Quote:
Originally Posted by jmeenach
Yes, same theory applies to OEM bolts. Here is the explanation from the Apex installation sheet:
As with all fasteners, wheel studs and bolts are held in tension, and slightly stretch when installed. After they have been cycled between full torque and then released several times, they eventually lose their elasticity and no longer properly hold tension. While this can’t be seen, it can be measured by precision instruments. Heat also works to embrittle and oxidize metal and brakes generate an incredible amount of heat under hard use. If you are a regular track day participant, it is our recommendation (echoed by many seasoned racers) that you replace your studs annually.
A few of us have been discussing the Schroth harnesses, so regardless of which comment convinced you to pull the trigger, I'm glad it did. Make sure you run a HANS with the harness, as you can cause more harm than good if you aren't using the system correctly. A factory 3-point allows your body to kind of "wrap around" the shoulder portion of the belt, which spreads load across your body. With a 4-point or higher, the inertia is sent to your neck (as your torso will have much less movement), which leads to basilar skull fracture.
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Preloading and unpreloading a bolt that’s preloaded to 65-75% of yield (different requirements at different gov’t facilities and aerospace companies) will NOT lose its “elasticity”. To lose “elasticity” you’d have to preload the fastener well beyond its yield strength but even then you don’t really lose “elasticity” when unloaded. The instantaneous stiffness/modulus is the slope of the stress-strain or load-disp curves (no longer a linear relationship between stress and strain). However, it’s still smart to replace studs and lug bolts because threads get damaged with repeated use and the load in the stud/lug bolt increases beyond its preloaded level due to thermally-induced loads/stresses.
Below is a picture showing a fastener being preloaded + an applied load that puts the fastener far past its yield strength and when unloaded it has a slope almost identical to the pre-yield stiffness/slope. However, each time it is re-preloaded, you start to get more displacement/strain until it reaches its ultimate strength and ruptures. The engineering stress-strain (or load-disp) curve shows the stress reducing with increased strain/disp because engineering stress always references the original unstressed cross-sectional area; however, if true stress-strain are used then you see the stress continually increasing until reaching the ultimate strength. This is because true stress is calculated using the instantaneous cross-sectional area which is decreasing with increasing load beyond the yield strength.
The calculations that I’ve performed on the 5-bolt wheel-to-hub bolt pattern shows approximately 10% of the applied lateral bending load is carried by the bolt/stud as an increased axial load (total axial load = applied load*% of applied load carried by studs/bolts + preload + thermally-induced load). It also, conservatively, shows the preload of grade 12.9 (and even 10.9) has sufficient preload from all five bolts/studs is sufficient to prevent the wheel from rotating on the hub (i.e., a lateral load + local bending due to shear are not carried by the studs/bolts). If the joint had gapped than the stud/bolt has to carry almost 100% of the applied axial load + shear load + bending moment due to shear and wheel applied torque..