Suzuki DR650 1996 on Shock
The DR650 shock can't cope with any additional weight over a 75kg rider, the stock spring is really light and the dampening is minimal. The shock is a 40mm KYB unit, compression adjustable only. While it's not the "trickest" shock out there it's a very reliable unit and easy to service and modify.
We have used complete shaft assemblies with pistons, Gold Valves and the like but it adds to the cost and did nothing for the performance we couldnt do with stock parts. After a lot of testing, both in the shop on the shock dyno and with selected customers, we have a shock setting that makes whoops out of Finke manageable, does not beat you up on an 800km day and makes the DR a whole lot more fun to ride. This revalve uses the original piston, and we think it beats the stuffing out of the American Race Tech piston and shaft conversion. We don't make the rebound adjustable as we don't think it's required and just adds to the cost. As the great Laurie Alderton said, "You don't need 40 clicks of adjustment; you only need one, the one that works!" Ours works. No gadgets.
We couple our shock mod up with a spring specially made for the DR650, so no spacer is required and you get plenty of preload adjustment.
DRZ400 Suspension Conversion Advice.
Some riders swap out the forks for DRZ400 units, but the triple clamps have to be swapped as well, because the fork size is 49mm. You need to run the 400 wheel too, as the axle is bigger, or make up a custom axle and spacers. After all that, the DRZ's fork springs are only 0.44'skg/mm, far too soft for most people, so you end up spending more money on harder fork springs. The stock 650 disc is bigger than the 400 too.
The DRZ400's shock is better, being a 50mm Showa. It needs quite a lot of reworking to the linkage knuckle on the 650 as it's narrower, requiring milling and a bearing that's not easy to get. Added to this, the spring series is different on the DRZ400 shock, so the stiffest spring we regularly stock is a 6.8kg/mm (20-115-68). Plus the shock needs a revalve to be as good as our modified stock DR650 shock. The advantages are that you have are a rebound adjuster and more oil capacity.
Think very carefully about what you want to achieve before diving in.