After a few seasons, you might notice some slippage in your Sidi shoes when you're charging all-out. It's time to replace the Ultra SL Straps across your feet and power on. Sidi's road and mountain shoes both depend on the Ultra SL Buckle and Strap system to keep you locked in and cranking. After a while, just like everything else, they can wear out. Good thing Sidi provides replacements to protect your investment. *Sold as a pair.