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There's a plastic disc between the cassette and rear wheel on my bicycle. It doesn't seem to have any effect on the actual drivetrain; but I've seen it on a number of bicycles.

What is it for?

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The spoke guard is there so that when your misadjusted/maltreated rear derailer shifts the chain off the top sprocket it doesn't jam in the spokes. This is the difference between being simply annoyed and being thrown off the bike (and possibly damaging the wheel). – Daniel R Hicks Mar 1 '14 at 19:02
@Daniel: If only I had known to search for "spoke guard". On the other hand if I'd have known that I'd not have had to ask the question :) – Billy ONeal Mar 3 '14 at 8:01
I'm convince that its real purpose is to work itself loose and then to rattle around clunkily until it drives you nuts. – RoboKaren Sep 19 '14 at 16:11
up vote 9 down vote accepted

From Sheldon Brown:

Spoke Protector

A plastic or sheet-metal disc that fits between the cluster and the right-side spokes of a rear wheel. This is intended to prevent the derailer or chain from getting caught in the spokes, possibly causing very extensive/expensive damage/destruction to the wheel, the derailer, and the frame.

A spoke protector is not a necessity on a bike that is well treated, because the derailer can't go into the spokes if it's properly adjusted and if it is not bent. Bicycles which are subjected to rough handling, however, are prone to getting the rear derailer bashed in, and in such a case, the spoke protector can prevent very serious damage.

And from purely personal anecdotal experience: I have a friend that doesn't like spoke protectors and has been pretty vocal about it. Faith would have it that a maladjusted XTR derailleur wrecked his wheel and destroyed itself. A spoke protector would have prevented just that.

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Why would one want to remove it? I mean, it seems to be a pain in the ass to keep clean, but beyond that... – Billy ONeal Mar 1 '14 at 19:17
Plastic is considered to be ugly by some, especially after a few years of sun discoloration. Some people do it to save weight. The old chrome ones looked much better. – Kibbee Mar 1 '14 at 19:23
Its also considered uncool by some people (I've found this is typically road cyclists who like to get dressed up in their spandex suits and what not). – Batman Mar 1 '14 at 21:47
Dirt and pebble tends to stuck between it and the cassette, making it sometimes allmost immposible to pedal. This ugly chunk of plastic should be broken and removed right after the bike purchase. – J-unior Mar 2 '14 at 13:33
I agree with @J-unior. If you treat your bike well and test changes to chain/cassette/derailleur on a repair stand instead of the road, it's an utterly useless, ugly chunk of crap. (I should add that on a mountain bike they make more sense since unseen damage is possible.) – Carey Gregory Sep 19 '14 at 0:00

Its a spoke guard. It prevents the chain from going between the spokes and cassette causing damage if you shift too far. This can only happen on badly tuned gears or old friction shifted shifters.

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Erm, I would hope that a friction shifter wouldn't be adjusted such that this would be necessary... – Billy ONeal Mar 1 '14 at 19:15
Even when using a friction shifter, you still have the high and low limit stops which prevent you from throwing the chain into the gears. Accidents happen though. – Batman Mar 1 '14 at 21:45

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