I use the Bike Peddler Eyeglass mirror, $12, Amazon, attached to my sun visor of my bike helmet. It stays put on rough roads and trails. Wide square mirror for a good view behind you and it can adjust for best viewing angle.
I use during the day and night when riding on bike paths for fun or vehicle roads to commute to work. Don't really notice it is there until you need to take a peek behind you.
The only issue I have is it can slide off your helmet sometimes if you do an one hand adjust on the fly (best to stop and adjust with 2 hands). It can also fall off your helmet if you put it down on a flat surface. I usually take it off every ride and place in hard eye glass case in my Topeak MTX rack mount bag.
I like the helmet mirror a little better because of its small size, cheap, I can see behind me no matter how the front wheel is turned, there is already too much stuff on my RadRover handlebars, and I know I would knock handlebar mirrors off my bike eventually.
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