These inserts are replacement inserts for the ones that would have come with your pannier originally. They will reduce the diameter of your Ortlieb mounting hooks from 16mm to enable their panniers to be fitted securely, without rattle or movement to 8mm or 10, or 12 mm diameter tubing, so no matter what type of rack you have, you'll be able to use Ortlieb's high quality, waterproof panniers.