DIY Rear Derailleur Adjustment for GRX RD-RX820 (12-speed 2x) on Gravel Bikes

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The Shimano GRX RD-RX820 [paid link] is designed specifically for gravel bikes, providing excellent performance in challenging terrains with its 12-speed 2x drivetrain. Proper adjustment of the rear derailleur is essential to ensure smooth shifting and optimal drivetrain performance. In this guide, we will cover the installation, adjustment, maintenance, and troubleshooting for the GRX RD-RX820, ensuring that your gravel bike delivers its best performance in all conditions.

Tools Required

  • 5mm Allen wrench
  • Phillips screwdriver
  • Chain checker tool
  • Chain lube
  • Lubricant for derailleur pivots
  • Clean rag
  • Cable cutter
  • Torque wrench (optional but recommended)
  • 10mm and 13mm wrenches (for rear axle)

Installation of the Shimano GRX RD-RX820 Rear Derailleur

Proper installation of the GRX RD-RX820 rear derailleur ensures longevity and precise shifting. Follow these steps for installation:

  1. Prepare the Bike: Ensure that your gravel bike is clean and stable. Set it on a work stand, allowing easy access to the drivetrain.
  2. Remove the Old Derailleur (if applicable): Use a 5mm Allen wrench to remove the bolts securing the derailleur hanger and the derailleur itself. Carefully remove the chain and the derailleur.
  3. Install the New Derailleur: Attach the derailleur to the derailleur hanger using the appropriate bolt and tighten it securely, but avoid overtightening. Use a torque wrench for precise tightening.
  4. Connect the Derailleur to the Chain: Thread the chain through the jockey wheels on the derailleur and reattach the chain to the bike, ensuring it runs smoothly through the front and rear cogs.
  5. Position the Derailleur: Ensure that the derailleur is correctly aligned with the cassette. The jockey wheels should align with the gear sprockets.
  6. Cable Installation: Secure the derailleur cable to the derailleur using the cable pinch bolt. Tighten it firmly using a 5mm Allen wrench.

Adjusting the Shimano GRX RD-RX820 Rear Derailleur

Once installed, the next step is to fine-tune the rear derailleur to ensure smooth and reliable shifting. Adjustments should be done carefully and methodically.

1. Set the Limit Screws

The Shimano GRX RD-RX820 rear derailleur has two limit screws that control the movement of the derailleur across the cassette: one for the high gear (H) and one for the low gear (L). These screws prevent the derailleur from overshooting and damaging the drivetrain.

  • High Gear (H) Limit Adjustment: Shift to the smallest cog and adjust the H screw so that the derailleur sits just next to the cog without rubbing against it.
  • Low Gear (L) Limit Adjustment: Shift to the largest cog and adjust the L screw so that the derailleur cage moves freely without overshooting the cog. Make sure the derailleur does not shift beyond the largest cog, as this could cause the chain to derail.

2. Cable Tension Adjustment

Once the limit screws are set, the next step is to adjust the cable tension. This controls how well the derailleur moves across the gears. If the derailleur is sluggish or hesitates when shifting, the cable tension is likely too loose.

  • Shift to the middle gear in the cassette and check the derailleur’s movement.
  • If the derailleur doesn’t shift to the next gear smoothly, increase the tension by turning the barrel adjuster counterclockwise.
  • If the derailleur shifts too quickly, decrease the tension by turning the barrel adjuster clockwise.

3. B-Tension Adjustment

The B-tension screw adjusts the position of the derailleur relative to the cassette. Proper B-tension is crucial for smooth shifting, particularly in the larger cogs.

  • Shift to the largest cog and observe the derailleur’s angle.
  • Adjust the B-tension screw so that the jockey wheel is 5-6mm from the largest cog.
  • This ensures the derailleur maintains a proper angle and can shift smoothly.

Troubleshooting Common Set-Up Issues

Even after careful adjustment, you might encounter some shifting issues. Below are common problems and how to resolve them.

1. Skipping or Sticking Gears

If your bike is skipping gears or if the derailleur is having trouble moving between gears, this is often a sign of poor cable tension or a worn-out cable.

  • Solution: Check the cable tension and adjust it using the barrel adjuster. If the cable is frayed or damaged, replace it with a new one.

2. Chain Drops Off the Cassette

If the chain drops off the cassette when shifting to the largest or smallest cog, this could be due to incorrect limit screw settings.

  • Solution: Revisit the high and low limit screws. Adjust them to ensure the derailleur doesn’t overshoot the cogs, particularly in extreme gear positions.

3. Slow Shifting Response

If your shifting is sluggish, it may be due to insufficient B-tension adjustment or a dirty derailleur.

  • Solution: Clean the derailleur pivots and re-lube them. Adjust the B-tension screw to ensure the derailleur is positioned correctly.

4. Ghost Shifting (Unintended Shifting)

Ghost shifting, where the derailleur shifts on its own without any input from the shifter, can occur due to several factors, such as a misaligned derailleur hanger or worn-out cables.

  • Solution: Inspect the derailleur hanger for alignment and straighten it if necessary. Replace any worn cables or housing that might be affecting the shifting.

Customizing and Optimizing the GRX RD-RX820 Rear Derailleur

To maximize performance on your gravel bike, consider the following adjustments for optimizing the GRX RD-RX820 rear derailleur:

1. Adjust the Jockey Wheels

If you want to fine-tune shifting smoothness, consider replacing the derailleur jockey wheels with higher-quality aftermarket options. Upgrading to ceramic jockey wheels can reduce friction and improve performance.

2. Use a Chain Catcher

On gravel bikes, chain drop is a common issue due to rough terrain. Installing a chain catcher can prevent the chain from dropping off the chainring when riding over rough terrain, ensuring that the drivetrain remains in good condition.

3. Choose the Right Cassette

The GRX RD-RX820 rear derailleur is compatible with 12-speed cassettes ranging from 11-34T to 11-42T. Depending on your riding style and terrain, choosing the right cassette for your needs can optimize performance. For rough gravel trails, consider using a larger cassette like 11-42T for a wider gear range.

Compatibility with Other Drivetrain Components

The Shimano GRX RD-RX820 is part of the GRX 11-speed series, but it is designed to work with Shimano’s 12-speed 2x drivetrain components. When considering compatibility with other drivetrain components, make sure to check the following:

  1. Shifters: The RD-RX820 rear derailleur is designed to work with Shimano GRX 12-speed shifters. Ensure that the shifters and derailleur are from the same series for optimal shifting performance.
  2. Crankset and Chainrings: The GRX RD-RX820 is compatible with Shimano GRX 2x cranksets and chainrings, offering smooth and reliable shifting across a wide gear range.
  3. Cassette Compatibility: The RD-RX820 rear derailleur is compatible with Shimano’s 12-speed HG 12 cassettes. Make sure your cassette is designed for 12-speed systems to ensure compatibility.
  4. Chain Compatibility: Shimano recommends using a Shimano 12-speed chain with the RD-RX820 to ensure compatibility and optimal performance.

With these guidelines, you can ensure that the GRX RD-RX820 works seamlessly with your gravel bike’s drivetrain, offering smooth and efficient shifting for all types of terrain.