4

BACKGROUND NOTE: The bike is a 1992 Univega MTB that originally came with 6-speed SIS with a triple chainring. I removed the SIS shifter when it broke, and went with a friction shifter. The bike is used as an all-purpose commuter/errands bike; I was having no problems with the "old-tech" setup other than a worn chain, and I have no plans to change the friction shifter on an old bike.

OLD SETUP
6-speed Sunrace Freewheel (straight-cut, 14-28); 6-speed chain (don't know the type).

NEW SETUP
7-speed DNP Epoch freewheel (beveled or ramped cut cogs, 11,13,15,18,21,24,32); KMC Z50 chain (1/2" x 3/32", 6-7 speed). The KMC chain box says "Indexed."

BOTH SETUPS USE:
Long-cage rear derailleur (Shimano Exage), triple chainring (28,38,48; Shimano Alivio, 18-24 months), and friction shifters.

THE REASON for the change:
chain wear and skipping, wanting two faster cogs and a lower hill-climber cog. The new chain was adjusted for length using the chain adjustment equation on Parktools web site. After installing the chain, I doubled-checked and eliminated any chain kinking. The rear derailleur is adjusted and shifting well with the new setup. When I re-installed the rear wheel, I placed a thin washer between the right-side axle nut and the dropout for chain clearance on the 11-cog but it might not be necessary.

THE PROBLEM:
The new chain is intermittently skipping in a "skip-forward" or half-shift" motion; less so when riding casually, more so when riding under load; less so on the 48-tooth chainring, appears to be more so on the smaller chain rings.

What's wrong with my new setup?

Improve this question
3
  • 1
    Assuming you're getting the friction shifter properly adjusted, most likely your chain is too long.
    – Daniel R Hicks
    Commented Dec 30, 2013 at 11:54
  • The chain could be too short for the amount of tension in the upper pivot. A tight chain will pull harder against the guide pulley, pulling it farther away from the sprocket, hiding misalignment issues. A looser chain is "tucked in" better by the guide pulley.
    – Kaz
    Commented Aug 6, 2014 at 22:45
  • I have a comment in addition to my answer and it is this: 11-32 is a ridiculous gear spread. Not all derailleurs will handle this range very well. The large sprocket means that your chain has to be quite slack if you want to engage that sprocket and your largest chain ring, and the derailleur has to pick up all that slack to be able to also engage the smallest ring and smallest sprocket without pulley clash. You do not need this. If you're on a hill where you feel you have to engage the 34 together with your smallest chain ring in the front, you might as well get off the bike and walk it.
    – Kaz
    Commented Aug 13, 2014 at 23:34

4 Answers 4

Reset to default
5

sounds like your chainrings are worn. Look at the rings and see if they are "shark fin" shaped rather than symmetrical - a google image search for "worn out chainring" will show examples including some really, really worn out rings. The old worn out chain likely wore out the rings as well.

I've had exactly what you're describing on a couple bikes and it was remedied with a new chainring up front.

Improve this answer
4
  • In 18-24 months, seems unlikely that the chainrings are worn out.
    – Batman
    Commented Dec 30, 2013 at 23:55
  • 1
    I disagree. I've worn out plenty of inexpensive rings in that amount of time.
    – OCary
    Commented Dec 31, 2013 at 0:43
  • @Batman - The fact the skipping occurs less on the large chainring supports OCary's original suggestion. Larger diameter rings will typically wear better than smaller diameter rings (assuming equal usage).
    – Rider_X
    Commented Aug 6, 2014 at 23:15
  • 1
    The fact that the skipping occurs less on the chainring is also a consequence of there being less leverage on the large chainring resulting in less chain tension. The jockey wheel will also droop more when the chain is on the smaller rings, resulting in less chain wrap. My POS bike has the original rings on it which are probably 10 years old, and show considerable wear: there is no skip/skip in the system after proper derailleur tweaks.
    – Kaz
    Commented Aug 13, 2014 at 23:27
3

I have experience with DNP freewheels (7 speed, 11-28) and KMC 6-7 speed chains.

I had exactly the same chain slip problem (literally word for word) with chain slipping and managed to fix it with great deal of attention to the derailleur setup.

You might think that your rear derailleur setup is good because of the evidence provided by good shifting. That observation might hold true for a different freewheel; for the DNP, the setup has to be ridiculously good, beyond merely exhibiting good shifting.

The DNP freewheels seem to be machined such that the chain skips easily. The result is that jockey wheel (upper pulley) has to be set up to track the freewheel's cogs very closely so that there is a large amount of "chain wrap" (the proportion of the cog's circumference that is meshed with the chain).

The jockey wheel position can be tuned with the "B-tension" adjustment in your derailleur. If the derailleur has no such adjustment, then you have to resort to drilling a custom hole for the spring through the body of the hanger, which is what I did.

Why the problem is worse when you shift the front derailleur to the lower rings is two-fold. One is that you have more leverage, so you're generating more chain tension when you mash the pedals. The other is that the increased chain slack will cause the jockey wheel to droop a little bit more, reducing the amount of "chain wrap". (Experiment: shift the rear derailleur to the smallest cog, and then observe the difference in the jockey wheel position when you shift the front derailleur.)

With the strict setup, there is no chain skip or slip under load whatsoever. I can stand on the pedals while climbing a hill and mash with confidence. On the highest two gears (smallest sprockets), I have a full 180 degrees of chain wrap: meaning that the chain entering the sprocket is parallel to the chain leaving: both are horizontal, and so about half the sprocket's circumference (180 degrees) is meshed with the chain. The jockey wheel is so close to the freewheel that it almost makes contact. This is a much tighter setup than what I see on the average bicycle (something I've been paying attention to lately when looking at any bicycle), yet the chain does not slip on the average bicyle. Now if I increase the B tension so that the jockey wheel droops by just an inch, the chain will start to show signs of slipping (climbing a hill, using the middle ring and second smallest sprocket). With confidence, I can pin the blame on the DNP freewheel.

On a different but related note, DNP freewheels will also wear out fast. I'm on my second one; the first one did not even last a year!

So, in spite of the fix (or workaround, if you will), the fact remains that the DNP freewheel will be garbage in about a year, and that the stock, original, heavily used freewheel on the bike (an inexpensive bike at that) never exhibited chain skip and slip problem regardless of the derailleur being out of adjustment, and that its teeth still appear taller than those of a brand new DNP freewheel.

If anyone knows where I can get a 7 speed thread-on-freewheel not made by DNP, in the range 11-25 to 11-28, I would love to hear about it.

Improve this answer
7
  • There are still many high quality thread on freewheels made today.
    – whatsisname
    Commented Aug 6, 2014 at 1:25
  • @whatsisname Do you have some links? What configurations are they available in?
    – Kaz
    Commented Aug 6, 2014 at 1:40
  • IRD probably makes the best freewheels out right now, but sunrace and shimano still sell 7 speed freewheels. However, I don't think you can get the ranging you want for that.
    – Batman
    Commented Aug 20, 2014 at 1:13
  • I've been having the same exact problem with NOS 46-34 chainrings, new KMC x8.99 chain and a 11-34 SunRace cassette with a vintage Suntour Arx derailleur. The vintage Suntour's knuckle puts the pulley well below and rearward of where a modern Shimano derailleur would, so not enough wrap to prevent slipping on the bottom 3 sprockets.
    – kfix
    Commented Sep 12, 2016 at 3:47
  • @kflix On top of that, let's face it, SunRace is junk. Probably on par with that DNP in the answer.
    – Kaz
    Commented Sep 12, 2016 at 3:52
1

Check the chain for stiff/sticking links. Especially where the chain was joined together. Make sure that all the links are able to move thru their range of motion correctly without binding. Sometimes, where the chain is joined, the "outer" side plates will bind. You can twist the chain 90 degrees to it's range of motion direction to free it. Dont twist too hard.

Improve this answer
0

Two thoughts, maybe with the washer the wheel is ever so slightly out of dish, meaning your chain-line may not be as straight as you would like it. The fix for this would be lose the washer.
The other may be a problem with your friction shifter holding it's spot over a longer chain. Was your chain the correct length before? It sounds like it likely is now but if it was to long before less tension would have less pull on the shifter. Is the shifter tight?

The chain ring items is surely worth looking at, see if worn out or bent. I haven't answered anything on this before so I may not be able to get back to you as I may never find this website again :).

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.