Editors Note: This article was written by Art’s Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances and breakdown component design. The original post can be found here.
Previously, we discussed the relationships between wheelbase, headtube angle, and BB height, and talked about how they affect handling and compared some different purpose-built rigs. Now it’s time to look at stack and reach measurements, and illustrate the best ways to tinker with geometry on your current bike.
An added element of complexity with mountain geometry comes from the complex frame designs. Top tube lengths can be difficult to measure due to sloping tubes, creating the need for effective top tube (ETT) measurements in geometry specifications. Effective top tube measures as the horizontal distance from the intersection of the seat tube and top tube, to the intersection of the head tube and top tube.
While this solved part of the problem with representing geometries, frames can still vary greatly without changing effective top tube. For example, two bikes with the same reach can have different effective top tube lengths. This is accomplished by decreasing seat tube angle, pushing the saddle back, increasing the distance to the handlebars, while the ETT remains the same.
Reach and stack measure the horizontal and vertical distances from the bottom bracket to the top of the head tube respectively. The purpose of these measurements is to establish a standard of comparison for fit across all brands. Similar reach and stack measurements between bikes will help to match the contact point engagement. These two measurements can be the same across multiple bikes, however, the wheelbase, steering characteristics, and numerous other factors can change underneath you. So while reach and stack will help you find a properly sized bicycle, you will still need to evaluate other geometries to arrive at the ride you’re looking for.