Ask a Tech – Should I Go With a Bigger or Smaller Frame Size?
Q) This could be very abstract sounding, but is it preferable for a rider between sizes to select the smaller size with a longer stem, or a larger size frame with a shorter stem? Is power lost in the smaller frame that is otherwise better distributed or better applied in the larger frame? – Christopher
A) Hi Christopher,
Your question is very common because it is one that many riders have felt they must face. I think your question is common for two primary reasons. First, compact frame geometries and sloping top tubes have laterally stiffened frames over their non-sloping, traditional predecessors. In a nutshell, all else being equal, a smaller frame with “tighter triangles” and shorter tubes are actually stiffer than a bigger frame and thus can transfer power more directly. Second, and probably most significant, not all bicycle dealers promote a fit first philosophy and the sizing of a bike becomes more abstract and less data-driven when there is not a proven protocol that corresponds to the selection process. Without a rider first fit philosophy, the bike’s needs will often be dictating your power and comfort instead of your needs determining how the bike can help you achieve your potential. Without a proven process to determine riding position first, left to choose on non-fit related variables, smaller often “sounds” better to many performance oriented riders.
This all being said, there is no reason to guess on this. A “fit first” approach to frame sizing yields greater accuracy in position and also will help clarify what frame size makes the most sense by showing how the rider’s position relates to the frame geometry before you buy. For example, I’ve seen some riders who swear by sizing down a frame and running really long stems, 130mm and greater, always assuming that sizing down is the right approach. However, sizing down can have some significant compromises for some riders. On a X / Y plane, the horizontal X coordinate is often achieved at the expense of the Y coordinate: sizing bigger or smaller not only changes effective top tube length (a factor of X) but it also changes headtube height (a factor of Y). In this scenario, unless you make the decision to run an excess of spacers under your stem (potentially compromising handling and stability in the process), the smaller bike may yield a lower than ideal position in order to properly adjust your Y coordinate.
A properly fit frame will be as close to the your ideal X and Y coordinates as possible while still being well within the normal set-up parameters the engineer who designed the frame was considering. A proper bike fit eliminates the guesswork in this regard. A qualified bike fitter has at their disposal the tools and data (geometry templates, motion capture, power data, physiology, kinesiology, interpersonal skills, etc.) to find that X / Y and commensurately and locate the best alchemical compromise between power and positioning.
So, in short, more power is almost always ideal, but how long it can be maintained in a certain position, and how comfortable that position is, is what really matters for most riders. Neither a smaller or bigger frame is better for everyone and generalizations in frame sizing are almost always risky. If your riding position is established first the best decision for you will likely become much more clear. Consider bike fit the holy trinity of comfort, power, and efficiency. A proper bike fit looks at the balance between these many variables and a professional bike fitter is your good shepherd to help you find that balance.
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