Chain Skipping Under Load?
 

Friends,

My recently finished 1983 Bianchi is a FANTASTIC ride (click here for detail), with the exception of one problem - chain skipping under load. A quick Google search brings up Sheldon's article (click here). The chain is a brand new KMC 8 speed. So, I guess it's the Suntour freewheel that I installed? What do you think?

NOTE:

- It ONLY skips under load
- It ONLY skips on the smallest rear sprocket

Here is a video of the issue:

Derailleur alignment?

Maybe I should elaborate. Stand directly behind the cog an look for any misalignment of the cog and the derailleur. Or bent derailleur hangar.

That was the first thing I checked. Looks pretty straight to me!
If it was a bent hangar, wouldn't it have issues in other gears as well?

I figured as much but it was the first thing that came to mind. I just had a problem with a new build. The bike was fine for 8 miles or so, small hills to rolling. Then first steep hill my chain started skipping and I actually went down in the road. The bike stopped abruptly and I didn't get un clipped fast enough. But, a quick adjustment and I was on my way.

I had a bent hanger that caused skipping in the largest cog only,...the others were fine.

throw away that old freewheel and get a new one. chains a freewheels need to be, IME, bought together, otherwise it's just asking for trouble.

derailleurs don't have the intelligence to just pick out one cog among the host of cogs to skip on. it's not likely to be hanger, or alignment problems.

Sounds like a worn cog on the freewheel.

This.
Skipping with a new chain on used cogs => worn out cogs.

I had this problem on my wife's bike when I put on a new chain. Tried adjustments and different cog spacers (the hanger was straight). The solution was a new smallest (12T) cog. Interesting that my Rohloff cog checking tool didn't indicate a problem, but riding it did.

If it's a used freewheel, then it's almost surely the classic new chain/worn sprocket issue. If so, a new freewheel is the only remedy.

Otherwise it could be a subtle trim issue, or a tooth profile issue. Tooth profile issues usually break in, but if it's bad you may not have the patience to live with until it does. Sometimes better lube helps, but it's really a question of wearing off the corners.

The reason you only feel the skip under load is because tension pulls the chain forward relative to the sprocket. That means an engaging roller can catch the tip of the tooth and ride up rather then sliding in. Wear breaks the tips down and solves the problem. If you have a Dremel, grinding the back corner of each tooth helps smooth the engagement solving the problem. Grind only the corner, and not any deeper than 2mm deep. This way you don't affect the working part of the tooth where the roller rides when engaged.

How used is the freewheel ?
The small cog is usually lightly used and the last to get worn out.
Perhaps the high limit screw or "too tight" cable adjustment is preventing the derailleur from shifting completely to the small cog ?

Thank you, everyone!

To answer a couple of questions:

The freewheels is a USED Suntour I bought on eBay. It looks like new, so maybe like [MENTION=158672]FBinNY[/MENTION] said, it's just not worn in. I am not sure I have the patience to do that ....

Should I just go with a brand new Sunrace? I recently read a thread that basically said that chains and freewheels are wear items, and new stuff is better than anything vintage. I am not a purist - I can sleep well, knowing that I have non-matching freewheel and chain.

ALSO, I have access to some Regina (America) and Suntour (Winner) - I am open to going that direction as well!

It's possible that most of the riding by the prior owner was on the smallest sprocket, so only that one is worn. In any case, even when all things are equal, smaller sprockets are always more prone to wear skipping than larger ones, because the teeth sweep into the chain on an angle, rather than nearly straight on.

You can do a quick diagnostic if you have a spare piece of chain. Wrap the chain half way around the sprocket, and tension it as if you were riding. Then peel the chain off from the bottom until only 5 teeth are engaged. Now pivot the chain onto and off that lowest engaged tooth, and see if it sangs on the corner. If so, it's either a worn sprocket (won't get any better) or a slightly too square tooth profile which will get better quickly.