For some reason, certain tyres can be very difficult to fit. Schwalbe Marathon Plus tyres have a certain noteriety for this but they are not alone. Sometimes it is not the tyre but the rim that can cuase problems- one particular tyre may fall on and off rim a but will need much heaving and puffing and gnashing of teeth to get the damn thing on rim b.
This is advice from Bryn, my LBS workshop manager (when did workshop managers start looking younger than coppers) on fitting BTFT’s (Bastard Tight Fit Tyres) such as Marathon Plus as a replacement (i.e. when you are at home, not mending visitations at the road side).
Work from the non drive side.
Get the first bead on and insert the tube (partially inflate if you need to but only very partially).
Starting adjacent to the valve, get the second bead started- push the valve deeper into the tyre to give room. Work around the rim, keeping the area you are working on away from you until the bead starts to get tight. Make sure the valve is still pushed deeper into the tyre and then squeeze the inserted beads together so they sit in the middle (deeper part) of the box section.
If you have access to a work bench, place the wheel flat on the bench with the cassette/ QR against the edge of the bench to give you something to lean into and roll the bead into the rim using the heel of your hands/ base of your thumbs.
Job should be a good un’ (I’ll let you know tomorrow after I have fettled a new Marathon Plus on to my commuter steed back wheel tonight).