msl109

Hi folks - just built up an older Cannondale 3.0 for my wife. Used an HG90 cassette, 13-30 in what looks like good condition, Shimano 105 (RD1050) RD. Wipperman chain.
The chain is slipping under power on the first 3 upper gears (13t, 15t, 17t). There also seems to be a bit of chain suck on the 13.
The chainline seems to be ok (see pic) and the cogs look to be in good shape (other pic). But when I adjust the RD tension, I get a slight improvement, less slippage. So I would normally attribute the slippage to low chain tension, but the tension feels right. If I remove another link, the RD will be stretched out past normal limits with the chain on on the 30t cog. I have thought to replace the 105 with a wider range derailleur (it's rated for 28t max, though I think this is conservative) so I could remove a link and still have the derailleur handle it. I'm just thinking I'm missing something obvious (?) Or I'm wrong about what I think is right ...
Thoughts? Appreciate any suggestions!

Chainline. Is the cage bent a bit inward? Significantly?

cog wear

alcjphil

A longer derailleur cage will not allow you to remove links from your chain. If the chain is too short for the big-big combination the cage will not change that fact

trailangel

Bent Derailleur Hanger!

alcjphil

According to OP: " Used an HG90 cassette, 13-30 in what looks like good condition,"
It is pretty much impossible to tell how worn a cassette is by looking at it. The true test is if the chain slips on some cogs

msl109

thought so. Possible culprit?

I think the derailleur cage itself may be bent.

msl109

Well, the chain is indeed too short for that, and that's why I can't see shortening it even more. But I'm not talking about crossing the chain all the way to extremes (large front to large rear)

Trakhak

In at least 9 cases out of 10, skipping on the smallest cogs means that the chain is worn out or the cassette is worn out or both are worn out. Install a new chain; if the skipping persists (which it almost certainly will), install a new cassette.

And get the derailleur alignment checked at a bike shop.

Once everything is squared away, start using the big ring and the middle of the cassette rather than exclusively using the middle or small ring and the small sprockets. The cassette will last much longer.

dsbrantjr

But if you or your wife does choose the big-big (and probably will at some point) combination with a too-short chain you will damage your derailleur and/or frame.

andrewclaus

Have you measured the chain for wear?

The hanger does look slightly bent in the photo.

msl109

True - and its about as tight as I'd want it as is. I think the suggestion that the hanger is out of line may be the situation - going to check into that and also try another cassette just to check it isn't a wear issue.

msl109

Thanks - you're probably right about wear. The chain is new, so that's not the issue. I'll replace the first few sprockets with ones I know have had no issues, have the alignment checked and see what happens.

msl109

The chain is brand new!
I don't see any significant rounding off on the cog teeth but I know that can be subtle.
the hanger may be slightly bent - will have it checked.
pretty sure the derailleur cage itself is out of line - probably happened when I rebuilt it.

Shimagnolo

^This.

And also, RD tension and chain length have nothing to do with a chain skipping on the cassette. Sometime back as an experiment, I installed a new chain on one of my bikes, and didn't remove any links at all. It has massive slack (on the lower chain) when on the small chainring and on the smaller half of the cassette, but functions perfectly.

msl109

Anyone want to chime in on this? Most of what I've read from professional sources says otherwise.

dsaul

The amount of "tension" the chain puts on the derailleur cage can have a slight effect on the distance from the upper pulley to the cogs, but that can be corrected with a B-screw adjustment. A slack chain may result in slower shifting, but it has nothing at all to do with the chain slipping on the cogs. If the chain is slipping under load, it is definitely a worn cog that is to blame.

Trakhak

Anything's possible. People fixing a flat for the first time and experiencing tube failures might indeed be buying defective tubes, but tubes from the same batch mysteriously never fail when installed by experienced mechanics.

Replace the cassette as soon as possible. Riding with a worn cassette can wear out a chain prematurely.

ClydeClydeson

if used cassette + new chain = chain slip under load, the cassette is worn.

There may be minor hanger misalignment but the problem is almost 100% definitely caused by worn cogs. If it still slips with a new cassette I will eat my hat.


(I own a hat made out of gingerbread so this is no big sacrifice.)

msl109

You get no gingerbread, then; you and the others who said it was a worn cog were right. The der. hanger wasn't out of alignment, but the cage was bent a bit. I realigned it, and that had some positive effect, but on the smallest cog, I can see the chain catch on the cog before releasing, and under force, it slips. In fact, when the crank is turned backward, the chain rides pretty much on top of the teeth. The wear isn't visibly obvious, but the chain action tells the tale. The rest of the cogs behave normally. So I'm going to swap the cog for one I know is good, and I believe that will do the trick. Thanks for the advice!