How bike size is measured?
So what does "different frame size" actually mean? And what does that frame size number, 13" or 25" or whatever, mean? It's fairly simple.
The frame size number comes from the length of the seat tube. The seat tube is that nearly vertical tube of the three big tubes that make up the "main triangle" of the bike frame. The seat tube has the bike's saddle attached at the top, and has the pedals and crank arms attached at the bottom. A short seat tube will make the pedals closer to the saddle; a long seat tube will make the pedals further away. The frame size number is the length from the center of the crank arm spindle (the axle that holds the two crank arms together) up to the top of the seat tube (where the saddle and seatpost are attached). On some bikes this is measured in inches, on others in centimeters.
But that's not the only dimension that changes for different frame sizes. As bicycle frames get taller, they also get longer. That means the distance from the saddle to the handlebars gets longer. This makes sense, since tall people don't just have longer legs than short people. They usually also have longer arms, a longer torso, and so on. So the bike frame also needs to get longer in every direction for a taller rider, not just longer from pedal to saddle. The top tube gets longer, which pushes the handlebars further away from the saddle. The head tube (the frame part that the fork attaches to) gets taller, so the handlebars will be higher up. All of these dimensions and more are fine tuned in every frame size, so the right size frame for a person's height fits well everywhere, not just in the saddle.
Dec 21, 2010 |