Today's entry is in response to a request to expand on my recent bike fitting, so here goes!
Next up, seatpost height. This is calculated using a couple of interesting measuring devices. One is a spring-loaded L shaped stick that measures your inseam. The above picture shows the formula used to figure the distance from the center of the bottom bracket to the top of the saddle. The other tool is a giant acrylic protractor that measures the angle of thigh to shin, using the knee as an axis. This measurement is taken when the foot is furthest from the saddle, which is not necessarily straight down. If your seat tube is angled, you would extend the crank to match that angle. The goal is 20-30 degrees, depending on flexibility.
Now, fore and aft position, meaning where your saddle is mounted on the seatpost. General lore dictates that when your crank is in the forward position, parallel with the ground, a plumb line from the base of your kneecap will line up with the ball of your foot and pedal spindle. There are several schools of thought on this, but this is the most widely accepted.
Ok, your butt's where it's supposed to be and so are your legs and feet. Next up is the torso. Again, this will depend on flexibility. Standing (off the bike), fold forward without bending your knees. If you can comfortably put your palms flat on the ground (without straining), you can ride with your bars lower than your saddle. I can comfortably rest my fingers on the ground so my bars are about the same height as my saddle. Less flexibility would require the bars to be higher than the saddle.
The bars, and especially the hoods, need to be reachable. It should be easy to ride with your elbows slightly bent and it should be easy to operate the brakes and shifters without straining. There's lots of adjustments to make this possible, including a little shim that goes in your brake lever to move the lever closer to the bar. A shorter or longer stem and stems with different angles can help accomplish the correct reach and height.
If you like riding in the drops like I do, you'll want a bar shaped for your riding style. Some folks love the ergonomic bars which feature a bend in the curve so it's easier to reach. Some like a shallower drop. Some swear by traditional bend. It's like choosing a favorite color. The width of the bars is dictated by your shoulder width - most common for men is 42cm and women 40cm.
When I bought my SOMA in 2007, I also bought a fitting. However, I was still recovering from a lower back injury and had a smaller annual mileage then, around 3000. I also weighed more than I do now. Since I've been riding more and climbing more and weighing less, my body and my fit have changed. Here's what changed specifically:
* Saddle moved several millimeters back, requiring a seatpost with setback
* Seatpost moved down, because the saddle was moved back, in effect my tush is further from the bottom bracket (which some refer to as "height")
* Bars moved back 10mm using a shorter stem, this again is due to the saddle moving back.
* Cleats moved alot - back about 3 millimeters. Bye bye, hot foot.
The bike continues to feel very comfortable and the knee acheyness that triggered my re-fit seems to be fading. I learned a lot about the complex field of biomechanics, or ergonomic fitting of a body to a machine. I also learned that it's ok to have different bikes fit you slightly differently. Your body may actually appreciate the change-up.
I am in no way an expert on these things, this is just what I learned during the process. You can do some basic guesswork and even jerry-rig your own plumb line, but there's no substitute for a real fitting with an expert. They usually include a year of follow up care too. Good luck and happy riding!