How do you compare one bicycle frame size to another? Geometry charts for bicycles can be quite confusing – there are so many numbers and measurements!

And not every brand shares every measurement. Or they may list them differently. For example, some brands may tell you Bottom Bracket Drop while others will tell you Bottom Bracket Height (they are different measurements). If you are trying to compare two different bikes to each other, this lack of information doesn’t help.

Fortunately, two numbers are truly consistent regardless of bicycle brand, make, or model: Frame Reach and Frame Stack (aka “Reach” and “Stack”).

  • Reach refers to the horizontal distance from the center of the bottom bracket (where the crankset is attached) to the center of the top of the head tube. Reach determines the bike’s cockpit length, affecting how stretched out or compact a rider’s position is on the bike. It combines with stem length to dictate how far forward the handlebars are.
  • Stack refers to the vertical distance from the center of the bottom bracket to the top of the head tube. It represents the height of the front end of the bike relative to the bottom bracket. Stack measurement is crucial for understanding how upright or aggressive the riding position will be. A taller stack generally indicates a more upright riding position, whereas a shorter stack typically means a more aggressive, aerodynamic position.
  • X/Y Coordinates: Essentially the center of the BB is your “X” coordinate, while the center of the top of the head tube is your “Y” coordinate.

Why do we use Reach & Stack to fit bikes?

Reach and Stack numbers became important when compact frame designs became mainstream. Prior to compact frames, bicycle frames had horizontal top tubes rather than the sloping top tubes we are accustomed to seeing today.

Up until the early 1990’s, bicycle frame sizes were typically determined based on a combination of the rider’s height and inseam measurement. However, the exact methods and standards for sizing varied between manufacturers and regions.

Sizing a rider to a stock, mass-produced bicycle mostly relied upon generalized concepts to get the rider in the “ballpark” of the correct size. Mostly, people used:

  • Rider Height: Manufacturers often provided size charts correlating a rider’s height to a specific frame size.
  • Rider Inseam: A rider’s inseam measurement was also used to determine the standover height, which is the clearance between the top tube of the frame and the rider’s groin when straddling the bike while standing.
  • Top Tube Length: While not always explicitly stated in sizing charts, the length of the top tube also played a role in determining fit. A longer top tube would result in a more stretched-out riding position, while a shorter top tube would result in a more upright position.

It’s important to note that bicycle sizing in the 1980’s, and early 90’s was not as standardized or precise as it is today. There was more variability between manufacturers and even between different models from the same manufacturer. Additionally, the focus was often more on general fit rather than the nuanced geometry considerations that are common in modern bike fitting practices.

Compact bicycle frame design, characterized by sloping top tubes and smaller frame triangles, became popular in the late 1990s and early 2000s. Compact frames offered several advantages over traditional frames with horizontal top tubes:

  1. Stiffness and Lightness: Compact frames could be made stiffer and lighter than traditional frames.
  2. Improved Standover Height: The sloping top tube of compact frames provided greater standover clearance, making it easier for riders to straddle the bike while stopped. This was particularly appealing for riders concerned about comfort and safety and made it easier for shorter riders to mount their bikes.
  3. Responsive Handling: The smaller frame triangles resulted in a stiffer overall structure, which contributed to more responsive handling characteristics, especially during aggressive riding and cornering.
  4. Aesthetic Appeal: Compact frames often had a sleek and modern appearance, which appealed to many cyclists seeking a more contemporary look.

One of the notable early adopters of compact frame design was the Italian bicycle manufacturer Bianchi, with their introduction of the Mega Pro XL frame in 1997. This frame, ridden by Marco Pantani in the Tour de France, helped popularize the compact design.

Effective Top Tube enters the room

As more brands launched compact frames, we saw a lot more variation in design and measurements. That’s when the concept of the “Effective Top Tube Length” became an important part of the frame size discussion. The effective top tube length (ETT) on a bicycle is an imaginary horizontal line measuring the distance between the center of the top of the head tube to the point where it intersects with the seat tube, measured parallel to the ground.

It’s important to note that the effective top tube length isn’t necessarily the same as the actual physical length of the top tube. Modern bike frames can have sloping top tubes or frames with unconventional shapes, and the effective top tube length provides a consistent reference point for comparing the fit and sizing of different bicycles.

How Reach, Stack & ETT work together for proper bike fit

So, how does ETT tie into Reach and Stack? As we have learned, compact frames have led to new concepts for frame design and how to measure those numbers. This has led to a wide variability between bike designs that has made it difficult for there to be a consistent measurement across the board using frame tubes alone.

The X/Y coordinates for Frame Reach and Stack can be measured on every single bicycle as those points are going to be consistent regardless of brand, make, model, or even material. Even comparing MTB’s and Drop bar bikes is a possibility due to the X/Y coordinates of the frames. Thus, Frame Reach and Frame Stack can allow a true “Apples to Apples” comparison between frame designs and sizes.

Adding ETT takes seat tube angle into consideration, because (all else being equal), a steeper seat angle will create a shorter ETT, and a slacker seat angle will create a longer ETT. But seat angle doesn’t affect Reach or Stack.

By looking at all three numbers, you’ll get a sense of how long a bicycle’s cockpit will be and easily compare one bike to another. When shopping for a new bike, compare these measurements to what you’re riding now, or to your fit data from a professional bike fit, to find the frames that will best fit you.

From there, you can use stem length, handlebar reach, seatpost setback, and saddle position to fine tune your riding position down to the millimeter!

Like this? Check out more from our Bike Design 101 series on our blog!