Thanks for re-posting. We cannot say with complete certainty what all it would take to make the 640 ADV wheel fit on the SuMo front end as we have not done this exact conversion.
What's known(according to parts fiche) is that although the front bearings, seals, and hub are different the axle used is the same. So the front hub with the spacers should "fit" in the front end, but from there some food for thought.
The main reason that an Adv wheel will not just fit on to the SM is because The 17" SuMo wheel is built with a x-lace(meaning from the hub to the rim, the spokes cross over the centerline of the rim) to afford more space for the larger brake caliper.
The 21" ADV wheel does not have a x-lace so when used with the larger caliper, the spokes will hit the caliper. Sometimes you can just grind off some of the caliper for clearance, but believe you will need to x-lace this wheel to make it work. This would require a custom rim and custom spoke lengths to accomplish.
To consider: if you were to have a x-laced front 21" or 19" rim laced to hub and mounted the 320mm rotor(only way to do it without making mods to the caliper). It should spin freely, but you will have a 320mm rotor on a 21" or 19" front wheel, which is a whole lotta brake for going off-road.
The only way to go to a smaller front rotor would be to make mods to the forks/caliper mount, make your own conversion kit, or go with KTM 640 ADV front forks. There are many conversion kits to go to oversized rotors, but very few if any that go the other way around.
We know of a few people who have the 950SM who put 19" and/or 21" front wheel on the SM and even after the time to make it happen, they regretted it because did not perform well off-road, was too heavy in the front end, etc. And one person was so fed up, he sold the bike, bought an Adventure and got SuMo wheels for it. Then the problem was solved.
Now the 2001 640 SM is not as heavy up front as the 950SM, but similar concepts and likely you will find that it was not worth the trouble.
Hope the above provides some clarity, feel free to let us know if you have other questions.
Originally Posted by BikerBobber
What was the answer to this? It never got posted...?
Sent from my mobile.