Seat Tube Length
What is Seat Tube Length?
Seat Tube Length is a measurement from the center of the bottom bracket up to one of several possible points along the seat tube axis:
Effective (HT Top): The “effective” or “virtual” length of the seat tube measured to where it intersects an imaginary horizontal line originating from the top of the head tube.
Effective (Center): The “effective” or “virtual” length of the seat tube measured to where it intersects an imaginary horizontal line originating from the point where the top tube intersects the center of the head tube.
Seat Tube Top (C-T): The “actual” length of the seat tube up to its very top. This is the most common seat tube measurement on modern bikes.
Top Tube Top (C-TTT): The length of the seat tube measured to the junction of the seat tube and the top of the top tube. This measurement is rarely used.
Top Tube Center (C-C): The length of the seat tube measured to the junction of the seat tube and the centerline of the top tube.
Why does it matter?
Historically, Seat Tube Length was used to determine the Labeled Size of a bike. In recent years, however, new bike designs and the advent of compact frame geometry have rendered this convention somewhat obsolete.
These days, seat tube length is still important in some cases:
To understand the lowest position of a saddle, especially important when sizing up to a larger frame or considering a bike with a classic, level top tube.
To determine the amount a saddle may be lowered when using a dropper seatpost.
Diagram Note
Bike Insights will use whichever Seat Tube Length measurements are provided to draw the best possible diagram for each bike geometry. For the best results, input all available Seat Tube Lengths from a brand’s geometry table when adding a new bike to Bike Insights.
Seat tube length types
Seat Tube Length, Unknown
If the brand Seat Tube Length is not specified, please use this option.
Seat Tube Length, C-T
The length of the seat tube from the center of the bottom bracket to the top of the seat tube. This is a less useful measurement than other seat tube measurements Center-TT Top or Center-Center because it does not precisely indicate the point where the top tube intersects the seat tube. It can be useful, however, in determining the length of seatpost needed to correctly position the saddle height for the rider.
Seat Tube Length, C-T of Top Tube (rare)
The length of the seat tube measured from the center of the bottom bracket to the junction of the seat tube and the top of the top tube.
Seat Tube Length, C-C of Top Tube
The length of the seat tube measured from the center of the bottom bracket to the junction of the seat tube and the centerline of the top tube.
Seat Tube Length, Effective (Unknown)
The “effective” length to an unknown point.
Seat Tube Length, Effective (Center - ETT HT Top)
The “effective” length as if the seat tube extended up to the point where it would intersect an
imaginary level top tube that is measured to the top of the head tube.
Seat Tube Length, Effective (Center - ETT Center)
The “effective” length as if the seat tube extended up to the point where it would intersect an
imaginary level top tube that is measured to the center of head tube at the top tube / head tube
intersection.