Most people who start biking don’t realize that bikes come in different sizes. However, if you have a tall person and a short person get on the same bike, the difference can be clearly seen. Either one can’t touch the pedals or the other will never be able to extend their legs.
The rule of thumb is to take a bike and stand over it, feet flat on the ground. It’ll be too big if your crotch can’t clear the top bar. It’ll be too small if the top bar is more than 2 inches below you. Now that you have a suitable bike for your height, you can make the following adjustments.
Seat height is really important because you can hurt yourself on your rides through improper forcing of your legs. It is an adjustment that all triathletes are familiar with because it can make or break your competition.
I’ve uploaded two videos on seat height. One is an easy, comfort-level adjustment. The second isn’t the greatest video quality but it is more technical and is by John Cobb, who used to design extremely innovative carbon fiber bike parts and is now exclusively doing bike seats. If anyone knows bike geometry, it would be him.
Stem length is a part where you can play with to take your handlebars nearer or farther away and is critical for an optimum ride.
The third video offers tips on where to have your stem. Now there are several things you’ve got to remember. The weight of your body is supported in three different places on a bike: your saddle supports your butt, your pedals support your feet and your handlebars support your hands. Put too much weight on your handlebars and your hands, which aren’t used to supporting weight like your butt or your feet, will hurt. As always, you have to play with the different settings to see what is optimum for your type of ride. An upright position is more comfortable but may not be the best for speed. Getting down low on your bike will offer for speed but it would depend on whether you plan to time trial or do triathlons on a regular basis and therefore need to be in that position.