What is the purpose and necessity of the shield installed behind the rear cassette on some bikes? I own three bikes and only two have this additional part. I’ve looked through several issues of your magazine and have not seen any used on the test bikes. Should I look into having one installed? It’s a 1986 Schwinn road bike that I bought new and have not had any problems without this shield.
The short answer is, it really depends on you and your needs. The shield is installed on many new bikes as a form of protection for the spokes until the derailleur is properly adjusted. During delivery a derailleur hanger can get bent or damaged, ,and if the recipient doesn’t notice it could lead to additional damage. Also, most bikes ship assembled but not adjusted or tuned. This means the limit screws on a derailleur are not set correctly. If the bike is run through its gears before doing this, there is a very high probability that it will send the chain past the cassette and into the spokes of the wheel. Under load, this could damage or break a spoke, but in the stand, it normally just leads to scratches on the spokes. This is where that plastic or on older bikes the metal shield is key. It helps shield the chain from damaging the wheel as much.
For us, we remove it because all of our bikes are tuned correctly before they ever hit the road. In many cycling circles, having the shield is frowned upon, but in all honesty, it doesn’t matter. The real issue is they are not always tight enough or get damaged from the UV exposure, and when riding, it can move and make noise. I personally think that on city bikes they make sense since leaning and locking your bike to things is common, and a bent hanger is fairly possible. Also, on vintage bikes, I think the metal ones are cool and offer more character.