NOTE: I received a lot of feedback about the accuracy of my earlier post, so I'm re-submitting this with the revisions made.
First time buying a road bike? Looking to buy a bike and are unsure of what to get? In this thread, I will try to lay down a few good tips to consider in finding your perfect bike, whether racing or triathlons are in your plans, or you just want to enjoy a nice and relaxing ride.
NOTE. This is a LONG read, but I constructed it so that you can skim over everything and find a particular item of interest. If any of the veterans have anything to add, please feel free!
Before anything, here are my two key rules for those just beginning to look:
1. Know your budget. Having a set budget on buying a new road bike will help you in knowing what you expect to get with your money. Buying your new road bike can be a needlessly expensive endeavor or a great savings, with enough planning on where it is you're looking to spend.
(EDIT: 8/21/08) It is important to also keep in mind that when purchasing a new road bike (especially from a bike shop), you will want to buy a few additional accessories to make your experience better. This includes, but is not limited to, a helmet, pump, jersey and cycling shorts, socks, etc. These can easily cost over $100, so make sure that your budget allocates some spending for these as well.
2. Know your motives. Why do you want to buy a road bike? Are you interested in seriously pursuing cycling, or is this going to be a summer activity? Knowing the reasons for your bike purchase will not only help shape your budget better, but will also help you find the right bike for you.
In addition, one of the most popular questions for new members of this forum is knowing what's a "good" road bike for them. If you know why you want to start cycling, then it's much easier to know what bike will be "good" for you. It may even be possible that a used bike will satisfy your needs, thus saving you money!
With that covered, here are more in-depth pointers to consider in your purchase:
1. Local bike shops are best! Buying your bike from the local bike shop gives you a sizeable advantage on selection and customer service that cannot be found in a large department store. Bike shop salespersons can fit the bike to your height and width and give you a ride better tailored to you. Furthermore, bike shops receive higher-quality bikes that are more likely to last longer and need less maintenance in the long-run. Also, most shops offer free labor and discounted parts upon purchase, which is usually never provided by department stores.
NOTE: If you do decide to buy your first bike from a big store, then it's HIGHLY recommended that you take the bike to a local bike shop for a full overhaul, as it likely that the store worker who assembled your bike may have done it incorrectly.
2. Do your research. Bike shop salespeople will tell you lots of things; that's their job. However, the onus is on you as a consumer to do a bit of research and find out whether their information is valid or not. Bike shops usually have a favored manufacturer, and they will attempt to push that brand out, especially over competing brands.
3. Frame materials and manufacturers. This is a big section, so feel free to skim.
a. Frame manufacturers
The main difference between frame manufacturers is their engineering of their frames (and price point). For example, for carbon fiber frames, Trek has their own patended process (Optimum Compaction, Low Void, or OCLV for short), Specialized has their own process, etc. Nobody will ever agree on a single best manufacturer; if that were the case, everybody would buy that brand!
b. Introduction to frame materials.
Another big difference between frames (and other components, but frames especially) is their materials composition. There are four stable materials used to build frames: aluminum, carbon fiber composite, titanium and steel.
Without digressing to materials engineering, here are some key differences between the main materials, borrowed from another website:
- One-third the density of steel, allowing the use of big tubes
- Easily formed into aero shapes
- Even cheap frames can be light
- Makes a light frame for a big rider
- Doesn't rust!
- One-third to one-half the strength of best steels and titanium (can break)
- One-third the stiffness of any steel, which requires larger diameter tubes
- Modest fatigue strength
- Not easily repaired or straightened
- Big, thin tubes means easy crash damage
- Best steel alloys are very strong
- Best stiffness overall
- Air-hardened alloys make ultra-high strength affordable
- Can be heavy - not the materials for big, light frames
- Half as dense as steel, making the lightest most resilient frames
- As strong as most steels
- Wont rust - no paint needed
- Good fatigue strength
- Makes a light frame for a large rider
- Half as stiff as steel (and known to be somewhat flexy)
- Difficult to repair
- Readily molded into exotic shapes
- Excellent fatigue strength; no rust
- Strength and stiffness are controllable
- Low density and high strength make very light strong frames possible
- Expensive raw material
- A bomb if poorly designed or made (too stiff or too flexible)
- Can be "notch sensitive" (prone to breakage)
c. Frame building process and cost
Another big contributor to the overall price of the bike is the manufacturing process used in creating the frame of the bicycle. As the complexity of the frame increases, its difficulty to manufacture also increases. This is most evident in modern aluminum frames.
When looking at the bike, take some time and investigate its corners. What you will notice is that the cheaper frames will have smooth, but cake-like corners and that higher-end aluminum frames will have very smooth corners that flow into each other. Some bikes (usually high-end) will even have cap-like ends (lugs) that attach the corners together. This is because the former is much cheaper to do, at the expense of aesthetics and (sometimes) quality.
I will discuss shortly how to determine which manufacturer is better for you.
4. Components. Like frames, components are often subject to endless holy wars over which is better (for reference, do a search on Shimano vs. Campagnolo vs. SRAM). Again, the differences lie mostly in engineering, price and location of manufacture. The choice is mostly a matter of preference, though I always recommend against buying the lowest end, as quality issues quickly become apparent (i.e. Shimano 2200).
Finally, the best way to find out which bike is best for you is simple:
5. GO FOR A TEST RIDE! When you buy your first car, you would drive it a little to see how it performs, right? The same concept applies for bikes. None of the differences and debates matter until you try it for yourself. Naturally, you will not be able to see every deficiency until you actually own the bike for a while, but at the very least it will help you weed out immediate problems and narrow down the choices for you.
Before you take the test ride, ensure that your bike is properly fitted to your body. I know that lots of smaller bike shops already do this, but I have been to one in particular that did a very quick fitting that left a lot to be desired.
I hope this post was helpful. If you have any questions, feel free to ask!
 The Care Exchange, Material Assets, last updated May 2006.