Bicycle Mechanics - Rear Chainguard on Cassete

So I've just changed up my cassette from a 12-23 to a 12-27 for a long tour and I had a question about the chainguard. It looks like the chainguard is for a 12-21 to 12-24 cassette and it doesn't come all the way out on the 12-27. Is this going to be a problem for me? Is the chainguard really neccesary? Does it actually hold the chain from slipping off the cassette or does it just protect from grease etc??

A chainguard only serves as a physical barrier to keep things out of the drivetrain. A chainguard has nothing to do with the actual functioning of the drivetrain.

I don't mean to insult you, but you're not talking about the rear derailer, are you?

I just realized that you're probably talking about the spoke protector, which is the plastic or metal disc that lies between the cassette or freewheel and the spokes. The spoke ptotector is there to prevent your chain from being overshifted into your spokes.

The spoke protector doesn't do anything under normal circumstances. If your derailer severely goes out of adjustment or your hanger gets bent, then it should prevent the chain from binding the rear wheel and possibly causing a nasty accident.

A replacement spoke protector will only cost a couple of bucks. I choose to run one, but it's really up to you.

Yeah, I guess I meant spoke protector not chainguard. I just got the cassette put on and don't really want the hastle of taking it back down to my LBS and getting it taken off, getting a new protector, and then getting the cassette put back on. If it's a neccessary safety thing, I'll do it, but if it justs keeps grease etc. off the hubs or something, it doesn't matter I guess. I'm just trying to figure out the intended function of the part.

Thanks

The job of the spoke protector is to keep the chain out of the spokes should the rear deraillleur overshift the inner-most cog. With a properly adjusted rear derailleur, it is impossible for the chain to overshift the cogs and wind up in the spokes. If the bike is crashed or the derailleur is in some way "tweaked", then all bets are off. Sending your chain into the spokes will almost certainly result in a rather painful crash as you're going to stop real quick.

Bottom line: if you take care of your rear derailleur, then the spoke protector isn't needed. If you want a little extra piece of mind, then leave it on.

If you're bike is adjusted properly, there shouldn't be a need for the spoke protector, you'll want to check regularly anyways spoke protector or not.

I usually just take it off. It can get in the way when the freewheel or cassette is coasting or pedaling backwards if the clips that hold is on breaks