The Evolution of the Fox 40: Axle, Dropout Design, and Wheel Standards Through the Years
The Fox 40 has long been a benchmark in downhill mountain biking. Although the name has remained unchanged, the fork itself has gone through several distinct generations. Over time, not only did the suspension internals evolve, but also how the front axle is mounted, how the dropouts function, and which wheel sizes are supported.
Understanding these generational differences is essential when working with wheels, axles, or axle-related components.
1. Early Fox 40 Generations (approx. 2002–2015)
The early generations of the Fox 40 were designed around the standards of their time, focusing on strength, simplicity, and reliability.
Wheel sizes
- 26” only
Axle mounting concept
- Traditional 20 mm x 110mm thru-axle
- The axle passes through the hub and threads directly into one fork leg
- The opposite dropout acts as a passive support
- No pinch bolts or secondary clamping system
Key characteristics
- Wheel alignment is largely determined by how the axle pulls the fork legs together during tightening
- Simple and robust, but sensitive to alignment tolerances
- Tightening the axle can introduce side-loads into the fork if the hub or dropouts are not perfectly aligned
These forks predate modern alignment-focused axle concepts. While extremely capable for their era, their dropout and axle architecture reflects the design priorities of early downhill racing.
2. Platform Redesign and Transitional Era (2016–2020)
In 2016, FOX introduced a major redesign of the Fox 40 platform, marking a clear break from earlier generations. This update coincided with changes in wheel standards and a shift toward improved chassis alignment.
Wheel sizes
- 27.5” (650B)
Axle mounting concept
- Still a 20 mm x 110mm thru-axle, but with a redesigned interface
- Introduction of pinch bolts on one dropout
- The axle is no longer the sole element responsible for alignment
What pinch bolts do
Pinch bolts allow the dropout to clamp around the axle, reducing reliance on axle tension alone. This distributes loads more evenly and improves stiffness under braking and cornering forces.
Why this mattered
- Reduced binding caused by forced alignment
- More consistent fork behavior after wheel removal
- Improved compatibility with stiffer frames and hubs
This generation represents the transition from traditional thru-axle concepts to the modern, alignment-controlled systems used today.
3. Modern Fox 40 Generation (2021–Present)
From model year 2021 onward, the Fox 40 entered its current generation, fully aligned with modern downhill standards and race demands.
Wheel sizes
- 27.5”
- 29”
Axle mounting concept
- Floating 20x110mm axle system combined with pinch-bolt retention
- One side of the axle interface is designed to float during installation
- After installing the axle, the fork is compressed to allow the floating side to self-align before the pinch bolts are tightened
What a floating axle does
A floating axle allows the fork legs to settle into their natural, low-friction alignment instead of being forced into position by the axle. This reduces stiction, improves small-bump sensitivity, and maintains smoother suspension performance under heavy braking loads.
Key characteristics
- Dropouts are specifically shaped for the floating and clamping interface
- Correct installation sequence is critical
- Axle, dropouts, and lower legs function as a single integrated system
This design prioritizes performance, consistency, and durability at modern downhill race speeds—but it also means compatibility with earlier Fox 40 generations is intentionally limited.
Why These Generational Differences Matter
Although all of these forks carry the Fox 40 name, they are not interchangeable across generations. Changes in wheel size, axle interface, and dropout geometry reflect evolving performance requirements and design philosophies.
An axle or adapter designed for the latest Fox 40 generation cannot simply be fitted to an older fork, as earlier dropouts were never designed to accommodate floating or clamping axle systems.
Closing Note
Today, our development work focuses exclusively on the latest-generation Fox 40, as it represents the current standard in downhill fork design, axle integration, and wheel compatibility.
For riders using older Fox 40 generations, adapter solutions may still be possible. These can be supplied on request, with designs tailored specifically to the axle and dropout architecture of the relevant generation.