xs650 said:
What is saving you, and to a lesser extent me from failed parts, is that automotive engineers put a lot of reserve strength into certain parts of a car.
No complaint there.
xs650 said:
It's their realization that cars get maintained by chimpanzees that keeps the lug bolts from failing when they are lubed.
I have philosophical problems with this statement. Certainly, everyone realizes that cars are maintained by chimpanzees. We've beat the dead horse here that the single biggest problem with lubing is the risk of someone using an impact wrench after we've taken the time to lube.
My anecdote is with my Ford. After I lubed the threads, I went years were I could tighten *and* loosen the lug nuts with a 4-way. I gave the car to an in-law to do struts, and when I tried to take off a wheel, I needed 6 feet of cheater bar.
There are at least two major advantages to lubing.
The first is for corrosion. It's scary how violently an impact wrench takes off these things when they're rusted. Pulling all that rust through the threads just can't be good for them.
The second major advantage to lubing is that it reduces the scatter of the preload. A rusty, unlubed joint can load up the torque wrench before reaching adequate preload. The lubed threads are more likely to give you the expected preload than the unlubed joint.
I don't see how these mythical automotive engineers can expect the joint to be dry and safe. Dry means corrosion, corrosion means deterioration and loss of strength. Dry also means there is an expectation of wirebrushing the threads before reassembly. I'm having trouble understanding how there can be an assumption of all-out neglect by the engineers---no lube, no wirebrushing, rampant overtorquing. At some level, they're relying on attentive people spending a little time to get the job done right. (And hopefully on the expectation that one or more bolts can be missing or broken and the car feels funny before a catastrophic loss-of-wheel.
Which reminds me of a story. Maybe 22 years ago---77 Ford Thunderbird, about 300 miles after a tire rotation done by a tire shop. I was driving home from college. I was completely oblivious to car problems, and as I rounded a bend on a 2-lane highway, all of the sudden the car felt weird. 1/4 mile ahead, a brand new little box store, new to the route. I pulled into the parking lot and looked the car over. Tires had air. Everything looked good. Slow circle to the left---nothing. Slow circle to the right---hmmm--clicking sounds. Walk around the car. Again. Hmmm. And then this shocking revelation, "OH MY GOD THAT THERE IS ONE LUG NUT HOLDING THAT WHEEL ON!" Four were missing. I'm thinking, "Where am I ever going to find lugs nuts here in the middle of nowhere?" I took a look at the store. I'd never heard of the place---"Wal-Mart". Oh man---I don't like the sound of that. Looks dodgy. It was way smaller than the ones today. I found the automotive section, and started looking around. There was a carded thing of 5 lug nuts for like $1.36. I took them outside and they fit the studs, but my spare tire wrench wouldn't fit the hex. So...back into Wal-Mart for a $4 chrome four-way that I had for 14 years or so, until someone grabbed it when I wasn't looking and I had to replace it for $8.
The fact that VW bolts have stretched because of lube is pretty shocking to me. I can only assume they were lubed and then overtorqued, hopefully with an air impact wrench. I'm looking forward to scurvy's torque test, but I would put up a few bucks to spring for straight up new bolts (or maybe a combination of new and old bolts), rather than just the locking style, just for the benefit of not having to answer the question, "Are the locking head bolts representative of the regular bolts?"
A really good basic book for threaded fasteners is "What Every Engineer Should Know about Threaded Fasteners: Materials and Design" by Alexander Blake. I've read it twice when some stuff at work came up. Anyone with serious design issues is going to need more depth, but...it's a quick read, and it's an awesome introduction to the topic.
The book:
http://books.google.com/books?id=TA...zG4zGyAT0mZxs&sig=LZh_3Nak4RfvXSTw5ptv9WaDqHE
The chapter about torque K's with various lubes:
"Formulas for Torque and Preload"
http://books.google.com/books?id=TA...ded+fasteners&sig=N1rPJL_aRCa-f4dfCC06rc6xPsU
(I think Google Books only gives you a limited number of page views, so don't go clicking all over the place.)