Broken Studs
11-05-2013, 11:30 PM
#92
I didn't want to do this, because it's too crude to be reliable, but finally decided to do some approximate analysis of ideal torque. Disclaimer: I'm not sure yet what this means, so don't make any changes to your practice based on what's here. Everyone should verify this somewhere else. And good luck doing that! No one wants to take the liability for making recommendations about altering something as fundamental as the torque that holds your wheels on. And no one has the resources to do this properly...
To do this right would require a whole bunch of additional information, time, and testing results that I don't have. But grinding through the old standby equation that relates torque to bolt force (T=kDF), and making a bunch of other assumptions from here and there, I found the following. For a 14 mm (0.551 in) unlubricated , uncoated bolt or stud (k=0.2) of grade 10.9 spec (yield strength of 136 ksi), acting over a tensile area of 0.1789 sq. in., I find that the "optimal" torque (based on 75% of yield) is 168 ft-lb. If you use a coated bolt, or perhaps use some lube, such that the k drops to 0.15, then the "optimal" torque drops to 126 ft-lb. Note that lube can drop the k value even further in some cases. thereby further reducing the torque to achieve 75% of the bolt yield stress. If you run these numbers for a grade 12.9 threaded fastener, the numbers climb to 197 ft-lb unlubricated and 148 ft-lb lubricated.
Obviously, the "optimal" torque based on bolt yield is highly dependent on what the steel strength is, as well as the value of k, which roughly equates to the coefficient of friction in between the parts -- something that's incredibly difficult to know or measure. Also, I use "optimal" in quotes here because the true optimal torque depends on more than just the fastener, but also the strength and stiffness of the clamped components (the wheel and spacer in this case) and the strength of the threads in the wheel carrier (and nuts, if studs are used). And of course the torque demand also depends on the loads put upon the studs during driving.
But despite all the missing parts here, I'm still getting a sense that the 96 ft-lb torque spec from the factory is "conservative" in that it may be less than ideal, especially for cars like ours that are on the track, with sticky tires, being operated at particularly high loads. If wheel studs (or bolts) start bending because of inadequate preload from being below and adequate torque level, fatigue is a real threat. And I think that's exactly what's going on here.
I have also confirmed that wheel bolts for the 2012 911 and Cayman (and perhaps the Boxster), which are black, have a Porsche factory torque spec of 160 N-m (118 ft-lb). And, to my knowledge they have the same 14 mm x 1.5 threads as the "old" bolts we all now use that have a 130 N-m (95.8 ft-lb) spec. I'm told those bolts fit on earlier cars too. Does that mean that if you put the new black bolts on your 996 GT3, that you should torque them to 118 ft-lb? And if so, should we be doing that -- either getting those black bolts and torqueing them higher, or finding bolts or studs of equivalent strength and running at a higher torque? Who knows, maybe those black bolts aren't actually any stronger than what we use, but Porsche has somehow decided that the old 96 ft-lb spec wasn't adequate anymore. Do you suppose we can expect a service bulletin from PCNA one of these days?
I'm getting very tempted to run my wheels, especially if I use studs, at a higher torque. I'm beginning to think that the old factory spec is marginal for our cars when they are run on the track. Even a modest increase, to, say, 115-120 ft-lb might be enough to help prevent the failures we've experienced, and I don't think it would risk overtorque damage to the studs, wheels, or other components. But I would want to make sure that the studs were of a known (and high) yield strength -- at least equivalent to a Grade 10.9.
Rob
To do this right would require a whole bunch of additional information, time, and testing results that I don't have. But grinding through the old standby equation that relates torque to bolt force (T=kDF), and making a bunch of other assumptions from here and there, I found the following. For a 14 mm (0.551 in) unlubricated , uncoated bolt or stud (k=0.2) of grade 10.9 spec (yield strength of 136 ksi), acting over a tensile area of 0.1789 sq. in., I find that the "optimal" torque (based on 75% of yield) is 168 ft-lb. If you use a coated bolt, or perhaps use some lube, such that the k drops to 0.15, then the "optimal" torque drops to 126 ft-lb. Note that lube can drop the k value even further in some cases. thereby further reducing the torque to achieve 75% of the bolt yield stress. If you run these numbers for a grade 12.9 threaded fastener, the numbers climb to 197 ft-lb unlubricated and 148 ft-lb lubricated.
Obviously, the "optimal" torque based on bolt yield is highly dependent on what the steel strength is, as well as the value of k, which roughly equates to the coefficient of friction in between the parts -- something that's incredibly difficult to know or measure. Also, I use "optimal" in quotes here because the true optimal torque depends on more than just the fastener, but also the strength and stiffness of the clamped components (the wheel and spacer in this case) and the strength of the threads in the wheel carrier (and nuts, if studs are used). And of course the torque demand also depends on the loads put upon the studs during driving.
But despite all the missing parts here, I'm still getting a sense that the 96 ft-lb torque spec from the factory is "conservative" in that it may be less than ideal, especially for cars like ours that are on the track, with sticky tires, being operated at particularly high loads. If wheel studs (or bolts) start bending because of inadequate preload from being below and adequate torque level, fatigue is a real threat. And I think that's exactly what's going on here.
I have also confirmed that wheel bolts for the 2012 911 and Cayman (and perhaps the Boxster), which are black, have a Porsche factory torque spec of 160 N-m (118 ft-lb). And, to my knowledge they have the same 14 mm x 1.5 threads as the "old" bolts we all now use that have a 130 N-m (95.8 ft-lb) spec. I'm told those bolts fit on earlier cars too. Does that mean that if you put the new black bolts on your 996 GT3, that you should torque them to 118 ft-lb? And if so, should we be doing that -- either getting those black bolts and torqueing them higher, or finding bolts or studs of equivalent strength and running at a higher torque? Who knows, maybe those black bolts aren't actually any stronger than what we use, but Porsche has somehow decided that the old 96 ft-lb spec wasn't adequate anymore. Do you suppose we can expect a service bulletin from PCNA one of these days?
I'm getting very tempted to run my wheels, especially if I use studs, at a higher torque. I'm beginning to think that the old factory spec is marginal for our cars when they are run on the track. Even a modest increase, to, say, 115-120 ft-lb might be enough to help prevent the failures we've experienced, and I don't think it would risk overtorque damage to the studs, wheels, or other components. But I would want to make sure that the studs were of a known (and high) yield strength -- at least equivalent to a Grade 10.9.
Rob
11-06-2013, 11:24 AM
#94
Not sure if my math is correct, but if the stud is torqued into the hub at 30ft.lb and then a 96ft.lb torque is applied to the nut as you tighten the wheel, does that equal a 126ft.lb load on the threads in the hub? It would seem so if you think about it. I'm just thinking what the load limit is on the hub threads.
11-06-2013, 12:31 PM
#95
Not sure if my math is correct, but if the stud is torqued into the hub at 30ft.lb and then a 96ft.lb torque is applied to the nut as you tighten the wheel, does that equal a 126ft.lb load on the threads in the hub? It would seem so if you think about it. I'm just thinking what the load limit is on the hub threads.
No, it's not additive like that. Whatever the maximum torque is that's applied at the nut end effectively governs the torque (actually the tensile force) throughout the stud. When you tighten the studs into the wheel carriers with, say, 30 ft-lb, it puts a tensile force just in the first few threads of the short end of the stud that's engaged in the wheel carrier. But when you put, say, 96 ft-lb of torque on a lug nut, it now stretches the stud between the first couple threads of the wheel carrier and the first couple threads of the lug nut. So the short end of the stud in the wheel carrier is now being pulled with a tensile force created only by the 96 ft-lb of torque, not by the additive sum of the two torques.
As to the load limit on the hub threads, I don't know what that is. It could be determined, but we'd need to know the material. My guess, though, is that it's plenty strong to handle 20% more torque. I've never heard of anyone having a failure there of any sort, which makes me think there's probably a large margin of safety in the hub threads. But of course, I'm not sure.
Rob
11-06-2013, 12:44 PM
#96
Rennlist Member
Not sure if my math is correct, but if the stud is torqued into the hub at 30ft.lb and then a 96ft.lb torque is applied to the nut as you tighten the wheel, does that equal a 126ft.lb load on the threads in the hub? It would seem so if you think about it. I'm just thinking what the load limit is on the hub threads.
__________________
I don't believe this is correct. If it were, then if you torqued a bolt to say, 30 ft.lbs., backed off and retightened, you would be torquing to 60 ft lbs. I think it would be more like 32 ft lbs. I think that if you torque a bolt to 96 ft lbs, it will be 96 ft lbs., no matter where it started if it started at less than 96.
OOPS, sorry, I was too slow.
__________________
I don't believe this is correct. If it were, then if you torqued a bolt to say, 30 ft.lbs., backed off and retightened, you would be torquing to 60 ft lbs. I think it would be more like 32 ft lbs. I think that if you torque a bolt to 96 ft lbs, it will be 96 ft lbs., no matter where it started if it started at less than 96.
OOPS, sorry, I was too slow.
Last edited by KOAN; 11-06-2013 at 01:37 PM.
11-06-2013, 09:44 PM
#97
Three Wheelin'
Great thread! Interested on the add torque idea but have no info to add, unfortunately.
Re studs, anyone have pos or neg exp with GLS Top Studs .. glstopstud.com .. ??? Thinking on cycling mine this off-season and these have been recommended. Thoughts?
Re studs, anyone have pos or neg exp with GLS Top Studs .. glstopstud.com .. ??? Thinking on cycling mine this off-season and these have been recommended. Thoughts?
11-09-2013, 06:50 PM
#98
We've run pressed in studs in Porsche cars for years. Never had a failure. Not one. We started using the screw in type studs in clients 996/997 GT-3 and 997 Turbo models. We've had a rash of failures. We have torqued the studs in proerly at 20 ft lbs, used loctite and always torqued the wheels. At first they admitted it was a material flaw. They sent us 60 new studs for free. A month ago we ordered another batch of new studs. Installed them properly. We had failures after just a few weeks. The last answer I received from the supplier was "Maybe I should try another manufactures product." This was not the answer I wanted to hear.
11-09-2013, 06:59 PM
#99
I have used RUF studs on my car for the last 3 years. They are made by H&R for RUF to the best of my knowledge. I have over 50 track days on them with zero issues. Knock on wood. Studs are torqued at 32ft.lb to the hub and wheel nuts at 96. I have a tendency to trust German made products over the stuff made here which may actually be sourced from China in many cases.
11-09-2013, 07:34 PM
#100
Three Wheelin'
We've run pressed in studs in Porsche cars for years. Never had a failure. Not one. We started using the screw in type studs in clients 996/997 GT-3 and 997 Turbo models. We've had a rash of failures. We have torqued the studs in proerly at 20 ft lbs, used loctite and always torqued the wheels. At first they admitted it was a material flaw. They sent us 60 new studs for free. A month ago we ordered another batch of new studs. Installed them properly. We had failures after just a few weeks. The last answer I received from the supplier was "Maybe I should try another manufactures product." This was not the answer I wanted to hear.
What pressed-in studs does your shop recommend/install, if you don't mind sharing?
TIA!
11-09-2013, 07:55 PM
#101
We've run pressed in studs in Porsche cars for years. Never had a failure. Not one. We started using the screw in type studs in clients 996/997 GT-3 and 997 Turbo models. We've had a rash of failures. We have torqued the studs in proerly at 20 ft lbs, used loctite and always torqued the wheels. At first they admitted it was a material flaw. They sent us 60 new studs for free. A month ago we ordered another batch of new studs. Installed them properly. We had failures after just a few weeks. The last answer I received from the supplier was "Maybe I should try another manufactures product." This was not the answer I wanted to hear.
I'm not convinced that the failure problem in newer cars has to do with whether the studs are pressed or screwed in, though. Do you feel otherwise? I think it has to do with either the wheel torque level on these newer, heavier, and more powerful cars, or maybe some problem in the manufacture of the aftermarket studs. To me, the latter seems less likely, because a number of different brands seem to fail, and they all fail at the same place and in the same manner -- fatigue where the stud enters the wheel carrier. (Have you seen failures of a different nature or in a different location?) Though fatigue resistance can be improved with certain material choices, I'd be surprised if different manufacturers of studs have all produced defective parts.
So here are a few more questions, at least one of which was asked by Cello, above. What do you do for the 996/997 cars you build and maintain for your customers? Do you use screw-in studs? Whose? What wheel torque do you use? Do you recommend changing the studs out after some period? There's no practical way, is there, to retrofit pressed-in studs into a 996 o 997? If there were, do you think it would help?
Your wisdom and perspective is appreciated.
Rob
11-09-2013, 09:51 PM
#103
We have a 996 GT-3..Pressed in studs, no failure. We have had multiple failures on the screw in type studs. Wheel nut torque has been 95 ft lbs. On most of the "press in" type wheel studs the hub has to be removed to perform the install.
More to come.
More to come.
01-11-2014, 09:43 PM
#104
I will be completing a 5-lug conversion in the coming weeks. Six of one, half dozen of the other, but I've decided to start with lug bolts. One of my trusted engineer/track buddies has used them for the past few years, and he concluded they are actually easier than studs when using the aluminum screw-in posts. I like the idea of the aluminum ball "washer" that prevents spinning against the rim; and when using high quality parts, bolts are about $6 less expensive per stud/nut combo for a savings of about $120. In the future, I will be able to form my own opinion when it comes time for refresh.
Here is what I've learned about studs.
First of all, I know that I used Verbus splined press-fit studs in my track RSA for over 10 years without so much as a whiff of a problem. I think the fundamental difference with smaller diameter threaded studs is the machined collar and adjacent start of the threads that go into the hub; I believe this is a necessary but weak point in the design relative to both the press-fit studs and the lug bolts.
Based on what I could gather, Apex studs are imported - although they never returned my request to clarify the country of origin. While I appreciate a good deal as much as the next guy, the unusually low price point relative to alternatives is another clue.
If I wanted to use studs, my preference would be to modify the hubs to accept standard press-fit studs. If using threaded studs, I would only use those verifiably made in the USA. One primary source appears to be MSI Racing in Charlotte (http://www.msiracingproducts.com/products.html). TC Kline also carries studs made in Charlotte.
My 3 cents… inflation ;-)
Here is what I've learned about studs.
First of all, I know that I used Verbus splined press-fit studs in my track RSA for over 10 years without so much as a whiff of a problem. I think the fundamental difference with smaller diameter threaded studs is the machined collar and adjacent start of the threads that go into the hub; I believe this is a necessary but weak point in the design relative to both the press-fit studs and the lug bolts.
Based on what I could gather, Apex studs are imported - although they never returned my request to clarify the country of origin. While I appreciate a good deal as much as the next guy, the unusually low price point relative to alternatives is another clue.
If I wanted to use studs, my preference would be to modify the hubs to accept standard press-fit studs. If using threaded studs, I would only use those verifiably made in the USA. One primary source appears to be MSI Racing in Charlotte (http://www.msiracingproducts.com/products.html). TC Kline also carries studs made in Charlotte.
My 3 cents… inflation ;-)
01-12-2014, 02:33 PM
#105
Technical Guru
Rennlist Member
Rennlist Member
According to the Factory Porsche 996tt/GT2 maintenance manual, the wheel bolt threads MUST be lubed prior to installation and torquing to 96ft/lb. The verbiage is copied below from page 776 of the manual:
"Always apply a thin coat of Optimoly TA (aluminium paste) on the thread of the wheel bolts, on the shank and between the bolt head bearing surface and spherical cap ring (under the head) −arrows− . Do not grease the spherical cap bearing surface −X− of the wheel bolts that faces the wheel."
I know this applies to wheel bolts but I would imagine the same would apply to studs/nuts.
"Always apply a thin coat of Optimoly TA (aluminium paste) on the thread of the wheel bolts, on the shank and between the bolt head bearing surface and spherical cap ring (under the head) −arrows− . Do not grease the spherical cap bearing surface −X− of the wheel bolts that faces the wheel."
I know this applies to wheel bolts but I would imagine the same would apply to studs/nuts.