byrd48

Hi,
I'm working on an old 5 speed bike with an axle mounted rear derailleur. When the chain is in the small cog and you apply a fair amount of force to the pedals, it skips. I have tried two different freewheels with the same result. I cannot get it to skip when the chain is in any other cog on the rear, only the 14 tooth cog. When comparing the derailleur position to other bikes, it seems to have fewer cog teeth engaged in the chain. Approximately 6 full teeth are exposed while others seem to leave only 5 teeth exposed. I've sized the chain properly. We have tried watching it to see where the skip is taking place, but it happens too fast. I did notice the lower wheel of the derailleur bounce when my partner was riding it, but still can't determine if the chain is slipping. Honestly, it feels like it's coming from the crankset, which has also been replaced. It's a square taper spindle, which looked fine when I replaced the crankset. At this point, I'm just out of ideas. It seems like the top wheel of the derailleur should be farther forward towards the front of the bike, as well as closer to the cog wheel, but there is no b adjustment.
Any ideas would be greatly appreciated.
Thanks!
Jon

FBinNY 

New chain or used?

If used measure it for stretch (wear). Use a 12" ruler, since chains are 1/2" pitch, the pins at both ends should line up the same with respect to the inch markings. Note how far beyond the mark the far pin is, and if it's more than 1/8" that's likely your problem.

Also set the bike in a corner with the front wheel against the wall so it can't roll. Have a friend stand on a pedal while you watch the chain on the small sprocket carefully. Have your friend apply load, up to his full weight in an effort to ride through the wall. The chain will creep froward on the sprocket. What you want to watch for, is whether it also creeps outward toward the tips of the teeth. If yes, the chain or sprocket (or both) are toast.

byrd48

Thanks, we measured the chain with a chain tool and it was ok. We will try riding through the wall.

wrk101

Try using a ruler, not a chain measurement tool.

CliffordK

A couple of thoughts.
If you have a "kink" in the chain (tight link), then it can cause the chain to skip, worst on the smaller cogs. Either crank slowly and watch the chain as it goes through the derailleur jockey wheels, or bend the chain with your fingers and crank, feeling for the kink.

A mal-adjusted derailleur will skip. Make sure the chain is coming off of the jockey wheel and up onto the sprocket perfectly centered. The jockey wheel will float a little bit, so it should also be centered.

Many "newer" derailleurs have a B-Screw adjustment to allow one to customize the derailleur body tension. Loosen the screw, and the body comes forward and the derailleur is closer to the sprockets. Tighten it for more clearance (important for the larger sprockets). I don't know if your "claw" derailleurs have this option. Hmmm, dug up an old Simplex Derailleur, and no B-Screw. But there are claw adapters for more modern derailleurs.

byrd48

Thanks, I tilted the bike in the stand to watch the chain as it entered the cog from the derailleur to see if it was catching on the neighboring cog, but the alignment was good. This derailleur mounts on the axle and has no fore aft adjustment and the frame does not have a derailleur hanger. We'll try the ruler measurement. Thanks!