Use this MTB Geometry Solver to compare and understand how key frame metrics—Reach, Stack, and Head Tube Length—interrelate with the overall Frame Index. Input any three known values to automatically calculate the fourth missing dimension, helping you dial in your perfect bike size.
MTB Geometry Solver
MTB Geometry Solver Formula
This calculator uses a simplified, illustrative formula to demonstrate the relationship between key frame metrics (Reach, Stack, Head Tube Length) and a theoretical Total Frame Index (Q) for comparative sizing.
Q = P + V + F
Formula Source: Bike Radar: MTB Geometry Explained | Pinkbike: Geometry Defined
Variables Explained
- Q (Total Frame Index): The total comparative score of the bike’s front triangle, expressed in millimeters (mm).
- P (Reach): The horizontal distance from the center of the bottom bracket to the center of the top of the head tube (mm).
- V (Stack): The vertical distance from the center of the bottom bracket to the center of the top of the head tube (mm).
- F (Head Tube Length): The physical length of the head tube component (mm).
What is MTB Reach?
Reach is one of the most crucial measurements for modern mountain bike sizing. It defines the horizontal distance from the bottom bracket (BB) center to the top center of the head tube. In simple terms, Reach tells you how long the bike is when standing up and out of the saddle, primarily affecting the stance and handling of the bike. A longer Reach generally results in a more stable bike at speed and requires a shorter stem for a balanced cockpit.
Reach is almost always paired with Stack, which is the corresponding vertical measurement. Together, the Reach and Stack define a specific point in space relative to the bottom bracket, which is critical for finding the correct fit for riders of different heights and arm lengths.
How to Calculate a Missing Value (Example)
Suppose you know the desired Reach, Stack, and Head Tube Length and want to determine the target Total Frame Index (Q).
- Start with the Formula: $\text{Q} = \text{P} + \text{V} + \text{F}$.
- Identify Known Values: Let $P = 470 \text{ mm}$ (Reach), $V = 615 \text{ mm}$ (Stack), and $F = 110 \text{ mm}$ (Head Tube Length).
- Substitute and Solve: $Q = 470 + 615 + 110$.
- Final Result: $Q = 1195 \text{ mm}$. This is the Total Frame Index needed for comparison.
Related Calculators
- Effective Top Tube Calculator
- MTB Stack vs. Reach Comparison
- Suspension Sag Calculator
- Tire Pressure Converter
Frequently Asked Questions (FAQ)
What is a good Reach for an MTB?
This depends heavily on your height, riding style, and the bike’s intended use. Generally, modern trail and enduro bikes for a average-sized rider will have a Reach between 450mm and 500mm. Taller riders may require 500mm+.
Does Reach affect standover height?
No, Reach is a horizontal measurement and does not directly affect the standover height (the distance from the ground to the top tube). Standover is mostly determined by the top tube’s shape and angle.
How is Stack different from Reach?
Reach is horizontal distance (forward/backward stretch), while Stack is vertical distance (handlebar height). They are the two coordinates that pinpoint the front end of the bike relative to the BB.
What if all my variables are known?
If all four variables (Q, P, V, F) are entered, the calculator will check if $Q = P + V + F$. If they are mathematically consistent, it reports “Consistent.” If not, it reports the mathematical difference.