One of the most important details when buying a bicycle is finding the right size frame for your body. When you have the right size frame you will be in the proper position, which improves cycling efficiency, power and comfort. You’ll be a better rider.
The size of a frame is determined from the length of the seat tube. This is the tube that connects the bottom bracket to the seat. The frame size you need relates closely to the length of your leg. Bicycle manufacturers measure the frame size various ways but there are two generally accepted ways.
Most road bicycles are sized by the distance along the seat tube from the center of the bottom bracket to the center of the top tube. This is call center-to-center. Some bicycles are sized from the center of the bottom bracket to the top of the top tube. This is called center-to-top. All measurements are in centimeters.
Top Tube Length:
The top tube runs horizontally from the seat to the headset. Frame size is the most important factor when choosing a frame that’s right for you but different manufacturers have different length top tubes for the same size frame. Women generally have shorter upper bodies than men and will need a bicycle with a shorter top tube. If your legs are very long or short in relation to the rest of your body you may need to either pay special attention to the top tube lengths from the various manufacturers or think about buying a custom-built bicycle.
Seat Tube Angle:
This is the angle formed by the seat tube and an imaginary horizontal line. For all-around road racing bikes, the suitable seat tube angle is about 73.5 degrees. Mountain bikers may want a little steeper angle – 74 or 74.5 degrees. Biking athletes prefer a steeper angle – 80 degrees, which gives the rider a more forward position. As your needs and riding style change you should pay more attention to the geometry of your specific machine.
Choosing Your Frame Size:
The frame size you choose is directly related to the length of your inseam. Here’s how to get the correct fit:
- Determine your inseam – stand with your back to a wall with a 1-inch thick book held between your legs, snug against your crotch and measure from the floor to the top edge of the book.
- If you used inches, convert it to metric by multiplying by 2.54. (For center-to-center, the frame size should be 0.65 of inseam. For center-to-top, add 1.5 centimeters.)
- In general, when you stand over a road bicycle there should be 1-inch of clearance between your crotch and the top tube with socks on and about 1 to 2-inches with shoes on.
Once you have the right sized frame for your body, the next step is the find the correct position on the bicycle (NEXT BLOG POST).
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