Chain rings slip when crank is turned

Question

I just pulled my old bicycle out of the garage. It's about 10 years old (a 21-speed mountain bike). It was working when it was put into storage.

When I tried to ride it today, the crank wouldn't engage properly to make the chain rings rotate; or more casually, when I pedaled nothing moved but the pedals themselves.

I flipped the bike over and if the rear wheel was spinning at a fair speed then the chain rings would engage when the crank was turned and the rear wheel had power driven to it as normally expected.

Is there a freewheel or freehub at the crank / chain ring junction that I should examine?

I read through a few dozen questions but I couldn't find any that addressed this specific issue; I hope this is a new question and not just a serious misunderstanding on my part!

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I'll post a video shortly, but to give further clarification...

1. The chain is on the gears; the chain does not slip
2. I can backpedal and the chains and gears spin, as normally expected
3. However, if I pedal forward, the gears which are located at the pedals will not spin; the pedals rotate freely. They rotate as if the thing they are supposed to be affixed to isn't attached.

Also, I'm certainly misusing terms here! By "chain rings" and "crank" I was referring to the areas of the bike as indicated in this diagram:

Answer 1

A quick search turned up this post, in which the OP is having the same trouble.

It sounds like your crank is swaged to the chainrings, as in this image:

"Swaged" means the "spider" (the group of radial arms that hold the chainring) is press-fitted onto the crank arm.

A better design is to manufacture the crank arm and spider as one piece, like this:

It seems that on your crank, the connection between crank arm and spider has failed. With the bike upside down and the rear wheel spinning, the load is reduced sufficiently for the crank arm to turn the spider and rings. However, it is just metal-on-metal friction that allows this to happen. As you have seen, if the back wheel is stationary, the crank arm spins independently.

You need a new drive-side crank. I'd replace the whole chainset.

Edit: It sounds like you searched and found the same pages I did, and have gotten confused by the suggestions about Shimano's "freewheeling cranks". Rest assured, your bike will almost certainly not have any weird gimmicky technology like that. Just sounds like a not-so-common failure of a poorly-manufactured crank to me. Swaged cranks are cheap and nasty (no offence intended), but you can replace them with a forged set without breaking the bank.

Answer 2

Less likely than the above, but there's also the possibility if you have removable cranks (where the crank arms can be pulled off of the spindle, like the common JIS tapered spindle cranks many mt. bikes have) that both crank arm have come loose from the spindle. Take a look. If you have that type then there's a dust cap in the center of the crank set you have to remove, under that there should be a bolt that holds the crank arm to the spindle. Check that to see if it is loose. If it is loose, it is best to remove both crank arms and determine what's causing it to be loose. You may determine the taper profile on the spindle is damaged, and the spindle (or entire BB set, or possibly both the BB set and the crank set ) needs to be replaced.