I spent three-ish weeks riding around Mexico’s Sierra Madre Occidental on roads worse than the fire roads in the San Gabriel Mountains with Bryan. Las Barrancas del Cobre. Para conocer el otro lado. It was incredible. It was also I think, somewhat beyond the original design specifications of our rigid frame bikes (a Surly LHT for me, and a Traveler’s Check for him). The bikes performed heroically though, and we got through the whole trip with nothing more serious than a flat tire, a lost bolt (for which a replacement was had), a broken chain, and some worn out brake pads. With some bone rattling descents taking half a day, and spanning 1400m vertically (not unlike the Mt. Wilson Toll Road, except steeper, and in much worse condition) I got very familiar with the different sizes of rocks and ruts and hills and other topographic obstacles, and what they would mean as far as the ride. I also had a lot of time to think about why the hell Bryan was already up on top of the hill ventilating his nether regions by prancing around in flip-flops and a turquoise sarong, while I was still hurling obscenities at the inch thick layer of obstacle obscuring volcanic ash dust covering the road and often obligating me to push the bike up a 10% or steeper grade.
Ultimately, I figure it comes down to the absorption spectrum of the bicycle and rider in question.