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Old 05-04-05, 11:40 PM   #1
genec
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something is jumping...

Have an older 80's Sannino with modern wheels... CAD ISO3 and 105 hubs, using a Hyperglide 7 speed cassette, with Shimano narrow HG chain and a friction shifter. Here is the problem... something slips on rare occasions when I really jump on it... such as just finishing a decent and then standing on the pedals to attack a new ascent.

Thought at first it was the freehub, and perhaps a pawl failing... but I changed that last night.... darn thing did it again today. The chainrings are in good shape, the cassette and chain are only 350 miles old. It has done it in the lowest three gears only... Never does it if I am in high gear (for instance 52-15 and just cranking hard)

Seems to be related to coasting a bit and then attacking. It is very intermittent... but the problem has dumped me once just as I attacked a short steep road... something slipped with a loud "clunk" and I went down hard... Ouch!

Just looking for ideas here... throw me some ideas at where to look closer. On the work stand, everything looks good... chain line is straight... teeth look good. I am baffled.
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Old 05-04-05, 11:58 PM   #2
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Is it possible that you think you have shifted a cog in the rear, but the der didn't actually move the chain over. Then when you push it, the shift actually occurs?
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Old 05-05-05, 06:38 AM   #3
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sound like it could be the chain not mated to cassette, thus skipping. or just a stiff link.
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Old 05-05-05, 07:42 AM   #4
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I've seen both of the above two things happen to people before, so those could definitely be the culprit.
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Old 05-05-05, 09:18 AM   #5
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Originally Posted by genericbikedude
sound like it could be the chain not mated to cassette, thus skipping. or just a stiff link.
It happens so infrequently, I bet is is a stiff link... the problem is that I just have no way to trust right now that I can stand on the pedals and attack. I don't think it is a shifting problem... the shifts are pretty snappy.
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Old 05-05-05, 11:13 AM   #6
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Know of any ways to find a stiff link? everything seems to be rolling smoothly in the shop.
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Old 05-05-05, 12:22 PM   #7
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The best way to find a stiff link is to just put the bike in a stand, and spin the cranks backwards and forwards until you see what is causing the problem. It could be your derailleur pulleys as well, if those are older and not in good condition--they could stick causing tension that pulls the chain into a different gear. Just spin the pedals while watching the rear derailleur and chain and cassette for a while, and see if you notice anything thats not moving in a perfect circle or parallel--you should be able to find the problem.
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Old 05-05-05, 02:35 PM   #8
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Originally Posted by martin_j001
The best way to find a stiff link is to just put the bike in a stand, and spin the cranks backwards and forwards until you see what is causing the problem. It could be your derailleur pulleys as well, if those are older and not in good condition--they could stick causing tension that pulls the chain into a different gear. Just spin the pedals while watching the rear derailleur and chain and cassette for a while, and see if you notice anything thats not moving in a perfect circle or parallel--you should be able to find the problem.
Yeah, been doing that... replaced the idler wheels on the drailleur with metal sealed bearing units... mindful of the spacing. I checked the chainline last night... just to make sure... arrow straight. Watching carefully while working backwards for any thing that seems different.

I'll try some different lighting on it and see if I see any links jump... sometimes a spot light instead of bright flourescent lighting can "enhance" a problem.

Thanks
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Old 05-05-05, 04:06 PM   #9
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Is it just jumping or actually shifting gears? If it's shifting it may be autoshifting but I doubt that is happening.
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Old 05-05-05, 04:21 PM   #10
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Is it just jumping or actually shifting gears? If it's shifting it may be autoshifting but I doubt that is happening.
Whoa... interesting thought... While it is not actually auto shifting... it does jump, I wonder if this type of mechanism is somewhat at play and just pulling the chain a bit off... This is a friction shifter... I'll check the cable routing and grease it.

What it feels like is the freehub loses grip on a pawl for a second, or the chain jumps teeth just for a second. There is a loud clunk and of course a sudden release of chain tension (enough to throw a rider... ouch!) I have looked, and the chain is on the same sprocket I had it on before this occurs.

I don't seem to be able to put enough force on it while I am in the saddle... only when I leave the saddle... and if I am just sprinting, it has not happened... but climbing hills... it has... slow, heavy, frame tweaking climbing.

Thanks
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