibrations and odd wear in the tires. Do yourself a favour, make sure the shop doing your tire work is up on torque spec’s and patterns and that they follow them, also cleaning the mating surfaces. Do a retorque as recommended by the tire shop, it’s usually free.
Till next time, safe travels.