I went through numerous mirrors on Brompton and other bikes, including a couple from B&M. My conclusion was that the need to have a good rear view and folding required a mirror that would easily rotate around the handlebar axis. If this amounts to the rotation of the part that goes inside the handlebar and holds the mirror, it amounts to loosening the hold and the mirror will wiggle itself out of the handlebar. You will have to constantly have to tighten the mount or lose the mirror which just turns into a pointless activity.
The next level is that you need to look for a mirror that rotates independently of the hold onto the handlebars. That basically takes you out of what is available for bikes. Next is that the rotation for folding should not impair the ability for the mirror to hold its orientation in regular riding. The required precision then takes you to CNC and prices that only motorcycle market can tolerate, with a mirror costing as much as an inexpensive bike. However, you at least solve the problem and the cost spreads over years of use. One solution are the
CRG mirrors. Personally, I like Arrow. There may be other similar brand choices, but I think that you need to look around motorcycles.
You may say that the mirror will break and my investment will be gone. Well, I have 2 such mirrors on my Brompton, it fell over numerous ties and the mirrors are fine. They have traveled with the bike in vehicles and planes. Finally, you can buy replacement parts for those mirrors and they have resale value, often offered at prices quite comparable to new, just like Brompton itself.
