BMW M3 Forum (E90 E92) - View Single Post

gills · 05-19-2020, 11:57 AM

Hey all,

I've covered the wheel stud topic ad nauseam in different threads in here so I'll try to be as succinct as possible (impossible....I type more than talk as dogbone can attest LOL).

The "lightness" of E36 and E46 cars seems logical relative to the E9x M, but these chassis' breaking studs in the series I run in (AER) is what prompted me to even bother offering a press-in stud converted BMW hub. And, we aren't even allowed to run anything beyond 180tw "street" tires. The majority of failures happening on the rear corners (think cycling brake AND engine torque) and more than 50% of them without wheel spacers.

I was growing tired of hearing about, and more importantly, worried for my fellow racers that experienced breaking studs after a pretty serious wreck (driver's ok) from losing a wheel in a very high speed section at Mid-Ohio (E36). Ironically enough, this same driver had the same exact thing happen in the same spot on the same track in a different car with a different team a year later (E90 328). You can see the incident as well as others on my website here (just about every stud brand is pictured in here, including almighty MSI and even a couple M14's):

https://www.core4motorsports.com/gal...mw-wheel-studs

Does session duration play a role? I'd say likely. Most guys tracking/racing are only pushing the car on track for 20-45 minutes at a time where they can check stud torque between sessions. AER/WRL/endurance events have 20-24 hours of total track time in a single weekend with the car on track for many hours at a time. BUT, there have even been cases of brand new studs installed for the weekend that broke within a few hours on the first day of racing.

Also, most don't check wheel stud torque during pit stops because internet lure has led us to believe that checking torque when "hot" is somehow more detrimental than the risk of having insufficient preload/clamping. And if you talk to anyone who's in the field of nut/bolt/fastening, insufficient preload is the single largest proponent to cause fastener failure; especially the failure mode that wheel studs fail (bending fatigue).

It is far more difficult than many think to truly overload a high strength M12 fastener with a cone/ball seat nut in an aluminum wheel by torquing them. In fact, you won't even get a permanent set/deformation in a grade 10.9 M12 stud using a cone seat nut if you torque them to 150 ft-lbs. And if a screw-in stud does break while torquing it or removing it from the hub, count your blessings because it was compromised in previous use and luckily didn't let go on you in the middle of turn.

Are wheel bolts actually more robust than screw-in studs? The answer is yes. This is specifically due to the very nature of how screw-in studs need to be manufactured in order for them to bottom out to prevent screwing in any further. Thread run-out regions, or in other words areas where threads start/end are stress concentrations. One of these areas of a screw-in stud just happens to always sit perfectly at a point of peak [bending] stress and where they break. You can clearly see that in the "broken stud gallery" I shared above.

dogbone mentions the F8x uses M14, but ironcially enough the GT4 car is supplied by BMW with "stepped" studs that are M14x1.25 that thread into the hub and M12x1.5 portion that you tighten the lug nut onto.

BMW GT4 stud:

BMW is basically saying that the clamping capacity of a M12 stud is sufficient for racing use at that level. The main difference here vs a M12 screw-in stud is having the larger M14 part in the hub provides a large increase in strength/material and resistance to bending fatigue failure (the GT4 stud has a couple of other features I won't bore you with).

An M12 press-in stud offers the same/similar effective diameter as an M14 thread where it is installed into the hub because threads have "roots" where the diameter necks down. An M14 thread root diameter is actually slightly smaller than the diameter of the M12 press-in studs we use where it presses into the hub. The M14 press-in stud we use is larger than both by a huge margin where it installs into the hub.

So, with the extra clamping capacity of a M14 studs, combined with huge increase in material size in a peak [bending] stress area, it will take a significant lapse in preparation and judgment or material defect to break one.

M14 press-in on right, M12 press-in on left:

05-19-2020, 11:57 AM	#32
gills Enlisted Member 83 Rep 32 Posts Drives: E36 M3 race car, E30 S54 swap Join Date: Oct 2019 Location: NY iTrader: (0)	Hey all, I've covered the wheel stud topic ad nauseam in different threads in here so I'll try to be as succinct as possible (impossible....I type more than talk as dogbone can attest LOL). The "lightness" of E36 and E46 cars seems logical relative to the E9x M, but these chassis' breaking studs in the series I run in (AER) is what prompted me to even bother offering a press-in stud converted BMW hub. And, we aren't even allowed to run anything beyond 180tw "street" tires. The majority of failures happening on the rear corners (think cycling brake AND engine torque) and more than 50% of them without wheel spacers. I was growing tired of hearing about, and more importantly, worried for my fellow racers that experienced breaking studs after a pretty serious wreck (driver's ok) from losing a wheel in a very high speed section at Mid-Ohio (E36). Ironically enough, this same driver had the same exact thing happen in the same spot on the same track in a different car with a different team a year later (E90 328). You can see the incident as well as others on my website here (just about every stud brand is pictured in here, including almighty MSI and even a couple M14's): https://www.core4motorsports.com/gal...mw-wheel-studs Does session duration play a role? I'd say likely. Most guys tracking/racing are only pushing the car on track for 20-45 minutes at a time where they can check stud torque between sessions. AER/WRL/endurance events have 20-24 hours of total track time in a single weekend with the car on track for many hours at a time. BUT, there have even been cases of brand new studs installed for the weekend that broke within a few hours on the first day of racing. Also, most don't check wheel stud torque during pit stops because internet lure has led us to believe that checking torque when "hot" is somehow more detrimental than the risk of having insufficient preload/clamping. And if you talk to anyone who's in the field of nut/bolt/fastening, insufficient preload is the single largest proponent to cause fastener failure; especially the failure mode that wheel studs fail (bending fatigue). It is far more difficult than many think to truly overload a high strength M12 fastener with a cone/ball seat nut in an aluminum wheel by torquing them. In fact, you won't even get a permanent set/deformation in a grade 10.9 M12 stud using a cone seat nut if you torque them to 150 ft-lbs. And if a screw-in stud does break while torquing it or removing it from the hub, count your blessings because it was compromised in previous use and luckily didn't let go on you in the middle of turn. Are wheel bolts actually more robust than screw-in studs? The answer is yes. This is specifically due to the very nature of how screw-in studs need to be manufactured in order for them to bottom out to prevent screwing in any further. Thread run-out regions, or in other words areas where threads start/end are stress concentrations. One of these areas of a screw-in stud just happens to always sit perfectly at a point of peak [bending] stress and where they break. You can clearly see that in the "broken stud gallery" I shared above. dogbone mentions the F8x uses M14, but ironcially enough the GT4 car is supplied by BMW with "stepped" studs that are M14x1.25 that thread into the hub and M12x1.5 portion that you tighten the lug nut onto. BMW GT4 stud: BMW is basically saying that the clamping capacity of a M12 stud is sufficient for racing use at that level. The main difference here vs a M12 screw-in stud is having the larger M14 part in the hub provides a large increase in strength/material and resistance to bending fatigue failure (the GT4 stud has a couple of other features I won't bore you with). An M12 press-in stud offers the same/similar effective diameter as an M14 thread where it is installed into the hub because threads have "roots" where the diameter necks down. An M14 thread root diameter is actually slightly smaller than the diameter of the M12 press-in studs we use where it presses into the hub. The M14 press-in stud we use is larger than both by a huge margin where it installs into the hub. So, with the extra clamping capacity of a M14 studs, combined with huge increase in material size in a peak [bending] stress area, it will take a significant lapse in preparation and judgment or material defect to break one. M14 press-in on right, M12 press-in on left: __________________ The ultimate wheel stud upgrade Last edited by gills; 05-19-2020 at 03:04 PM..
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