Combo 1: Scorpion Pro FIM (90/90-21) front, Trakmaster II (100/100-18) rear.
My route took a mix of paved, dirt, and rough bush roads full of ice, snow, knee-deep puddles, and tons of mud. After about an hour of ripping around I got a pretty good feel for how this combo rides.
Trakmaster II: Tractor-like grip in the slop. No problem ploughing through oozing, semi-frozen mud that was almost a foot deep in places. This is a definite strong point for the tire.
However, this tire is just too small for the back end. On harder surfaces it felt like I had a swivel caster on the back end. Turn-in was twitchy, the ride was harsh given the small diameter, and my top-end was limited to about 88 km/hr with my current 13/47 gearing. I also found that the superb straight-line grip would almost cause my back end to outrun the front in the deeper and stickier sections of mud, and I had to ease off the throttle to avoid swapping. I really wanted to like this tire and it would be great for a pure woods bike. Just not the right rubber for the Roaming Rally, where I need to keep up with two team mates who totally shred the MX. Maybe the 110/100-18 would be a better fit for the WR. [In fact, it is!]
Scorpion Pro FIM: Wow. What a treat having front-end control in the mud! Not buzzy at all on the pavement. The real test came on the next two rides.
Combo 2: Scorpion Pro FIM (90/90-21) front, MT43 (4.00-18) rear
This combo is the bomb.
First test was about a 120 km ride into Ottawa on 1/3 dirt, 2/3 paved roads, with loaded bags. Cold, windy day at the limit of my comfort for the distance. I took this ride a lot easier than the mudfest on the day before, not wanting to grind down the rubber too hard.
The MT43 gave me back my top end, allowing me to reach 100 km/hr no problem despite the 50 km/hr gusts. But I was shocked to see how much it wore down: easily 1-2 mm on the centre knobs. I was running about 22 psi and had loaded bags on which probably explains some of the wear.
The Scorpion was great. Still no buzzing in the bars, OK braking on pavement, predictable cornering. No significant signs of wear compared to the MT43.
Second test was about 90 km of mixed pavement, dirt roads, and full-on dirt riding--again in deep water holes, over rocky terrain, and through snow and ice. No bags this time. I aired down the front and rear to 18 psi and had no problem on the technical terrain. Mud and water crossings were point-and shoot: this is where the Scorpion really shone, allowing me to pick a line and stick to it, with the MT43 tracking straight as an arrow through the slop even if it did lose a bit of forward traction. My riding buddy Rob is a pro and he pushed me harder than I would've done on my own. I was at my skill limit but I never felt like my bike or tire were at their limits. What a blast!
Wear on the MT43 was significant. Looks like I've taken off about 1/8"-3/16" just this weekend! At least the trail riding evened out the wear across the knobs. But it looks like I'll need to replace the tire in the next month if I keep riding like this. I'll be lucky to get 1000 km out of it. On the other hand, the Scorpion now just looks nicely broken in.
For a loaded rear the MT43 has my vote. It's a supple, sticky tire that handles square edges and loose rocks beautifully, and on my light bike is surprisingly competent in the mud. At lower pressures it would just flow over these obstacles like an amoeba. The tradeoff is poor wear.
The Scorpion Pro also has my vote. Great tracking, great breaking, predictable cornering. Wear remains to be seen. In any case there's a Scorpion Rally on backorder which I'm curious to try.