sortieavelo

Hi,

Just upgraded my 1995 trek zx6500 with suspension fork and while I was at it cleaned hubs, replaced BB, new tires, etc.

Took it out on the trail and rear derailleur/chain was slipping to smaller cogs. It doesn’t do it on the stand—shifts great & holds shift. it shifts fine to larger cogs.

but on the trail, it was slipping to smaller cogs. Teeth on crank are fine, cassette is newish, and chain is fine (I used chain wear tool to confirm).

is this a barrel adjuster type adjustment?

John Nolan 

New chain? Could be the sprockets are worn and need to be replaced.

Miele Man

Looks to me like you have some badly worn cogs and maybe a worn chainring. I'd replace the cassette and the chain and then if you still have skipping under load, I'd replace whichever chainring it's skipping on.

Cheers

Andrew R Stewart 

Please better define "slipping to smaller cogs".

Are you saying the chain shifts onto a smaller cog when riding but not when in the repair stand. Or are you saying the chain skips over the teeth of the smaller cogs (but stays on that cog)?
A major difference in cause and solutions. Andy

JohnDThompson 

Chain skipping under load on small sprockets is usually a sign that the chain is less worn than the sprockets it tries to engage. The effect is greater on lower tooth count sprockets because wear on those sprockets is concentrated over fewer teeth. That's why many people replace chain and sprockets at the same time.

sortieavelo

Hi Andy, yes, it shifts fine up and down on the stand and holds shifts through all gears. While riding, it holds shifts when I shift to larger cogs, but when I shift to smaller cogs, the chain after a revolution or two will jump down to one smaller cog. LBS says its not the teeth on either crank or cassette, and they measured the chain and said it's fine.

Thanks!

cyccommute 

That sounds less like a problem with teeth on either the chainring or cogs and more like a cable or derailer alignment problem. Check the derailer alignment. You can quickly check it just by sighting if the jockey wheels are in a straight line with the cassette.

You should also check the cable tension. Shift the rear derailer to the highest gear and see if the cable is loose. It should be taut but not super tight.

Andrew R Stewart 

Well that is quite a different issue then most here think you are experiencing. One would think that the LBS would have made sure the indexing adjustments (cable tension) is proper when they assessed the chain and cog conditions. Seeing an off indexed situation should be easy when viewed from behind the bike and when test shifting.

A few possibilities- Cable friction, or the lack of it, is vital for the shifting to index in both directions and when in the smaller cogs (with the lowered cable tension when in the smaller cogs). So free cable movement needs to be checked for. Besides the usual cable fraying chance there's the aspect of housing wear and how the housing ends are dealt with. I've found that the last loop of housing, from the stay to the der, can often have a lot of internal wear (unseen) and replacing that portion of housing can sometimes help a lot. The housing ends need proper capping and fitting in the frame/der stops. Both metal and plastic end caps can break down and allow housing strands to poke through.

The STI shifters of that era are prone to becoming gummed up with the factory grease losing its oils and the remaining soap base is sticky. Again this sticky lever action is more noticeable in the higher effort gears for that same reason of cable friction in the smaller cogs can be a problem. A thorough solvent flushing and relubing usually revives them. Replacing cables and casing is a good choice at this level of service.

Shimano rear ders saw a reduction of the parallelogram return spring's strength/tension back in the 1980s, Shimano sought a lighter lever pull effort. With the spring loaded levers just coming to the market then the cable had less tug from the der but more push from the lever (and the lack of push that Grip Shifters have is one issue with them). But sometimes this return spring can soften it's pulling ability over time and the effect is greatest when in the smaller cogs. It's hard to feel this loss of spring tension but one can get a better feel for this by detaching the cabl;e from the der and moving the der through its range of travel. The der will lose its rapidness of movement as it returns back to those smaller cogs. If this is at play a replacement der is called for.

A neat way to better assess if the problem is from worn/miss matched cog/chain wear is to, again, remove the cable from the der. Then take an old cable and thread it through the der's adjusting barrel and attach it to the anchor bolt. Position the der/chain on the cog where the problem seems to be and adjust this shorty cable, with the barrel adjuster, to center the chain on that cog. Now the bike can't shift into another cog when riding. Test ride and if the skipping issue remains then it's likely chain/cog wear. If the problem goes away then it's likely a cable friction one. Back in those days when the early Al frames were patterned after steel tubing dimensions frames could flex enough to tug on the cable with each pedal stroke. With a non indexed lever this tugging would slightly move the lever abd bit by stroke the lever would play out cable and the bike being a stupid system thought you were wanting to shift, and it would shift onto a smaller cog. By removing both the real cable's friction and the frame flex assessing the remaining contributions to function becomes easier.

Thanks for making it clearer what type of problem you actually are having. Andy

sortieavelo

Andy,

A million thanks for that--it all makes sense. In fact, the shifters *do* feel a bit sticky now that you mention it. I'm going to solvent/relube those, and try your shorty cable advice on rear mech. New shifter cables and housing/housing ends, too.

If that doesn't do it, then I'll focus on chain/cassette wear.

Really appreciate your thoughts!

Chris

ThermionicScott 

Teeth on crank are NOT fine. I see shark-finned teeth on each ring. Someone rode this bike with a worn-out chain.

SpedFast

I would start with a quarter turn out on the barrel adj. and then check cable tension and cable friction. Per OP It shifts to a larger cog and stays put but when shifting to a smaller cog it jumps an extra one under load. I experienced that when climbing hills and it only took a quarter turn out on the barrel to get it right. Not saying anyone is wrong here, but that is a cheap and easy couple of things to eliminate before buying parts and throwing money at it.

sortieavelo

thanks for this. When you say quarter turn out, you mean counter clockwise, right? And counter clockwise tightens tension, correct?

cyccommute 

I think you are seeing something that isn’t there. I can see machining marks on the teeth all around all of the rings. The divots that the red arrow point to indicate very little wear.

ThermionicScott 

That's always possible! And I know about the various ways modern chainring teeth are shaped to aid shifting -- the ones that are cut shorter, or have part of the side or back cut away... believe me, I've spent time trying to calm newbies who worry that they've somehow damaged their chainrings to produce those artifacts.

What I'm referring to here are the teeth where the pressure face has worn from half of a "U" shape, to a diagonal, so that a chain roller could slip off under pressure. I've marked a few with red lines (perhaps "shark fin" was a misleading word):


It's subtler than some of the worst pictures we see here, which is why the OP is still able to ride the bike much of the time, and the slipping only happens when he dials it up...

Andrew R Stewart 

I have to agree with Stuart that the rings don't look to be worn enough to suggest skipping on them, IME. Andy

SpedFast

Correct-when you shift up in gear it's the spring in the rear derailleur that pulls the chain to the next (smaller) cog and is stopped by the tensioning of the cable. Not enough tension and it will jump to the next gear when you stress it, if not before.

alcjphil

I have another suggestion to offer. If the bolt holding the freehub body onto the hub is a bit loose, the movement of the body under load might allow the chain to shift to different cogs when on the smaller ones

cyccommute 

Here’s a picture of what is stated to be an NOS STX chainring. The red arrows point to odd shaped valleys that are similar to sortieavelo pictures. The green arrow points to a rather sharp “shark finny” tooth that is, again, similar to what can be seen in sortieavelo pictures.

Miele Man

Okay.

Is it possible that there's excessive bottom bracket flex for some reason?

Cheers

mitchmellow62

There's lots of knowledge and experience on this forum but I would approach the problem by the KISS principle. Start with the simplest solution first. It is not unusual for one to get perfect shifting on the stand but have some slow shifting or even auto shifting in actual use where forces are greater. Try the cable adjustment first. Alignment next. Parts replacement last. I have only had one rear derailleur with a weak parallelogram spring. The shift to the smallest cog is slow while all the other shifts are fine. It is worst when the chain is on one of the smaller chain rings.

Andrew R Stewart 

Sure but... As I mentioned with enough frame flex the cables (not running down the exact center axis of the tubes) will see some tugging and if the lever has too little friction to hold against this tugging then the lever can migrate and in time move far enough to play out enough cable to cause an up shift. This issue pretty much has gone away because of the levers being indexed; the detent is a pretty large "friction" factor to be overcome by the tugging. I doubt this condition is at play with the OP's bike. I mentioned it for the benefit of others who might have non indexed levers. Andy

bikemike73 

On the LARGE chainring I can see 4 teeth worn about 50%.

Location looks to be opposite of each other , but 2 in a row.

I believe it is the chainrings. if mine looked like these, I would change them .

Hope this helps, and Good Luck !

cyccommute 

Those aren’t worn…at least not excessively. The “2 in a row” is the clue. Those are purposefully flattened as part of the ramp and pin shifting aid. You can see the same pattern at the 12 o’clock position in the picture I posted. They are a feature, not a bug.

bikemike73 

Learn something new everyday

I have never heard of " ramp and pin shifting"

What is it?
What does it mean ?

Thank you for the info. Much appreciated

ThermionicScott 

Ah, gotcha. I wasn't too worried about the tooth valleys themselves... can't imagine what purpose is served by those teeth I don't like, but I can see them on other NOS rings now. Weird.

And after rereading the OP, the problem was "auto-shifting" the whole time. Oops, sorry for the tangent, folks!