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Frame Size and Geometry – Get it right!


When it comes to buying a road bike,

The most crucial decision is to first choose the correct geometry and then the right size. This process can be overwhelming due to the multitude of options available. In this article, we will explore the key factors to consider when selecting the right geometry and size for your road bike.

Frame Size Guide: Can We Trust It?

Bike manufacturers often provide size guides to help you determine the appropriate frame size based on your height.

BMC sizing chart
Ribble sizing chart

While these guides can be used as a starting point, it’s important to note that they are not always accurate. Some brands, like Origine or Canyon, may ask for more information during the sizing process, but even then, the guidelines remain somewhat limited in their precision.

Origine questionnaire to size a bike

Frame Size: Can We Rely on It?

It’s worth acknowledging that different brands have their own sizing conventions, and frames of the same size can still vary in dimensions. For instance, a 54cm frame from one brand may not be the same size as a 54cm frame from another brand. Therefore, it’s crucial to approach these guidelines with caution and recognise their limitations.

Comparing a Merida and a Cervelo race bike both in size 54

The image above (from Bike Insights) shows two bikes given as a size 54 and, from the overlay, it is clear they are completely different bikes. 10mm difference on the stack and 15mm on the reach are an important difference which means you could end up with a bike that is way too big for you (or too small depending which one you had first).

Understanding Bike Geometry

When selecting a road bike, it is crucial to have a solid understanding of its geometry. Bike manufacturers often provide tables with a range of measurements, but what exactly do these numbers signify? Factors such as the head tube angle, seat tube angle, head tube length, and top tube length all play a significant role in determining your riding position and overall comfort. It’s important to recognise that different geometries may be more or less suitable for individuals based on their unique body proportions, flexibility, and fitness level considerations.

Endurance, Race, or Aero Bikes?

The choice between endurance, race, or aero bikes can be a challenging one. However, for most people, an endurance bike is often the best option. Endurance bikes are designed with a more relaxed geometry that suits a wide range of riders and riding styles. On the other hand, many aero bikes require the addition of numerous spacers to achieve a comfortable riding position, which can compromise the bike’s intended aerodynamic benefits and visual aspect. When selecting a bike, it is crucial to align its geometry with your intended usage, allowing you to prioritise both comfort and performance for an optimal riding experience.

Bike Insight comparing a TCR and a Defy in size M

Above, you can see a comparison of two Giant bikes. The TCR is part of the race category while the Defy is part of the endurance category. The Defy has a much more “relaxed” geometry with a shorter reach (-11mm) and higher stack (+22mm). The difference means that the Defy allows for a longer stem and less spacers in order to achieve the same measurements to the shifters.

Reach and Stack: Important but Not Everything

Reach and stack are two crucial measurements used to assess the sizing of a bike frame. Reach refers to the horizontal distance between the bottom bracket and the centre of the head tube, while stack represents the vertical distance between the bottom bracket and the top of the head tube. When considering reach and stack, it’s important to note that they provide a helpful starting point but are not the sole determining factors for a proper bike fit.

The reach and stack measurements to the shifters is a more critical measurement for achieving a proper fit (or at minimum, to the centre of the handlebar). Factors such as the head tube angle, stem angle and length, handlebar reach, and handlebar shape all affect the position of the shifters. They influence the extension or reduction of your reach, and the overall ergonomic setup of the bike.

Screenshot from Origine website when sizing a bike

By prioritizing the position of the shifters, you can ensure a comfortable and efficient fit on your road bike. Remember, achieving a well-tailored riding position contributes to enhanced performance and enjoyment during your cycling endeavours.

The Importance of Body Geometry

Above all, your body geometry is the most critical aspect to consider when choosing a road bike. Each person is unique, and their bike needs to be tailored accordingly. Two cyclists of the same height may require different-sized bikes or even bikes with different geometries. This is why it’s crucial to prioritise getting a professional bike fit before purchasing a bike. The mantra “fit first, buy later” (or Get A Bike Fit, then decide – no pun intended) holds true in ensuring that your bike is specifically suited to your body and riding style.

In conclusion,

choosing the correct frame size and geometry for a road bike is a vital step towards enjoying a comfortable and efficient riding experience. While manufacturer size guides can be helpful, they are not foolproof. Understanding bike geometry, considering your body proportions and flexibility, and getting a professional bike fit are all key elements in finding the perfect bike for you. Remember, every rider is unique, and a personalised fit will enhance your enjoyment and performance on the road. Happy riding!


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