It is often said they self tighten. But given the wheels are on splines I cant see how. There is a bit of circular momentum from the ears... but that acts in opposite direction when accelerating and braking... I suppose the braking force is higher than the accelerating force which would net out as a tightening force. But the mass of the ears is pretty small I would have thought it had minimal effect?
By the way, this is an innocent question.... not got an axe to grind
The mechanics of this are quite simple, but absolutely not what you might expect.
The self-tightening effect comes about because there is a deliberate diameter mismatch between the splined shaft and the hub that sits on it, such that the wheel is moving eccentrically around the splined shaft even as both rotate (if you ever had a 'Spirograph' kit as a kid, this will be familiar). The wheel nut is of course held by the threaded section at the end of the splined shaft.
Even with just a one thou (.001 inch) difference, the relative motion of the nut to the wheel will be 0.003 inch with each wheel rotation or about one turn of the wheel nut in 3 miles driven if you have your shafts/wheels on the wrong side of the car. Of course, you have your wheel nuts the right way round, so the eccentric motion serves to tighten the nut which, at a certain clamping force, will prevent any further oscillation around the splines.
