I've heard that dirt bikes have less long-term reliability than street bikes. Assuming this is true, is it primarily due to higher stress levels in dirt bikes as opposed to inherent design? Or maybe a combination of both? Let's try this experiment: Compete a 250 cc 4-stroke trail (not motocross) dirt bike against a 250 cc 4-stroke street bike. Same rider. Let's meet half-way stress level wise. We'll only be doing gravel and hard-packed dirt roads with varying terrain (straight-aways, hills and ruts). Assume both bikes go through their fork compression full ranges but neither bike bottoms out on the forks. Assume both bikes vary speed from low to high but neither bike redlines out on RPM. Over long term riding under these conditions, which bike would be expected to have better long-term reliability?