Was doing some work on the Corolla tonight, took one wheel off and noticed that the keyed security nut was unusually tight, but not far from the correct torque. Later when putting the wheel back on, I was doing the initial tightening with the wheel in the air and turning freely with the gearbox in neutral, and it felt like the lug key had slipped off of the keyed nut, it started to spin freely. It felt like I was having trouble getting it lined up again. So I go to carefully engage it by hand, and it falls off and I see this:
That was a genuine Toyota-brand Land Cruiser wheel stud, put on in early 2016 after I had some no-name extended studs fail on track. I'm pretty sure they've never been tightened with an impact gun since then or meaningfully overtightened by any means. Something I only noticed after taking the pic was the darker area in the sheared section (toward bottom-left of pic) with a nasty spot of rust. It looks like this stud was cracked for a while before it broke, allowing rust to start in the crack. I guess I'm lucky I found it with tools rather than the hard way, but this is an unexpected expense I certainly could've done without right now.
Any idea why this would've happened? Will a relatively small crack really make the stud so easy to pull in two?
I've only ever had studs break like that on me two times.
Both were on my Sequoia.
IDK, maybe it's a Toyota SUV thing. Like frame rust......
ShawnG
PowerDork
10/5/19 10:06 p.m.
Overtorqued and then failed just a little more every time it was tightened.
You can see the line every time it failed a little more.
Check the depth of the keyed nut. If it's too shallow it'll bottom on the stud and twist it off with surprisingly little torque.
Surprising, I wonder if something happened at a shop that I don't know about. I can say I've definitely never found these lugs on at more than 100ft-lbs, well under the ~200ft-lbs most cars are driving around with. Could a pothole strike have initiated a crack in one stud?
I think when I send back my first batch of first-world money to fix up this car, I'll allocate some for a set of ARP extended studs.
Edit: Keyed nut has the same dimensions as the others and they're far from bottoming out, these are holding thick alloy wheels.
That rusty spot is the initial crack, then the lines that move away from there are called beach marks and they show a progressive opening of said crack. Finally, it went far enough that it failed the rest of the way all at once, where the broken surface moved away from flat and is rougher at about a 45 degree angle. Textbook example of that sort of failure.
I'd carefully inspect all your other lugs, possibly with dye penetrant, or just replace them for peace of mind.
Vigo
MegaDork
10/6/19 10:53 a.m.
This is not an accusation but one thing to keep in mind with discussions of 'proper' torque is that it only applies to clean dry undamaged threads between the two originally-specified types of metal. For example, find a super slippery lube and you'd probably be able to tear a stud in half before hitting the factory torque spec. So, a torque spec only helps you if everything else is already right.
Sucks that this happened but it happened in the best possible way!
einy
HalfDork
10/6/19 11:45 a.m.
GameboyRMH said:
Surprising, I wonder if something happened at a shop that I don't know about. I can say I've definitely never found these lugs on at more than 100ft-lbs, well under the ~200ft-lbs most cars are driving around with.
I am truly curious - what passenger cars specify 200 ft-lbs of torque on wheel nuts? I have had a few cars in my 35 years of car ownership, but never one with more that 100 ft-lbs specified.
einy said:
GameboyRMH said:
Surprising, I wonder if something happened at a shop that I don't know about. I can say I've definitely never found these lugs on at more than 100ft-lbs, well under the ~200ft-lbs most cars are driving around with.
I am truly curious - what passenger cars specify 200 ft-lbs of torque on wheel nuts? I have had a few cars in my 35 years of car ownership, but never one with more that 100 ft-lbs specified.
I think he's saying most people have their nuts ugga-dugga'd to the nearest ugga
einy said:
GameboyRMH said:
Surprising, I wonder if something happened at a shop that I don't know about. I can say I've definitely never found these lugs on at more than 100ft-lbs, well under the ~200ft-lbs most cars are driving around with.
I am truly curious - what passenger cars specify 200 ft-lbs of torque on wheel nuts? I have had a few cars in my 35 years of car ownership, but never one with more that 100 ft-lbs specified.
None that I know of, but that's what most tire shops and many cheapo mechanic chains will zip your lugs up to with an impact gun. They may even check that they can't be tightened further using a torque wrench at the correct setting aferward 
TheRX7Project said:
einy said:
I am truly curious - what passenger cars specify 200 ft-lbs of torque on wheel nuts? I have had a few cars in my 35 years of car ownership, but never one with more that 100 ft-lbs specified.
I think he's saying most people have their nuts ugga-dugga'd to the nearest ugga
Bwahaha that's another way of putting it 
I've had two break on me, also on a Toyota. However those were definitely over-torqued by a shop. Both broke while I was taking the nut off to work on brakes.
Our Edge Sport requires 160ft-lbs of torque for the 21" wheel options. I thought that was crazy high but its in the service manual.
wspohn
Dork
10/7/19 12:44 p.m.
GameboyRMH said:
None that I know of, but that's what most tire shops and many cheapo mechanic chains will zip your lugs up to with an impact gun. They may even check that they can't be tightened further using a torque wrench at the correct setting aferward
The only shops I would go to may zip the nuts on with an air gun, but only until they are barely snug and then use a torque wrench.
Some of them use 'torque sticks' that yeld at a given level by twisting, at which point the air wrench ceases to tighten any further.
RX8's got it.
Look at the fracture surface. See the little lines there, like a sandy beach with about 2" of water being hit with small waves? That is called Beaching.
Starts with a small fissure, then the unbroken parts beat together making a wave, then the failure gets bigger... like using a spoon to beat the edge of a quarter to make a ring. It goes on until ultimate failure.
The final blow was at rwo o'clock in your photo, see the little mountains, not beaching. Classic brittle failure, no cup & cone. Look at 7 or 8 o'clock, see the oxidation? It's been open for a while.
Dan
Vigo
MegaDork
10/7/19 2:56 p.m.
Saron81 said:
einy said:
I am truly curious - what passenger cars specify 200 ft-lbs of torque on wheel nuts? I have had a few cars in my 35 years of car ownership, but never one with more that 100 ft-lbs specified.
At my old school we had a chart on the wall with lug nut torque by make/model. Sometimes students would look at it and notice there was one line that was ~440lbft under Porsche, and then i got to explain what center lock wheels were and Google a bunch of Porsche pictures for a minute or two. I always liked when people would notice that number..
tester
New Reader
10/7/19 7:14 p.m.
irish44j said:
I've only ever had studs break like that on me two times.
Both were on my Sequoia.
IDK, maybe it's a Toyota SUV thing. Like frame rust......
The only lug I have ever broken was on a Toyota Celica. It’s a Toyota thing.
FWIW, the lug nut looks like it is pinched. This could be from a single overtorque event, or just normal wear that happens because there is no support there at its thinnest part except for the threads on the stud, that it is now galling.
And people wonder why I appreciate the engineering behind lug bolts and shanked lug nuts.
914Driver said:
RX8's got it.
Look at the fracture surface. See the little lines there, like a sandy beach with about 2" of water being hit with small waves? That is called Beaching.
Starts with a small fissure, then the unbroken parts beat together making a wave, then the failure gets bigger... like using a spoon to beat the edge of a quarter to make a ring. It goes on until ultimate failure.
The final blow was at rwo o'clock in your photo, see the little mountains, not beaching. Classic brittle failure, no cup & cone. Look at 7 or 8 o'clock, see the oxidation? It's been open for a while.
Dan
And in reply to RX8-
I was going to suggest fatigue- but you guys seem to know more about this- what would a fatigue crack propagation look like vs, the beaching?
Also- do either of you think the beaching was caused every torque event, or from road loading? Based on the pulling apart load, I would assume that crack propagated via a tightening event....
Other than a specialized materials class I took way, way back- I've not had to deal with material failures....
In reply to alfadriver :
If the stud ever saw side loading, it is because all of the other lug nuts were loose and the wheel was rattling around flexing the stud. Or, one lug nut torqued up before the others are snugged. This puts a huge side load on the other studs when the tapers try to center in the wheel that now cannot move, plus the wheel often eventually loosens up when the wheel does work its way centered in the tapers, and sometimes leads to the first example. (And leads some people to say "See! 77ft-lb ain't enough, they keep loosening!" and then think stalling the impact gun is the only way)
Either way, it's a result of bad technique.
One other thing that can cause side loads on the stud is cheap wheels with a screwed up bolt pattern. I've seen it twice in the nearly 4 decades I've been messing with cars so it's rare but it does happen
