I have bought a couple of new schwalbe tyres for my cheap mountain bike. The rims say 26 X 1.5/1.75 559 X 19 on each of them (no ISO number I can see). The tyres are 26 X 1.5 (ISO number 40-559). I am having trouble centering the tyres (both of them), they seam very hard to get right, the tyres are schwable marathons. Is there a knack to centering tyres, can anyone give me some tips on this?