Lug bolts breaking/coming loose

[gr330zhp]

Whats up guys? Havent been on the forum in a few weeks, back home in Greece on vacation.

Been caught in a dilemma over here, doesnt relate to my ZHP though. We have a 2008 W211 Mercedes Brabus E200 Kompressor Sport. Car has been great, about 40k on it, 6 speed manual.

I noticed yesterday, rear wheel, 2 of the lug bolts were missing, this shocked me as I had washed the car the day before and wasn't like that. Look closer an notice the bolts had actually broke off. Brought the car to our Mercedes shop about 2 km away. Car drove fine. We examine it and 2 more bolts broke in the process of removing, one on the same wheel, one on the opposite side. He removed the remains and we see that they weren't OE bolts. Ordered new bolts from Mercedes today(wont be in til tomorrow or following day) and he replaced them with some he had at the shop. Left, went home, took the car back out and notice I hear some noise when entering potholes or bad road surfaces. Stop and see a lug from the front was completely gone! Watched him torque them after and everything. All the remaining ones were tight.

Tomorrow morning I go to get the 20 new bolts installed. But any idea what is going on here?! Have plans too and dont want to drive the car til its sorted out

Thanks guys

-Stathi

[Avetiso]

Could the threads somehow be damaged that they can't hold the bolts well?

Nice to have you back. I hope you enjoyed the trip.

[gr330zhp]

Could the threads somehow be damaged that they can't hold the bolts well?

Nice to have you back. I hope you enjoyed the trip.

Well we checked them today and the bolts seemed to all thread in fine :dunno thing is it is random too, was the rear wheels, now missing from the front.

And thanks! Still overseas right now haha.

[BCS_ZHP]

Someone over torqued all bolts in the past, stretched the bolts and weakened them. Now even though you're torquing them properly, the bolts are failing because their stress failure level has already been compromised. You're on the right track, replace all of them, torque to proper specs and you should be good to go.

[wsmeyer]

Any chance someone lubricated the threads?

[yod88]

Someone over torqued all bolts in the past, stretched the bolts and weakened them. Now even though you're torquing them properly, the bolts are failing because their stress failure level has already been compromised. You're on the right track, replace all of them, torque to proper specs and you should be good to go.

+1, pay attention to the new bolts as well...hopefully the hubs are ok and it was just the bolts...my question is why did the bolts break in the first place if you werent removing the first two you found?


Any chance someone lubricated the threads?

dont think this is the culprit here as they are breaking them off when trying to remove...some lubrication wouldve helped in this case


hope your trip is going great Stathi, tell the family i say hello!

-Navid

[danewilson77]

Someone over torqued all bolts in the past, stretched the bolts and weakened them. Now even though you're torquing them properly, the bolts are failing because their stress failure level has already been compromised. You're on the right track, replace all of them, torque to proper specs and you should be good to go.

This.


Any chance someone lubricated the threads?

Applied thread lube should minimally affect the torque spec (by only a few ft-lbs I would guess)

[wsmeyer]

Unless it is specified otherwise, torque values are always given for a dry interface. When they specify a torque, i.e 87ftlbs for our lugs, it's not the torque that is important it is the tensile force that that torque will apply to the lug that is important. The ratio is dependent on the diameter of the lug and the thread pitch but if you leave those as fixed it's going to come down to the torque on the lug - the friction loss.

If you have a rusty lug that is hard to turn the friction will be higher and torquing it to 87ftlbs will result in less tensile force on the lug than they call for.

If you have any sort of lubricant on the treads the friction will be less than anticipated and the same 87ftlbs of torque will yield a much higher tensile force on the lug.

The friction between the lug and the hub is also what keeps the lug from backing out so when you see missing and broken lugs lube on the threads would be the first thing I would look for.

[gr330zhp]

Installed all the new lug bolts and drove about 150kms. All went fine! Will keep an eye on it next couple days


Sent from my iPhone 5.

[BCS_ZHP]

They always say to recheck lug nut torque after 50-100 miles of driving, you did 90 miles today so time to recheck the torque.