+1......include your year and model next time for better/faster responses.
Except for this particular question, it's the same answer for
all Ducatis that don't have single-sided swingarms.
If you think about what that spacer is supposed to do, it should be obvious that you want the maximum area against the swingarm, especially if you have an aluminum swingarm. The tapered part only goes against a bearing that is very small.
It's too bad that Ducati designed the rear wheels so that the spacer could be inserted backwards. I can still remember back in 1998 when I had purchases a larger rear sprocket for my 1997 M900. I had been warned by the dealer about which way the spacer went, but managed to put in backwards anyway. I could freely spin the rear wheel until I tightened up the axle bolts, then everything got tight. I loosened and tightened them several times, but the wheel didn't want to turn when the bolts were properly tightened.
I finally took it back apart, then I remembered what the dealer had mentioned. Swapped around the correct way had the wheel turning freely with everything properly tightened up. Once I had experienced the problem, I never tried to put the spacer in backwards again. I'm sure I've installed Ducati rear wheels at least twenty times.
So, the answer for all Ducatis with two-sided swingarms is to put the flat side against the swingarm and to be sure you check for smooth rotation as you tighten up the axle bolts.
