Bottom bracket preload and chainline represent two of the most misunderstood yet critical aspects of drivetrain setup. When these are optimised, shifting improves, efficiency increases, and component longevity extends. When neglected, the opposite occurs—poor shifting, increased wear, and reduced performance. Understanding how to properly set preload and optimise chainline for different crank systems is essential for serious cyclists.
Preload refers to the tension applied to the crank spindle through the bottom bracket bearings. Proper preload eliminates axial play (side-to-side movement of the crank) while minimising bearing drag. Too little preload allows play, which causes creaking, poor shifting, and accelerated bearing wear. Too much preload creates excessive bearing drag, reducing efficiency and potentially causing premature bearing failure.
Chainline refers to the alignment of the chainrings relative to the cassette. Optimal chainline places the chainring(s) directly in line with the corresponding cassette cog(s), minimising chain angle and improving shifting and efficiency. Poor chainline forces the chain to run at an angle, increasing wear, reducing efficiency, and causing shifting problems.
Different crank systems have different requirements for preload and chainline:
Shimano Hollowtech II Cranks:
- Use the plastic preload cap on the non-drive side to set preload
- Tighten until slight resistance is felt, then back off slightly
- The left crank pinch bolts actually secure the crank, not the preload cap
- Chainline is typically fixed by the spindle length and bottom bracket shell width
- Standard road chainline is 43.5mm, mountain bike is 47.5-50mm
SRAM DUB Cranks:
- Similar preload adjustment to Shimano but with different cap design
- DUB spindles are narrower, requiring specific DUB bottom brackets
- Chainline varies by crank type—road, mountain, and 1x specific cranks have different chainlines
- SRAM provides specific chainline measurements for each crank model
Campagnolo Ultra-Torque Cranks:
- Unique system with Hirth joint (splined connection) between crank halves
- Preload is set by the central bolt that joins the crank halves
- Critical to tighten to proper torque specification—too loose allows play, too tight can damage the Hirth joint
- Chainline is typically fixed but can be adjusted with different bottom bracket spindle lengths
FSA MegaExo/BB386EVO Cranks:
- External bearing system similar to Shimano
- Preload cap on non-drive side sets initial tension
- Chainline can be adjusted by using different bottom bracket cups with varying offsets
- Particularly relevant for bikes with non-standard bottom bracket shell widths
For modern 1x drivetrains, chainline optimisation is particularly critical. With only one chainring, the chainline must position the chainring ideally relative to the entire cassette range. Most manufacturers provide specific chainline recommendations for 1x systems—typically 47-52mm depending on frame design and intended use.
Australian riding conditions influence chainline considerations. Riders who spend most time in the largest cassette cogs (climbers, those in mountainous regions) might benefit from slightly inward chainline optimisation. Riders who spend more time in smaller cogs (flatlanders, criterium racers) might prefer outward optimization. However, deviating too far from manufacturer recommendations can cause more problems than it solves.
Electronic shifting systems (Di2, AXS, EPS) are particularly sensitive to chainline issues. While these systems can compensate for minor chainline variations through trim functions, optimal chainline still provides the best performance. In fact, electronic systems might mask chainline issues longer than mechanical systems, making regular verification even more important.
For most riders, following manufacturer recommendations for preload and chainline provides the best results. Deviations should only be made for specific reasons (frame compatibility issues, unusual riding conditions, or specific performance goals) and should be done with understanding of the trade-offs involved.