Road-legal knobblies wear faster than Road or All-Road tyres – the reward is secure grip on loose surfaces which may be more important on your trip. Traction is OK on the road where there’s usually plenty to spare anyway, but at the cost of smoothness, noise and braking. Up to a point modern ABS systems may compensate for a knobbly’s reduced traction on road but that won’t stop you sliding off on a wet hairpin bend. I’d designate these tyres as 50/50 road/dirt use; they do the job on the highway and work better than most in the dirt, including sand and mud.
Tyres shown here left and right include Continental’s TKC 80, the Michelin T63, Michelin Desert and Pirelli MT21.
The other day I spent the weekend in Wales with my WR (left; with Mitas Rockriders) on its original Bridgestone TW 301/2s and expected the worst. They were at least 8 years old but they managed everything except perhaps wet grassy ruts just fine. Not a single slide in 150 miles. And with a dense knob pattern ought to last OK and made no noise that I could hear.
There are scores more examples of full-on, balls-out, off-road racing knobblies. But on a big, loaded travel bike their aggressive tread patterns with tall, widely spaced knobs will wear very quickly on leading edges, causing cupping to give a horrible ride on the road, as well as gripping poorly and squirming in bends. Knobs may even break off at high speeds when a tyre gets hot. They can work on a light and low-powered bike like the CRF250L left (Maxxis Desert) but Conti’s popular TKC80 as well as the Kenda and Mitas clones have shallow knobs for just that reason.