Wheel Nuts

Why would I want to discuss the correct tightening of the humble wheel nut?

Well you see, I just completed a job where I had to replace the front and rear disc pads on a Mitsubishi Magna. To do the job, the wheels have to come off. But before I get into the nitty gritty of it, let me take you back a few steps to the lead up of this job being undertaken. The vehicle was taken in for a service to the dealership where upon removal of the wheels they discovered that the disc pads required replacement. They refitted the wheels and informed the customer of the extra work required. The customer got a high end price to do the job and decided to call me for a quote. I undercut the quote and got the job. Well, I struck a bit of trouble as soon as I tried to remove the wheels. There was no way that I could remove any of them with a wheel brace. So what would happen if the owner got a flat tyre? They would be stuffed, to put it simply. They would have to call RACQ or a mobile mechanic to undo the wheel nuts for them. That's just not good enough.

Far to many auto businesses use an incorrectly adjusted impact gun to tighten the wheel nuts and this was exactly the case here. The customer will be none the wiser until they get a flat tyre and have to try to loosen the nuts with the wheel spanner supplied with the vehicle. I own an impact gun and they are so handy for undoing nuts & bolts, especially tight ones but I never use them to torque up a wheel nut. I will set the impact gun setting to 1 or 2, just to nip it up and then use a wheel brace for the final torque up. That way if the customer ever gets a flat, they will be able to undo the nuts using the spanner supplied in the vehicle.

Now back to the task at hand. I pulled out my breaker bar with a single hex impact socket and still could not break the nuts on the right hand side, however I did manage to loosen all the nuts on the left side. Back to the right side, I added a 12" extension bar into the end of my breaker bar to increase the leverage length. With all my strength I managed to loosen all the nuts but one. I knew then it would never come out, even when I sprayed the nut and stud with a releasing agent. The only way I was going to remove the wheel was to keep undoing the nut until it sheared the stud. So that is exactly what I did.



Oops, this is what happens when the nut is over-torqued and friction seizes onto the wheel rim. The nut locks itself to the stud thread as you try to undo it, at the same time it has also locked itself to the matching taper in the rim hub. If the nut did come off without shearing the stud, the damage caused would still require the replacement of the stud and nut due to thread damage.

what it looks like after the wheel was refitted.

                                                      OK, the end of the story. My assessment is that the wheel rim, all the studs and all the nuts on this wheel need to be replaced at a minimum. The reason the rim needs to be replaced is because the taper is damaged from friction weld of the nut. I guess compensation from the business would be in order.

Now I would like to cover these and some more consequences of over-tightening wheel nuts. Some are apparent immediately and some are not. The studs that are used to hold the wheel on are high tensile grade studs. The nuts have a taper machined on one end and this taper is also built into the wheel rim. You will have noticed of course that there is no sort of washer between the nut and rim. When the two tapers meet and the nut is torqued sufficiently, the nut will not come loose. However when the nut is tightened further, several things happen and all are not ideal:

  • The tapers on the wheel and nut can friction weld together (as seen above with the resultant damage)
  • The stud is stretched beyond its yield length and if it did not break this time then it could fail while driving
  • The nut thread and stud threads are distorted and will not screw together as they should. The nut should be able to be screwed up with the fingers all the way to the rim and should not be excessively loose on the stud.
  • the wheel rim hub is distorted and with possible taper damage
  • of course you won't be able to remove the wheel with normal wheel braces

So, what is the right way to fit the wheels to the hubs. First I will tell you how you should do it because all you will have is simple basic tools. Now use the tools that came with the car or use a wheel brace if you have one. Without adding any sort of extended lever to the tool, hand tighten the nuts till there is no movement of the wheel, then just use the wheel brace to nip them up a bit tighter in a crisscross pattern, then lower the vehicle until the tyre touches the ground. Now using your arm and hand muscles only, continue to tighten the nuts using the same pattern to about as tight as you can get them without getting a hernia. DO NOT stand on the brace. You should now be able to also remove the wheel using the same tool with about the same amount of effort. Sometimes it may require a little more effort but nothing that you should not be able to handle. Now the car can be fully lowered. For the ladies this will be sufficient for a little while and what I would suggest is that you get a tyre business to help you determine if you have done them up tight enough. If you have not, then you may use an extension handle on your wheel spanner to help you next time. A little extension is always handy to have in the tool kit anyway. Once you get the feel of how tight they should be then you won't go wrong again.

For the technical people, you may like to use a torque wrench, but who uses them, honestly. I know I had to when working as a mechanic in the Army because some of the heavy vehicles had to be torqued otherwise the wheels came loose. For a street car it will be difficult to find a torque value stated anywhere in your manuals. I eventually found a few and they varied somewhat, depending on the wheel size & weight of vehicle. A small car would be in the area of 110Nm (81 ft/lbs), a bigger vehicle was 130Nm (96 ft/lbs).

My last point I can make here is when you take your car in to get new tyres, try taking one wheel nut off with your wheel spanner when you get home. If you had to add an extension on of more than six inchs to the brace to loosen it, then I would suggest the nuts are over torqued.