DIY Guide to Replacing Derailleur Cables for GRX RD-RX820

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Replacing the derailleur cables on your Shimano GRX RD-RX820 [paid link] is a crucial maintenance task to ensure smooth shifting performance on your gravel or road bike. Over time, cables can stretch, fray, or become corroded, leading to poor shifting performance and potential damage to the drivetrain. This guide will walk you through the step-by-step process of replacing the derailleur cables, from removing the old cables to installing and adjusting the new ones.

Tools Required

  • 5mm Allen wrench
  • Cable cutters
  • Phillips screwdriver
  • 10mm or 11mm wrench (depending on your bike)
  • Small adjustable wrench (optional)
  • Derailleur hanger alignment tool (optional)
  • New derailleur cables
  • Lubricant (cable grease)
  • New cable housing (optional, if replacing)

Step-by-Step Guide to Replacing Derailleur Cables for GRX RD-RX820

1. Preparing the Bike

Before starting the replacement process, make sure your bike is in a stable position, either by placing it in a repair stand or flipping it upside down. This ensures you have easy access to the drivetrain. Shift your bike into the smallest gear on the rear cassette and the front chainring to relieve tension from the derailleur.

2. Removing the Old Derailleur Cables

Begin by loosening the cable anchor bolt on the Shimano GRX RD-RX820 derailleur using a 5mm Allen wrench. This will release the cable tension, allowing you to remove the old cable from the derailleur.

Next, locate the cable stop on the frame, which is where the cable housing enters the frame. If necessary, use a Phillips screwdriver to remove any frame stops or guides that are in place. You can now pull the derailleur cable out of the cable housing.

If you are replacing the housing, cut the old housing with a pair of cable cutters. Make sure to remove any leftover cable housing pieces from the frame.

3. Installing the New Derailleur Cable

Lubricate the new derailleur cable with some cable grease to ensure smooth movement through the housing. Begin threading the new cable through the cable housing, starting from the frame’s cable entry point, and move towards the derailleur.

Make sure to feed the cable through the housing correctly and avoid any kinks or sharp bends, as these can cause friction and negatively affect shifting performance.

4. Attaching the Cable to the Derailleur

Once the cable is in place, feed it through the Shimano GRX RD-RX820 derailleur. Ensure the cable is properly seated in the derailleur’s cable guide. Secure the cable by tightening the cable anchor bolt with the 5mm Allen wrench. Be sure to pull the cable taut to eliminate slack, but avoid overtightening, as this could damage the cable or derailleur.

5. Adjusting the Cable Tension

With the cable attached, it’s time to adjust the tension. Shift the derailleur through all the gears and check for smooth movement. If shifting is sluggish or skipping, adjust the barrel adjuster located on the derailleur or the frame’s cable stop to fine-tune the cable tension.

If shifting is too stiff, loosen the tension by turning the barrel adjuster counterclockwise. If shifting is too sloppy or underperforming, tighten the tension by turning the barrel adjuster clockwise.

6. Replacing the Cable Housing (If Necessary)

If you’re also replacing the cable housing, cut the new housing to the proper length using cable cutters. The housing should be long enough to allow smooth movement but short enough to avoid any slack. Install the new housing into the cable stops on the frame, ensuring there is no binding or friction along the way.

Lubricate the new housing before inserting the derailleur cable to reduce friction. Be sure the cable housing enters and exits the frame smoothly, allowing the derailleur to function without resistance.

7. Final Adjustments

After ensuring the derailleur cable is properly installed, double-check the shifting performance. Shift through all the gears, both on the front and rear, to ensure crisp and accurate shifts. Make small adjustments to the barrel adjuster as needed.

If you notice that the derailleur is not shifting properly in certain gears or making noise, it may be worth checking the derailleur hanger alignment. Misalignment can affect shifting performance, and in some cases, it may require realignment using a derailleur hanger tool.

8. Test Ride and Fine-Tuning

Once all adjustments are made, take your bike for a test ride. Pay attention to the shifting performance during different gear changes. If necessary, return to the bike stand to make additional fine-tuning adjustments to the cable tension or derailleur position until you achieve optimal shifting performance.

Troubleshooting Common Set-Up Issues

Poor Shifting Performance

  • Cause: Cable tension is too loose or too tight.
  • Solution: Adjust the barrel adjuster to fine-tune the tension for smooth and precise shifting.

Skipping Gears

  • Cause: The derailleur hanger is misaligned or the derailleur is out of adjustment.
  • Solution: Use a derailleur hanger alignment tool to ensure proper alignment and adjust the derailleur limits if necessary.

Shifting Delays

  • Cause: Friction in the cable housing or worn cables.
  • Solution: Lubricate the cables and housing and check for any damaged areas. Replace any frayed cables.

Customization and Optimization for Better Performance

For optimal performance, consider upgrading your Shimano GRX RD-RX820’s cable and housing to high-end, friction-reducing options. These can reduce shifting resistance and improve durability, especially when riding on gravel or in wet conditions.

Additionally, fine-tuning the derailleur’s limit screws can help you prevent the chain from overshooting into the spokes or dropping off the cassette when shifting into extreme gears. Setting these properly ensures smoother transitions and enhances overall performance.

Compatibility with Other Drivetrain Components

The Shimano GRX RD-RX820 derailleur is designed for gravel-specific riding but is compatible with Shimano 10- or 11-speed road drivetrains. It features Shimano’s Shadow RD+ technology, which helps to reduce chain slap and improve shifting in rough conditions. It is important to ensure that your cassette, shifters, and crankset are compatible with the 10- or 11-speed configuration for seamless integration.

Before installing, check the compatibility of the derailleur with your existing drivetrain components, as using the wrong gear range could affect shifting performance.