Axle Path

Summary

Axle path refers to the specific trajectory a bicycle’s rear axle follows as the suspension compresses. It plays a fundamental role in how a full-suspension bike responds to terrain, pedals under load, and behaves while braking. Different axle path designs affect traction, efficiency, and overall ride feel across disciplines from XC to downhill.

Key Facts

  • Category: Concept
  • Also known as: Rear wheel path, axle trajectory
  • Defined by: Suspension layout and pivot configuration
  • Determines: Rear wheel movement under compression
  • Impacts: Pedaling efficiency, bump absorption, braking behavior
  • Used in: All full-suspension mountain bikes
  • Related to: Anti-squat, anti-rise, leverage rate, instant center

Overview

Every full-suspension bike has an axle path — the arc or curve the rear wheel follows as the suspension compresses. While it might seem like the wheel simply moves up and down, in reality, the motion is more complex, especially with multi-link designs. The direction and shape of this path directly influence how the bike rides.

In early mountain bikes with single-pivot suspension, axle paths tended to be simple arcs centered around the main pivot. These created predictable but sometimes compromise-laden ride characteristics. As suspension systems evolved, designers began to manipulate axle path more deliberately, using virtual pivot points and multi-link systems to control how the wheel moves through travel.

Today, axle path is a core element of suspension kinematics. It’s one of the first things suspension engineers look at when defining a new frame, especially for enduro and downhill bikes where traction and impact behavior are critical. By tuning the axle path — often in combination with leverage curves and anti-squat targets — brands can create bikes that pedal crisply but remain active over rough terrain.

How It Works

The rear axle’s path is determined by the location and motion of the rear triangle as the suspension compresses. In a single-pivot design, the path is a clean arc around the pivot. In multi-link designs, however, the axle path can shift from rearward to vertical to slightly forward as the bike goes through its travel.

Key Factors Influencing Axle Path:

  1. Main Pivot Location: The pivot’s height and distance from the bottom bracket set the starting point of the path. Higher pivots allow for more rearward motion.
  2. Linkage Configuration: Designs like Horst Link, VPP, DW-Link, and others change how the rear triangle rotates and translates, modifying the curve.
  3. Instant Center Movement: In systems with a moving instant center, the effective pivot location shifts throughout the travel, altering the path in real time.
  4. Suspension Travel: More travel typically allows more axle path variation, especially rearward or vertical movement.
  5. Frame Architecture Constraints: Designers must balance ideal axle paths with packaging limits (chainstay length, tire clearance, drivetrain interference).

Common Measurement

Axle paths are visualized as a 2D curve on an X-Y axis. Vertical movement represents compression; horizontal movement shows how much the axle moves fore or aft. These curves are generated with suspension kinematics software and are standard in engineering and technical marketing presentations.

Axle Path Types & Effects

1. Single-Pivot Arc

  • Description: Rear wheel follows a fixed-radius arc around a single pivot.
  • Effect: Predictable but less optimized for chain growth, pedal kickback, and terrain compliance.
  • Used in: Simpler trail bikes, entry-level full-suspension platforms.
  • Drawback: Less tunability. May lead to harsher bump absorption under pedaling or braking.

2. Rearward Axle Path

  • Description: As suspension compresses, the axle moves backward before transitioning vertically or slightly forward.
  • Effect: Improves bump absorption, especially at high speeds, by allowing the wheel to “get out of the way” of impacts.
  • Used in: High-pivot designs, downhill bikes, and some enduro rigs (e.g., Commencal Supreme, Forbidden Druid).
  • Trade-offs: Increases chain growth and requires idler pulleys to manage pedal kickback.

3. Slightly Rearward → Vertical Path

  • Description: A gentle rearward curve in early travel, transitioning to vertical mid-stroke.
  • Effect: Balances small bump compliance with manageable drivetrain dynamics.
  • Used in: DW-Link, VPP, Maestro systems.
  • Advantage: Offers a good compromise between pedaling efficiency and active suspension.

4. Vertical or Slightly Forward Path

  • Description: The axle moves straight up or slightly forward through compression.
  • Effect: Minimizes chain growth and pedal kickback, but reduces bump compliance.
  • Used in: Some XC bikes prioritizing efficiency.
  • Drawback: May feel harsher in rough terrain due to the wheel moving into the bump.

Notable Implementations

  • Forbidden Druid (High Pivot): Axle moves significantly rearward, improving square-edge bump performance.
  • Santa Cruz V10 (VPP): Tightly controlled axle path tuned for downhill compliance and acceleration.
  • Pivot Firebird (DW-Link): Rearward then vertical path for efficient climbing and plush descending.
  • Specialized Enduro: Uses an advanced FSR layout to optimize axle path and braking performance.
  • GT Fury (Idler-equipped): Rearward path with idler pulley to reduce chain growth and improve suspension independence.

Related Terms

References

  • Linkage Design Kinematics Software Documentation
  • Pinkbike Tech: “Understanding Axle Paths”
  • BikeRadar Suspension Deep Dive Series
  • Engineering analyses from Commencal, Pivot, Santa Cruz
  • Forbidden Bikes Tech Papers on High Pivot Layouts
  • Fox and RockShox Suspension Setup Guides
  • Technical interviews with suspension engineers (Bike Magazine archives)
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