Bike Size vs Frame Geometry: How to Choose the Right Bike

Choosing the right bike is the most important step in finding a comfortable and efficient riding position. Many riders and bike shops only focus on frame size when buying/selling a bike, but size alone doesn't tell the full story. 

In reality, frame geometry has a much bigger impact on how a bike fits. Two bikes both labelled as the same size can fit completely different depending on their geometry.

In this article I'll explain how bike sizing works, why geometry matters, and how a professional bike fit can help you choose the right bike for you.

Why Bike Size Matters

Riding the wrong size bike can lead to discomfort, poor power transfer and reduced control.

A bike that is too large may leave you stretched out and struggling to control the bike. A bike that is too small can feel cramped and limit your ability to produce power efficiently.

The correct bike size helps to create a good starting point for:

• Comfort on longer rides
• Efficient power transfer 
• Better bike handling 
• Reduced risk of injury
• Improved pedalling dynamics

However, frame size alone doesn't determine how a bike will fit and ride.

Why Frame Geometry Matters More Than Size

Two road bikes labelled "56cm" can fit and feel completely different to ride. This is because bike brands design frames with different geometries for different types of rider and disciplines.

Frame geometry refers to the angles and lengths of the tubes that make up a bicycle frame. These measurements effect the bikes riding characteristics as well as how it will fit.

Some key geometry measurements are:

Stack - the vertical distance from the centre of the bottom bracket to the top of the head tube

Reach - the horizontal distance from the centre of the bottom bracket to the centre and top of the head tube 

Seat tube angle - the angle of a bicycle's seat tube relative to the horizontal plane

There are other key measurements that will affect how a bike will fit and the riding characteristics of the bike but these three can be a good way to get an idea of the type of bike you are looking at. 

For example a road race bike will generally have a lower stack, longer reach and steeper seat tube angle encouraging a more aerodynamic riding position. Whereas an endurance road bike will typically have a higher stack, shorter reach and slacker seat tube angle, creating a more upright position. 

Historically most 56cm road bikes regardless of brand used very similar geometries and had a top tube that measured 56cm making it very easy to distinguish between bikes. Move to modern road bikes which generally use compact frames with sloping top tubes and a 56cm frame won't actually have a top tube that measures 56cm. Manufacturers now give an effective top tube measurement which is an imaginary horizontal line and they all have their own geometry and this will differ between models.

Why Online Size Charts Are Only a Starting Point

Most bike brands will provide a sizing chart based on rider height and potentially their inseam. Whilst these can be helpful, they don't account for individual body proportions or your level of mobility.

Two riders of the same height may have very different anthropometry (body proportions). These factors will heavily influence the ideal riding position and therefore which bike and size is correct. 

Can a Bike Fit Fix the Wrong Bike Size?

A professional bike fit can make significant improvements to a rider's position by adjusting a rider's saddle height, setback, stem length, handlebar width etc. However there are limits. If a frame's geometry is fundamentally wrong for a rider it can be difficult to achieve an ideal position. 

This is why many riders benefit from getting a bike fit before purchasing a new bike, to make sure they bike the right bike for them. With modern bikes it is becoming more and more important to get a fit first and buy later as bike often come with one piece handlebars resulting in little to no ability to adjust their fit without huge cost (often more than the cost of a bike fit).

Choosing the Right Bike for Your Position

When choosing a bike it's important to consider more than just frame size.

This is why many riders benefit from getting a bike fit before purchasing a new bike, to make sure they bike the right bike for them. With modern bikes it is becoming more and more important to get a fit first and buy later as bikes often come with one piece handlebars resulting in little to no ability to adjust their fit without huge cost (often more than the cost of a bike fit).

A pre purchase bike fit removes the guesswork when buying a new bike. Rather than choosing a bike based only on height charts and saddle height, the fit process looks at your anthropometry, flexibility and riding goals to determine the position that works best for you.

From this we can identify the correct bike and frame size that will that position to be achieved comfortably. A Pre-purchase bike fit will also help determine the best components for your bike, including:

• Crank Length
• Handlebar Width
• Stem Length
• Saddle Choice and Position
• Cleat Setup 
  

At Bucky Builds & Fits we can carry out pre purchase fits and would love to discuss building the best bike for you and help you get the most out of your riding, whether you're just getting started or you've been riding for years we're here to help you. 

 

 - Bucky Builds and Fits, Bike Fitting Cambridge -