Inside, the Technomicro liner material is plush and perforated for airflow.An external ankle brace yields a bit more protection than the Sidi Vertigo, though a bit less than the Sidi Vortice, which has a liner boot as well as a more advanced strap mechanism at the instep and top of the shin. Build quality is as good as I have seen anywhere, and better than most— typical for a premium Italian brand such as Sidi.Using Euro numbering, the ST Air boots match my usual size 45; they slide right on and fit just like the last few pairs of Sidi boots I’ve owned— perfectly. They look great, feel wonderful, and work well. Fortunately, I’ve never crashed wearing a pair, so I will rely on anecdotal reports that are almost always positive.A bit stiff when new, after wearing the ST Air boots for a couple of rides I found that ankle and toe flex became easier, especially on the left boot. There, the range of motion is critical to allow the boot to get under the shift lever and release it far enough between gear changes for the ratcheting shift mechanism to pick up the next gear. A sturdy toe protector saves your foot from any marks.In addition to the air flowing through the perforated leather, there are also closeable air vents above your instep. Open them up on the hottest days and close when needed.Sidi’s patented Vertebra System is designed to protect the Achilles tendon area in concert with thermoplastic resin ankle protection. The heel cup, toe pads, and other bolt-on pieces are all replaceable and the boots can be resoled.Hope for the best, prepare for the worst, and look good the whole time— the Sidi ST Air boots are an all-around excellent boot for the street and the track.