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Old 09-19-06, 11:44 AM   #1
pgoat
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Help diagnose my shifting woes pls

1993 Marin Pine Mountain MTB, 46/36/26 up front, 13-30 7-spd HG in rear. XT Front der, DX rear, XT rapidfire shifters.

I just replaced the original chain, as I measured at least 1/16" of stretch, maybe more like 1/8". The bike was used a few years off road, then stahsed til 2005, when I started commuting on it. It might have 1000 miles on it tops. The 36T chain ring was replaced a few years back as I had bashed the original while offroading. I now only use the bike for street commuting with slicks.

The last few years when I approach a hill and shift to the granny 26T I would start freewheeling madly. I'd have to shift up to the 13 (smallest rear cog) to 'catch' the chain. There was no noise, just free and clear spinning as if the chain had disappeared! Once I got it going on the 13 cog I could shift down to the larger cogs for an easier gear but as you can imagine, this makes for frustrating climbs.

I inspected the cog and chainwheel teeth and they look okay, so I bought a new SRAM PC-68 chain and removed a link or two to match the old chain length. Last Sunday I spun some clipless peds on and took the mtb to a metric century in hilly LI for its debut with new chain, assuming the granny gear would make the day more pleasant. Wrong - First steep hill I shifted, and this time got the same freewheeling sensation, but with an awful grinding sound (chain on metal). Unlike the old chain, I couldn't shift it to 'catch' no matter what, and as I couldn't unclick on the hill, i spazzed and fell - luckily into some soft roadside leaves I managed to steer over towards (ahhh! Nice!). Unfortunately a minute later while trying to click in uphill I went over the other way onto the road (ow!).

so i am rather perturbed....

the other shifting issue is a hesitancy when shifting to larger rear cog (you usually need to put pressure on the big right lever after the 'click' to get the easier lower gear). up shifts are still pretty crisp. This is the same with new chain as with old.

The new chain is 'skipping' at times - usually in middle ring, middle cogs - maybe a worn cogs there? or chain slack? The old chain did not do this.

I am wondering if my cogs and or chain rings are worn even tho they look okay? do i need a der adjustment? cable stretch? (I had the cables replaced and the bike tuned about 7 yrs ago, hadn't used it much until last year, it just sat in storage).

One final oddity - I was comparing my mtb to my wife's and realized my derailleur is a short cage model - not the typically seen long cage like on most touring or mtbs, tho I believe it says "max 30t" on the body. ...... could it be the chain is too slack because of this? Would I need to remove a link? fwiw I do try and keep a str8 chainline when shifting (ie, small cogs with big front ring); I only did the little cog in granny gear as a last ditch bail to get the chain 'on' when about to climb.
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Old 09-19-06, 12:08 PM   #2
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Likely that your granny is worn beyond redemption. Replace it. Notably, your 36T was replaced and is OK, and the biggest ring, too. Maybe when it was ridden in its younger days, much time was spent in the granny?

If your chain was closer to 1/8th overlength than 1/16th on measuring -- 12 complete links = 12 inches for a new chain -- then also likely you will need to replace the sprockets.
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Old 09-19-06, 01:00 PM   #3
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that's what I am afraid of - tho they certainly look okay. No shark fin shape, etc that my eye can see, anyway.

I am wondering if this is the case is it worth getting the cassette removal tool? I don't have a bench vise, btw. I could just bring it to the LBS of course. Maybe get a new set of cables and a der tweak while it's there....I have so little patience for such adjustments.

THIS is why I rode a SS for many years.....and why the MTB sat unused.
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Old 09-19-06, 01:06 PM   #4
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Sonds like the freewheel/hub is worn/damaged.
I doubt a 23t cog will wear significantly before a 13t cog.
If the cog teeth do not have burrs on them then the problem is internal.
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Old 09-19-06, 01:07 PM   #5
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Get the LBS to do it if you aren't sure or don't have the patience (which is OK). Shark-fin shape is a last stage in tooth wear, but the symptoms occur for a more technical reason. The original chain wore together with the teeth and the overall pitch of chain and teeth changed (from half-inch between the rollers to slightly more). When you put a new chain on, that chain has a half-inch pitch, but the teeth are slightly wider than that, so the rollers on the chain tend to ride up over the teeth, causing chainslip.
The measurement guide usually is 1/16th of an inch over 12 inches is fine to only replace the chain; 1/8th of an inch over means the chain AND reawr sprockets needs replacing, and perhaps the chainrings. The chainrings depend on whether they are made of aluminium alloy or steel, the latter lasting longer. The symptoms you portray in the middle chainring might also suggest that you need to replace the that as well.
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Old 09-19-06, 01:10 PM   #6
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Originally Posted by powers2b
Sonds like the freewheel/hub is worn/damaged.
I doubt a 23t cog will wear significantly before a 13t cog.
If the cog teeth do not have burrs on them then the problem is internal.
If the bike was used for a few years off-road, then 23T wear is more likely than 13T.
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Old 09-19-06, 02:39 PM   #7
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Thanks for all that guys

fwiw I mainly used the middle 36T ring all these years, so it's not impossible the new one is worn. I rarely use the 46T ring. The 26T granny ring was only used on real steep dirt climbs until '05, when I started using it on some steep climbs to baby my knees while tired at the evening commute with the extra weight of chain and backpack (eg, Manhattan side ramp of queensborough bridge - ugh....big difference when I am rested and unladen on a road bike on a Saturday or Sunday morning)

I think from what I see and hear and what you are telling me the explanation that old chain and cogs wore together for 13 yrs makes the most sense.

Maybe I'll keep the 26/36/46 arrangement up front but get a smaller spaced 'corncob' in back to handle the street stuff. Been wanting to try that anyway....unless I am better off raising the front der and getting bigger set of chainrings instead?

I'll prolly take it to a shop. I actually wouldn't mind replacing the casette myself but adjusting everything is not my forte.

Observing Murphy's law however, two days after I spend $$ on a tune up it'll get stolen from outside my office bldng

finally, does my 'too short' rear derailleur/too loose chain theory hold no water at all?
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Old 09-20-06, 07:38 AM   #8
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There was no noise, just free and clear spinning as if the chain had disappeared ! Once I got it going on the 13 cog I could shift down to the larger cogs for an easier gear but as you can itmagine, this makes for frustrating climbs..
I've just rebuilt a 7 speed rear cassette that had this very problem. I found the original grease had hardened and was sticky holding the pawls off the ratchet teeth. With everything cleaned and regreased it now works like a charm, very smooth and precise action. The same thing had happened to the shifter pawls preventing the indicater from moving and no gear or chainring changes. Same problem, same cure. It was not a difficult job just very fiddly with all those tiny free running balls.
If you attempt this I would suggest making a drawing of everything as you disantle it to help getting it back together correctly.
This bike had been junked after a long storage and that was all that was wrong with it.
Regards
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Old 09-22-06, 09:14 PM   #9
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so I did one 48 mile ride with the new chain, then used it on three days' commuting (9 miles each way). Coming home thursday night there was an awful skipping glitch every three revolutions or so.

Upon arriving home my guess proved correct - twas not the cogs but the chain - one link is totally frozen stiff on the new PC-68. Can't budge it. Tried flexing, pushing the pin out, no good.

Ironically I slapped the old chain back on for Friday's commute and while it took the usual coaxing to shift now and again the bike was imminently more rideable.....sigh
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