Heavy tracking? Check your hubs/ bearings!!

[kyippee]

This whole press-in vs. thread-in discussion is great and aligns with what I've previously said. I personally stick with regular bolts because our cars don't come with press-in, and people regularly question why I don't switch to thread-in.

I just replace my wheel bolts regularly and make sure the wheels I use fit properly with the bolts I use

[Ngilbe36]

Originally Posted by roastbeef

How sure are you of that? I'm not 100% convinced it's not as strong or stronger. I want to do more digging, but I've never come across anyone that has personally had an issue. I've seen plenty of steel studs break, too.

If you were able to guard against galling, is there really more risk than a steel stud?

The general rule I have been taught is that for anything 8.8 and above, you need steel. Ti and Al cannot withstand the loads/forces, sheer or tensile.

Again that's a general rule.

[Chris_PDX]

Originally Posted by NRBRNG

Hey.
I know we all check things diligently after every track day, however some things are only really possible to check when other things are being replaced/ or start of season.
As part of new rotors/ calipers going on, I already had 4 new hubs inbound. Look at what i found on a my front hub/bearing...! The crack was all the way around except for about 1.5 inches. This was not there when I last replaced the rotors (< 2 years ago...) it was a new OEM hub/bearing 5 years ago...
Needless to say, this would have been an epic off if it failed on me- of course under heavy braking. Car is 100% track and runs slicks.

Still, CHECK yours when you can!

As part of my preventative maintenance, I had already ordered all 4 new hubs with press-in studs from Core4 Motorsports https://www.core4motorsports.com/pro...onversion-hubs
They come with a good reputation and use OEM hubs and options for press-ins. (No affiliation).

BTW, I will be posting some EXCITING news of my new project build and pics in a few weeks. More soon!
Cheers, Mark

I had the exact same thing happen to me last year at the Skip Track Day on my E46. Martin recommended the Core4 hubs with press in studs so we went that route. I had sheared several wheel studs previously on the front as well (oddly, the same day Eddie ditched your car outside T7 for the same issue). Hopefully no more of those shenanigans!

[gills]

Since people didn't grow tired of my engineering rants....

The general rule for nuts and bolts is that the strength/hardness of the nut is a couple of points weaker/lower than the stud/bolt threads. MSI manufacturers their lug nuts and wheel studs to this standard.

This is so that the threads in the nut yield a bit more than the bolt threads to ensure that there's even pressure distribution across a larger group of threads. The first 6 or 7 threads of a tensioned nut/bolt takes up more than 90% of the load. This also ensures that the nut threads are first to yield/strip instead of the bolt/stud threads in the case that they are overloaded.

If you end up using a harder nut than bolt/stud, the pressure distribution converges to less threads on the bolt/stud since it stretches more than the nut. The same load over a smaller area = higher stress.


BMW wheel hub material strength is softer than bolt grade 10.9 and about equal to 8.8 specifications, so grade 10.9 is the sweet spot. Like I said in one of my previous posts, going to a 12.9 or higher grade thread-in with BMW hub material strength doesn't abide by that general rule. This will only really come into play if you get to the optimum amount of tension that a 12mm grade 12.9+ bolt/thread-in stud can generate.

Press-in studs have no reliance on hub material strength to generate tension due to how the head of the stud generates tension from the back side of the hub. This ultimately makes for a stronger joint as well since it encompasses more hub material when everything gets clamped.

[dparm]

Thanks, gills. What I am still trying to find data on is "how do I know it's time to replace studs" -- the aftermarket would love for you to buy new ones, and shops would also love it so they can make a few bucks. I understand the benefits of a longer stud making wheel changes easier, or being necessary with certain wheels to ensure the nuts have enough thread. I'm not really worried about either of those use cases.

My factory studs are grade 10.9. As I understand it, ductile steel used in wheel studs (or any bolt) are typically torqued to ~90% of the yield strength of the steel. Any loads less than the yield strength result in the bolt working in its elastic region. This means if you release the load, the bolt returns to its original length and zero stress. If a bolt is “stretched” it means that it was stressed past its yield strength. The trick in spec’ing a bolt or stud is understanding the worst case loads it will see. If the load ever exceeds the yield strength, then the bolt gets permanently longer. This means that since it is longer, you also lost some of the clamping force and the nut is loose.

So in other words, if you follow the GM torque spec (100ft-lbs), you are keeping it in the elastic region and the lifespan is virtually infinite (excluding outside factors like corrosion, impact damage, etc)?

[gills]

dparm

We all want to know a true hard data point on when it is actually the right time to change wheel studs, but there's not one single answer. You can call wheel stud manufacturers yourself and they will beat around the bush. I've tried. Far too many variables. One thing that I also have never seen in factory service manuals is recommended service intervals of when to change wheel studs. If they did, perhaps you could extrapolate from that for a track application. Typically they all mention to just inspect the wheel nuts and threads to make sure they aren't stripped/deformed.

Then some people claim that stud life should be measured in torque cycles, but I haven't found any technical articles or testing that shows studs should be timed out from torsional fatigue. Fatigue is measured at a minimum of 1000 cycles. I think it's safe to say no one is coming close to that, barring constant, heavy, untrained impact gun tightening.

Perhaps they're referring to potential thread damage causing an excessive amount of torsional strain/stress while tightening? A little lubricant (or dry lubricant coating like MSI studs) making sure things turn smoothly and prevent galling goes a long way in this case. Torsional stress from applied torque is pretty small compared to the tensile stress due to preload when threads are working correctly.

And to be clear, when I say preload, I mean the amount of clamping or tension the stud is undergoing once it's tightened and BEFORE any external loads are applied to it.


So I'm glad that you pointed out what GM recommends. It should be noted they recommend 100 ft-lbs for a grade 10.9 12mm stud. Nissan also recommends 97 ft-lbs on their grade 10.9 12mm studs for the GTR (I recently found out the GTR NISMO model actually use 14mm. Torque of 144 lbs-ft). These are typically dry condition torque values.

Aftermarket BMW thread-in stud sellers/makers are recommending torques from 70 lbs-ft to 90 lbs-ft for studs that are grade 12.9 and above. Why?

You're correct that for a fastener that is "re-used" the optimum working preload will be when it's tightened to 90% of yield strength. At this preload is when a fastener has the greatest fatigue life also. I can tell you right now, basically no one is getting to that point using typical tightening torques. Even at GM and Nissan torques with 10.9 hardware, but they are erring on the safe side with a dry torque recommendation, because if someone does put on lubricant to lower friction, the torque will still be safe (and will get you closer to 90% of yield).

Also, something that is misunderstood in the case of external tensile/axial loads (loads applied along the axis of the stud) is that the stud sees ALL of this load PLUS the preload/tension from being tightened. To make things more confusing, the reality is that a stud only sees a small portion of tensile/axial loads and the portion it sees is directly related to the relative "stiffness" between the parts it's clamping and itself.

The parts you're clamping (in this case wheel, rotor, hub, and maybe spacer) compress because they're aren't infinitely stiff and have a "spring rate". All metals have a spring rate and deflect when subjected to a force. The stiffer the parts you're clamping, the lesser the portion of externally applied load the stud undergoes. The stud will typically see less than 20% in the case you're clamping metal components, where the clamped components themselves take up more than 80%. The ratio worsens as stud/bolt preload decreases and starts to become detrimental to fatigue life.

As I said in my previous post, the way a press-in stud encompasses more of the hub actually makes for a "stiffer" joint vs using internal threads. A stiffer joint loads the studs less, which is paramount for fatigue life.

If you couldn't tell by now, the topic of bolting/fastening is pretty complex so I apologize if this is very dry/technical. Most of us just look at a nut and bolt and say "it's just an effing nut and bolt!" Luckily, there's pretty large safety factors for things like keeping the wheel on your car, so more often than not, things don't happen. Until, that is, you introduce things with design flaws (thread-in studs).

If this stuff is interesting to you at all and you want it to make more sense, I'd highly recommend reading this basics on bolting: https://www.boltscience.com/pages/basics1.htm

[Chris_PDX]

Originally Posted by gills

dparm

We all want to know a true hard data point on when it is actually the right time to change wheel studs, but there's not one single answer. You can call wheel stud manufacturers yourself and they will beat around the bush. I've tried. Far too many variables.

I know that the guys at AR Motorsport recommend replacing them every year for a heavily tracked car. I didn't ask him about the ones with your hub.

I do 17-20 days a year, though not all of those are full day driving (free laps while coaching, for example). So I average 20-30 hours of actual track time per year. I always just assumed that studs are a consumable item and should be treated as such. And they're cheap enough to replace just once a year.