Helmet strap loosens as I ride then the day after becomes too tight Well, the title sums it up. It's a cheap (~50 EUR) KTM helmet. This is reproducible, is happens on every ride: if I adjust it to fit well at the beginning of the ride, it's loose after a while. If I tighten it during the ride then it is way too tight at the start of the next ride.
Two One hypotheses:


The straps are getting wet with sweat and loosening, then constricting again when they dry. Nylon does in fact absorb water and stretch when it gets wet, so this is the more likely cause. 


2. Exertion causes vasodilation. If you've had the sensation of your necktie getting tighter under stress, this is the cause. Your body is pumping more blood to your head and the veins are expanding.