Wheel studs- worth checking and/or replacing
02-18-2014, 10:07 PM
#46
Yikes, that's just bizarre. Subscribed.
I've taken a big interest in the reason for stud failures on GT3s; if you haven't seen the thread, it's here:
https://rennlist.com/forums/996-gt2-...ighlight=studs
It would be very helpful to see closer pictures of the stud fractures, especially those that haven't been smeared by later damage. The studs that are still in the hub probably have the greatest chance of providing clues.
I, too, don't understand why the outboard stud portion and nut stayed retained in the wheel (and drum remnant), and why the drum fractured as it did. The stud holes in the drum should have pass-through clearance, so the nuts and outboard stud pieces should have flown off once they failed, and I don't understand why they took the drum out too. There should be no particular load on the drum at all (at least none that would cause it to fail like that). I'm wondering if there wasn't proper clearance between the drum holes and studs. If that as the case, it could explain the problem. I've seen this happen on other cars -- where there's too large a stud OD in a hole that's too small in the drum, and it gives a false sense of proper torque by holding the drum away from the hub just a tad. This can cause a relaxation of torque over time and can ultimately lead to stud fatigue failure. If that's the case, you may see mushrooming of the drum surface around the stud hole. This could also explain why the broken off studs stayed in the wheel and drum -- because they were trapped by the drum. It will be interesting to know how tightly the broken studs were trapped by the drum (and of course, why they were trapped by the drum).
Anyway, very interesting. Please update if you get more information.
Rob
I've taken a big interest in the reason for stud failures on GT3s; if you haven't seen the thread, it's here:
https://rennlist.com/forums/996-gt2-...ighlight=studs
It would be very helpful to see closer pictures of the stud fractures, especially those that haven't been smeared by later damage. The studs that are still in the hub probably have the greatest chance of providing clues.
I, too, don't understand why the outboard stud portion and nut stayed retained in the wheel (and drum remnant), and why the drum fractured as it did. The stud holes in the drum should have pass-through clearance, so the nuts and outboard stud pieces should have flown off once they failed, and I don't understand why they took the drum out too. There should be no particular load on the drum at all (at least none that would cause it to fail like that). I'm wondering if there wasn't proper clearance between the drum holes and studs. If that as the case, it could explain the problem. I've seen this happen on other cars -- where there's too large a stud OD in a hole that's too small in the drum, and it gives a false sense of proper torque by holding the drum away from the hub just a tad. This can cause a relaxation of torque over time and can ultimately lead to stud fatigue failure. If that's the case, you may see mushrooming of the drum surface around the stud hole. This could also explain why the broken off studs stayed in the wheel and drum -- because they were trapped by the drum. It will be interesting to know how tightly the broken studs were trapped by the drum (and of course, why they were trapped by the drum).
Anyway, very interesting. Please update if you get more information.
Rob
02-19-2014, 11:01 AM
#47
Rennlist Member
Thread Starter
My wrench Dan @ The Speed Gallery just reminded me of when the 2 little screws that hold the rotor to the wheel carrier backed out of one of my rears wheels. When I tightened the wheel I could see it didn't fit properly and called him over to look. He removed the wheel and sure enough, those 2 little screws had backed out just enough to keep the wheel from fitting tightly against the rotor hub. It would be easy to miss and going out like that could easily cause enough vibration to break off studs but also could break off the little screws leaving a void between the wheel and the rotor. Either way could be catastrophic....
Hope to post a video today of the incident.
02-20-2014, 12:04 AM
#50
Rennlist Member
Thread Starter
02-20-2014, 10:40 AM
#51
Rennlist Member
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02-22-2014, 11:30 AM
#55
Nordschleife Master
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I can't remember ever reading about 5 studs braking off simultaneously. Occasionally one will creak when torqueing, but all 5 infers something more than fatigue to me. Mine are over 4 years old with well over 100 hours and they're the longest I could find at 100mm from C-Doc. I always over-torque them to 100lbs. and I haven't lost one yet....
02-22-2014, 01:54 PM
#56
Rennlist Member
hi, glad you are ok , you said iy was 7cup rear rotors so could it be a failure due to the difference in the diameter of the five holes, biggers on 7 cup than those for the five studs ?
We sometimes use, for the cups, 997 road rear rotors and have the five holes re-drilled to the good diameter.
We sometimes use, for the cups, 997 road rear rotors and have the five holes re-drilled to the good diameter.
02-22-2014, 03:13 PM
#57
David,
I sent you a PM.
Questions about this. When the failure occurred, how long had the car been driven (in total) since the last time that wheel was torqued? Had you run a bunch of practice sessions and qualifying since the last retorque? You said it had last been torqued to "just over 90 ft-lb." What torque level did you use and what kind of torque wrench? Had it been calibrated recently? Looks like a right rear wheel came off and the predominant direction of the track you were running was clockwise, is that correct? Do you typically run on clockwise or counterclockwise tracks?
From your photos (especially the one in your first post) I can see fatigue on at least one of the studs, maybe two of them. It could be that others fatigued as well, but I can't be sure from your photos (hence have asked you to send me the original images so I can have a better look).
One likely scenario is that you had been fatiguing those studs over a long period of time (for unknown reasons so far), but none of them had fatigued enough to show up (i.e., break off) during your last retorque. Then during your race weekend, one stud broke while driving, then another (and each stayed trapped for yet other unknown reasons). These failures may not have been able to be sensed by you. Then as you continued to drive, the load on the few remaining studs exceeded their strength (which may have been compromised by fatigue cracks as well), and those remaining studs "unzipped" and the wheel separated. You said that 15 minutes into your race, you felt a vibration from the wheels ("clag" or so you thought). That was probably the point at which your wheel was held on by two studs or so, which lasted half a lap until the wheel broke off.
The big questions in my mind still are 1) why did the studs fatigue and 2) why did they remain trapped in the wheel/drum. And the answer may show that these symptoms are related. Fatigue happens when there is insufficient torque (bolt stretch, actually) and the studs go into bending during operation. Perhaps your wheels were running in an undertorqued condition during a substantial portion of their life, which caused progressive damage. But why?
You'll see from my posts on the other thread that I suspect the factory torque spec of 96 ft-lb is marginal for 996 and later cars, especially when aftermarket studs and nuts are used instead of the factory lug bolts. Factory lug bolts have a built-in spherical-head "washer" that may allow a greater portion of torque to be converted into bolt stretch, rather than lost in friction. I confirmed yesterday with the local Porsche dealer that all 2012 and later 991s, Boxsters, and Caymans use wheel bolts that are interchangeable with 996 and 997 cars, yet they've raised the torque spec to 118 ft-lb on the newer cars. To date, I've not seen any documentation that 996 or 997 owners are supposed to raise the torque spec for their cars, and frankly, I doubt we ever will. But it kinda makes me wonder what's going on...
Rob
I sent you a PM.
Questions about this. When the failure occurred, how long had the car been driven (in total) since the last time that wheel was torqued? Had you run a bunch of practice sessions and qualifying since the last retorque? You said it had last been torqued to "just over 90 ft-lb." What torque level did you use and what kind of torque wrench? Had it been calibrated recently? Looks like a right rear wheel came off and the predominant direction of the track you were running was clockwise, is that correct? Do you typically run on clockwise or counterclockwise tracks?
From your photos (especially the one in your first post) I can see fatigue on at least one of the studs, maybe two of them. It could be that others fatigued as well, but I can't be sure from your photos (hence have asked you to send me the original images so I can have a better look).
One likely scenario is that you had been fatiguing those studs over a long period of time (for unknown reasons so far), but none of them had fatigued enough to show up (i.e., break off) during your last retorque. Then during your race weekend, one stud broke while driving, then another (and each stayed trapped for yet other unknown reasons). These failures may not have been able to be sensed by you. Then as you continued to drive, the load on the few remaining studs exceeded their strength (which may have been compromised by fatigue cracks as well), and those remaining studs "unzipped" and the wheel separated. You said that 15 minutes into your race, you felt a vibration from the wheels ("clag" or so you thought). That was probably the point at which your wheel was held on by two studs or so, which lasted half a lap until the wheel broke off.
The big questions in my mind still are 1) why did the studs fatigue and 2) why did they remain trapped in the wheel/drum. And the answer may show that these symptoms are related. Fatigue happens when there is insufficient torque (bolt stretch, actually) and the studs go into bending during operation. Perhaps your wheels were running in an undertorqued condition during a substantial portion of their life, which caused progressive damage. But why?
You'll see from my posts on the other thread that I suspect the factory torque spec of 96 ft-lb is marginal for 996 and later cars, especially when aftermarket studs and nuts are used instead of the factory lug bolts. Factory lug bolts have a built-in spherical-head "washer" that may allow a greater portion of torque to be converted into bolt stretch, rather than lost in friction. I confirmed yesterday with the local Porsche dealer that all 2012 and later 991s, Boxsters, and Caymans use wheel bolts that are interchangeable with 996 and 997 cars, yet they've raised the torque spec to 118 ft-lb on the newer cars. To date, I've not seen any documentation that 996 or 997 owners are supposed to raise the torque spec for their cars, and frankly, I doubt we ever will. But it kinda makes me wonder what's going on...
Rob
02-22-2014, 03:46 PM
#58
Nordschleife Master
See attached. If there is no step in a brand new stud, then you can see evidence of fatigue cracking around the circumference in all of them.
Shiny = new damage
Rusty = Old (Fatigue) Damage
Shiny = new damage
Rusty = Old (Fatigue) Damage
02-22-2014, 04:09 PM
#59
Rennlist Member
Thread Starter
Interesting discussion.
The studs have been raced for over 3 seasons (might be 4) with no issue until last w/e.
They were likely torqued that morning before the race. The day before I ran a 30 min sprint and 15 min practice and 15 min qual.
The Cup brakes have been on for 1.5 seasons.
I'm out if town right now but I think it will be interesting to look also at the other wheel, especially the rear left as it gets most of the workout on the tracks I frequent (RA, Sebring and Barber). Really glad this did not happen on T12 or T1 at RA.
The studs have been raced for over 3 seasons (might be 4) with no issue until last w/e.
They were likely torqued that morning before the race. The day before I ran a 30 min sprint and 15 min practice and 15 min qual.
The Cup brakes have been on for 1.5 seasons.
I'm out if town right now but I think it will be interesting to look also at the other wheel, especially the rear left as it gets most of the workout on the tracks I frequent (RA, Sebring and Barber). Really glad this did not happen on T12 or T1 at RA.
02-24-2014, 12:27 PM
#60
Drifting
Agree that it looks like fatigue, at least on the top 3 studs in the hub picture. The oxidation means they were probably cracked for a while before the failure occured. The fatigue crack propagates to the point where the area left isn't enough to deal with the loads, then it fails by tensile or shear overload. The hubs are designed with a factor of safety and shouldn't need all of the studs to carry the load. But with 3 of them cracked ~50%+ through the area on that one side, and the increased loads of racing, it makes sense that they all let go.
It looks like that step is a shoulder for where the stud mates with the hub. That sharp corner is a stress riser and if possible it would be better to find studs that have a good sized radius there, even if you had to chamfer your hubs to give it room. It seems like this particular failure is unique to the screw-in stud design. The stock bolts, or traditional press-in studs don't have that sharp corner to concentrate the stress, which is right at the location of peak stress. Also when you torque a press-in stud or wheels bolt the load is reacted at the head of the stud in addition to the threads. A proper torque load should be significantly higher (tensile) than the loads the stud sees in service. The problem I see with these type of studs is that every time you remove the nut for the wheel there is potential for it to loosen up the stud, and there is no head on the backside to stop it from loosening. Over time things corrode, and threads gall, which would worsen the situation. The only way to be sure would be to loosen and re-torque the studs themselves before installing a wheel, which is impractical. Wheel bolts are a pain with frequent tire changes but if you are going to continue with these studs I would add checking the stud torque to your race prep checklist. I'd also consider the studs and nuts to be consumable and replace at some interval, like every season or every X races or hours.
It looks like that step is a shoulder for where the stud mates with the hub. That sharp corner is a stress riser and if possible it would be better to find studs that have a good sized radius there, even if you had to chamfer your hubs to give it room. It seems like this particular failure is unique to the screw-in stud design. The stock bolts, or traditional press-in studs don't have that sharp corner to concentrate the stress, which is right at the location of peak stress. Also when you torque a press-in stud or wheels bolt the load is reacted at the head of the stud in addition to the threads. A proper torque load should be significantly higher (tensile) than the loads the stud sees in service. The problem I see with these type of studs is that every time you remove the nut for the wheel there is potential for it to loosen up the stud, and there is no head on the backside to stop it from loosening. Over time things corrode, and threads gall, which would worsen the situation. The only way to be sure would be to loosen and re-torque the studs themselves before installing a wheel, which is impractical. Wheel bolts are a pain with frequent tire changes but if you are going to continue with these studs I would add checking the stud torque to your race prep checklist. I'd also consider the studs and nuts to be consumable and replace at some interval, like every season or every X races or hours.