There are always lots of posts by people who have a bike that isn't a tourer and want to know if they can tour on it. Other people want to buy a bike to tour, but want it to do other things as well. They're considering a "compromise bike" - one that can tour, but also commute, do centuries, etc. - so they're considering a non-tourer that can be used to tour.
I've opined many times that you can tour on almost any bike, because I've done so at various times in my life, for various reasons, and had some wonderful trips.
However, I thought it might be interesting for people to state why an actual "touring bike" is best for touring, provided you agree with me that this is true. If you don't agree, feel free to weigh in - I'm not trying to prove anything, just get some discourse flowing.
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I love to take long, self-supported tours - enough so that I invested in a "real" touring bike. I bought a Surly LHT frame (before they offered the complete) and built it up. I chose it based mostly on the number of positive comments on this forum, as well as the reasonable price. There are many other tourers that I've read about that I'd consider as well if I were buying one today.
When you get a true touring bike, such as the LHT, you get all the extra fittings. It has eyelets for racks and fenders front and rear, three water bottle mounts, a fitting for carrying a couple of spare spokes on the chainstay, a chain hanger, and even a pump peg. Just those fittings alone make it desirable for a tourer; you don't have to worry about how you're going to attach things.
Another feature a tourer gives you is longer chainstays, which means the rear panniers will be a little further back, which means your heels won't hit them on each revolution. I have size 14 feet, so this is important to me. On an old 10-speed I toured on in the 70's I had to keep my feet steady. If they slid backwards a centimeter, they hit the panniers.
Another nice feature on tourers is the ability to take wider tires. 23cm tires might be nice on a road bike, but on tour you need cushioning - both for your body and for the bike, to prevent pinch flats, broken spokes, etc. I have 32cm tires on my LHT which roll nicely but are very comfortable. If I was touring on a softer surface (the Katy Trail, dirt roads that are part of some routes, etc.) I might even go larger. I think I can put up to 50cm tires on my LHT.
I had issues with broken spokes on a road bike I used for touring in the 90's. I started out fine, but broke my first spoke halfway through the tour. Towards the end the wheel was completely worn out and I was breaking spokes every couple of days. I think it's vital that a tourer, have strong wheels, especially for someone carrying a big load. I like 36-spoke wheels with excellent hubs, strong rims, and double-butted spokes. Most of the good-quality tourers have strong wheels
Handling is another issue. On the non-touring road bikes I've toured on, there has been some whippiness (exacerbated by the fact that I have large-framed bikes - I'm 6'4".) Unloaded they've been fine, because that's what they were designed for, but load them up, get going fast (like down a hill) and they've shimmied so badly that I thought something would break. My LHT exhibits none of this, no matter how fast I've gone, and I'm sure other tourers are similar.
Another aspect of handling is how "twitchy" the steering is. You want to be able to keep a straight line, which can be difficult when you're climbing a mountain pass with a big load, going only 4 or 5 miles per hour. When I first road my LHT unloaded around home, the handling felt just a little weird. I was used to my road bike. However, on tour I love how it handles.
Comfort is big on tour. At home I go on long rides, but seldom on successive days, and never day after day for weeks. On tour that's exactly what I do. Discomfort starts to mount with each passing day, until you might be forced to take a rest day just to give your body a chance to stop hurting. I set my LHT up so that it's as comfortable as possible. One thing I can't change is the geometry of the bike. On the LHT the geometry choices are made in part for comfort; on my road bike they're made for quick handling and speed; comfort isn't much of a concern.
The brakes on my road bike work fine for stopping me and the bike; they wouldn't fare as well stopping me, the bike, and 30 pounds of stuff. My LHT has cantilever brakes, which have more stopping power. I'm considering switching to V-brakes before next summer, because I love them on my mountain bike. I couldn't do either on my road bike.
One last factor to mention is the peace of mind. I'm a worrier on tour; I can't help it. I'm almost always by myself, which adds to my worrying. I worry about whether my bike will break. I worry about finding a campsite. I worry about finding a place to buy food before nightfall. My LHT has completed three long tours with nothing breaking. Since I trust my bike so much, my worrying has diminished considerably. If I was on a less-sturdy bike I might worry that something would break (something unrepairable) in the middle of nowhere. It's nice not to worry so much.
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So, I think my touring bike is best for touring. Can it do other things as well? Sure. I think it would make a great commuter. It can carry a load, either on the racks or in panniers. It has fender mounts for commuting in inclement weather. It's a little heavier than my road bike and the gearing is a lot lower, so I can say it's not as fast. However, for commuting I don't think that's a big deal.
I've ridden my LHT in centuries, and on recreational rides around home. It's fine for both, and very comfortable. Sure, I prefer my road bike, but if I could only afford one, the LHT would be very acceptable.
If I were a racer, I certainly wouldn't want to ride my LHT, so that's about the only thing it's highly unsuitable for.
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To sum up, you can tour on almost anything, but if you're going to do much loaded touring, there are good reasons to consider a "real" touring bike.