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Last Changed 3/23/2009 |
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Loose wheel lug nuts create a very dangerous condition. There is
the extreme chance that if driven long enough, the lug nuts could
come off of the hub studs and the wheel rim could come off the
axle. More likely, the wheel rim, loose on the hub, will wear the
hub studs, causing them to fail. Tight lug nuts is especially
important to trailer where the wheels are subjected to more
side loads from turns during parking. Aside from the risk of loosing a wheel, the probable outcome of the wheel rim being loose on the hub, is a lot more flexing of the tire carcass, causing heat and breakdown of the tire. No matter the result, loose lug nuts is a very bad condition. There is a lot of misunderstanding about torque and wheel lug nuts. Many believe that if the lugs nuts are loose, then enough torque was not applied to the lug nuts. This is not true. In fact, over torqueing can be part of the reason for the loose lug nuts. More importantly, over torqueing lug nuts can over stretch the hub studs and lead to the failure of the studs. |
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First a little background. The threads on the hub stud and in the lug nut are actually ramps wrapped around in a circle. When torque (twisting force) is applied to the lug nut, the rising of the ramp causes the stud to be stretched. This stretch is what causes the lug nuts to stay tight. The stretched stud is like a spring and pulls the nut towards the hub holding the wheel rim tight. | |||||||||||||||||
Wheel hub typesThere are basically two types of wheel hubs determined by how the wheel is centered on the hub. |
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In reality, especially with the weight of the wheels we have on our RVs, you will probably have to use a wrench just to get the lug nuts seated and spinning the wheel will probably be out of the question. The chance of a lug nut not being fully seated could result in a condition like the center drawing in the picture above. You can apply full torque and the nut will not seat. You can over torque the lug nuts and still not seat the lug nut that is off center. Again, driving down the road will allow the wheel to settle. Again, the best thing is to torque the lug nuts, drive a bit, and re-torque the lug nuts. A couple of times would be appropriate for stud piloted wheels. When I was in the ARMY, I saw truck drivers use a three foot breaker bar and stand on the bar to torque the lug nuts on a truck (several hundred pounds of torque) and fail inspection the next day with loose lug nuts. I would use the torque wrench, drive the truck down the runway, and re-torque until the nuts were found to not be loose. After that, the nuts stayed tight until the next time the wheel needed to be removed. Over torqueing is done every day on wheel studs. Each size stud has an appropriate torque. That torque will stretch the stud the amount necessary to keep the lug nut from turning. If you over torque, you over stretch the stud and weaken it. Even using a torque wrench you can over-torque a stud. Once a lug nut has reached the proper torque, if you use the torque wrench again, the clicking action of a snap torque wrench will apply a slight amount of over-torque. Click enough times and you are over-torqueing the stud. So the next time you see a mechanic using a air/electric power nut driver, and he applies power until the clicking limit is reached several times, you just watched a stud being stretched. There are torque wrenches that use a bending indicator that do not increase the torque when you check the torque level. The snap torque wrenches work without your having to look at it when torqueing. The objective is to torque the lug nut until the wrench snaps. One snap is enough. Lubricating the ThreadsSome people like to put lubricant on the threads so that they are easier to remove. You actually make things worse doing that. With lubricant on the threads, you can put more stretch into the stud with the same amount of torque. The lubricant will eventually evaporate and when you go to remove the lug nut, you have the extra stretch tension to break loose.Likewise, if the threads are cruddy, that can induce more friction and cause the stud to not be properly stretched. When torque values are given, they are for dry threads. Torque Values
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Disclaimer: The information in this site is a collection of data we derived from the vendors and from our personal experiences. This information is meant as a learning guide for you to make your own decisions Best practices and code should always be followed. The recommendations we make are from our personal experiences and we do not receive any compensation for those recommendations. |
