Top Tube Length

What is Top Tube Length?

The length of the top tube is commonly measured three ways:

Actual (Center-Center): Measured along the length of the top tube from the center of the seat tube to the center of the head tube. This method of measurement has fallen out of favor in recent years and has largely been replaced by the Effective top tube measurement. It can still be a useful input for more precise bike diagrams on Bike Insights.
Effective (HT Top): The length of an “imaginary” level tube extending from the center of the seat tube axis to the top of the head tube.
Effective (TT Center): The length of an “imaginary” level tube extending from the center of the seat tube axis to the point where the top tube intersects the center of the head tube.

For modern bikes, effective top tube measurements are more commonly used since they offer a more consistent way of measuring the perceived horizontal length of the top tube on bikes having compact frame geometry.

Top Tube Lengths

In the example above, two bikes have top tubes with varying degrees of slope (compact frame geometry). This results in two different center-center top tube measurements, 543mm and 536mm. However, the effective top tube length remains constant at 561mm. Since both bikes will place the rider in the same position (given the same overall saddle height), effective top tube length is the more consistent way to compare the two.

Effective Top Tube can hide underlying design issues

As a bike design is scaled down to fit smaller riders, Toe Overlap (the tendency for the rider's toes to catch on the trailing edge of the front wheel when wide turns are taken) can become a problem. One approach bike designers take to mitigate this is to scale down a bike by shortening the Top Tube Length without shortening the Front Center proportionally (or at all in some cases). This can mitigate Toe Overlap with the downside of placing the rider too far forward on the bike relative to the bottom bracket (or require that the rider adopt a more setback seatpost which negates the move towards a smaller size).

Seat Tube Angle

In the example above, the orange bike appears smaller since the seat tube and top tube have been shortened. Indeed, the effective top tube measurement is shorter on the orange bike and the default saddle position will be closer to the handlebars. In reality, however, the fore-aft positioning of the saddle should be determined relative to the bottom bracket and it’s likely that the orange bike would require a seatpost with significant rearward offset in order to fit properly. At this point the effect of the shortened Effective Top Tube would be negated and both bikes would fit and handle exactly the same. Unfortunately, these types of scenarios are hard to identify when doing a test ride of a bike and they can easily result in a smaller rider ending up with a poorly-fitting bike.

One quick way to check for this design compromise is to see if there is a sharp jump in seat tube angle on the smaller sizes of a bike model. If so, beware that the fit may likewise be compromised. This does not always mean that the design is “bad” but caution should be exercised to make sure that a proper fit can be achieved. Stack and Reach are the preferred measure of a bike’s true size in this case.

Why does it matter?

Bike Insights will use whichever top tube length measurements are provided to draw the best possible diagram for each bike geometry. However, it’s important that we know which method the brand uses for their measurements.

In cases where it is not made explicit in a brand’s geometry table, sometimes this can be determined from a diagram accompanying the table. Note that bike geometry tables frequently fail to distinguish between Effective (HT Top) and Effective (TT Center), so this typically must be verified using a diagram. In the case of a bike with parallel head tube and seat tube angles, these values will be the same. But if the head tube and seat tube angles differ, the two effective top tube lengths will measure slightly differently due to converging or diverging angles.

Top tube length types

Top Tube Length, Effective/Horizontal Unknown

Top Tube Length, Effective/Horizontal Unknown
The length of an “imaginary” level tube to unknown points.

Top Tube Length, Effective/Horizontal (HT Top)

Top Tube Length, Effective/Horizontal (HT Top)
The length of an “imaginary” level tube extending from the centerpoint of the top of the head tube and the centerpoint of the seat tube axis.

Top Tube Length, Effective/Horizontal (Center)

Top Tube Length, Effective/Horizontal (Center)
The length of an “imaginary” level tube extending from the centerpoint of the head tube where it joins the top tube and the centerpoint of the seat tube axis.

Top Tube Length, Actual C-C

Top Tube Length, Actual C-C
Measured along the top tube from the center of the seat tube to the center of the head tube.

Top Tube Length, Unknown

Top Tube Length, Unknown
Unknown measurement of length of the the top tube.

Related Pages