Have you ever noticed a small piece of plastic, typically transparent, behind the derailleur on a bike and wondered what it’s for? This little piece of plastic is known as a dork disc and it plays an important safety role in preventing the chain from falling into the spokes of the wheel, especially if the derailleur isn’t adjusted properly.
In this blog post, we’ll explore the reasons why bike manufacturers add a dork disc behind the rear derailleur and the pros and cons of keeping it on your bike. We’ll also discuss when and how to remove this plastic disc if necessary. So, if you’re curious about this small but significant bike component, read on!
What is the piece of plastic behind the derailleur on a bike?
The piece of plastic, generally transparent, found behind the cassette on a bike is a dork disc. It prevents your chain from falling off on the wheel side and prevents the drivetrain mechanism to get caught by the wheel if the derailleur isn’t adjusted properly.
Why do bike manufacturers add a dork disc behind the rear derailleur?
Bike manufacturers add a little piece of plastic behind the derailleur because it is required by law in the US. According to the Consumer Product Safety Commission (CPSC) on commercial practices, bicycles must be sold with a derailleur guard:
“Derailleur guard. Derailleurs shall be guarded to prevent the drive chain from interfering with or stopping the rotation of the wheel through improper adjustments or damage.”Consumer Product Safety Commission (“CPSC”), § 1512.9 Requirements for protective guards.
It used to be the norm on children’s bicycles, but nowadays all bikes are concerned with plastic derailleur guards.
The intent of this law is to protect cyclists from having an accident. If the drive chain interferes with the wheel spokes, it can cause an accident by blocking the wheel in a matter of seconds. The rider will likely lose control of the bike and fall off. This can have even more dangerous consequences if it happens when sharing the road with other vehicles or at higher speeds.
Note that such an accident can only happen if the derailleur is not adjusted properly. Especially if the limit screw is not set up properly, meaning the screw that limits the movement of the derailleur. If you have never experienced it, a limit screw badly adjusted can lead to the bottom part of the derailleur getting in contact with the wheel, and the chain falling off against the wheel.
Pros and cons of keeping the protective plastic disc on the derailleur
- Prevent damaging your rear derailleur if not set properly
- Prevent your chain from going into the spokes and not damage the spokes
- Avoid an accident if the derailleur is not set properly
- Difficult to clean
- Can make a rattling noise if the plastic starts bending and touching the spokes
- Cheap plastic that can breaks (particularly true when you fall off on a MTB)
- A properly adjusted bike, well and regularly maintained, doesn’t benefit from having a dork disc
Can you remove the dork disc?
If you keep your bike correctly maintained, by doing an annual revision in a bike shop and regular maintenance on your bike, then you don’t need a dork disc. Your derailleur is adjusted correctly and checked regularly to prevent any risk of accidents and injuries.
But if you are new to cycling without much understanding of how to adjust the rear derailleur, I would recommend keeping the dork disc as a precautionary measure.
Of course, if your dork disk gets bent, or starts rattling on the spokes of the wheel, then there is no point to keep it.
- If you do long-distance bike packing, I would recommend keeping the dork disc on. The damage can be very important to the bike if something goes wrong, and depending on where you are cycling it might be a nightmare to get the spare parts you need.
- For children, I would recommend always keeping it on as a safety precaution, even if you maintain their bikes on a regular basis.
- If you are a mountain biker, and you fall off from time to time. Be careful that the hanger of the derailleur did not get bent. Else next time you shift, it will enter in contact with your wheel. So always check your bike derailleur straightness after a fall, and adjust the limit screw if needed.
On a similar “do I need” / “rules related” topic, you may be interested in:
How to remove the dork disc behind the derailleur?
To remove the dork disc, follow these steps:
- Remove the rear wheel from the bike
- Unscrew the quick-release axle
Note: if you have a thru-axle, you just need to unscrew it, remove the axle, and then the wheel is free to be removed from the bike.
Now we need to remove the cassette.
Assuming your cassette is a Shimano / SRAM cassette. You will need a lock ring tool and a chain whip to change your cassette.
- Engage the lock ring tool into the splines & notches of the cassette
- Hold the biggest cog in the clock-wise direction with the chain whip tool
- Turn the lock ring tool in the anti clock-wise direction to untighten the cassette
- Once it is turning freely, keep unscrewing the locking ring until you can remove it
- Slide the cassette off the freehub body
- Remove the dork disc
- Put back the cassette on the freehub body
- The freehub body has a number of notches on it. There is one wider gap on the freehub body that you need to locate that will help you find the position to slide the cassette on.
- Add the locking ring and tighten it manually
- Insert the lock ring tool
- Place the chain whip on the biggest cog to prevent the cassette from turning (opposite direction to when you removed the cassette)
- Tighten the lock ring with the tool until you no longer hear a clicking sound
- Put the quick-release skewer back in place
- Re-assemble the wheel onto the bike