Caps and hats under helmets
Summary: Baseball caps and other caps can be ok under a helmet if the helmet is still fitted correctly, but that takes extra effort.
Baseball caps under helmets
Baseball caps under a helmet pose the same problems as thicker hairdos. The thickness of the cap and its sweatband, as well as the bill of the cap, interfere with the fit of the helmet, causing it to ride higher than it should and perhaps leave some of the head unprotected. In addition, many baseball caps have a steel ball in the button on the top. That ball causes all the same problems as the beads mentioned above. Caps without the ball are less problematic. Just fit the helmet with the cap on, and make sure the fitting ring or pads and the straps are properly adjusted. Of course, if you ride without the cap you need to adjust the helmet again. We question whether most riders will take the time to do that.
If the cap being tried out seems likely to push the helmet back on the head in an impact, leaving the forehead exposed, that requires either better adjustment of the straps or trying a new cap.
People wear other caps under helmets, mostly to either keep warm or protect against sunburn. Most of them are bald riders. We have a page for them with tips for bald riders on sun exposure. We also have a page up on hairstyles and helmets.