Chain growth in mountain biking refers to a phenomenon on full suspension bikes where the distance between the rear axle and the bottom bracket's center lengthens as the rear suspension compresses. This lengthening effectively increases the chainstay length (or rear center) and can impact the bike's drivetrain performance and rider feel.
Understanding the Mechanics of Chain Growth
On a full suspension mountain bike, the rear wheel is not rigidly fixed but moves through an arc as the suspension absorbs bumps and impacts. Due to the design of the suspension linkage and pivot points, this movement can cause the rear axle to pull away from the bottom bracket. When this happens, the chain needs to accommodate the increased distance, leading to what is known as chain growth.
Imagine your bike's chainstay length as a flexible ruler. As the suspension cycles, this "ruler" momentarily stretches. The chain, which connects the front chainring to the rear cassette, must be long enough to account for this maximum extension.
Why Does Chain Growth Occur?
Chain growth is primarily a consequence of the suspension design. Different linkage designs on full suspension bikes influence how the rear wheel travels:
- Pivot Location: The position of the main pivot relative to the bottom bracket and rear axle dictates the arc of the rear wheel. Designs that cause the rear axle to move further away from the bottom bracket during compression will exhibit more chain growth.
- Wheel Path: The specific path the rear wheel takes through its travel (e.g., more vertical, more horizontal, or a combination) is crucial. A more rearward wheel path during compression often correlates with increased chain growth.
Impact on Riding Experience
Chain growth isn't just a technical detail; it has tangible effects on how a bike feels and performs:
- Pedal Kickback: This is the most common and noticeable effect. As the suspension compresses, the chain pulls on the chainring, causing the cranks to rotate backward against the rider's pedaling motion. While often subtle, severe pedal kickback can be jarring and lead to a feeling of resistance, especially on square-edged bumps or during hard landings while pedaling. Learn more about Pedal Kickback in MTB (external resource for general knowledge).
- Drivetrain Strain: The constant stretching and releasing action puts additional stress on the chain, rear derailleur, and other drivetrain components, potentially increasing wear over time.
- Shifting Performance: In extreme cases, rapid suspension compression combined with significant chain growth could momentarily affect shifting precision, though modern drivetrains and well-designed suspensions generally mitigate this.
Suspension Designs and Chain Growth
Different suspension designs handle chain growth in varying ways:
| Suspension Design Type | Typical Chain Growth Characteristics | Common Examples |
|---|---|---|
| Single Pivot | Can have moderate to high chain growth, depending on pivot placement. | Santa Cruz Bullit (older), Orange Five |
| Linkage-Driven Single Pivot (LTSP) | Varies greatly, often optimized to reduce chain growth and pedal kickback. | Specialized FSR, Trek ABP |
| Multi-Link (e.g., Horst Link, VPP) | Generally designed to manage or minimize chain growth and pedal kickback. | Yeti Switch Infinity, Santa Cruz VPP, DW-Link |
| High-Pivot | Often designed with inherent rearward wheel path, leading to significant chain growth, but usually mitigated by an idler pulley. | Commencal Supreme, Norco Range |
Mitigating Chain Growth
Mountain bike manufacturers employ several strategies to manage or counteract the effects of chain growth:
- Suspension Linkage Optimization: Engineers meticulously design pivot locations and linkage ratios to create a desired wheel path that balances chain growth with other performance characteristics like anti-squat and braking performance.
- Idler Pulleys: High-pivot suspension designs, known for their rearward axle path (which typically results in high chain growth), frequently incorporate an idler pulley positioned near the main pivot. This pulley reroutes the chain, effectively isolating the chain's upper run from the suspension movement, thereby eliminating pedal kickback and its associated chain growth issues.
- Chain Length: Proper chain length is crucial. A chain that is too short will bind or put excessive stress on the drivetrain at full suspension compression, while one that is too long can cause shifting issues and chain slap. Most manufacturers recommend setting chain length at full compression.
Understanding chain growth helps riders appreciate the complexities of full suspension design and how these intricate systems work together to deliver a smooth and efficient ride on diverse terrain.
