Where should I mount a mirror?

As mentioned in this answer, I use a helmet mounted mirror. I get a good field of view behind me and have gotten used to using a quick side-to-side glance to see everything behind me. The mirror has a mount that adheres to my helmet, and the mirror arm snaps on and off for travel.

I tried a handlebar-mounted mirror prior to that. I found that I couldn't really see all that much behind me and couldn't see laterally very well (because it was attached to the handlebars). This was on a road bike where I generally ride on straight, flat ground. But I imagine the problem would be worse on a mountain bike.

A mirror is a great addition to a cyclist's toolkit. While all cyclists should be able to look around behind them without one, a mirror lets you do this more often and safely. And a mirror is no substitute for looking behind you anyway. Here's a summary of the kinds of mirrors I've used.

Glasses-mounted mirrors like the take-a-look mirror are great for giving you a wide, easily adjustable field of view. Pivoting your head means you can "sweep" the area behind you, and the mirror is close to your eye so it's a fairly wide field of view. However, they're only usable if you wear glasses or goggles, and some mirrors won't mount to some glasses, particularly glasses with thicker temples like some non-prescription sunglasses.

I find that the take-a-look is a great mirror, but it tends to pull my glasses off just a bit over time. Adjusting it is merely an annoyance that I've decided to live with, since the mirror is otherwise excellent. (My glasses are also extremely light, and this would happen with any mirror, although the take-a-look is fairly heavy for a mirror.)

• Upside: Great field of view, keeps the mirror off the bike. Inexpensive.
• Downside: A little more finicky to adjust than bar-mounted mirrors, easy to forget.

If you don't wear glasses, or prefer not to put a mirror there, you can get a helmet-mounted mirror instead. Most of these attach to your helmet with adhesive pads. The only one I've had that I cared for attached to a helmet's visor that was designed to accept it. Look at helmets specifically for commuters if you want one of these, I loved it but had to get rid of it when I replaced the helmet.

• Upside: The mirror is always there.
• Downsides: More vibration, smaller mirror, and you're locked into that helmet if you want to keep the mirror. Harder to adjust than any other kind of mirror, since your head is always moving and your helmet moves a bit on your head - unless your helmet is really tight.

Handlebar-mounted mirrors are generally larger in area and have a curved surface. (Think "objects in mirror are closet than they appear".) The ones meant for drop bars usually mount with velcro or another attachment system, and the ones for flat bars will generally be screwed onto the side of the bars. I've never found one I liked for drop bars, but the flat-bar mirrors work quite well.

• Upside: The mirror is always there, and the permanently mounted ones are easier to adjust than any other system.
• Downside: A little clunky, they make your bike wider (an issue if you need to take your bike on the train or if you have a folding bike), and you need to look down to see it, taking your eyes off the road. Can be snapped off if the bike falls or grazes a wall. (Some are advertised as "folding on impact", though.)

Recommendations

Outside of the ubiquitous take-a-look mirror, it's difficult to give specific product recommendations. But there are only a few kinds of mirror available, and even the most expensive mirrors are still fairy cheap.

I'd avoid plastic eyeglass-mounted mirrors, as they tend to break. But a good eyeglass mirror will give you the best rear visibility.

If you want to avoid eyeglass mirrors: In your situation, with flat bars, I recommend a mirror that screws into the sides of the bars. You can put on one or two mirrors, depending on your situation. For example, in countries where you ride on the right, a single mirror on the left side of your bar will cover almost all situations. Having a second one on the right is nice, as it covers your "blind spot", I consider it a luxury.

In the end, everybody has different preferences with mirrors. You really need to try a few different kinds and brands and see what you prefer. There is a silver lining, though; many cyclists have unused mirrors. Chat up some cyclists if you have a club or friends who cycle, and someone will likely offer to lend you a mirror of give it to you outright.

Yeah, I've tried them all, and keep coming back to the helmet mirror. An eyeglass mount is less stable and doesn't project out from the head as far, so it's hard to see around a bulky helmet (and it seems all helmets these days are bulky).

I've tried handlebar mirrors of several varieties, but on my touring bike with drop bars no mirror I've tried is able to see around a set of panniers. Plus you don't get as good a field of view with the handlebar mirror.

Keep in mind that the closer the mirror is to your head the wider your field of view (for a given size mirror). And a head-mounted (helmet or glasses) mirror has the advantage that you can rotate your head up/down and left/right to see something not immediately in your field of view.

The main down-side of a helmet mirror is its relative fragility. I've most recently (for maybe 5 years) used a Third Eye helmet mirror that attaches to the helmet with an adhesive Velcro pad. It's never gotten knocked off or broken, but it does get knocked out of alignment just about daily. And I have to be fairly careful when I lay my helmet down to not damage the mirror. I'm sure that there are some newer models that are better, but they all have this fundamental problem.

Drop bar one works well for me.

It provides good visibility and I feel is safer in a crash vs. one that is mounted to your helmet. Note, I have crashed with one on the handlebar after catching a longitudinal crack in the road and did not have issues, but I worry that a helmet/glass mounted mirror could pose a safety hazard in the case of a crash.

https://amzn.to/3mCoIVF

You could use this if you have straight bars. I have no experience with this, but it seems like it should work.

https://amzn.to/3ri4l3g