Start a bicycle-mirror purchase with handlebar shape and available space. A bar-end fitting depends on the tube’s internal diameter; an external clamp depends on the diameter where it will sit. These measurements are not interchangeable.
The Zéfal Spin 25 illustrates the compact drop-bar option. Its mount fits a 16–22 mm opening, the mirror area is 25 cm², and it can go on either side. It adjusts on three axes and folds beneath the handlebar.
The Zéfal Espion E80 clamps onto a 22–25.4 mm handlebar and has an 80 cm² mirror area. Separate left and right versions are offered; the manufacturer excludes road drop bars. Its folding function lets it return to the previous position.
Before buying, check that the mount will clear the brake lever, grip and other equipment. Assess visibility in your normal riding position: a larger surface cannot help if your arm blocks it. A mirror supplements observation, but checking the blind spot before manoeuvring remains necessary.