Size

Historically, there were three common ways of measuring the size of a bike:

Length of seat tube measured from the bottom bracket to the top of the seat tube.

Length of seat tube measured from the bottom bracket to the junction of the seat tube and the centerline of the top tube.

Length of seat tube measured from the bottom bracket to the junction of the seat tube and the top of the top tube.

More recently, compact frame geometry (shortening the seat tube while keeping the same functional geometry of a bike in terms of fit and handling) has made it difficult to compare the size of bikes with differing designs. For example, a compact-geometry bike with a 52cm seat tube could fit the same as a classic-geometry bike with a 60cm seat tube.

Classic vs Compact Geometry — In the example above, the orange bike with classic geometry is effectively the same size as the blue bike with compact geometry despite the different lengths of seat tubes. The difference will be made up for with a longer seat post on the blue bike.

In response to this inconsistency, some brands report the “effective” seat tube length as if the seat tube extended up to the point where it would intersect an imaginary level top tube.

Increasingly, the trend is for brands to move to a descriptive, nominal sizing scheme such as “Small, Medium, Large, etc…”. However, this approach is also not standardized, so one brand’s Large may fit like another brand’s Medium and so on.

The state of bike sizing today is quite fragmented and can be confusing at best and misleading at worst. A brand’s Size description is necessary for us to refer to a specific bike geometry but it’s not a good solution for accurately conveying and comparing the sizes of bikes.

Feeling overwhelmed by sizing? The Bike Insights Compare view is a great way to cut through the confusion and instantly see how the size of bikes differ.

Seat Tube Length

What is Size?

Why does it matter?

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