Strange skipping problem
 

This weekend saw some bad storms come through so I took the downtime to do a much needed tuneup on my bike. Took off the chain and did a scrub and clean on it, the crank and the cassette. I was having some mild skipping and jumping problems before the cleaning. Now what is happening is that when I shift from lower (bigger) to higher (smaller) it ships from 6 right to 8 (8 speed cassette). If I shift from 8 back to 7 it stays there and doesn't skip out. Only coming down the cassette.

Does this seen like a cable tension problem? If I adjust the barrel a few clicks either direction it seems to mess up all the other working gears.

Old 99 Diamondback, low end components all the way around, RD is a Shimano 8515. Aside from all the low end stuff this thing had been bomb proof.

I suspect that you may have a sticky shift cable. You may have compensated for it a bit by biasing the trim so most gears shift OK, but it isn't predictable. Other things that can cause this are poor hanger alignment, worn RD pivots or pulleys, worn chain, and/or poor RD adjustments .

Try adjusting the RD trim for the most outboard trim that still downshifts (to bigger) OK. Then after each upshift pluck the bare wire away from the downtube like a guitar string, and see if you can work through all the gears this one at a time this way. If you can get it shifting uniformly when plucking after each upshift, but not otherwise, then it's definitely a cable/housing or possibly (but less likely) a sticky shift mechanism

Well, not sure exactly what fixed the problem, but seems to be working. I lubed up the cable/housing with a few drips of triflow on my fingers. I also turned the rear barrel back in all the way and started shifting through the gears and the were all working fine now. Maybe I posted to soon. My only concern, and doesn't really seem to be an issue thus far is that there is about a half inch of play in the cable when on the smallest cog. That is to say I can pull the cable slack away from the frame a half inch. I cant say I remember if this is how it was previous or not. If I turn out the barrel so much as 1 click it messes everything up again.

Of course the real test will come when I actually get it out and test under load.

That's fine, lateral-flex of the cable isn't an indicator of longitudinal excess. The very last click of the lever actually releases more cable than the others. This is a difficult shift for the derailleur because the spring-preload is low with the spring not very wound up. It's then the function of the outer limit-screw that determines the derailleur position.

Perfect, I hope to finish reassembly tonight and get it back on the road tomorrow for test.

Got it out yesterday. While it is a better than it was before the cleaning it still seems to have issues under load. There it's a fair amount of hesitation going both up and down the cassette. I think I'm thing to order a new cable and chain and see if that helps. But at least it's ridable for the time being.

Replace the housing with the cable, best to replace the chain and cassette together.

Don't spend any money, until/unless you know what's actually wrong. Without a proper analysis of the problem and the cause, you're likely to waste money by replacing perfectly good parts without solving anything.

As I say in my signature " an ounce of diagnosis is worth a pound of cure". the bike is ridable, so this is the time to try to better determine if this a trim issue, oe a chain wear issue. Once you know the cause you'll know where to throw the dough (if necessary).

I'll look at it some more before dropping any money. The cables and housings seem fine. That is to say there isn't any drag and I did lube up everything. I don't believe I have ever changed the chain, cassette, or cables on this bike. I do know the rear wheel isn't exactly true and I can see a slight wobble in the cassette when the wheel turns. I also notice there is some play in the actual derailleur where it attaches to the hanger. If I have some time I will try to get some video if that would help.

Wheel trueness won't be a factor, and Cassette wobble is perfectly normal (within reason), so that isn't a factor either.

Some play at the RD mount is also normal, and you can pretty safely disregard.

The key is how the problem manifests.

consistently skipping under high load, but rarely otherwise, points to chain/sprocket wear
skipping, or more specifically auto shifting, or trying to shift, more randomly, and not uniquely when under load is usually an RD trim issue, but, can also have other causes, for example it could be housing length on full suspension bikes.

You can also confirm a chain related issue by simply measuring the chain for stretch, and if this is the original chain and cassette that would also rule out the cassette, but if not the first chain, then the cassette could be worn from the older chain, which BTW is a common cause of skipping.

Unless you have some dough you need to spend, or time is critical, give yourself as much time as you need to narrow down the most likely cause.

Also, cable-friction tends to only cause laziness and 2-gear shifts going from the big to smaller cogs where it's only spring-tension that moves the derailleur. Your hand can apply a lot more force than that spring and can overcome pretty much any cable-friction when going from small to big cogs.

Since you have issues with the RD shifting up AND down the cogs, there's something else going one, perhaps in addition to possible cable-friction. First do the chain-wear measurement that FB suggests. It's a very quick and simple test.

Then eyeball the bike from the rear for vertical alignment of the rear-derailleur and hanger: Park Tool - Rear Derailleur Hanger Alignment. The rear-derailleur cage should be vertical.
http://bikebooboos.com/images/derailleur%20hanger.gif
http://bikebooboos.com/images/derailleur%20hanger.gif

It's possible if you have play where the derailleur attaches to the hanger, that can cause poor shifting. Or sometimes the hanger isn't bolted to the frame securely as well.

Hanger seems straight by eye. No tool to tell for sure. For sure the chain and cassette are original from 99. Strange thing is that when I measure the chain, with the zero mark on the center of a pin, the closest pin to the 12" mark is at 12 1/32 to center. Am I measuring this correctly? Seems odd to me that there would be that little to no stretch after all this time. I never did beat on the bike and never had anything major happen with it until this shifting problem. This doesn't seem very major, just something I must be missing or overlooking. I did notice that the barrels for the RD are tight on both sides. Tight to shifter body and tight to RD body, not sure if that is going to matter.

Here is a link to a high res pic I took from the rear of the bike, but it's upside down because I dont have a work stand.
http://www.ericksaint.com/images/biking/hanger.jpg

I'll try to setup a camera on the back and shift through the gears and grab some video. Not sure if the symptoms will be the same will no load.

Thanks a bunch for all the help so far, I really appreciate it. It's one of the reasons I had donated I think before I ever made a post here.

Must be stretched a full link + 1/32 a link ;). If it were my bike the cassette and chain would have been replaced once per year. That little stretch (is it a regular/daily rider?) over so long time is indeed rare...I would go and replace the casette and chain just as a regular maintenance thing. Not that it will necessarily solve this problem.

Just cus I didn't see it mentioned yet, are there any stiff links in the chain? to me it sounds like cable stretch or friction in the housing, but you seem to have ruled those out. If barrel adjustments don't work and the housing and cables have also never been replaced I would probably just start doing some of the regular maintenance things and see if it stops, if not at least you've eliminated those possibilities.

Then again, I'm not slow to spend money on things..

EDIT: and you are measuring right...should be an inch from pin to pin and no more than 1/8th inch stretch over 12 inches.

It isn't necessarily that the chain has little stretch even after 12 years. Chains don't stretch over time, they stretch over miles. There are lots of bikes that age with less than 3k miles on them, so the condition shouldn't surprise you. OTOH, just in case, did you put a bit of tension on the chain to ensure that all the slack was taken up before measuring?

Another thought - is there any play in the RD cage pivots or between the jockey wheels? such that something is slipping side to side..

The bike is the only one I have had since I bought it new. But admittedly my riding has gone in spurts over the years, I'm guessing there are no more than 2k miles on the bike. And that would most likely be the high side. I didnt initially put any extra tension on the chain, but I did it again while pulling on the chain and got the same reading.

Here's a quick video. But of course it doesn't really seem that bad. Was definitely worse out on the trail. At the end you can see the wobble I was talking about in the cassette.

http://youtu.be/SIlQvCP3uN8

Not sure what the hell happened. I didn't make a single adjustment from Tuesday's ride. Today's ride was 16.70 miles and not a single skip, tick or other problem. Shifted through the whole cassette several times just to test. No idea, hopefully it stays that way.

Again thanks for all the help everyone.