DIY Rear Derailleur Adjustment for GRX RD-RX820 (12-speed 1x) on Gravel Bikes

Posted on
in Uncategorized

The Shimano GRX RD-RX820 [paid link] (12-speed 1x) rear derailleur is specifically designed to handle the demands of gravel biking, offering smooth, reliable shifting under the roughest conditions. Ensuring that this component is properly adjusted will not only enhance the shifting performance but also improve the overall efficiency and longevity of your drivetrain. This guide will take you step-by-step through the process of adjusting your GRX RD-RX820 rear derailleur.

Tools Required

To perform the rear derailleur adjustment, you’ll need the following tools:

  • 5mm Allen wrench
  • 2mm Allen wrench
  • Phillips screwdriver
  • Torque wrench (optional but recommended for precision)
  • Chain checker (optional)
  • Bicycle stand (recommended for ease of adjustment)
  • Chain lubricant

Understanding the Shimano GRX RD-RX820 Rear Derailleur

The Shimano GRX RD-RX820 (12-speed 1x) rear derailleur is a direct mount derailleur designed for gravel bikes, specifically built to withstand the unique challenges of off-road riding. It features:

  • Wide-gear range compatibility: Optimized for a 1x drivetrain, providing reliable shifting across a broad range of cogs.
  • Shadow RD+ technology: Offers an integrated clutch system to reduce chain slap and improve chain retention.
  • Larger pulley wheels: Designed to minimize friction and improve chain engagement over rough terrain.

Properly adjusting this derailleur ensures smooth, precise shifting, even when you’re navigating uneven surfaces or dealing with dirt and grime.

Step 1: Check the Drivetrain Condition

Before making any adjustments to the rear derailleur, ensure that your drivetrain is in good condition. This involves inspecting the chain, cassette, and derailleur hanger for any visible damage or wear. A damaged hanger can prevent proper derailleur alignment, leading to poor shifting performance.

  • Chain condition: Use a chain checker to assess the wear. Replace the chain if it is too stretched.
  • Cassette wear: Ensure the teeth on the cassette are not excessively worn or damaged.
  • Derailleur hanger alignment: A misaligned derailleur hanger can cause issues with shifting, even after adjustments.

If everything is in good condition, you can proceed to the adjustment process.

Step 2: Set the Bike on a Stand

For better access to the derailleur and easier adjustment, place the bike on a stand. This will allow you to freely rotate the pedals and shift through the gears without interference from the ground.

Step 3: Adjust the B-Screw

The B-screw controls the derailleur’s distance from the cassette. Setting this correctly will help optimize the shifting performance across all gears.

  • Shift the bike to the largest cog.
  • Use the 2mm Allen wrench to turn the B-screw. The derailleur should be positioned so that the jockey wheels are about 5mm away from the largest cog.
  • If the derailleur is too far from the cassette, the shifting may become sluggish or inaccurate. If it’s too close, it may cause noise or rubbing.

Make small adjustments and check the position until you find the correct alignment.

Step 4: Adjust the High and Low Limit Screws

The high and low limit screws control the range of motion of the derailleur. Adjusting these screws ensures the derailleur cannot move beyond the smallest or largest cog, preventing chain drops and excessive derailleur travel.

High Limit Screw Adjustment

  • Shift to the smallest cog in the rear.
  • Identify the high-limit screw, marked with an “H” on the derailleur.
  • Adjust the screw using a Phillips screwdriver to ensure the derailleur aligns perfectly with the smallest cog. The jockey wheel should be directly in line with the cog.
  • If the derailleur is too far from the cog, shift performance will be less precise. If too close, the chain may rub against the derailleur.

Low Limit Screw Adjustment

  • Shift to the largest cog.
  • Locate the low-limit screw, marked with an “L”.
  • Adjust the screw to ensure the derailleur doesn’t shift past the largest cog. The jockey wheel should align with the largest cog.
  • A well-set low limit screw prevents the derailleur from dropping the chain off the cassette when you shift into the largest cog.

Step 5: Fine-Tune the Cable Tension

Proper cable tension is essential for smooth, crisp shifting. Too much tension can make shifting sluggish, while too little tension can result in poor shifting performance, particularly under load.

  • Shift to the middle cog.
  • Use the barrel adjuster, located where the cable enters the derailleur or at the shifter, to adjust the tension. Turning the barrel adjuster clockwise increases tension, while counterclockwise decreases tension.
  • With the bike in a stand, gently rotate the pedals and observe the shifting. Fine-tune the tension until the derailleur moves crisply from one cog to the next, with no hesitation or skipping.

Step 6: Test the Shifting Performance

Once you’ve made the necessary adjustments to the limit screws and cable tension, test the shifting performance by shifting through all the cogs. The derailleur should smoothly and quickly shift up and down the cassette without hesitation or noise.

  • Shift from the smallest to the largest cog and back again, ensuring smooth transitions in both directions.
  • Pay attention to any skipped gears or rubbing sounds, which could indicate an issue with cable tension, limit screw settings, or derailleur alignment.

Step 7: Adjust the Chain Length

If you find that the chain is either too tight or too loose after adjusting the derailleur, you may need to adjust the chain length. A correctly sized chain ensures smooth operation and prevents damage to the drivetrain.

  • Shift to the largest cog and the chainring.
  • If the chain is too tight, remove the excess links using a chain tool.
  • If the chain is too loose, ensure that the rear derailleur is able to handle the increased chain length and tension.

Step 8: Lubricate the Chain

After making all necessary adjustments, it’s a good practice to lubricate the chain to ensure smooth operation. Apply chain oil to the inside of the chain while slowly rotating the pedals. Wipe off any excess oil with a clean cloth to prevent dirt buildup.

Troubleshooting Common Issues

If you’re still experiencing issues after completing the adjustment process, consider the following troubleshooting tips:

  • Skipping gears or hesitation in shifting: Check cable tension and limit screws. Incorrect tension can lead to sluggish or skipping shifting, especially under load.
  • Chain rubbing against derailleur: Adjust the B-screw to increase the derailleur’s distance from the cassette. Check for derailleur hanger misalignment.
  • Shifting not reaching the largest or smallest cog: Double-check the high and low limit screws. If the derailleur is not aligning with the cogs, re-adjust these screws.

Comparison with Other Rear Derailleurs

When comparing the Shimano GRX RD-RX820 with other gravel-specific rear derailleurs, such as the SRAM Force 1 or Shimano Deore XT, the GRX model stands out for its unique gravel-focused features. The GRX RD-RX820 is built specifically for off-road use, offering a more robust clutch system (Shadow RD+) and increased pulley size for smoother operation on rough terrain. While other models may offer similar performance, the GRX’s design specifically addresses gravel-specific needs like chain retention and smooth shifting on uneven ground.

Compatibility with Other Drivetrain Components

The Shimano GRX RD-RX820 (12-speed 1x) is designed to be used with Shimano 12-speed cassettes, such as the 11-42T or 11-46T cassettes, and a 1x chainring. It is important to ensure that the rest of your drivetrain is compatible with this system, particularly the shifters, which should be Shimano GRX 12-speed compatible.

The derailleur is also compatible with various Shimano road and mountain bike 12-speed components, but the clutch mechanism is designed to excel in gravel conditions, making it more suited for off-road riding than other 12-speed road-specific derailleurs.