FifthWheel

What is the cause of a rear hub free wheeling? In other words the sprockets are turning but apparently the rear sprocket is not catching on the hub. Not a matter of just a couple of turns. I switch gears bounce it around and if finally catches. The bike is a 21-speed. Does it on more than one gear. Not all the time. I'll be riding along then it free wheels. The bike about 6 or 7 years old but very low mileage. Wife bought it to pedal around with our youngest on the back but didn't use it much. It is a Fuji Sandblaster with a Shimano Alivio shifter on the back.

Wil Davis

It sounds like the pawls inside the freewheel are sticking. One solution might be to squirt some light lubricant into the mechanism to free them. Be wary of using WD-40 as a lubricant - it might be ideal for cleaning and removing the cause, but you will need to use a light machine-oil to lubricate the freewheel.

- Wil

HillRider

The freehub body is either dirty or extremely dry inside and the pawls that control the drive aren't catching reliably.

To lube it properly, you will have to remove the axle and the freehub body and drip a lot of light oil into the gap at the back of the freehub body after removing the rubber seal. See Park Tools website for instructions or plan on taking it to your LBS.

You can try to lube the freehub body while it is still on the wheel by dripping a lot of oil into the front of the freehub around where the axle protrudes but you are more likely to contaminate the hub bearings than you are to effectively lube the freehub.

Svr

The ratcheting mechanism is skipping. The practical solution is to replace it. You'll have to determine weather you've got freehub or freewheel system. See here:

https://sheldonbrown.com/k7.html

HillRider

If it's Alivio, it's a cassette freehub.

mcoine

Wow, a shimano freehub that is failing!?

DannoXYZ 

Hmmm, someone could've stuffed it full of grease. In the winters, the grease thickens and causes skipping pawls. The most effective solution I've found is to remove the freehub from the wheel, remove the cogs, and drop into ultrasonic cleaner filled with degreaser (don't tell the GF). Freehub comes out sparkling clean with nice crisp clicks of the pawls. Re-lub with heavy oil and you're set to go.

HillRider

It doesn't sound like it's failing. It sounds like it's dirty. Clean and lube it and it will be good for thousands of miles more.