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?
asked Mar 1, 2014 at 18:42
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4The 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 HicksCommented Mar 1, 2014 at 19:02
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2@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 ONealCommented Mar 3, 2014 at 8:01
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13I'm convince that its real purpose is to work itself loose and then to rattle around clunkily until it drives you nuts.– RoboKarenCommented Sep 19, 2014 at 16:11
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The plastic ones like to deform and start rubbing on the derailleur too...– Brian KnoblauchCommented Nov 17, 2018 at 18:43
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1The colloquial term is 'dork disk', which indicates how a lot of bikers view them.– Monica Apologists Get OutCommented Jun 6, 2019 at 14:00
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4 Answers
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.
answered Mar 1, 2014 at 19:07
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1Yeah, mainly they're considered uncool. As to discoloration, you can always buy a new one every few years. Commented Mar 1, 2014 at 22:14
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1@user1049697 - Or it might signal that you're prudent. Commented Mar 2, 2014 at 13:12
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6Dirt 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-uniorCommented Mar 2, 2014 at 13:33
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1@user1049697 derailleurs can get clobbered quite easily, including in bike sheds when you might not even know anything had happened. And mine certainly came in handy on the first test ride after changing the cassette and chain.– Chris HCommented Mar 3, 2014 at 9:09
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7I 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, Until you happen to be climbing hard, and the chain drops off inside the largest cog when you shift because your frame is flexing. So no, the claim "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" is complete bullhockey. Ragging on spoke protectors is just more Cat 6 MUP racer elitism. Commented Jun 5, 2019 at 21:02
The other reason for the guard is to help keep chain oil off the brake disc. Once the pads are fouled with oil the brake is useless. Sloppy oiling of the chain can cause a spray of oil droplets to migrate to the disc more easily then you might think. The guard does help prevent this. It’s worth keeping it on the bike if you have rear disc brakes. And oil conservatively, slowly turning the crank for a bit then wipe off excess oil before riding.
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Hi, welcome to bicycles! Can you explain how chain lube would migrate sideways onto the rotor? I've certainly had to wipe lube off the rim (rim brakes) when I got a bit over-enthusiastic, but I've never had it spread horizontally!– DavidWCommented Oct 4, 2021 at 22:24
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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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1Erm, I would hope that a friction shifter wouldn't be adjusted such that this would be necessary... Commented Mar 1, 2014 at 19:15
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2Even 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.– BatmanCommented Mar 1, 2014 at 21:45
It's a dork disk.
It's called that because a lot of cyclists are dorks and therefore remove the disk. Eventually, due to rear derailleur damage, the rear derailleur gets out of adjustment and causes the chain to get between the spinning wheel spokes and the stationary cassette. You get spoke damage that way.
In contrast, non-dork cyclists have a completely different approach: they not only leave the dork disk there, but also buy these disks for self-built wheels. One disk doesn't cost much but can prevent lot of damage due to chain getting between the spokes and the cassette.
