I’ve researched this a bit but haven’t tried it so bear in mind this is all hypothetical until someone does it.
The DR650 forks are 43mm in diameter and internally a basic damping rod design, this means they have no external adjustments.
Many Suzuki Sports and Sports Tourer style bikes fitted with standard damping rod forks came with fork caps containing an adjuster that when turned moved a rod up and down in the top of the fork, adding or removing pressure onto the top of the internal fork spring.
According to my research some Suzuki Motorcycles with a 43mm staunchion that had these preload adjustable fork caps included the first generation RF900 and early DL650 V-Strom and there maybe others. As long as the thread pitch of the fork caps on the donor bike are the same as the DR they should screw straight into DR650 forks.
If anyone has some spare 43mm preload adjustable fork caps lying around I could give them a try on my bike.
‘@MarkH, your next mod?
This has been looked into on DRRIDERS.com and the thread pitch on the DR650 is different and so Pro Cycle ended up having some made up.
Well there you go, there’s not much out there that hasn’t been done before.
Other option is adding air valves, some have drilled and tapped top cap to add an air valve. Running 6 – 12 psi with no problems.
http://drriders.com/topic5237.html
An Australian company is now making adjustable fork caps for the DR650, cost is $189AUS.
http://www.mscmoto.com/suzuki-dr650-preload-adjuster-caps
Features:
20mm of preload adjustment
1mm per turn with a 14mm spanner
Adjuster nut stays at the cap as mechanisim is all internal.
CNC Machined from billet aluminum
Anodized for durability and better thread life
O’ring pre installed
Spacer tube can be cut down to suit emulators
Fits Suzuki DR650 1996 to 2016 with 43mm forks
These will also will fit other bikes using a 43mm conventional fork design with a M40 x 1.0mm thread.
MSCMOTO also make quite a few other tasty bits for the DR650 and DRZ400.
One of our Adventure Riding NZ Facebook group members has had a not so great experience with MSCMoto it seems.
Aliexpress are now advertising some adjustable Preload Fork caps for the DR650
https://www.aliexpress.com/item/33043627083.html?spm=a2g0s.8937460.0.0.34612e0eZX6puR
looks good, maybe a group buy direct?