I have the same problems on my 56 Mk2 Sq4. When 2 up, even with a slender passenger, and hitting a hole or bump, the rear tire catches the inner part of the mudguard, bringing the rear wheel to a sudden stop. Not so good for the transmission.
As stronger springs are obviously not available, i thought packing out the original springs with a tubular packing piece to increase the preload by a certain margin. On too soft front forks this often did the trick.
Any comments to that bodge ?
Ariel 1930 model G special, Ariel SQ4 1957, Norton Commando 750, Norton Mercury, Norton 99, Norton N15CS, Velocette Venom, Velocette Thruxton, Velocette LE x2, Morgan F Super, Calthorpe Ivory 500, Yamaha sr500, Moto Guzzi Falcone