I have 26x2.00 knobbly mountain tyres. Is it OK to change just the rear tyre to a totally smooth treadless 26x1.25 to maximise the contact of the roller on a Subaru Robin engine? Would there be a balance problem? Or does the front tyre have to be changed as well? What is the grip of a treadless tyre like on a normal lightly rain-wetted road?