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What Chainline Is Boost?

Published in Bicycle Drivetrain 2 mins read

Boost chainline refers to a specific range of distances for the chainring relative to the bike's centerline, typically between 51 mm and 53 mm. This measurement is crucial for optimal drivetrain performance and compatibility with the Boost hub standard.

Understanding Chainline

Chainline is a fundamental measurement in bicycle drivetrains, particularly important for single-chainring (1x) configurations. As defined by the Boost standard, the chain line is:

  • The distance between the centre line of the frame and the centre line of the chain ring.

An accurate chainline ensures efficient power transfer, reduces chain wear, and minimizes cross-chaining issues, leading to smoother shifting and a quieter drivetrain.

Boost Chainline Explained

The Boost standard is a newer specification for bicycle wheels (hubs) and transmissions. It was developed to increase wheel stiffness and allow for features like shorter chainstays or greater tire clearance. To accommodate the wider Boost hubs (typically 148mm rear spacing), the chainring's position on the crankset needs to be moved outwards to maintain an ideal chainline.

For a Boost setup, the ideal chainline value is 51-53 mm. This outward shift ensures that the chain maintains an optimal alignment with the wider hub and cassette, particularly when utilizing the larger cogs on the cassette.

Boost vs. Non-Boost Chainline

To highlight the difference, it's useful to compare the Boost chainline with the traditional, non-Boost standard:

Feature Non-Boost Chainline Boost Chainline
Ideal Value 48-50 mm 51-53 mm
Hub Spacing Typically 142mm rear Typically 148mm rear
Purpose Standard MTB/Road Wider, stiffer wheels, more tire clearance

This table clearly illustrates the difference in chainring offset required by the Boost standard compared to its predecessor.

Practical Considerations

Achieving the correct Boost chainline is essential for several reasons:

  • Drivetrain Efficiency: Proper alignment reduces friction and optimizes power transfer.
  • Component Lifespan: Minimizes wear on the chain, chainring, and cassette by reducing excessive cross-chaining.
  • Shifting Performance: Contributes to smoother and more reliable gear changes, especially in the extreme gears.
  • Frame Clearance: Ensures the chainring clears the chainstays and provides adequate tire clearance, which Boost frames are designed to offer.

Bike manufacturers achieve the Boost chainline by designing specific cranksets or chainrings with a different offset compared to non-Boost versions. When upgrading or replacing components, it's crucial to select parts compatible with your frame's Boost or non-Boost standard.