AdoAdoAdo

Hello there. If I am moving along on my bike, and I keep the pedals stationary, the rear cassette continues to rotate and the chain subsequently becomes loose between the front and rear cogs. I am not knowledgeable of these mechanics, but my guess is that the casette has 'stuck' itself to the rear axle and needs to be lubricated, seeing as it shouldn't rotate if the pedals are not moving the chain across it. Would I be correct, and how would I go about solving this problem? Thank you!

FBinNY 

Yes, you've diagnosed the problem correctly, and welcome to the forum. The freehub (not the cassette per se) isn't ratcheting as freely as it should and is being pushed forward by the wheel. .

As you note, it's usually a lubrication issue, but might also be a corrosion issue, or on some hubs related to a bearing adjustment. Before taking it to the bike shop, remove the wheel and see feel the resistance to spinning the cassette backeard, it should be very slight. Next, remove the cassette if you have the tool, and see if the freehub has an oil hole, or try to run some light oil into the gaps between the spinning and stationary parts.

If you can't get it spinning backward freely, or if there's still drag when you hold the freehub body (or cassette) and spin the wheel forward you'll need to visit the LBS for service. In most cases disassembly, cleaning and lubrication does the trick, so you'll be paying for labor only, not having to buy parts.

BTW- if you're not mechanically declined, this isn't a very difficult job, and there are a number of tutorials available on the net, but you'll need a few special tools.

AdoAdoAdo

That's a start, then. How would I fix this? Do I simply lubricate where the friction is? I'll have to have a good look in there, it's quite compact.

EDIT: Just seen your edit. Thank you, I'll get working on it!

FBinNY 

As I said you might get away by wicking some oil into the gap between the freehub and hub. The ratchet mechanism is on the back end toward the hub, so you'll want to remove the cassette first for easier access, which requires a cassette remover and chainwhip. (unless your bike is 20 years old or so, and possibly has a freewheel see here).

If you want to try the quick and easy, lay the wheel on a table cassette side down, and use an eyedropper to put some oil on the back gap, and let gravity and a bit of turning of the wheel carry it down. Before you do so, clean the area behind the cassette using a strip of rag shoeshine method both to see what you're doing and avoid carrying dirt into the mechanism with the oil.

Here's a tutorial showing freehub service/replacement on a typical hub, though some models are different. There are others out there if you search for freehub service or maintenance. Take your time to read through a few until you have a sense of it, then decide if you want to tackle the job. You might also consider bringing the wheel in for a quote on the job so you have a basis of comparison for the job vs. buying the various tools needed.