Hi, I recently bought the Schlumpf Speed Drive but ordered the ‘maintenance tool set’ instead of the ‘installation toll set’, meaning that I did not receive the big ring nut wrench. Fortunately, I managed to unlock the big groove nut with a wrench coming from an old BMW motorbike tool set and which is used to adjust the spring preload of the rear shock absorbers.
It is similar to this one:
So, if you need one, ask to your friends who own a motorbike or buy one at a motorbike shop (ask for a shock wrench).