Chain wear tool r cwt a chain wear tool is a quick and handy device that will tell you if your bicycle chain needs replacing through wear and tear a chain will stretch and if left for too long could result in poor performance from gear changes and be potentially dangerous if the chain slips the r cwt is designed to give you a quick guide if your chain is worn simply by seeing if the tool slots into the chain you can tell if the chain 075 on the tool or both the chain and cassette nee