rykoala

Bike is a 1990 ( +/- a year) Trek 820. Got on the bike last night, its just now getting in rideable, and I went to really pound on it to accelerate. Cranks went straight down, chainring just sat there. Upon inspection I found that the cranks can turn independatnly of the chain ring.

How can I fix this?

DieselDan

You've got one of those old cranksets Shimano made with the freewheel in the the crankset, rather then the rear hub. (I forgot the name Shimano gave it) In your case, the freewheel is shot, and you're in need of some serious parts and repairs. New crankset, new rear hub, new freewheel or cassette.

rykoala

Say what? No the freewheel is definitely in the rear hub, not in the crankset, unless I'm missing something. If the freewheel was in the crankset then the whole geartrain would turn all the time.

Are you pulling my leg?

Retro Grouch 

I'm not sure what-all you'll have to replace, but it could get ugly.

Crankset for sure. A modern crankset will probably require a narrower bottom bracket. I seem to remember that those "front freewheel" bikes actually had a freewheel in back also, but I don't think that it worked quite the same as a standard freewheel so you'll probably need one of those too. Finally, since you're getting new sprockets front and rear, you don't want to wear them down prematurely by riding with a cruddy old chain so you ought to replace that too. That's going to take you up over $100.00 for parts if you go with all new stuff. A cheaper alternative might be to find a "donor" bike that has a bent fork or a tacoed wheel and work a parts swap.

rykoala

Good lord, so much for a free bike to beat around...

I have a parts bike with a crankset/bb that'll probably work, as its of the same vintage and also shimano. I'll try just replacing that first....

Could someone explain this dual freehweel thing to me? It doesn't make sense, I remember this bike *always* 'skipping' on me when I went to really pound on it, I thought it was the chain all this time.

Thanks guys, I really appreciate the help.

rykoala

Ah, I should mention this: It DOES turn under no load, but as soon as you put any load on it then the chainrings stop and the cranks keep turning. Tested this with the bike upside down.

Tom Pedale

If you have a swaged crank, which means that the crank spider is press-fitted to the right crankarm instead of being one piece of forged/ cast metal, it's possible that the right crank arm has detached from the spider with the result that when you pedal, the crankarms move, but the chainrings don't. You'll have to replace the right crank/chainwheel assembly. Most likely, it will be cheaper to buy a whole crank (left as well as right hand arms). If you're going to buy the whole set, this gives you the opportunity to change chain ring sizes and crank arm lengths if you're not happy with present setup. If changing chainring sizes, check to make sure the new size(s) are compatible with your current shifting setup.
Also, make sure that new crank will fit present bottom bracket axle without fit problems such as chainrings whacking chainstays on either or both sides or taper on new crank being different than taper on present bottom bracket axle.

Historical note: The earlier post referencing the freewheel incorporated into the crank was the Shimano Front Freewheeling system (I don't think you have this system since it's way before 1990). At any rate, as you know, you should be pedaling when you shift. With this system, you could shift while coasting since the front chainrings were moving. The idea was that the learning curve for beginners learning to shift a bike would be shortened. In real life it didn't work out that way and the this idea faded away after a few years.

rykoala

Tom, thank you. This makes alot of sense and I for sure don't have the system mentioned in above posts. I'll go to the LBS and kindly ask them to pull the cranks for me, which they've offered in the past, and I'll also have them pull the cranks on my parts bike. Swap should work fine ;-)

Tom Pedale

Have fun with your new setup!

rykoala

I shall! This is the same bike I used to ride to school when I was a kid. Its a tad bit small now but I think its going to make a great SS beater in the near future ;-)