Comparing bike geometry can be confusing, especially when you’re trying to understand how different bikes will fit and feel. While terms like “effective top tube” are often used, they can be misleading. To truly compare bike geometries and predict riding position, focusing on stack and reach provides a more accurate and reliable approach. This article delves into why stack and reach are superior metrics for bike geometry comparison, particularly when considering handlebar position relative to your body.
The core advantage of using stack and reach is their direct relationship to your contact points. Stack and reach precisely define the vertical and horizontal distance of the handlebars from the bottom bracket. This is crucial because the bottom bracket’s position is fundamentally linked to your pedaling and overall body position on the bike. By knowing the stack and reach, you immediately understand where your hands will be in relation to your legs and torso.
From there, achieving the correct saddle position relative to the bottom bracket becomes the next critical step. This is primarily determined by the seat tube angle. A steeper seat tube angle positions the saddle further forward relative to the bottom bracket. While seat tube angle is a key factor, seatpost setback also plays a significant role, especially in bikes with aerodynamic seatposts where adjustments might be limited. Remember to consider any headset spacers used on the bike, as these will affect the final stack height and handlebar position.
Using effective top tube as a primary comparison metric falls short because it doesn’t directly account for the seat tube angle. Effective top tube is measured horizontally from the head tube to the intersection of the seat tube and a virtual horizontal line extending from the top of the head tube. This measurement is influenced by the seat tube angle, meaning bikes with different seat tube angles can have similar effective top tube lengths but drastically different handlebar positions relative to the rider.
Consider the Look 795 (size M) and the Giant Propel (size M) as a practical example. The Look 795 has a shorter effective top tube (526.2mm) compared to the Giant Propel (550mm). However, when you examine the stack and reach figures, the handlebar positions are remarkably similar (Look 795: 386mm stack/552mm reach vs. Giant Propel: 383mm stack/545mm reach). This discrepancy arises because the Look 795 employs a very steep seat tube angle combined with a seatpost with significant setback. In contrast, the Giant Propel has a slacker seat tube angle and a straighter seatpost. Despite the effective top tube suggesting a significant size difference, the actual riding position, defined by stack and reach, is quite comparable.
If you were to rely solely on effective top tube for sizing, you might mistakenly conclude these bikes are vastly different sizes. However, by focusing on stack and reach, you gain a much clearer and more accurate understanding of how a bike will fit and how your body will be positioned on it. Therefore, when comparing bike geometries, prioritize stack and reach for a more meaningful and practical assessment of bike fit.
