On the Tiara it seems no matter how hard a set the left axle nut it wants to eventually wiggle loose, so much so that I took a 14mm socket with me on the ride today (it loosened twice).

When I look at the nut I see the little "locking nubbins" (not knowing exactly what to call them) but they seem awfully smooth to me. Is this my problem? Should I try to get a new one at an LBS or try ar lock washer in between the nut and the crank arm?

Ideas? Common problem?