A derailleur guard is a steel cage, mounted proud of the rear gear, that might prevent damage to a rear derailleur when it suffers impact, or as a result of mistreatment such as the bike being dumped on the deck after a ride.
Health Warning
We have to admit that we are ambivalent about derailleur guards. While a derailleur guard is useful in theory, the fact that the wheel track nuts or QR secures it in place causes misgivings. A derailleur guard can't help but interrupt the interface between the frame and wheel nut. This means that the wheel nut will be less secure, and a less secure wheel is potentially more dangerous than a whacked rear derailleur. Having said that, derailleur guards are usually reserved for kids' bikes where the rider will probably not have the strength to loosen a wheel while riding.
