With a reputation for pursuing excellence in bike fit; I get this question a lot. I figure that there are more of you out there asking that same question - so if I can help; here's some thoughts on the subject:
- Bike size can be a complex issue as different bicycle brands can use different sizing methods. A 56cm in one brand could be considerably larger or smaller in another brand depending on the standard of frame measurement the brand uses. There are at least 3 common standards for seat tube measuring which can each vary by over 1 cm on the same frame yet still denote the relative "size" of the frame.
- Most bicycle frames consist of up to 8 separate tube sections welded or bonded together. A bike with a 56cm seat tube could be made to fit a variety of body sizes depending on the lengths chosen for the other 7 frame tubes yet the 'frame size' based on the seat tube length for all possible variations would still be called a 56cm.
- The length and relative angles of the 7 tubes not included in the "size" description are as critical to the correct fit of a given rider. The dimensional variations of the other tubes can result in geometrical changes to the overall frame effecting specific riding characteristics along with rider positioning. It's possible to get the right "stand over" size but the wrong geometrical specification for your intended use and physical needs. Additionally, some bikes are intended for competitive or high performance riding while others are designed for comfort over the long haul. The right size in a bicycle intended for a different application than you aspire to will result in an improper bicycle purchase.
- Comparing one brand to the next can become the source of a splitting headache at times. Considering all of the variables outlined above; while it is difficult, a few assets make it more manageable. Accurate frame geometry data, a good grasp on your ideal riding position, and an expert in the applied science of bike fitting and "fit transfer" (adapting a riding position from one bike to another as best as mechanically possible). However, a simple sizing can occasionally help narrow the herd by discovering what collection of the above outlined characteristics will best lead to your ideal riding position.
- Your comfort on your bicycle is tantamount to enjoying your riding experience. Given the variables in sizing of the modern bicycle; consulting with a professional is far more accurate than assuming all sizes are similar across multiple brands. Even when comparing brands within one store; small differences can be found which may mean that a different size in brand "a" fits better than the size which fit best in brand "b". The best method of determining correct size is to first conduct a fitting and establish the ideal riding position so that data can be applied to the bicycles being considered. Bike shops will often require an appointment and should charge for this service; but as those who have experienced an expertly performed fitting will attest - it is the best money you can spend on your cycling habit; no matter how serious you are or want to be.
While the answer is not a simple one; we hope you find this helpful. Contact me and let me help you discover your new bike!
Thanks for reading.