MTB Geometry Calculator
Analyze how fork travel and frame changes impact your mountain bike handling.
Horizontal distance from BB to head tube center.
Please enter a valid reach.
Vertical distance from BB to head tube center.
Angle of the steering axis.
Angle of the seat tube from horizontal.
Use positive for longer fork, negative for shorter.
64.0°
75.0°
442 mm
628 mm
+7.5 mm
Formula: Angle change is approximately 1° per 20mm of axle-to-crown change. Stack and Reach are adjusted using trigonometric offsets based on the head tube angle.
Geometry Shift Visualization
The blue line represents current geometry; the green line represents the calculated new geometry (exaggerated for visibility).
| Metric | Current Setup | New Setup | Difference |
|---|
Table 1: Comparison of current mountain bike geometry vs. modified setup using the mtb geometry calculator.
What is an MTB Geometry Calculator?
An mtb geometry calculator is a specialized tool used by mountain bikers, frame builders, and mechanics to predict how modifications to a bike will alter its handling characteristics. Mountain bike geometry is defined by several key measurements, including reach, stack, head tube angle, and bottom bracket height. Even a small change, such as increasing your fork travel by 10mm or installing offset bushings, can significantly shift how a bike climbs, descends, and corners.
Who should use an mtb geometry calculator? Primarily, riders looking to “overfork” their bikes (adding travel) or those considering a mullet setup (29″ front / 27.5″ rear wheel conversion). A common misconception is that adding 20mm of travel only makes the front end higher. In reality, it slackens the entire bike, reducing the reach and moving the center of mass rearward, which can affect front-wheel traction on steep climbs.
MTB Geometry Calculator Formula and Mathematical Explanation
The physics behind the mtb geometry calculator relies on trigonometry. When you lengthen the fork, the bike rotates around the rear axle. This rotation is what changes the angles and heights.
The core rule of thumb in the industry is that 20mm of axle-to-crown height change equals roughly a 1-degree change in head tube angle. However, our calculator uses more precise calculations based on the following variables:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Reach | Horizontal distance from BB to head tube | mm | 400 – 520mm |
| Stack | Vertical distance from BB to head tube | mm | 580 – 650mm |
| Head Angle | Angle of the fork relative to horizontal | Degrees (°) | 63° – 68° |
| Seat Angle | Angle of the seat tube relative to horizontal | Degrees (°) | 74° – 78° |
Mathematical derivation: To find the new reach, we calculate the arc of rotation. The reach decreases because as the front of the bike rises, the head tube moves backward towards the rider.
New Reach = Old Reach – (Fork Change * sin(Head Angle))
New Stack = Old Stack + (Fork Change * cos(Head Angle))
Practical Examples (Real-World Use Cases)
Example 1: The Trail-to-Enduro Conversion
A rider has a trail bike with a 140mm fork and a 66° head angle. They decide to install a 160mm fork (+20mm). Using the mtb geometry calculator, we see the head angle slackens to 65°, providing more stability on descents. However, the seat tube angle also slackens to 75°, which may make steep climbing slightly more difficult as the rider’s weight shifts over the rear axle.
Example 2: Adding Offset Bushings
Offset bushings effectively shorten the rear shock eye-to-eye length, usually by about 5-6mm. This can slacken the head angle by roughly 0.5°. While small, this lowers the BB height significantly, improving cornering stability but increasing the risk of pedal strikes on technical terrain.
How to Use This MTB Geometry Calculator
- Find Your Current Specs: Visit your bike manufacturer’s website to find your current reach, stack, and angles.
- Enter Base Data: Input these values into the first four fields of the mtb geometry calculator.
- Input Your Modification: If you are adding a longer fork, enter the difference in travel (e.g., 20). If shortening, use a negative number (e.g., -10).
- Review the Shift: Observe the “New Setup” column. Pay close attention to the Reach and Seat Tube Angle.
- Evaluate Handling: A slacker head angle (lower number) is better for descending; a steeper seat angle (higher number) is better for climbing.
Key Factors That Affect MTB Geometry Results
- Axle-to-Crown Length: Not all forks of the same travel have the same length. A 160mm Fox 36 may have a different A-C than a 160mm RockShox Lyrik.
- Tire Volume: Switching from a 2.3″ to a 2.6″ tire increases the radius, raising the BB and affecting the trail.
- Sag Settings: Geometry is static. Once you sit on the bike, the mountain bike suspension setup changes these numbers dynamically.
- External Headset Cups: Using an “AngleSet” can change the head angle without affecting the seat tube angle as much as a fork change does.
- Wheel Size (Mullet): Swapping a 29″ rear wheel for a 27.5″ wheel drops the rear of the bike, slackening angles by about 1 degree.
- Frame Compression: Older frames may flex under load, though this is rarely factored into a standard mtb geometry calculator.
Related Tools and Internal Resources
- mountain bike suspension setup – Perfect your air pressure and damping after changing your geometry.
- mtb frame size guide – Find the right starting point for your height and riding style.
- dropper post length calculator – Ensure your seat height remains correct after a geometry shift.
- mountain bike tire pressure – Adjust your grip to match your new head tube angle.
- crank length calculator – Lowered your BB? You might need shorter cranks to avoid pedal strikes.
- mtb handlebar width guide – Compensate for reach changes with the perfect bar width.
Frequently Asked Questions (FAQ)
Many manufacturers allow a +/- 10mm change, but always check. A 20mm change often exceeds the designed stress limits of the head tube.
It makes the steering feel “heavier” and more stable at high speeds but can make the bike feel floppy or sluggish on slow, tight climbs.
Yes, reach is the standard for modern mtb geometry calculator usage because it describes the bike’s feel while standing on the pedals.
Trail is the horizontal distance between the tire’s contact point and where the steering axis hits the ground. More trail equals more stability.
While the trig works, road bikes have different handling priorities where small changes have larger impacts on high-speed aerodynamics.
It is a bike with a 29-inch front wheel and a 27.5-inch rear wheel, designed to combine front-end rollover with rear-end maneuverability.
A lower BB lowers your center of gravity for better cornering, but increases the frequency of pedal strikes on rocks and roots.
Because the bike rotates around the rear axle. As the front goes up, the head tube tilts back toward the seat, effectively shortening the horizontal reach.