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Thanks for all the input guys, I switched to studs after I had some "fancy" bolts from the previous owner snap at the track 2 years ago. I was on R comp coming out of 80mph corner and heard a loud pop as I was accellerating out of it. The pop was 2 bots snapping and shooting off into the weeds, luckily I didn't.
I did studs immediately after have been crazy about rechecking torque since.
Upping the torque from 96 to 118 per a post I'd read from John last year has eliminated almost any nut rotation after a single re-torquing.
That just proves you're a much faster driver than they are. I'm slow on the other hand and have to cheat to keep up by using a faster car.
Meant to ask earlier, were most/all the studs you broke on the rear axle or also the front? Obviously the rear sees the most load.
All on the rear, which as you rightly pointed out, take significantly more load than the front. Always on corner exit as your feeding the gas in. They break, you go violently sideways, get it under control and limp to the side of the track. Its never a fun experience and partly why I am just not willing to try anything else other than bolts now.
I've used several brands of studs and have had them break, but have had great luck with MSI. I should change them out soon. Where is a good source for H&R? I may try those next.
I use MSI studs/nuts
2 year interval
not had one break
I do NOT use a gun to put bolts on. I do use one to remove
Torque to 110.
I DO use a gun to put them on; the Ridgid 18v impact driver I use just happens to torque nuts to 100-105lb. Makes quick work of wheel swap or on/off when you only need to move torque wrench a little bit.
I use MSI studs/nuts
2 year interval
not had one break
I do NOT use a gun to put bolts on. I do use one to remove
Torque to 110.
Same here.
Many failures (not Matt's, thats just speed/work .. ) are caused by bad installation technique. Many over torque to hub and overstretch the threads which starts the failure process... I have a good .pdf doc somewhere on torque values on install, etc., that was created from info provided by a Mfgr and seconded by a very good race shop and if I can locate the thing I will post it up...
Be careful with Porsche as porsche use radius chamfer on the wheels and nuts vs angled chamfer for other brands.
Therefore nuts need too match the rim. If you buy porsche aftermarket rims, verify seating, but most will be to Porsche spec as they assume you will reuse the bolts.
So if you buy other rims not specific to Porsche, you can't buy Porsche branded stud and nuts chamfer won't match and you will have incorrect load path.
Be careful with Porsche as porsche use radius chamfer on the wheels and nuts vs angled chamfer for other brands.
Therefore nuts need too match the rim. If you buy porsche aftermarket rims, verify seating, but most will be to Porsche spec as they assume you will reuse the bolts.
So if you buy other rims not specific to Porsche, you can't buy Porsche branded stud and nuts chamfer won't match and you will have incorrect load path.
Exactly, the inexpensive and popular Forgestar use conical bolts/nuts vs the Porsche standard ball seat.
Also there's lots of Chinesium crap marketed as "performance" product. I trust the studs I've used 2 years no problem and the H&R brand Hound has been using without issue. The Mevius brand conical lugs snapped on me at the track running F14 Forgestar.
I run studs and if I break one I will send for anyalasis. Here is what's Micro structure report looks like for recast and micro cracks in inconel base material in turbine blades for industrial gas turbines.
detailed met inspection is always a way to know for sure what the exact failure mode is!! This inspection was at 500 power.
Torque your wheels to 118 ft.lbs. Your stud breakage woes will go away... 96 is a carry over from the old air cooled days when Porsche used aluminum lug nuts and seems too low allowing for some flexing of the studs eventually leading to their failure (sort of like flexing a paperclip back and forth). In 6 years of heavily tracking my car and torquing to 118 I've never had a stud failure. I only use German H&R studs... Lot of the fancy "Nascar" type studs have dubious origins. Know exactly what you are putting on your car..
To be clear, I torque the stud in to the hub at 32 ft.lbs and wheel at 118. Incidentally 160NM (118 ft.lbs) is what Porsche switched to on 991 and later cars using the same 14x1.5mm lug bolt size.
I agree with powdrhound on this and have been following these concerns for years (and have posted about it several times). The reason for every failure I've seen (my own and many others, on different brands of studs) is fatigue. The failure surface is typically flat and very smooth across most of its face, with a small area of "final fracture" on one side. This is an indication of a fatigue crack that initiates on the perimeter an grows slowly across the face of the stud material, often over many hundreds of thousands of cycles of bending in the stud, which occur as the car is driven. It's also an indication that the normal tensile stress is quite low in the stud, otherwise it would fail before the crack grows across 90% of the diameter. Fatigue is caused by excessive bending stress, often because of insufficient "preload" in the stud -- which is the result of insufficient lug nut torque. This allows the stud to flex and bend more than is optimal. There can also be other factors that influence fatigue that such as the material specs, heat treatment, how the threads were made (cut vs. rolled), and the integrity of the clamped joint (the wheel and hub stiffness, etc.), but the big one -- and the one you can control -- is lug nut torque.
The failures I've seen have not been the result of overtorque of either the lugnut on the stud or of the stud in the hub. If overtorque had been the cause, the fracture surface would have a different appearance. Failures have also been known to happen with different brands of studs. Though I've chosen MSI studs for my car, and I liked what I could learn about their manufacture, I haven't accumulated enough time on them to provide any longevity data yet.
The problem with studs, as I see it, is that they use traditional lug nuts of the type that Porsche used prior the Boxster and 996. The use of traditional lug nuts on 996s increases the friction between the nut and the wheel when compared with the factory lug bolts that have a captured washer-like "bearing" to bear on the wheel. Hence, the 996 factory wheel bolts, when torqued to the time-honored 96 ft-lb recommended spec, create more bolt preload (or clamping force) than when traditional lug nuts (on studs) are used. Conversely, traditional lug nuts "absorb" more of the 96 ft-lb torque in friction at the nut wheel interface, and less of the lug nut torque goes into stretching the bolt and clamping the connection. When you now add in the much more powerful 996 cars on sticky tires, like the GT3 and TT, this becomes important, because these cars put more load into the wheel connection. In fact, it's likely that even with the factory bolts with washers, the 96 ft-lb spec was getting marginal. This was shown by the fact that in 2012 Porsche changed the recommended torque spec to 118 ft-lb, even though the bolts were identical to those used for the 996, 997, and Boxsters. Why is this? We haven't been told, but I suspect Porsche realized that the 96 ft-lb spec was no longer adequate for the higher loads that the newer cars imposed on the wheel bolt connection. And that was for the use of the factory wheel bolts, with those captured washers...
So if you're running a GT3 on sticky tires, in a track setting where loads are higher, (as I suspect those reading this forum are doing), and if you're using studs and traditional nuts, you're risking being significantly undertorqued at 96 ft-lb. I endorse what powdrhound is doing -- and use a higher torque level. I don't have a specific number, but it seems 118 ft-lb would be a reasonable place to start.
Also, if you *ever* find after driving your car that your wheels have lost torque for any reason, you are risking failure to run on those studs again (or bolts, if you're running them). It's not good enough to just tighten them down. If you run low on torque, those studs were subjected to bending stresses and a crack may have formed and started to grow. You should replace all five studs on that wheel as soon as possible. Also if you ever break one stud on a wheel, the other four should be replaced because they likely were overstressed as well.
It seems that stud failures most commonly occur on rear wheels that are on the outboard side of the principal track rotational running direction. So if a car is most often used on a track that runs counterclockwise, the right rear wheel studs will be the most vulnerable because they receive the highest cumulative loading for several reasons.
I can't explain why mmuller says he's had studs fail even when torqued to 118 ft-lb; maybe something else is going on there. But I've also heard of people breaking factory wheel bolts. So nothing is 100% certain and there will always be outliers, but the mechanical and metallurgical evidence, as well as Porsche's higher torque spec on newer cars, suggests that a higher torque level will help prevent these failures.
Though it's fine to carry extra studs with you to the track, they will be of little use to you if you break one because it's not easy to extract the broken part. The stud often breaks off flush with the hub and requires drilling out and the use of a extraction tool. I suppose you could carry all the tools to extract a broken stud, but it would be a pain to do so.
Finally, I don't believe that properly manufactured wheel studs that are run at the proper torque ought to be a frequent replacement item. If they need to be replaced every year or two, something is wrong with the system. The broken studs I've seen all indicate that the bending stress is out of control; if the bending stress can be reduced, the stud should have an infinite life. The best way to achieve that is to increase the torque level to keep the bending stress in check.
The first photo shows a typical stud failure -- flush with the drive flange surface. The second shows a close up of the stud fracture surface. A fatigue crack started on the right, probably at the root of a thread, and progressed from right to left. The little bent "tab" is was the last part to fail. The third is a close up of the mating fracture surface of the stud in the drive flange.
02-16-2017, 01:40 PM
