At some point in your life, you're probably going to buy a bike online - either through an outfit like November that only sells online, or via eBay, CraigsList, or your local cycling listserv. When you do, there are a couple of obstacles to overcome. The first is whether or not the bike's ride characteristics suit you, which in most cases is a binary question: is this bike any good, or not? We're confident ours is more than "any good" and we go to great lengths to try to dispel the myths around what we believe are meaningless points of differentiation between many high-end racing bikes. Our entire business model is predicated, in fact, on the belief that bikes at the high end are far more similar than they are different, and racers should pay for the entree, not the jazz band in the corner playing to lure them into the restaurant.
Picking the right size bike is an altogether separate concern. To some people a frame size choice is shrouded in the same mysticism, and the perceived fear from making the wrong choice is paralyzing. Relax. 1) It isn't that hard to pick the right size bike. I'll show you how, below. And b) even if you're "wrong" it's hardly ever a catastrophic failure, particularly on bikes with many different sizes (ours has 8). There's probably a size that works better than others, but there may well be 2 or 3 sizes of the same bike that would work for you if you set it up with the right stack height, stem length and saddle height and setback.
1. Test ride your own bike.
I bet you've already done this. You know what size bike you're on, and have a good sense of whether or not it fits you well. If you like your current fit, all you need to do with your next bike is to try and replicate it. In doing so, the geometry parts that are most relevant are Effective Top Tube Length, Head Tube Length, and Seat Tube Angle. Here's why:
Effective Top Tube Length: Most bikes that are sized with a number (eg, 56) which sometimes refers to the seat tube length and sometimes refers to the top tube length. Huge difference. Because your saddle height is easily adjustable with the post, the height of the frame's seat tube doesn't contribute much to fit. What does contribute is how far you have to reach from your seat to your cockpit. Effective Top Tube length is the heart of that measurement. You can still tweak it with stem length and handlebar reach (which is why different size frames can work), but you want an Effective Top Tube that gives you the option of going a little longer AND a little shorter if you need to. If your current bike fits great but your stem is really short you might want a slightly shorter frame to lengthen the stem. Long stems give relatively mellower steering, shorter stems give quicker steering. It's a geometry thing.
It's common to see people jam their saddles way back in order to get a longer reach, or put their saddles forward to shorten the reach. Don't do this. Your saddle height and setback are the starting points of bike fit. They work together (moving the saddle back also makes it effectively higher), and the first thing you should try to do is get your saddle at the right height and the right fore/aft spot. The most common (but not necessarily most correct) fore/aft position is when the front of your kneecap is directly over your pedal spindle when you are clipped in the pedals and the crank is at the 3 o'clock position.
Head Tube Length: This is a largely overlooked dimension in frame size, but it's pretty important. A bike that has the same effective top tube as the one that fits you great, but has a head tube that's 25mm shorter will either significantly change your position, or require you to run a tall stack of headset spacers (which you could do with alloy steerer tubes but have to rein back with carbon). Once you choose a bike with the right effective top tube, make sure you check the head tube length against your current ride (including spacers) to make sure it's within 10mm or so. If it's not, you'll either need to add spacers or take them away, depending on where the delta is - does your setup allow you to do that?
Seat Tube Angle: Most seat tube angles fall between 73 and 74 degrees for mid-sized racing frames, a little steeper for small frames and shallower for large frames. But sometimes you'll find frames with more upright seat tubes, which are designed to roll the rider forward over the bottom bracket and split the difference between a traditional road position and the TT position. A steeper seat tube angle moves your saddle forward relative to your pedals, and shortens your effective top tube. How much is determined by how high your saddle is above the top tube. The higher you go, the more that steep angle is amplified - in some cases it can be a full cm, which is halfway to another frame size. It's the kind of SAT geometry that makes my head spin, so I can't offer you a specific formula. But you do need to be cognizant of a seat angle that's different from the one you have now.
2. Throw a leg over a different bike with comparable geometry.
If the geometry of the bike you are on now is nothing like the geometry of the bike you are considering, and you can't test ride the one you're considering, simply test ride one with a geometry like the one you're looking at. In our case, we deliberately chose geometry comparable to that of the most commonly raced bicycles. It's proven and racers are comfortable on it. But the added upside is that if you can't test ride our bikes for fit, you can throw a leg over a Cannondale SuperSIX or CAAD9/10 or a Specialized Tarmac and get a pretty good approximation of fit. For example, a size 54 Cannondale CAAD10 has a seat tube of 54.5cm, head tube of 14.0cm and a seat tube angle of 73.5. Our Wheelhouse in a size 4 has a 54.5 seat tube, 15.5 head tube and a seat tube angle of 73.5. Our head tube is 1.5cm higher, which means if you have 1.5cm in spacers on the C'Dale, you can take them out on of the Wheelhouse and have a very comparable setup. (But as to my point on Head Tube above, if you run the stem all the way down in the CDale already and can't take out 1.5cm, you might want a smaller Wheelhouse which you'll stretch out with a longer stem to get the same fit.).
Stems also come with different rise/decline angles. There are a lot of +/- 6 degree stems out there, but some are +/- 10. Different stem rises can put your bars higher up or lower down, but be aware that a 110mm stem with a 6 degree decline will act shorter than a 110mm stem with a 10 degree decline. The closer your stem is to horizontal (on a 73 degree head tube a -17 degree stem would be horizontal), the longer its effective length. Beware of that.
3. Visit a bike fitter.
Some bike fitters will help you figure out what size bike to buy by pre-fitting you. They can take measurements and back into your ideal frame size(s). It's not a full fit of course since you don't have the bike yet, but it does prepare them for giving you the best possible fit by helping ensure that the frame you buy will accommodate the riding position that makes the most sense for you.
Even if you don't intend to buy a bike on the interwebs, determining for yourself the best size you need in advance of test riding at your LBS is a very good idea. Some shops have excellent staff very capable of helping you select an appropriate size, but even the skilled ones don't know what you do about your riding position from your own experience or previous bike fits. Anyone can put you on a bike that's not a perfect fit, but you're the one who has the greatest incentive to get it right.