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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?

Billy ONeal
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4 Answers4

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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.

user1049697
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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 01 '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 01 '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 01 '14 at 21:47
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    Yeah, mainly they're considered uncool. As to discoloration, you can always buy a new one every few years. – Daniel R Hicks Mar 01 '14 at 22:14
  • And it might signal that you don't care or can adjust your derailleur properly because you need "protection". – user1049697 Mar 02 '14 at 08:16
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    @user1049697 - Or it might signal that you're prudent. – Daniel R Hicks Mar 02 '14 at 13:12
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    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 02 '14 at 13:33
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    @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 H Mar 03 '14 at 09:09
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    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 00:00
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    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, 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. – Andrew Henle Jun 05 '19 at 21:02
  • @J-unior 'looking nice and clean' by not having that 'useless, ugly chunk of crap' will not save you when your stays flex and/or your RD hanger gets bent.

    I always suggest to people to buy black-colored dork disks if the discoloration of translucent ones puts them off.

     – Gregory Leo Jun 06 '19 at 09:20
  • @GregoryLeo it's been almost 4 years since I have changed my RD hanger last time and I'm far from being accurate rider. IDK about road riding, but in modern MTB frames and derailleurs you are most likely to bash your derailleur than the hanger. – J-unior Jun 06 '19 at 19:31
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    Having recently had an RD go into the back wheel even with such a disk, they're not a complete solution. The disk smashed and the chain chewed up the spokes; I only just unclipped in time to get off when the back wheel and pedals locked up. – Chris H Jun 07 '19 at 12:31
  • @J-unior That's on your case though. It's your choice if you want to remove your spoke guard, but what I don't like is this stigma that dork disks are 'bad', which forces starting riders to remove theirs in order to 'not look bad'. I also don't get your last sentence. your hanger never gets bashed. it's the RD being bashed that bends the hanger. – Gregory Leo Jun 07 '19 at 16:00
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    @ChrisH If I were you, I'd get the aluminum ones. Another alternative that aggressive downhillers do is to insert a large cog on the freehub body before placing the cassette to act as a 'chainguide' and block the chain from slipping further in. Heck, even some downhill drivetrains have cassettes with built-in AL dork disks. – Gregory Leo Jun 07 '19 at 16:05
  • @GregoryLeo I've never seen aluminium ones, but as I'm rebuilding the wheel tomorrow I'm hoping to fit a plastic one from a beater bike. – Chris H Jun 07 '19 at 16:20
  • @ChrisH just cut out one from a 3mm delrin or PVC sheet. It's sturdier than the flimsy, nylon protectors. – Gregory Leo Jun 07 '19 at 19:50
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    @GregoryLeo 2mm polycarbonate ought to do the trick and I've got a piece just about the right size – Chris H Jun 07 '19 at 21:31
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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.

Brad de Leon
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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! – DavidW Oct 04 '21 at 22:24
  • Welcome to the site - thank you for your answer. Please do take a moment to browse the [tour] and learn how the site works. – Criggie Oct 05 '21 at 02:04
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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.

Kibbee
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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 01 '14 at 19:15
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    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 01 '14 at 21:45
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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.

juhist
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