I have to second Trimapa's recommendation for a rear shock upgrade of the Duke. Although many owners do use the bike successfully with the OEM rear shock, I concur with Trimapa that such an upgrade is indeed "mandatory" if the owner wants to exploit the full capabilities of the Duke's marvelously agile chassis. The OEM shock, a classic "cost-cutter special", is simply not up to the level of performance offered by the rest of the chassis. When cold, the shock's damping is extremely harsh in compression; When the shock heats up from proximity to the exhaust cannister, the rebound damping practically disappears. The One-Size-Fits-All progressive rate spring (which is really not a proper weight match for any rider) is devilishly difficult to access in order to adjust preload. All in all, the OEM shock/spring is just not a worthy complement to the brilliant chassis.
The most cost-effective shock upgrade for the Duke is a Japanese shock adopted from an early-generation Yamaha R6. The following thread from the RC Forum details the research and experimentation by RC390 tuner Matt Schreiner of Formula 390 that resulted in what has since become a popular and economical suspension upgrade.
Shock Swap with 99-02 R6
Matt recommends that the R6 donor shock be upgraded with a straight-rate spring matched to the rider's weight, as well as a threaded preload adjuster. A number of Duke owners, (eg., Mikebet et al) have found that the R6 shock can be successfully utilized with the original yellow spring and ramp adjuster from Yamaha, making this the ultimate inexpensive upgrade. Straight or modified, the R6 conversion has proven extremely popular and effective. A huge upgrade over OEM. I was an early adopter, and I continue to be nothing but pleased with the R6 shock. Others please comment.
For those who want the latest and best technology, there are a number of current-production shocks from leading suspension makers that will fit the Duke/RC. Among the new shocks available are Ohlins, JRi, Penske, Wilbers, Nitro, YSS, M-Shock, K-Tech, Elka, and others. A plethora of choices. Comments from Duke owners who have fitted new aftermarket shocks are welcome.
(Front fork upgrades will be addressed in a separate post.)