Seriously thinking of 21" front and back on WM4 rims. Just done 4k miles in NZ on the "A" and front end feels very positive and reassuring it did have 20" rear when we first had it which gave it long legs (higher gearing) since 99 on 19".. Avon Trialrider 90/90 x 21" recommend WM4 better support for tire run at 10 psi lower pressure giving very much softer ride,down side is more difficult turning bars whilst stationary.
The only info I now require from AVON is can I use it as a driving tire ! Which way round I fit it arrow front or backwards. As on the "B" rear 410 x 18 Trialrider on WM5 (3") (tread 10.5mm) things clear easily but big space under guard and all ways looking for another gear with a 22 and 46 teeth sprockets.
How much tread on a rib 3.6 mm , as aposed Trailrider 5.6, double the wear mileage , no brainer as some one mentioned lately, OK for the image look and riding out for the occasional Sunday lunch out at the local.