Some modern 12-speed drivetrains, especially when paired with different frame or bottom bracket styles, often create chainline issues. Riders may find that the front derailleur cannot be adjusted enough for smooth shifting. One simple and effective solution is to install a spacer behind the drive-side crank arm.

What the spacer does

Adding a washer between the crank arm and the bottom bracket spindle moves the crank outward slightly. This small adjustment can dramatically improve the chainline, allowing better alignment and smoother shifting across the cassette and chainrings.

Thin or thick? Use what fits

Most installations only require a thin spacer, like an 18-gauge 1-inch washer. These are easy to find at any hardware store. If you still notice poor alignment after that, a thicker option is available. The goal is to adjust the drive-side spacing without compromising crank arm engagement or safety.

What to expect on the non-drive side

When you shift the crank outward on the drive side, you may see a few exposed threads on the non-drive crank arm. This is normal and safe — the crank arm will still clamp securely onto the spindle.

Chain and ring compatibility tips

JCOB chainrings generally shift better with Shimano 12-speed chains. Riders using SRAM's "Flat Top" chains may notice slower shifts, especially on steeper ramps. If you’re experiencing hesitation in shifting, try switching to a Shimano chain for more consistent engagement.

These small adjustments can solve a lot of frustration. A $1 spacer can be the difference between clunky and crisp shifting — especially on tight 12-speed setups.