USE's shims are made to 100mm length and are longer than other shims on the market. This gives your seat post much better support inside the frame. All shims are designed with a top lip to stop the shim sliding into the frame. USE Seatpost Shim Features Rolled Aluminum shim - the very thinnest shims are rolled from sheet metal. 25.4mm shims need to be so thin as they are etched down in acid to the correct tolerance and are too thin to anodise (show as silver). Machined Aluminum shim - machined to very high tolerances from 6082 T6 Aluminum and then black anodised to protect from corrosion. Machined Plastic shim - larger size shims are machined to very high tolerances from filled engineering thermoplastic UPVC. This keeps the weight down, as the shims get thicker.