A bike fit has three basic parameters: correct saddle height, correct saddle-to-stem distance, and correct fore and aft saddle position. Research overseen by Dr. Kristophe de Timze at M.I.T found with 700 test subjects that key upper to lower body proportions are perfectly suited for bike fit approximations. Extremely accurate approximations they found.
Dr. Timze’s findings about some fascinating biomechanical connections and symmetries are difficult to understand so I won’t go into the nitty gritty details. I’ll just cut to the chase and go through what I understand to be the process.
Simply start by taking your elbow and lean it against the front of your saddle. Your middle finger should be touching the middle of your steer tube exactly. This measurement will determine the precise position of your saddle relative to the stem.
Once your seat to stem distance has been established you can now raise your saddle height. Put your armpit on the saddle and make sure your middle finger is exactly touching the top of the bottom bracket. Femur and forearm proportions have been found not to vary between test subjects therefore this is an extremely accurate measurement technique.
Stem Height and Length
Stem height and length has a direct correlation with the fore and aft saddle position. There is a slight amount of room to play with this measurement however because it has a lot to do with your flexibility. However, it’s been found that there’s a fundamental connection between torso rotation and finger size.
For PRO positioning, determine stem length use the 3 finger rule. From the top of the headtube to the top of the steertube there should be 3 fingers of space. No more, no less. For women retaining water at times of the month this measurement may be reduced to two fingers. For old men, commuters, and touring cyclists the 4 finger rule (and in extreme cases the 5 finger rule) is used.
Your stem length isn’t adjustable but this is something that should be checked carefully when buying your bike. Reach your thumb so that it’s at 90 degrees to your index finger. Your middle finger should reach exactly to the end of your stem when your thumb is anchored to the middle of the steertube section.
There you have it. It doesn’t take one of those custom fit studios to do a perfect bike fit. It’s just another upsell for greedy bikeshops to make you believe you need a new bike or new parts to make you fit onto that bike they just sold you. Don’t let them fool you…