Today, I rode road bike and I felt a loud vibration from rear wheel. I looked at it and it turned out the chain was loose and banging on bike's frame. The chains aren't off track, but it seems that the plastic chain guard was jamming the sprockets that causing rear derailleur hang to act weird. The plastic chain guard is loose and the holder was broken.

I went to local bike shop and they don't have in stock and the employee decided to take one off from brand new bike and gave it to me. That got me puzzled.

So, my question is - is chain guard really necessary? Is it done a job prevent from chain falling into between wheel and sprocket? Will it harm anything if I don't use it?

The chain guard isn't met for keeping the chain on the chainrings or rear cog but more for keeping clothing out of the chain . there be no harm done if you don't use it . Only harm will be to your pant leg if you don't use it .

Do you mean the spoke protector on the rear wheel?

The spoke protector will keep the chain from cutting the spokes if because of a mis-shift the chain falls between the large cog and the spokes. Don't ask me how I know this.

Yes, from the description, it seems that the OP was referring to a spoke protector. If not he needs to clarify.

However, if it was the spoke protector it seems that he has the cause and effect backward. It's job is to protect the wheel and derailleur form ill effects of over shifting into the wheel. Those can be the chain jamming between the cassette and spokes, the chain cutting spokes when jammed there, or the RD engaging the spokes and getting torn off the bike in the process.

I suspect that what's he's seeing now is the after effects of the spoke protector sacrificing itself to save the other parts. Odds are that the sequence was, the RD hanger getting bent inward. possibly by the bike falling on it's right side. Then at some later time, he shifted to low, and the RD overshot, either hitting the spoke protector, or sending the chain into it. Either way, that broke the spoke protector, and left him where he is now.

To answer the question as asked, no you don't need one and plenty of us don't have one, but the consequences of RD misadjustment will probably be much more severe without one. It's like doing a high wire act without a net, perfectly safe if you're good enough, a poor decision if you're not.

Yes, spoke protector. I don't know what it called and I was just guessing.

Here's an image of spoke protector loose.


The spoke protector that got loose and somehow it jammed the cassette. If I pedaling it, it runs normal, but when I stopped pedaling (cruising) the chain would go waving and banging on chainstay that got my attention. I stopped and checked it, I noticed that if I backpedal it, the chain would pull rear derailleur and top chain would get "sagged". And then I noticed the spoke protector was loose. It's possible that it broke while I rode (I have run across some rough road).

So, it's like 50/50 chance. Don't the front derailleur would prevent chain from it go over last sprocket?

No it would not, especially if your rear derailleur has gotten bumped or whacked with a branch and is pushed in towards the wheel.

It is not necessary per se. Many remove it for appearance's sake. But keep in mind that there is an increased risk of damage to the bike of it is removed. You have to stay on top of your hanger alignment to be sure of not having your chain drop off the backside of your cassette and damage your spokes.

Two colloquial terms you'll sometimes here are "pie plate" and "dork disk". That latter is a little bit unfair, but it gets thrown around a lot. The part is there for a reason, and removing it, while visually appealing, does increase the risk of damage to the spokes.

I've only ever heard it referred to as the rookie ring.

If you feel your bike overshift into the spokes, stop pedaling, stop the bike, and manually put he chain back on the cassette. If you shift to a smaller cog and pedal, hoping it'll pop back on, you're going to have a bad time.

'Spoke Protector' and you aint got many spokes lose one or 2 & you're walking..

add some holes and you can zip tie or tape it to the spokes to quiet the rattle ..

Do you live in a perfect world? are you feeling lucky?






....

That plastic thing is the first thing taken off on any new bike I buy, along with all the warning stickers that are all over bikes now. I like to live dangerously.

I was thinking the same thing when I read OP, and my answer was therefore going to be, "Yes, you need it." A broken spoke protector is merely annoying. That broken spoke protector indicates that something's misaligned or maladjusted. Broken spokes, broken derailer, or broken derailer hanger (aka the things that a spoke protector can be sacrificed to protect) are ride-stoppers.

I have Metal ones on the cranksets, but this is not about that, in spite of the title..