The cycling world certainly has gone through a "carbon revolution" in the last 10 years. It is now possible to get a full carbon road bike for under (sometimes well under) $1000. These bikes have distinctive tube shapes, and paint to match.
So what are the upsides of having a carbon bike? Well, carbon bikes can be lighter than other materials. Certainly manufacturers have been able to make the average carbon bike lighter than the average steel, titanium, and possibly aluminum bike. But it is often not the case all the time -- there were many very light aluminum bikes in the day that still hit the 15 or 16 pounds mark. You paid for them, but you pay for these light carbon bikes as well.
Where carbon is supposed to have a leg up on aluminum is in ride quality, feel, road dampening; whatever you want to call it. I think in general it succeeds here. On very long rides, I have noticed less vibration on the carbon bikes I have owned and this, I believe translates into less fatigue. I think this has mostly to do with the central nervous system. Just as being stimulated with loud music or airplane noise can induce fatigue, so to does the buzz on a harsh road bike cause this "central fatigue" (ever notice how LOUD it is on an airplane even when no one is talking -- all that ambient noise has been shown to make you tired.)
This "dampening" quality is very dependent on how the carbon is laid up. In my experience, there are some carbon bikes out there that are just as uncomfortable as the harshest aluminum. If the layup schedule is too rigid or if the epoxy/resin content is not tightly controlled, it is my belief that it can negate the benefits of the carbon in this respect.
So why else do the manufacturers use it?
The main reason now is that it is cheap. You can get carbon fiber cheap, and have it molded into tubes or into a complete bike very inexpensively in Asia. You can teach a heck of a lot more people how to lay carbon material into a monocoque mold or how to wrap joints for tube-to-tube construction methods than you can teach people to weld a perfect joint.
What is being limited are the number of sizes.
This is partially a function of the molds they use to create the bikes. The molds are the expensive part, so if you can get away with 4 or 6 molds (and therefore sizes) rather than 12 sizes you are saving money right off the bat.
The other factor that has precipitated fewer sizes is compact or sloping geometry. With a sloping top tube many more people can physically throw their leg over the top tube to do the "standover test". It's not that more people can fit the bikes, it's that more people can APPEAR to fit the bikes. Back 10 or 15 years ago, before sloping geometries were common, the standover test would at least exclude a few people from being able to buy a certain frame -- their inseam was just not tall enough to comfortably straddle the bike. The standover test, then as now is an awful way to size a bike, but at least back then, there was a certain amount of restriction in bike selection.
I know it sounds like I am really "down" on the manufacturers of today, and...er...well, I am. I should be ecstatic, because as long as they keep doing what they are doing, I will continue to sell a consistent stream of custom bikes to clients who have no chance of fitting a stock bike (this pool of clientele continues to grow every year), as well as doing a few hundred bike fits a year to try and correct for ill-fitting machines.
It doesn't make me happy, however -- I just think about all the people that threw in the towel on cycling because they could not get comfortable on their bike. That stinks. That's not good for anyone's business.
I'm afraid that the story gets a bit worse. There is one more aspect of bike construction that is falling further behind as well. It has to do with the forks. Nowadays it is commonplace to have an entire line of bikes with 2 different offsets, and many are now going to only one fork offset (or rake).
The same reason applies -- a new rake means a new mold, so if you can build just one or two, you're saving money.
This problem is one that has come more to my attention in the few years I have been using the Retul system. After we do the bike fitting we can use the wand, which is called the Zin, and log in the exact dimensions and build of the bike. It is accurate to about 0.2 mm on most bikes and it not only gives you measurements to the bar and seat position, but also gives you the stack and reach of the frame, and the rake and trail of the fork, among many others. You see, you can't always look up the rake and trail of your bike make and model. Many of the manufacturers will asterisk (*) out the fork rake and/or head angle on one or all the models, claiming it is "proprietary" information. I didn't really think much of it until I started to "Zin" some of these bikes only to find out that the rake measurements that were asterisked out were actually the same as the size below it, where it was stated. If it is the same, why the need to hide it? The only reason I can think of is that they don't want you to know it's the same fork.
Why would they not want you to know? Well, we need to do a quick discussion about rake and trail. What is trail?
Trail is the distance between the following points:
1. if you drop a plumb line directly from the front axle to the ground and mark this spot on the ground - basically the tire contact patch
2. project a line through the center of the head tube, and at the same angle, of course, as the head angle until it reaches the ground; mark this point on the ground.
The way you manipulate the amount of trail on a bike is to alter the head angle and/or the rake of the fork.
A simple explanation holds that more trail means more stable (and more "cumbersome" in the extremes) and less trail means less stable (and more "twitchy" in the extremes). Lennard Zinn of velonews.com has written ad nauseum on all the variations in their technical Q&A section -- I definitely recommend reading some of those for more information.
In my experience, for a road bike, a trail measurement close to 60 mm seems to work quite well -- it is a nice balance between stability and yet still have easy to initiate turns.
So now imagine two bikes of the same make and model, just different sizes. The first bike is a 48 cm bike with an effective top tube of 49.1 cm, and the other is a 61 cm bike with a 60 cm effective TT. The smaller bike may fit someone that's 5'2" and the larger perhaps 6'4". When you think of the ways these bikes can be manipulated to fit these drastically different sized individuals you might immediately think of the effective top tube, the head tube length, and the seat and head angles. There are, of course more things to consider, but lets consider only these four things.
Of course the top tube and head tube measurements would have to be scaled up for the bigger bikes and made smaller for the smaller bikes, so we expect those measurements to change. Would the head and seat angles change?
I think it's safe to assume that, in a perfect situation, the seat and head angles would have to change some. Mostly to conform to the limits of 700c wheels -- some of these limits have to do with keeping the wheelbase down or limiting the amount of toe overlap with the front wheel, among other things. As you look on the geometry charts for all of the major stock bike (and by that I mean not custom) manufacturers, you can see this play out -- all of these variables change through the sizes.
Where this breaks down is when you see the fork rake listings (if they even show them). They are very often all the same or at best there are only two different rakes. So if everything about the bikes of different sizes is changing except the rake, and changing rake and head angle are the two main ways of manipulating the trail (and therefore handling) of the bike, then it becomes obvious that they are building the bikes to fit the fork, rather than to fit the rider.
They have excluded rake changes from the equation entirely so they are trying to manage the handling of the bike through the head and seat angle (I know many will say "chainstay length" as well, but have a look on the websites at how many of them change the chainstay lengths through the sizes -- nearly no one) while also making sure to manage toe overlap.
So who wins? Well, the bike companies do for one. But there are certain individuals who can buy these bikes and make out just fine. Generally the forks are made for the "in the middle" sizes -- the 54s and 56s mostly. You can see this bear out on the geometry charts again if they list the "Trail" on the bike you'll see that around the middle sizes the trail comes close to the magical 60 mm number and at the larger and smaller end of the spectrum, it diverges further and further away.
Now there are many out there who will say that no rider can feel the difference between 2 or 3 mm in fork rake and the perhaps 2-3 mm change in trail that happens, and for the most part I would agree with them, EXCEPT when the rider has had to alter the positioning on their bike. What do I mean?
I mean if you have a 25 mm setback seat post, and a 110 mm stem with 6 degrees of rise -- so basically your seat and bar position fall easily within the parameters for contact points that the manufacturer had intended, then changing the fork rake either direction 2 or 3 mm you will likely not feel like much. You are "draped" across the the frame as was intended (read as: your weight is distributed closely to how it was intended to be) and the gross affect of the rake change is muted somewhat.
However, if you had to apply a high rise stem (> 15 degrees of rise) or a particularly short stem (< 90 mm in length) or slide your seat nearly all the way forward or all the way back on the post, then you are more likely to fall near the edge or fully outside of the intended weight distribution continuum of the bike, then an ill-fitting fork offset will cause problems: speed wobble, twitchiness, inability to easily reach for a water bottle, difficulty arcing a smooth turn, etc etc etc.
So if you're having trouble with the handling of your bike, and you're not sure how to fix it, consider how your bike may have been constructed - take a closer look at the geometry on the manufacturers website. Look at your fit -- where are you bars positioned? seat? Hopefully, with some minor changes it can be remedied. If it cannot, dig until you find the culprit and resolve to not let that happen on the next bike.