KOAN

My first stud failure happened when I was removing a lug nut, I don't remember which wheel. I was using the longest (80 mm) rolled thread stud available, with essentially no unthreaded portion...just a small collar at the base. It was the end of their first season, so I changed them all. They were red high temp thread locked in. One other one broke as I was removing it. I used BBS, CCW and Champion wheels, all different thicknesses.
I went with a different brand, which was silver instead of black, with all new lug nuts. A different coating or metal compound. These were the ones that broke while I was on the track, 2 on the left rear. In all cases, the studs were fully engaged in the wheel carrier, and had no dead space collar that would allow the lug nut not to be snugged into the wheel, were hand torqued to 96 ft/lbs.
The last 2 Porsche track cars I had were older and had factory studs. In approx 8 years, never had a problem. I've not had a problem with Porsche bolts on the GT3, before or after the stud experience. I hypothesize that the metal in the aftermarket studs my be more brittle than the porsche bolts, but don't know this to be true. I use my impact gun to remove the bolts and the nuts, but hand tighten all of them.
The fact that so many stud failures have been reported here is useful, as it increases the sample size, and makes personal experience interesting, but perhaps not as statistically meaningful.

LVDell

This is what I would agree with more than any other hypothesis. Driver error would not be a hypothesis as it is more of a confounding variable that we need to attempt to control for.


We would need to quantify "so many" as that has no value. Maybe time to start from the beginning and employ a good old fashioned survey/poll?

Rob S

I have to respectfully disagree here.

Lubricating the studs, whether it be with anit-seize, oil, or any other lubricant should not be a problem. If it were, there would be many more failures than there are.

What's happening in a bolted clamped joint, such as a wheel connection, is that torque (which is relatively easy to measure) is being used as a substitute for bolt axial force (which is difficult to measure). If we could measure that force directly, we'd be better off (and we do sometimes measure stretch - which directly relates to force - for critical connections like connecting rod bolts). But the relationship between torque and bolt force is not as direct or certain as we'd like it to be. Still, for 99.9% of mechanical applications, measuring torque is good enough. That's because despite the variations imposed by slightly out of calibration tools, the presence of lubrication, or the roughness of the threaded parts, a torque spec is "good enough" to assure a perfectly safe and serviceable connection. It's true that a lubricated connection will result in greater bolt tensile force than an unlubricated bolt at the same measured torque, but that difference is very unlikely to be the cause of this failure or others. The overall design of this wheel connection, including the torque spec, should be robust enough that it's just not that sensitive to the state of lubrication.

That said, we don't know what caused the failure of the studs described by the OP or others in this thread. To the OP: if you have really good close-up macro photos (or even photos from a microscope), please post them. If fatigue was the cause, it may be very evident from the photos, though a more in-depth look might still be required. And if bending fatigue was involved, then lack of sufficient torque is more likely the culprit than any form of overtorque. If there is an under-torque condition, there may be other clues as well -- including rubbing contact or galling of the lug nut/wheel or wheel/hub contact areas.

Burger

Torque for bolt axial force isn't generally done in the same manner. (Correct me if I am wrong here, not my area of expertise)

In the assembly of many fasteners, say for example engine internals, the bolt axial torque process that can be used for a 'wet' or lubricared connection would have you torque the fastener to well below it's failure point, then turn the bolt or nut a specific degree past that point, which would be determined by the thread pitch, in effect stretching the fastener a measurable amount to secure the connection while being within the limits of the components.

With a 'dry' torque spec of 95 ft/lbs, a 'wet' torque spec would need to be lower and have a follow up rotation to be safely within the fasteners tolerances.

I have used Vorschlag stud kit on a car in the past and they offer spec for both conditions-

http://www.vorshlag.com/product_info...297976b8b00346

Specifics on the Vorshlag 80mm Wheel Stud:

Material: 30MnB3
Heat Treatment: Hardened and tempered to 10.9 class
Tensile Strength: 150,000 psi
Hardness HV: min 320, max 380
Elongation after breakage: 9% min.
Max Torque for Lug Nuts: 180Nm (132 lb-ft) - recommended torque on the lug nuts (NOT the stud during install) using anti-seize is 80 lb-ft
Install using Red Threadlocker (Loctite 262 or equivalent) to secure the studs to the wheel hub.

I have also witnessed someone spin the heads off of Porsche wheel bolts when they applied anti-seize to the ball portion of the bolt.

KOAN

These Vorschlag studs appear to be cut rather than rolled. Do you know if that is true? I have been told by some friends who are auto design engineers that this is an important difference.

Burger

This I do not know. I have not heard of any of their studs breaking, if that is of any consolation.

JohnPG

Francois,

I know that there are differenct nut types, as you mention sperical and conical. I've left that detail to my mechanic. I TRUST he has paired my wheels which are all CCW's with the correct nuts. I had mentioned the different types of nuts as as a possible cause of stud failure. If the nuts don't match the seating profile of the wheel, could that be cause of stud failure? I don't know. Some of the postings today have come from folks far more qualified to answer this question.

We've all witnessed either first hand on our cars or on cars of folks we drive with enough of these failures to be concerned.

The question remains, is it user error or product defect?


On a lighter note. How long is the line and are you serious about a Cup?

Rob S

Torque on a bolted connection is an indirect measure of axial force and nothing more. It is an alias -- and a rather crude one at that -- for this measure. It doesn't matter if the joint is at a wheel or inside an engine.

I don't want to belabor this, but I want to be sure to be heard about the bottom line here: lubrication on wheel fastener threads is usually fine. I want to make sure that people don't rush out and clean off their wheel studs and nuts thinking that they might be in trouble.

Often, vehicle manufacturers are silent on the issue of wheel stud lubrication. In some cases, there may be specific recommendations, but it varies by car manufacturer, wheel manufacturer and fastener manufactuer. And the recommendations from one may conflict with the other -- especially when using aftermarket parts. Wheel connections are so robust that they generally are not critically sensitive to the presence of wheel fastener lubricants. It's true that for a given torque level bolt preload increases with lubrication, but generally, it would not increase to the point of causing damage to the fasteners or wheels.

From looking at the Vorschlag stud recommendations, their specs do not appear to conflict with what I've said. They show a *maximum* torque of 132 ft-lb, but they don't specify lubed or dry. They also specify a *recommended* torque of 80 ft-lb lubed. Those two values are not necessarily equivalent. And their technical instructions specifically instruct to use anti seize and to torque to 80-85 ft-lb. But those recommendations are not focused on the GT3 -- let alone one used on the track. So in addition to their rather sloppy documentation, it becomes difficult to know whose/which specs to trust.

I don't know why wheel studs seem to be failing in the field, but I wouldn't assume that they're necessarily failing for the same reason in each case. Each one needs to be analyzed independently before anyone can jump to conclusions that there's a worrisome trend. Let's start with the OP's stud. If we had a really clear pictiure of the fracture surface, that would be a start. But a careful inspection of the rest of the parts -- the wheel, the nut, the hub mating surface, and the other studs would be helpful too. And even then, it might require a full metallurgical workup on the broken stud to find the answer. Meanwhile, keep your wheels torqued properly, check them frequently, and report on any failures.

Boinkus44

Update.

Had a chance to talk to the race shop (Fiorano Racing in Toronto) that installed the studs originally. Confirmed that they were installed with hand torque tightness down to the shoulder and set with red loctite - like they have been doing on their race cars for many years. FWIW, they have never had a stud fail on a race car or track cars that they prepare except as a result of an impact, and they have used the Apex Comp races studs many times. We're talking dozens of race/track vehicles over the years and almost exclusively Porsche.

I have not yet had a chance to talk to Apex about the failure yet, but hopefully tomorrow. In the meantime, here's a couple of additional pics that may provide soem additional insight into the failure. I can say that even though it may not be evident from the pics, there is clear evidence that the stud had been cracked for a period of time prior to failure due to the corrosion present on the cross section. Apologies for not owning a camera.........

FFaust

It appears that there is actually more to it than "conical or spherical":

<<<Lastly, an important safety note. Many other retailers are selling "ball seat" Nuts for Porsche. But what does this mean? Ask them if they are R12, R13 or R14 radius nuts, you'll most likely draw a blank stare. M14x1.5 VW/Audi nuts are commonly sold as "ball seat" nuts for Porsche.... yet they will NOT seat properly on an OE Porsche wheel !>>>

From: https://rennlist.com/forums/7780945-post19.html

Serious? I would never joke about something like that However, still plenty to learn and do with the 996, so it ain't for just yet. Also, I would love to follow everyone's advice and keep this car forever, and just add a cup, so that makes it even less of a short-term project. Sorry.

All aircraft wheels where the wheel halves are bolted together are lube-torqued. This is the only way to ensure consistency all around, and from wheel to wheel.

Here are some pics of my first failure.

StephenB

Some thoughts.

First a quote by Carroll Smith: "Since threads are, by definition, exceedingly efficient stress risers, it behooves us to give the bolt manufacturers a bit of a hand in avoidance of fatigue failure."

It looks like your failure occurred at the root of the first thread. I would suspect this is the common failure location of others as well. This could be indicative of stress due to utilizing the studs as a means of locating device (bending stress). A 3" long stud used to guide the wheel on can load the top stud. When using wheel bolts, the hubcentric extension axle bears all of this locating stress.

I respectfully disagree that lubricants do not play a part in over stressing a bolt or stud. For example, collodial copper with a friction factor of 0.11 would reduce the required torque by approximately 45%.

Aircraft manufacturers specify dry or lubricated torque values and A&P's strictly follow these values. Take a quick look at the following article and the AC mentioned:
http://www.aopa.org/members/files/pi...0/anp0009.html

Also, Engineer To Win by Carroll Smith has excellent discussions on the topic of fasteners, stud design, stress, fatigue, rolled vs. cut, etc.

Given all of the above, I would treat the studs as disposable item that do have a fatigue life and the hubs were not designed for studs. Inspect all new studs for any scratches near the radius before installing. New studs/bolts should be a few times to reduce the friction factor before final torque.

KOAN

I'm really learning a lot. Thanks to everyone for their insights.
It would seem to me that the stress put on studs by torque would also be vastly different depending on the way the threads are surfaced, that is, rolled vs. cut, as well as the surface and composition of the nuts. Apex recommends using new nuts when studs are changed, suggesting that they deform over frequent use to a significant degree.

The idea that stress caused by using the studs as locators would apply bending stress, and would explain the most common location of the fractures would also explain why it is happening less frequently with bolts, which cannot be used as locators. Too bad, as one of the main advantages of studs is to use them as mounting locators. But it is an intriguing explanation.

LVDell

How much stress does a 40-50 pound wheel/tire place on a stud? I don't profess any knowledge in engineering so I can't even take a guess why this would be a high stress situation.

KOAN

Qualitatively....I think that the effective stress would depend on the surface area of the fulcrum, and how gently the wheel is placed on the first stud, in addition to other factors like the length of the lever arm ( stud length sticking out of the wheel carrier). I have no idea what type of stress a stud should be able to withstand, but I would guess it is temperature dependent, and that each metal type, depending on its tempering, is different.

Rob S

From some quick calculations for a 14.5 mm stud of grade 10.9, assuming a 3 inch bending length, a 50 lb wheel hanging on the very end of one stud would create bending stresses where the stud meets the hub that are about 10% of the stud's yield strength. Design guidelines often suggest keeping design loads below 80% of yield. Doing so, the hanging wheel still only approaches 12% of the allowable load, which is a very modest number. There would be some magnification of stress from the thread root, and there could be some impact loading if someone slammed the wheel downward on the end of the stud. But these factors would not likley be enough to change the conclusion that it is highly unlikely that using the studs as locating devices while mounting a wheel has any consequence that's relevant to this issue.

Also, I didn't say or mean to suggest that use of lubricants cannot play a part in overstressing a stud. The use or misuse of lubricants can be absolutely critical in some applications. Take, for example, the very strict guidelines ARP uses for their rod bolts, which requires the use of only their lubricant and several burnishing torque-ups before final torque. But this is not a rod bolt -- wheel studs are not designed to run as close to the edge of their performance envelope as rod bolts. So the use of lubricant, or lack thereof, is not likely to be nearly as critical in this application.

Still, the most likely reason for stud failures of the type we're seeing (at least, for the two pictured in this thread) is inadequate preload resulting in excessive bending loads during operation that cause fatigue at the base of the stud. But whether and why it seems to be plaguing studs more commonly than bolts is not clear.