"It just goes to show you, it's always something." - Roseanne Roseannadanna
On my first DR650 I substituted a 18" rear wheel from a DR350. I liked that conversion because it allowed me to run a wider variety of tires than the stock 17" wheel and it was MUCH easier to repair trailside than the obstinate DR650 wheel with that damn safety bead. It also provided a tad more ground clearance and although subtle, I thought the bike actually handled better off-road with the 18" wheel.
The conversion is very straightforward since the early DR350S cush drive hub is the same part as the current DR650 hub. The only difference being that the 650's brake rotor is larger and must be substituted. So, since I already had some of the potentially required components for this conversion in the barn, I told myself that whenever a 350 wheel appeared for a reasonable price I would snap it up and convert my current bike also. That happened last week and I bought the wheel, some fresh bearings, a new brake rotor and a set of new sprocket bolts.
One of the items I had on hand was a nearly new IRC TR8 4.50x18 tire. I really like the TR8 in that it provides a great balance of off-road/on-road performance and durability for a reasonable price. However, being an old school tire (aspect ratio of ~100) it is quite tall and that is where the rub (quite literally) comes in. I bought this tire originally to run on my DR350 but frankly it was as little too wide for that bike so I replaced it with a 4.00x18 which worked much better.
This is a BIG tire. The mounted 4.50x18 TR8 is approximately 2" larger in diameter than the stock DR650 wheel with a 4.60x17 Shinko 244. One inch of the increase is, of course, due to the larger rim. The second inch is due to the higher profile of the TR8. That 2 inches (1" change in radius) results in ~8% taller gearing or approximately the same as 1 countershaft tooth so I knew I would have to change sprockets to keep my effective final drive ratio the same. I normally run 14/42 gearing so, no problem, I thought, another of those on-the-shelf items was a 45 tooth rear sprocket and a spare sprocket carrier hub.
The 18" TR8 is a much larger tire than the 17" Shinko 244
4.60x17 Shinko 244 radius to ground ~12 3/8"
4.50x18 TR8 radius to ground ~13 3/8"
So, I bolted up the 45T sprocket, installed fresh bearings, installed a new 650 brake rotor and mounted the TR8. Yep, it looks awesome.
Mounted DR350S wheel with 45T 525 sprocket and ProFactory rotor
BUT, the extra three teeth on the rear sprocket means I have to run the chain adjustment at the minimum settings on the adjustment cam. And, the tire hits the swing arm before the axle is pushed all the way forward in the swing arm slots so getting the chain on the sprocket is a challenge. Most importantly, with the chain adjusted properly there is insufficient clearance between the front of the tire and the swing arm. It looks okay just sitting in the garage but at around 60 mph the tire grows enough to rub on the swing arm. Not good.
Only a millimeter or so of clearance to the front of the swing arm
So, as much as I would like to run the 4.50X18 TR8 it really doesn't fit. I could probably make this work by adding some links to the chain or running 13/42 sprockets so that the axle could move back in the adjustment slots a bit but I'm not sure it would be suitable even then because the larger diameter tire might strike the bottom of the fender at full compression of the suspension plus I don't know how much the tire will grow at 60+ mph. I'm pretty sure it will be fine with a lower profile tire (80 or maybe 90 aspect ratio) and that's likely what I will do but damn, I really like those TR8s. So, if you're considering this change be aware you can't run just any 18" tire without some complications.
Oh well, like Roseanne said, it's always something...
POST SCRIPT: I have since replaced the stock 110 link chain with a 112 link D.I.D. The extra 2 links makes it possible to run the 45T rear sprocket without a significant issue. I normally run 14/45 with the 18" wheel and tire and when running the stock 17" wheel I run 15/45. These sprockets approximate 14/42 gearing with a 17" stock wheel/tire. I find this gearing to be suitable for 90% of my riding. For VERY rugged or tight single track I will sometimes drop 1 tooth in the front.