I am no expert on this topic. I have investigated articles and discussion threads about the topic to find out what "magic" is performed in the various fits. What I have learned is that before approaching a fitter, you need to ask yourself: what do you want the fitter to do. What are your expectations as in the article http://www.slowtwitch.com/Bike_Fit/R...ions_3595.html

Generally, there are a range of fits that are available that may or may not meet what is outlined in the above article. In the lower part of the fitting price range, you bring your bike to the bike shop and it is put on a trainer. The fitter watches you as you pedal and advises you based on his experience whether your saddle is the appropriate height and whether your reach to the bars, hoods and drops are satisfactory. Some adjustments may be made to height and reach. Some recommendation may be made as to whether a different handlebar eg. one with a shorter reach and drop may be made. This type of fit will aim at making your bicycle more comfortable and more efficient to ride.

Because a key question may be what size bicycle should I purchase, a more extensive fit may be in order and this will be higher in the price range. To help understand the sizes of bicycles and the differences between the geometry of different bicycles, you need to have some basic knowledge about geometry. Without this knowledge, you may not understand what the fitter is telling you or why he is telling you. If you are not aware of it, you need to know what is meant by frame stack and reach. These are the (x,y) coordinates that come out of a bike fit where one uses a fit bike like Retul or Guru or some other make. Knowing your (x,y) coordinates allows you and the fitter to look at published data and geometry charts that show the frame stack and reach measure of particular frames. In this way, you can match the data and identify the size of bike that will fit. The fit should be precise enough to identify stem length, angle and the amount of spacers under the stem. It should be precise enough for you to know what is your measured position on the bicycle. Some fits in the upper price range will study your motion on the fit bike with a camera (and this will contribute to the cost of the fit). Some fits will determine your best position on the bike by using power output measurements. To know what you are going to get from the fit, you need to talk to the fitter. Both you and the fitter have to have the same understanding. An additional aspect of the fit may involve positioning the cleats on your shoe. This also may be priced separately. As the fit becomes more detailed or intensive with power and motion data, then the price of the fit will increase.

To understand stack and reach, you may want to look at http://www.slowtwitch.com/Bike_Fit/G...ature_190.html and http://www.slowtwitch.com/Bike_Fit/G...mbers_193.html