It's not uncommon to increase the torque/preload a bit when used on a track. I unusually use some blue/medium thread locker and torque to spec which helps retain it and also effectively lubricates the threads on install, increasing the preload seen by the bearing a hair.
Are you super sure that you properly torqued that nut the first time? Are you running a good wheel bearing? it could be a faulty part, or a couple beers too deep. I think thread locker and/or more torque is a safe bet, but I don't think the tires we are limited to will allow us to exceed the preload of that nut and unload it. It happens on high aero cars with slicks but it seems hard to believe in Lemons... and even in those high aero/slick applications they usually increase torque and the problem goes away. They're not tapered roller bearings, so I don't think a cotter pin/safety wire will do any good. Once it loosens enough to rely on the pin/safety wire you've already unloaded your bearing and are heading to failure.
For day 2, you probably started some wear on day 1 driving without preload, which propagated to failure on day 2.
Make sure your running good parts here (plenty of places you can cheap out, wheel bearings/hubs is not one of them). FAG, SKF, NSK are all good brands... the cheaper brands always seem to come with thinner hub flanges (can be as thin as half the OE thickness), but "fortunately" cheap wheel bearing will probably fail before the flange. Make sure you are using a trustworthy torque wrench, blue loctite, and good parts and I'd be surprised if this happens again.
Full Ass Racing
#455 Piñata Miata - 1990 Miata
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