A Bike that’s Just Right
The first thing you should do is stand over the top tube of the bike that is, in between the saddle and the handlebars. While standing over it, raise the front end of the bike. You should have a couple inches of clearance before the top tube comes into contact with you. In former days, general cycling knowledge was that as long as your feet could touch the ground even if you were literally sitting on the top tube the bike was an acceptable fit. Many people undoubtedly learned the hard way that such a bike was not a good fit. I recommend about two inches or five centimeters as a safe amount of stand over clearance. This should give you a safe cushion in the instance that you come forward off of your saddle quickly. The frame size you want to purchase is most likely the largest size which leaves you enough stand over clearance.
Once you have found a bicycle that you can stand over, the next step is to have a seat on the bike. You should raise the seat so that your foot is flat and your knee is slightly bent at the bottom of the pedal stroke. The seat is too high if you strain or extend your foot to reach the bottom of the pedal stroke, and the seat is too low if you are far from extending your leg at the bottom of the pedal stroke. This picture shows a friend of mine modeling how far your leg should be extended at the bottom of the pedal stroke. Notice that his foot is flat and his knee is still bent just a little bit.
Now that you’re sitting on the bike, the next thing to consider is how far away the handlebars are from the saddle. You will be spending many hours in this position, so it needs to be comfortable. The pitfalls to avoid are being so far away that your elbows are locked before you reach the bars and that they are so close that your knees hit the bars or your hands on the bars at the top of the pedal stroke. If the frame fits and the seat can be adjusted correctly for you, this is probably the right bicycle for you. The stem can be exchanged to move the bars higher or lower, and closer to you or farther away from you to make your position more comfortable. However, the stem can only move the handlebars so far.
If you fit as far as stand over clearance and seat height vertically, if you will, but you are a long way from fitting front-to-back horizontally, you should pass on this bicycle and do more research. Ask the salesman to measure the seat tube length, seat height from the bottom bracket, and top tube height for you, as well as the measurement from saddle to stem. This information will be important when you go home to do your research. Like we noted earlier, manufacturers publish their bike geometries. You would do well to find a bicycle with measurements in line with what you brought home from the bike shop, taking into account the areas where that particular model fell short or long. With the number of different manufacturers and models out on the market, chances are you will find a bike that will fit you, and probably one that you can test ride in a shop not too far from where you live.
This is enough information for you to find a bicycle that will fit you comfortably and keep you injury-free. After you have ridden the bike for a while enough time to become accustomed to the position of the new bike, most likely several rides over a couple weeks’ time you may notice that you would like the bars just a touch closer or farther away. No problem. The shop where you purchased your new ride will more than likely swap out the stem for free if you do it soon after purchase, or at least for a nominal charge. They will also probably check over the cables and adjustments on your new bicycle, as things tend to settle in over the course of several outings