I would rec'm first trying a pair of Continental TKC80 knobbies on your stock 19" and 17" wheels. IMO if you need a more dirt worthy bike than with this set-up on the Vee then you probably have the wrong bike.
I think oldturtle has the right idea, but if you still want to do it then I think you want the forks and wheel from the DR650. I've seen one in person and I think he also raised the rear to keep the geometry somewhat within reason.
There may be some threads on the subject on ADVrider.
Some folks buy a bike to ride. Others buy to tinker. Over on Advrider there is no shortage of guys building frankenbikes just for the fun of it. A few have done it here too, some with amazing results. I like the creative innovation even if I agree that the Strom is not a great offroad platform to start with.
need 21 "because in here we have many bad roads, also a slightly longer travel suspension would provide me more comfort and last but not least an easier travel in the sand and mud and ground clearance a little high. I thought the cheapest for doing these tests is a fork DR650/750/800.sorry for english !!!
Do you really think having a 21" wheel will make it a dirt bike? A 500 lb dirt bike. Just get some TKC's it will work great. I'll sell you another set of wheels, you can have one set of street tires and a set of TKC's already mounted ready to change over. It takes me about 45 min to change to my dirt tires.
The benefit of spokes over stock rims, IMHO, is primarily a more flexible rim. The difference between a 19 and a 21" rim with similar offroad tires is not really that great, honestly. If you're not hitting a lot of rocks and or are not worried about the supposed fragility of the cast wheels, I'd put TKC80's on and see how well you can fare.
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