Aceoffix brompton seatpost reducer, 34.9mm to 31.8mm.During installation, the protrusion is aligned with the groove at the back of the seat tube, and then the outer side of the reducing sleeve needs to be coated with glue or 3m double-sided tape.The package comes with 3M double-sided tape, which can be fully adhered when inserted into the seat tube after 6 hours.