<====The winning bike from Trans Iowa V5
Choosing a bicycle for a gravel or back road event can be a bit confusing. If you have been around any of these events, and are just casually looking about, it would seem that about any type of bike will do, and in fact- it will! Truth be known, almost any type of bicycle can be ridden on gravel roads, back road dirt pathways, double track, or what have you. The thing is, if you are competitive, or want to be comfortable on a long adventure, there are better choices in bikes over others.
Let's start with some basics. Traveling gravel or rough back roads will usually rule out road bike tires. I'm talking anything under 30mm in width and having little to no tread. Of course, as I said above, you could ride a road bike tire/road bike, but it isn't optimal for several reasons. First of all, road tires typically have very little volume, are fragile in rocks, and prone to pinch flatting. Bigger, tougher, more voluminous tires on sturdy wheels is a better choice for gravel roads. Added to this is the fact that road bike calipers limit mud/debris clearance. I've seen bikes come to grinding halts that were hung with road brakes when other bikes cruised right through using cantis/disc brakes. So usually a full on road bike isn't ideal.
Base your choice on body weight, (lighter guys can get away with skinnier tires, lighter wheels, and lighter bikes), and type of terrain to be ridden. (Stay away from road racing bikes with limited tire clearances or tight fitting side pull caliper brakes) Carrying a bunch of stuff? Make sure your bike can deal with it! Rack mounts, water bottle mounts, and other features may figure into your final bike choice.
<===A group of Salsa Fargos- excellent all terrain travelers.
Secondly, if your event/ride is long- like counted in the tens of hours/hundreds of miles long- you will want to pay particular attention to rider comfort. Obviously a good fitting bike is paramount, but you may want to raise the handle bars a bit over your standard road position. You may want a different saddle, and grips/bar tape choice is a really big deal. Basically you are going to want to experiment with contact points and the equipment that is specific to those points. Only by some trial and error will you accomplish finding the right equipment for you.
I usually would only change one thing, go for a three plus hour ride, evaluate, and adjust if necessary. Your specific "thinkering" routine may look different, but I recommend experimentation to find products that work.
Saddle sizing is now available, and I highly recommend taking advantage of that to dial in a proper width saddle. Also remember that weight of components is less a concern. Comfort comes first. Grips and bar tape are also very important. I highly recommend Ergon grips for flat bar users, but do experiment here. Also there are interesting findings that are indicating padded/gel bar tape for drop bars is not a good idea. Building up the diameter of bar tape using multiple layers seems to be better here. Again- experimentation is required to find what works best for any individual.
<===This bike with a higher handle bar position, bar ends on flat bars, and 2.0 inch rubber make gravel travel much more enjoyable- for me! Your mileage may vary!
So, what have we learned? First off- traditional, modern road racing bikes are not ideal for gravel grinding. Contact points are very, very important on longer gravel road rides. Pay attention to what works and what doesn't by experimenting on several long gravel/back road rides. Dial in your set up.
Now, a word or three on some specific bike choices that I have observed working well on gravel road events.
Cyclo-cross Bikes: These bikes can be the fastest, best choice in gravel grinding- but......
Cyclo-cross bikes in their purest form are all out, efficient racing machines meant for short events. Real, very traditional cyclo-cross bikes won't even have water bottle bosses! This is not ideal for gravel grinding. Fortunately there are several bikes that blur the lines between true cyclo-cross, road, and adventure type riding. A great, workman-like rig to exemplify what I mean is Surly's CrossCheck. Stick to something like that bike and you can't really go wrong.
Mountain Bikes: These can also be great gravel grinders, but........
Mountain bikes typically are heavier, shod with knobby tires that are 2 plus inches wide, and may have shock systems that are typically overkill on most gravel grinders. You can certainly change tires, and a fully rigid bike is actually a good thing on gravel roads. My advice would be to get a 29"er, which is really just 700c wheels fitted with wider tires and a frame and fork that accept the wider meats. Wide, 2 inch tires with minimal tread like Stan's Raven, WTB's Vulpine, and Nanoraptor models excel at gravel travel. You may not be the fastest guy in the event, but you will be protected from pinch flatting more so than skinnier tired bikes are, you'll be more comfortable on rough roads, and you'll have an advantage on really rough, or muddy sections. I've seen lots of mountain bikes- typically 29"ers, do well at gravel road races, most notably the Fisher Paragon and Rig single speed models, modified with rigid forks.
Adventure Bikes: This is kind of an odd category, but it is being defined into three distinct types of bikes: The "monster-cross" bike, the randonneuring bike, and the 29"er based off road touring type rigs. Used to be that "monster-cross" bikes, many times based upon the aforementioned Surly CrossCheck, were showing up a lot at these events in highly user modified states. Usually shod with 38-42mm tires, drive trains featuring single speeds to "1 X" type set ups, to internal geared set ups, and flat bars, H-Bars, or drops, these bikes almost defined gravel grinding for a time. Then we had the Salsa Cycles Fargo come on the scene and in that bikes wake we are seeing the introduction of several drop barred, 29 inch wheeled, adventure rigs that really lend themselves well to the gravel culture. Finally, randonneuring rigs, like Velo-Orange's "Polyvalent" all-purpose 650B rig are also great gravel rigs, (minus the fenders, perhaps!) and can yeild an excellent balance between rider comfort and speed over rough roads and gravel.
Which is right for you? Depends on if your aim is to go as fast as possible, (then a cyclo-cross type variant), or if you want to finish and enjoy the ride, (adventure bike), or if you want to be practical, (mountain bike that you can use elsewhere on trails). There is no "wrong" choice.
The big thing is to get out and grind some gravel- on whatever bike that you might have!