Like all the best questions in life, the ‘what size frame do I need?’ opens up not just a single can of worms, but almost a whole shelf.

Because of the scope and scale of the topic of bike geometry - and its relationship to the size that fits you as the rider, I've put together this article, as the first in a mini-series on the subject. Bike geometry - regarding the 'what size' question, is best understood with an understanding of its component parts. Geometry can be broken down into three top level areas. In this little series I’ll be expanding the constituent parts and go on to expand the relationships between them.

What is geometry?

To begin, it's probably prudent to explain the three top-line headings and offer a brief summary of how they break down individually.

  • Geometry for bike handling
  • Geometry for bike sizing
  • Geometry for bike fitting.

Geometry for bike handling: This is to do with how to design a frame to distribute the mass of the rider in such a way that it gives the desired handling characteristics.

Geometry for bike sizing: Creating a range of sizes of bike frames to fit a large cross-section of bicycle riders maintaining the ability of riders to make the most of the bikes capabilities

Geometry for Bike fitting: Fitting is the positioning of a rider on the bicycle - for comfort, performance or somewhere in between and is a goal-driven activity.

The circular geometrical relationship


All three areas of geometry overlap and are intimately intertwined in both cause and effect. Having an understanding of each individually, and the relationships between them will allow you to make a much more informed decision and know why the choice of frame size you make is the correct choice for you and the cycling you’d like to do.

Handling and Sizing

A bike designer will generally try to design a bike frame with a specific weight bias front to rear, in order that the detail and nuance of the geometry have the desired effect. Making sure there is room for a certain sized rider to fit on the bike and get their centre of gravity sitting at, for example, a 60/40 front to rear weight bias on a certain type of bike is taken care of by increasing the ‘length and height’ of each frame size. Ensuring that that size rider gets the same type of ride feel from the bike is taken care of by adjusting chainstays, the drop of the bottom bracket below the wheel axles, head tube lengths, fork rakes, and a multitude of other details.

Sizing and Fit

Sizing is all about with the scale of your body. Fitting is to do with perfecting what you’re trying to make your body do whilst cycling; supreme comfort, ultimate performance, or somewhere in between - these are accomplished through bike fitting. It's important to remember that some parts of the balance between sizing and fit are pretty much set in stone if you want the best experience. From this fixed point, working out where the handlebars should go can be destined by your goal. Low and stretched, more upright and closer, or anywhere in between. You can obviously make a smaller frame fit with a longer stem, or vice versa, but it will have a very definite effect on the handling - which leads us nicely to completing the circle.

Fit and Handling

As we first said the riders centre of gravity is what a frame designer plays around with when designing geometry for handling. The riders personal fit can have a defined effect on this. A longer stem on a smaller frame will shift the centre of gravity forward on the bike, affecting the handling of the bike. While this may not always be a bad thing for a given rider, it's important to understand it does have an effect, and will change the way the bike riders compared to it's intended designed characteristics. The obvious answer would be to change size frame - and thus the circular geometric relationship is complete.

There is no right or wrong answer to any of the points mentioned in the three stages. But it's always good to know how a choice in one area will have a knock-on effect somewhere else. When we discuss sizing with customers, we will always start from an intended ride characteristic of a frame - and make suggestions. From there we’ll advise along any given path explaining pro’s and cons (from a design intention perspective).

In the next installments of this frame sizing mini-series, we’ll expand each of these areas out in a bit more detail.

Until then, always remember - every day is a school day.



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