So, what makes a mountain bike a mountain bike? What makes a road bike a road bike? Is there any physical difference?
In the old days, road bikes had 700c wheels (or 27 inch wheels, if they were of a certain age/pricepoint), and mountain bikes had 26 inch wheels. But, in the days of 29er mountain bikes and 26 inch wheeled Surly Long Haul Truckers, that is no longer an acceptable definition. (That definition was the death of the Bridgestone XO series, in the early 90s. With a frame built along the lines of classic road racing bikes, and 26 inch "mountain bike" wheels, the XOs were neither fish nor fowl to the marketplace of the time.)
I made a point, for quite a while, of riding cross bikes on "mountain bike rides". I joined in with all my friends, who were riding the latest and greatest in suspension technology and huge tires, and rode along on my 700x35c tires, on a totally rigid road frame, with drop bars and cantilever brakes.
People would say to me, "You need a mountain bike to ride this trail." To which I would reply, "Then, I guess if I ride this trail, then my bike is a mountain bike, by definition."
Some of them got it. Some did not.
When I had lived here about a year, a friend from Ohio and his wife came out on business, and stayed with Valerie and me. John and I went mountain biking, one day, because he was as thrilled with the possibilities as I had been when I first visited.
At that time, I had only two bicycles; my Specialized M2 StumpJumper and a Schwinn Super Sport road bike from about 1988. The Schwinn had road bike gearing (52x39 chain rings, and a 13-25 cogset). I had installed some skinny 700x28c Specialized cyclocross tires on it, because we lived in a house 3 miles from the nearest paved road, and it made the occasional commute easier to have the knobs.
So, when John and I went to Waterton Canyon, and rode up the Colorado Trail and did the Roxborough Loop, I let him ride the M2, and I took the Schwinn. As far as I'm concerned, that bike was a "mountain bike" for that day. I was riding single track trails in the mountains. I was on a bike. Therefore...
Anyway, that's one of the reasons I have a hard time with chauvinism within the bike world. To me, bikes are bikes. Some are better for certain uses than others. But, any bike can be ridden anywhere that any other bike can be ridden. It might not be as easy to ride, or as fast as another bike might be. But, it will get you there.
What makes a road bike a road bike, or a mountain bike a mountain bike?