Lascauxcaveman 

But what surprised me, is it shifts to and from the large (34T) cog without complaint, and it's the small one (13T) it won't play with nicely - chain skips all but the lightest load. Is it just a matter of chain length in this case? The chain is really loose when it's running on the smallest cog. (Standard 52/39 chain wheels up front)

The other thing that surprised me is when it's running on the big cog, it seems to still have some give in the spring; as if it's not completely maxed out at 34T.



It's a 1980ish Shimano 600 Arabesque medium cage RD. Original freewheel it came with was a 5 speed 14 -28 (worked fine with that one); the one I'm experimenting with here is a 6 speed, only one tooth smaller on the small cog (13 - 34).

I'm pretty inexperienced with tuning RDs, having only changed them out on a few bikes, and know the long-cage ones seem to always soak up any range I try.

Road Fan

What are the tooth counts on your crankset?

PatTheSlat 

It's my experience that chain skipping is almost always caused by a worn chain and/or freewheel/cassette.

oldskoolwrench

Based on the photo and your description I would venture to guess that the 13t cog is worn out. It appears as if there's wider spacing between the teeth on the 13 as compared to the 28t cog (2nd position).

I'm surprised that the short cage DR can shift cleanly up to the 34t without the upper pulley catching the 34! A long cage DR will alleviate the chain 'sag' in the small/small combination.

Lascauxcaveman 

Naw, they're both pretty fresh in this case; the freewheel is one I got NOS and has less than 1000 miles on it, the chain and chainrings is 1980ish shimano 600 (original to the bike it's on) from a barn find with very little use.

The chain is notably loose in the small/small (39/13) combo. But it skips under load in the big/small (52/13) combo as well.

Road Fan

I'd start with taking three links out of your chain to get the cage to lay over to maybe 45 degrees when you're on the small-small. That will tighten the chain in the 39/13, possibly enough to eliminate slack. While the chain is off shift the rear derailleur so you can verify that the cage lines up with the small cog when it should and same for the large cog. If it can be limit set so both of these condiitons are met, you have a chance of settin up good shifting. If it can't, you might have not enough spacers on the drive side of the wheel axle.

Basic advice is to try to set this up by the book first, and then see if it works better. It's not at all clear you have things positioned correctly. Bikes will usually ride and shift after a fashion when not set up right, but will shift better when right.

Sorry, make it four links, it's gotta be an even number!

Sixty Fiver

Derailleur capacity is calculated by adding the difference between the smallest and largest chainwheel and adding that to the difference between the smallest and largest rear cog and is independent of the maximum cog size a derailleur can handle... most derailleurs have published ratings of their capacity and maximum cog.

The capacity of a 600 GS (long cage) is 34 with a max cog size of 34, your 600 has a capacity rating of 28 and a max cog of 28.

You drivetrain has a 34 tooth rating and the derailleur you have, which has shorter cage than the GS cannot match the needs of the drive for capacity, I am surprised it will handle the 34 without kissing the cog and when it drops down to the 13 it does not have enough take up when you are running the 39.

If you set up the drivetrain so that it had no slack at the 13 tooth cog , it would not be able to handle the 34 with the 52.

I would agree that the 13 tooth cog does look worn more than the rest of the cogs as well.

You need a long cage derailleur and a new freewheel.

Glennfordx4

This combo shouldn't work out on paper with that RD, but going from a 14-28 to a 13-34 the chain should have the RD over tensioned when on the 34 & under tensioned when on the 13 if the chain was cut correctly. Did you check the chain for wear with a chain checker, I know you say the FW doesn't have a lot of miles but looking at the pictures it looks worn on the 13 ( it could just be the picture I suppose ) which is what will cause it to skip as will a worn out chain on a good cog will do, check both. Working with used chains and cogs that didn't wear together can cause a bunch of grief (been there done that) that's why it's always better to replace them at the same time or at least keep the mating chain & cogset together.


Glenn

Lascauxcaveman 

Perhaps I should have said in my OP that I'm not determined to make this setup work; in fact, it was just loaner-wheel situation where I wanted to compare the ride of the brittle, hard-as-rock tires the bike came with the cushy Schwalbes I had sitting around waiting to go back on another project.

But since I also had my little screwdriver with me, I figured, "What the hell, let's see if this old medium cage derailleur likes that big ol' 34T cog" and backing off the screw a turn, was very surprised that it did. And I was equally surprised it didn't work with the smaller 13T cog. I spent the next 20 minutes making sure the "high" adjustment on the RD was set to line up the RD to the small cog, back-and-forth, back-and-forth, still no go. Skipping like a little Catholic girl on the way home from Sunday school. "This is ass-backwards," I thought to myself, riding off for a 20 mile jaunt undhindered but for a lack of that top gear.

The "worn cog" theory didn't occur to me, since I bought that freewheel NOS and haven't used it all that much. To my admittedly untrained eye, it looks pretty fresh, but is very skippity on the small (13T) cog, on this bike. On the 3 other bikes I've had it on - all with long cage RDs - it's fine. In fact, it's "SunTour Perfect". Here's a better photo:


And here's the original 14-28 freewheel, marked Shimano UG, maybe a little more worn? But this is the one that happily works in all gears:


I did an informal tension test with the chain riding the small/small combo, and while it seemed pretty loose, it wasn't visibly sagging and the tension seemed similar to other bikes I have sitting around with the chain on small/small.

Anyway, I thought it'd be a fun thing to bring before the collective wisdom of my fellow C&Vers who are more experienced with this stuff to see what they thought of what I thought was a backwards situation.

ThermionicScott 

This RD has a B-screw, right? I wonder if by backing it out so much that it can clear the 34T, it doesn't have enough wrap on the smallest cog and that's why it skips.

eschlwc

sheldon taught me never to ride in the small/small combo. when i first started riding again, i did this all the time. now i know to look for a better gear with more efficient chainline.

dddd

I think that the UG chain may have premature wear, i.e. stretch (they wore quickly for me).

Perhaps also the UG chain's width won't clear the thicker suntour cog in the adjacent next-larger pos'n?

Also check the chain's tension on the big-small combo, and the amount of wrap around the smallest cog in that combo.

Lastly, if the smallest cog has wear that is allowing the chain's rollers to reside in an ever-so-slightly "advanced" position under load, the chain's rollers may be hanging up on the tooth's driven-side tip corner, causing the familiar "failure-to-engage" and resultant skipping under load.
And, if the driven-side tip corners are beveled off with a 3/8" dia Dremel stone, at a 30-degr angle to the driven face of the tooth, a 1mm bevel will typically correct any failure-to-engage under load. I've done this to many a worn sprocket's teeth, while still installed on the bike, to readily correct any skipping under load after installing a new chain on a used freewheel. I've also done this using a bench grinder holding the loose cog in my hands. A tiny bevel makes a huge improvement.

TiHabanero

You say it works fine on three other bikes. I am assuming you are moving the chain along with the FW from bike to bike? If not, take one of the chains from one of those known good bikes and move it to the bike that has the skipping problem. If it skips, I'll be a skippity monkey's uncle.

pastorbobnlnh 

In my experience, you have a rare Suntour Perfect freewheel with the 13T small cog. I've personally handled 100s of Perfects and have had just one 13T Perfect.



The reason I mention this is that if the 13T is worn or bad, it might be almost impossible to find a replacement cog. But it is my guess that your 13T is fine, as has already been mentioned, it's having too many links in the chain. The challenge is that if you shorten the chain to work on the 13T it probably won't play nice with the 34T while using the medium cage Shimano 600.

So another option might be to replace the 34T cog with a 32T or a 30T.

Lascauxcaveman 

I remember shopping eBay for quite awhile before I saw this one. At $50 (with the shipping) I'm sure that's the most I ever paid for a freewheel.

As I mentioned before; I'm not determined to make this setup work, just wanted to see what others said about the 14T being hunky-dory and the 13T being unusable. And to express my surprise that the 34T worked as well as it did, with actually a little room to spare. Right now I'm liking dddd's theory about the slight differences between the Perfect cogs and the UG chain, and will file away his tip on beveling the driven faces, next time run into a cog that actually is worn.

Here's one more shot, with the chain on the small cog, does that look like enough wrap, generally speaking?

jeirvine 

+1:

owenmyers

I have a 13t replacement cog for Suntour.

dddd

There's no B-screw on these older Shimano derailers, and the springs are clearly very relaxed in the small-small combo as shown.
But the OP doesn't use the small-small and states that the problem occurs in the big-small, so something other than chain tension and wrap seems to be to blame.
Not that increased tension and/or wrap alone wouldn't reduce or eliminate skipping, ...it just might.


I wonder if it would be possible to put enough load on the chain (enough to make it skip) with the bike on the stand, so that any failure-to-engage could be observed visually.
That is how I first verified the true cause of a new chain skipping under load on a well-worn freewheel cog, by watching the chain rollers hanging up on the protruding driven-side corner of the worn cog tooth that the chain was trying to engage, leading to successive rollers riding atop the tips of the next several teeth.
Note that the wear was near the base of the cog teeth, causing the corners up at the tips to effectively protrude into the chain roller's path during engagement into the space between cog teeth.

dddd

Note that these early Perfect (and ProCompe) freewheels had a larger threaded diameter for the small cog(s), so any 13t Suntour cog you find will almost always be from one of their later freewheels with the smaller thread diameter.
Earlier Shimano Freewheels also had a larger thread diameter than their later offerings, and I've never seen one of those with a 13t.

Lascauxcaveman 

Both points correct, in this case. I think I'd try shortening the chain if I determined to make this work (there was still some give left on the 34T cog). Or I might try it anyway, if I find a chain that I don't mind sacrificing. If I do, I'll post an update.

DiegoFrogs

In the case of a 13 tooth (odd numbered) cog, I'd expect it'll ride up on the inner plates, which occur every two revolutions of the freewheel.

I had a problem where I was bouncing the chain off of a Pro Compe recently, and after trying new pulleys I ultimately witnessed the chain lifting onto a slightly twisted tooth. Since the offending cog was an even-numbered cog, half the time it wasn't happening (i.e., that tooth only sees outer plates and doesn't lift), which made it very difficult to troubleshoot and see in the workstand.