XY Bike Calculator User Guide

Understanding the XY Position Calculator

The XY Position Calculator is a tool designed to help cyclists and fitters compare bike geometries based on handlebar and saddle positions. Using X (horizontal) and Y (vertical) coordinates, you can precisely determine how different bikes will position the rider.

What are X and Y coordinates?

In the XY Position Calculator, we use a coordinate system where:

  • The X-axis represents horizontal distance
  • The Y-axis represents vertical height
  • The origin (0,0) is at the bottom bracket (BB) of the bike
Diagram showing XY coordinate measurements on a bicycle

As shown in the diagram above:

  • Saddle X (SX) - The horizontal distance from the bottom bracket to the saddle rail center
  • Saddle Y (SY) - The vertical height from the bottom bracket to the saddle rail center
  • Handlebar X (HX) - The horizontal distance from the bottom bracket to the handlebar center
  • Handlebar Y (HY) - The vertical height from the bottom bracket to the handlebar center

Target Positions

Target positions are the known or desired X and Y coordinates for your saddle and handlebar positions. These values can come from:

Fit Bike Measurements

If you've had a professional bike fit on a fit bike (like a Retül, Guru, or similar), the fitter would have determined your optimal saddle and handlebar positions. These X and Y coordinates represent your ideal position.

Current Bike Measurements

If you're happy with your current bike position, you can measure the X and Y coordinates of your saddle and handlebar positions. This allows you to find a new bike that matches your current riding position.

By entering these target positions into the calculator, you can evaluate how closely different bike geometries will match your preferred position.

Understanding the Results

The calculator produces several important results that help you evaluate bike fit.

Position Differences

When the calculator displays the handlebar X and Y positions, it also shows the difference between the calculated position and your target position:

  • → 5mm longer means the handlebar X position is 5mm further forward than your target
  • ← 3mm shorter means the handlebar X position is 3mm closer than your target
  • ↑ 8mm higher means the handlebar Y position is 8mm higher than your target
  • ↓ 2mm lower means the handlebar Y position is 2mm lower than your target

These differences help you understand how closely the bike geometry matches your target position with the given stem configuration. The color coding (red and blue) provides a quick visual reference—the same colors are used in the calculator itself.

Bar Reach Needed

The "Bar Reach Needed" value accounts for small differences between the calculated Handlebar X position and your Target Handlebar X. It suggests how much handlebar reach you would need to achieve your exact target X position.

For example, if your calculated Handlebar X is 5mm shorter than your target, a handlebar with 5mm more reach would help you achieve your exact target position.

Setback vs STA

"Setback vs STA" indicates how far back (or forward) the saddle rail center would be positioned on a straight/zero setback seatpost:

  • Negative values (e.g., -24mm) indicate the saddle is set back behind the seat tube angle line
  • Positive values (e.g., +5mm) indicate the saddle is set forward ahead of the seat tube angle line

This measurement helps you determine what type of seatpost you'll need:

  • Large negative values may require a setback seatpost
  • Positive values or small negative values might work better with a zero-offset or forward seatpost

BB to Rail

The "BB to Rail" value is particularly useful for bikes with integrated seat masts or specific seatpost designs (like Trek or Giant ISMs). It measures the distance from the bottom bracket to the saddle rail along the seat tube axis.

This value helps determine:

  • Whether a position can fit within a size's minimum-maximum range for a specific seatmast system
  • The appropriate seatmast cap or seatpost length needed
  • How much adjustment range you have available

Practical Usage Tips

Here are some practical tips for getting the most out of the XY Position Calculator:

Comparing Multiple Bikes

The calculator allows you to add multiple bikes for side-by-side comparison. This makes it easy to see which bike geometry comes closest to your target position with the least modification.

Exploring Stem Options

You can adjust stem length, angle, and spacer height to see how these changes affect the handlebar position. This helps you find the optimal stem configuration for each bike geometry.

Note on Stem Height

"Stem Height" refers to the height of the stem clamp on the steerer tube. This measurement typically ranges from 38-42mm on traditional stems.

With the advent of internal cable routing systems, some modern stems (like the FSA NS ACR stem) have a very short clamp section at the steerer tube and non-traditional shapes. For these stems, the calculator's results may be off by a few millimeters since they don't follow conventional stem geometry.

If you're working with a bike that uses an integrated cockpit or non-traditional stem, be aware that small variations in the calculations might occur.

Note on Spacer Height

"Spacer Height" typically includes everything from the top of the frame at the headtube to the bottom of the stem, including the headset bearing cover/compression ring.

It's important to know that some brands incorporate the headset bearing cover into their published stack and reach numbers, while others don't:

  • Specialized Tarmac SL8: Has the transition spacer included in the geometry; only count actual spacers in your spacer height measurement.
  • Specialized Tarmac SL7: Does not include the transition spacer in geometry; count everything between the frame and stem.

When in doubt, check the manufacturer's geometry chart notes or contact them directly to confirm how spacer heights should be measured for a particular model.

Saving Your Data

Once you've created a setup that works, use the Save button to store your configuration for future reference. This is particularly helpful when testing multiple potential bike options.

Printing Results

The Print button generates a printable report of your bike comparisons, making it easy to share with clients or keep in your records for future reference.