I’ve been asked many times what type of bike is best for cycling the C&O and the GAP. That is a hard question to answer. The real question is what type of bike you will feel comfortable riding considering the trail conditions. The C&O has rocks and tree roots that can make the ride a bit bumpy. Think of them as nature’s speed bumps. Therefore, I would recommend a bike that has tires that are at least 700x32s. I’ve used 700×32 tires but when I do, I’m usually riding my bike with a suspension fork and a suspension seat post. A mountain bike is a good choice also. When I ride my mountain bike, I prefer to ride 26×1.5 or 26×1.9 inch tires. Nevertheless, if you are riding a bike with narrow tires, it may be a good investment to purchase an inexpensive suspension seat post (about $20).
In addition, if there has been rain recently here may be large puddles on the trail. So also consider what size and type of tires you would feel comfortable riding through deep soft muddy puddles. I generally ride with a road type tread, even on my mountain bike. I don’t think knobby tires are necessary. But if you feel more comfortable with knobby tires, then ride what you feel most comfortable with.
Finally, consider installing fenders on your bike, if you don’t already have them. Even with fenders, things can get a bit messy if the trail is wet. If the trail is not muddy, fenders will keep you and your bike much cleaner. Riding the C&O and the GAP should be a GREAT experience for everyone. But the key to an enjoyable ride is knowing what you feel comfortable riding considering the trail conditions and being properly prepared for the riding conditions.