Summary
Axle-to-crown is a measurement used in bicycle design to indicate the vertical distance from the axle of a fork to the top of its crown. It plays a key role in determining a bike’s front-end geometry, including head tube angle, bottom bracket height, and handling feel — especially when switching between suspension fork sizes or rigid forks.
Key Facts
- Introduced: Standardized in MTB geometry charts by the early 2000s
- Category: Concepts
- Also known as: A-to-C, ATC
- Used by / Found on: All suspension and rigid fork specifications
- Common range: ~370 mm (rigid gravel forks) to 580+ mm (long-travel MTB forks)
- Impacts: Head tube angle, stack, bottom bracket height, fork rake trail
- Unit of measurement: Millimeters (mm)
- Official website: Not applicable
Overview
Axle-to-crown is a foundational geometry measurement in bicycle design, particularly in mountain biking and gravel disciplines where fork travel and front-end height significantly affect fit and handling. Defined as the straight-line distance between the center of the front axle and the top of the fork crown (where it meets the frame’s head tube), this number helps determine how a fork interacts with a given frame’s geometry.
For rigid forks, the axle-to-crown dimension essentially sets the head angle, stack height, and trail. For suspension forks, the measurement is based on the axle-to-crown at top-out — i.e., when the fork is fully extended without compression. Designers use this measurement as a key reference point when designing frames or selecting replacement forks, because it directly impacts how a bike rides.
A longer axle-to-crown measurement raises the front end, slackens the head angle, and increases stack height, which generally improves descending stability but may reduce climbing efficiency or steering responsiveness. A shorter measurement has the opposite effect, sharpening the handling and lowering the rider’s center of gravity.
Whether you’re upgrading forks, swapping from rigid to suspension, or analyzing geometry charts, axle-to-crown is the critical baseline for predicting how geometry will change.
Why It Matters in Geometry
The axle-to-crown number serves as a vertical lever that can subtly or significantly reshape a bike’s ride characteristics. Because it controls the fork’s contribution to frame angle and height, even small changes — 10–20 mm — can impact multiple geometry variables.
Here’s how it connects to core geometry metrics:
Head Tube Angle
The most immediate effect of changing axle-to-crown is a change in head tube angle (HTA). Increasing axle-to-crown slackens the head angle — often by ~0.5° for every 20 mm added. This shifts handling toward downhill stability, reducing twitchiness but also slowing low-speed responsiveness.
Bottom Bracket Height
Raising axle-to-crown raises the front of the bike, which in turn slightly raises the bottom bracket (BB). This can reduce pedal strikes but may also increase the feeling of “sitting on top” of the bike rather than in it.
Stack Height
Since stack is measured as vertical distance from the bottom bracket to the top of the head tube, a taller fork (with longer A-to-C) increases effective stack. This can improve comfort and fit for riders seeking a more upright position — but may also reduce front-end pressure, especially for aggressive riding.
Trail
Fork trail is a product of head angle and fork offset. A longer axle-to-crown length increases fork trail indirectly by slackening the HTA — contributing to more stable, centered steering but slightly reducing agility.
Suspension-Corrected Forks
Many rigid forks are built with axle-to-crown lengths that “mimic” the top-out height of a suspension fork they’re meant to replace. These are called suspension-corrected forks, and they allow a rider to convert to rigid without dramatically altering the intended geometry of the bike.
This is common on:
- Touring and bikepacking rigs
- Steel hardtails converted for cargo or utility use
- Gravel bikes using rigid carbon forks built to MTB geometry
Changes Over Time & Suspension Travel
Axle-to-crown measurements have evolved alongside mountain biking itself. As fork travel increased, so too did the length of forks at full extension. Frame geometries adapted to these changes, and axle-to-crown became one of the key design constraints.
Here’s a rough guide to axle-to-crown by travel category:
| Fork Type | Travel | Axle-to-Crown (approx) |
|---|---|---|
| Gravel Rigid | 0 mm | 370–420 mm |
| XC Suspension | 100–120 mm | 480–500 mm |
| Trail / All-Mountain | 130–150 mm | 520–550 mm |
| Enduro | 160–180 mm | 550–580 mm |
| Downhill | 200 mm | 580–600+ mm |
Manufacturers often adjust crown designs, stanchion lengths, and lower casting dimensions to fine-tune these numbers — especially when trying to maintain consistent geometry across travel levels.
It’s important to note that axle-to-crown is not always proportional to travel. Two 150 mm forks may have slightly different axle-to-crown measurements depending on design priorities (e.g., offset, internal air chamber layout, axle standard, or chassis stiffness goals).
Notable Use Cases / Comparisons
- Rigid to Suspension Conversions: A rider converting a gravel bike to a suspension fork may see their head angle slacken by 1–2 degrees and their BB height increase by up to 15 mm. Choosing a fork with a matching axle-to-crown helps maintain balance.
- Fork Travel Upgrades: Going from a 140 mm to a 160 mm fork typically raises the A-to-C by 10–20 mm, altering handling in a way some riders love (more stability), and others find disorienting.
- Bikepacking Fork Swaps: Steel forks with bottle mounts often maintain the axle-to-crown of the bike’s stock fork to avoid compromising frame handling with a heavily loaded front end.
- Dual-Crown vs. Single-Crown Forks: Dual-crown downhill forks often have longer axle-to-crown numbers not just because of travel, but due to chassis overlap between crowns, impacting frame compatibility and fit.
Related Terms
- Head Tube Angle
- Stack
- Suspension Travel
- Fork Offset
- Suspension-Corrected
References
- RockShox Fork Specifications Database
- Fox Racing Shox Technical Setup Guides
- Surly Bikes: Geometry Charts and Fork Compatibility Docs
- Cane Creek Fork Fitment Support
- Geometry Geeks Fork Measurement Archive
- Pinkbike: “Does Travel Ruin Your Bike’s Geometry?” Feature