I went with a 2.15" x19" front wheel on my DR650. I am currently using a Metzler Karoo 2 in a 110/80x19 front size. With one inch lowering links on the rear it had a reduced castor effect on steering and the street biased tread pattern combined to cause the front tire to oscillate from side to side when traveling on soft sand/gravel over hardpack.
I lowered the fork tubes in the tripple clamp approximatley 1/2", which helped straightline stability and left the flickability very quick on firm surfaces. Felt good in the corners on tarmac and stable on firm dirt roads. There was still some instability on looser dirt surfaces.
I then went to 1& 1/2" lowering links. It now is quite stable on sand, gravel and loose off road surfaces. On street it is not as flickable but has good straight line stability and is stable in corners. I can change line mid corner.
One thing I have noticed is that there is more stability with wider rims. A 2.5' is more stable than this 2.15 and a 3.5" rear rim is more stable than a 2.5" on the rear.
Recently purchased a new to us 110/80x19 Pirelli Scorpion Rally front tire. It is over 1" taller than the Karoo 2, although it also has the lower FIM size knobs. When it is installed l should be able to return the fork tubes to their normal position.
Would like to hear from anyone with more geometry knowledge on the effects of changing the fore/aft pitch of the bikes when changing sizes and the effect of tire width on traversing soft or mixed terrain.
Even the vintage motocross racers go to 21" front tires on their 500s and 650 sleds, while we are in effect going the opposite way with our wide 19s. I do notice on this bike that it may float on top to a degree but find it better than tlhe narrow 21s digging in. Alternate views? Thanks, Tom