Does anyone know what is the smallest tyre I can put on the wheels? There are no markings on the rims. They came with 26 x 2.1 tyres, and I've put 26 x 1.75 on at the moment, but I'm too "scared" so go with anything lower than that for fear they won't fit the rims. The decal on the wheels say DDM-1 if that is any help.