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PostPosted: Mon Mar 18, 2013 1:11 pm 
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The picture's good enough to see the chainrings are fine, let's get a close up picture of the cassette teeth.


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PostPosted: Mon Mar 18, 2013 1:16 pm 
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Sounds like chain slipping and re engaging when I true to observe the problem while riding (difficult as several times I nearly rose into a lamp post car (add object here) it seemed as if the chain wasn't engaging with smaller cogs.
I did spray it to lube it with that pfte spray before riding


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PostPosted: Mon Mar 18, 2013 1:16 pm 
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Ah - fair point, if it's a rapid-rise rear mech, i.e. low-normal so that with no cable tension it sits extended with the chain on the largest sprocket, the cable mounting is correct; increasing cable tension will pull the derailleur over toward the smaller sprockets. There's nothing wrong with these derailleurs and, as Anthony says, there's no reason to change it, but it's worth tinkering with it to make sure all four adjustments are correct:

- high-limit screw: stops the derailleur shifting beyond the smallest sprocket and into the gap between cassette and frame.

- low-limit screw: stops the derailleur shifting beyond the largest sprocket and into the spokes - v important!

- B-screw: adjusts the tension on the spring holding the top jockey wheel away from the sprockets - if this is too low, the wheel will sit too close to the sprockets and "chatter", especially in the lower gears (bigger sprockets); if it's too high, it will pull the wheel too far away from the sprockets and shifting will become vague as there's too much gap between the jockey wheel and the gear it's meant to be engaging.

- barrel adjuster: this adjusts the indexing so that the top jockey wheels sits exactly below the sprocket required. If it's too far one way, the chain will rub against the next-biggest sprocket and occasionally 'ghost-shift' down a gear; if too far the other, the chain may not make much extra noise but will occasionally 'ghost-shift' up a gear onto the next-smaller sprocket, especially under load. This can feel a lot like the chain is slipping, but actually it's very rare for a chain to literally slip over the sprocket teeth unless it's very worn or you're putting a lot of force across a tiny sprocket.

It can't harm to give the chain a bit of oil, but be wary of over-oiling it if you use a wet lube, make sure you wipe the excess off with a rag: too much oil sitting on the chain's surface can pick up dirt and grit and end up leaving the chain coated with a nasty grubby oily clag that, in the worst cases, can accelerate chain wear as it's basically covered in a clay-like grinding paste!


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PostPosted: Mon Mar 18, 2013 1:23 pm 
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Some useful information and photos at Park Tool's page, including a section on low-normal mechs.


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PostPosted: Mon Mar 18, 2013 1:25 pm 
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Pierre wrote:
Ah - fair point, if it's a rapid-rise rear mech, i.e. low-normal so that with no cable tension it sits extended with the chain on the largest sprocket, the cable mounting is correct; increasing cable tension will pull the derailleur over toward the smaller sprockets. There's nothing wrong with these derailleurs and, as Anthony says, there's no reason to change it, but it's worth tinkering with it to make sure all four adjustments are correct:

- high-limit screw: stops the derailleur shifting beyond the smallest sprocket and into the gap between cassette and frame.

- low-limit screw: stops the derailleur shifting beyond the largest sprocket and into the spokes - v important!

- B-screw: adjusts the tension on the spring holding the top jockey wheel away from the sprockets - if this is too low, the wheel will sit too close to the sprockets and "chatter", especially in the lower gears (bigger sprockets); if it's too high, it will pull the wheel too far away from the sprockets and shifting will become vague as there's too much gap between the jockey wheel and the gear it's meant to be engaging.

- barrel adjuster: this adjusts the indexing so that the top jockey wheels sits exactly below the sprocket required. If it's too far one way, the chain will rub against the next-biggest sprocket and occasionally 'ghost-shift' down a gear; if too far the other, the chain may not make much extra noise but will occasionally 'ghost-shift' up a gear onto the next-smaller sprocket, especially under load. This can feel a lot like the chain is slipping, but actually it's very rare for a chain to literally slip over the sprocket teeth unless it's very worn or you're putting a lot of force across a tiny sprocket.

It can't harm to give the chain a bit of oil, but be wary of over-oiling it if you use a wet lube, make sure you wipe the excess off with a rag: too much oil sitting on the chain's surface can pick up dirt and grit and end up leaving the chain coated with a nasty grubby oily clag that, in the worst cases, can accelerate chain wear as it's basically covered in a clay-like grinding paste!



Pierre thanks for the tips.
Ill spend some time on it on the morrow and see what happens.
I've never had this problem with any other bike hence I assumed its the rear mech.
My three bikes (in my sig) have normal rear mechs and shift beautifully.
This 'rapid ride' mech is something new as are the shifters hence I was automatically concerned about them rather than the freehub.


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PostPosted: Mon Mar 18, 2013 3:29 pm 
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ibbz wrote:
I did spray it to lube it with that pfte spray before riding

Doesn't work. Nothing else but chain oil, and one drop of chain oil on each roller, nowhere else, just the rollers.


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PostPosted: Mon Mar 18, 2013 4:21 pm 
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Not entirely sure what you're getting at, Anthony. Any lube is better than nothing, and a lack of lube will not make the chain "slip off the chainrings" if the derailleurs are properly adjusted. A dry chain will make a bit of noise and in extreme cases it will have stiff links which will not like shifting or will jump around the cassette and jockey wheels, but it would be pretty obvious if this was the problem.

As Sheldon Brown says, factory lube is great. There's no reason to remove it and the best way to re-lube a chain is to turn the pedals backwards while holding the lube bottle upside-down and dripping oil into the inside face of the chain, i.e. the cog-contacting face (not talking about "sides" of the chain so you don't think I could mean left or right side of the chain, I mean the inside of the loop rather than the outside), so that it works its way in. One drop of oil on each roller is sometimes excessive, it's important to wipe the excess off with a rag.


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PostPosted: Mon Mar 18, 2013 10:06 pm 
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Pierre wrote:
Not entirely sure what you're getting at, Anthony. Any lube is better than nothing, and a lack of lube will not make the chain "slip off the chainrings" if the derailleurs are properly adjusted. A dry chain will make a bit of noise and in extreme cases it will have stiff links which will not like shifting or will jump around the cassette and jockey wheels, but it would be pretty obvious if this was the problem.

As Sheldon Brown says, factory lube is great. There's no reason to remove it and the best way to re-lube a chain is to turn the pedals backwards while holding the lube bottle upside-down and dripping oil into the inside face of the chain, i.e. the cog-contacting face (not talking about "sides" of the chain so you don't think I could mean left or right side of the chain, I mean the inside of the loop rather than the outside), so that it works its way in. One drop of oil on each roller is sometimes excessive, it's important to wipe the excess off with a rag.

I am speaking from personal experience, which contradicts both of your suggestions. I have had a bike be totally unrideable when in the countryside miles from anywhere, and back to perfection when a kind passer-by let me use his oil. I adjusted nothing, but the bike went from total chain-slip to running as sweet as a nut. Admittedly that chain was very dry indeed though. I have also had a similar, but not quite so extreme, experience with so-called factory lube, whatever Saint Sheldon may say.


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PostPosted: Mon Mar 18, 2013 10:31 pm 
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Checked the jockey wheels for wear or damage? I had a similar thing...and was a mis-shaped jockey kicking the chain out slighly causing a slip.... took me ages to work it out and not sure how it happend to this day.


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PostPosted: Tue Mar 19, 2013 12:26 am 
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Is the mech hanger straight? Without any load the chain will spin fine, but under load it may slip as the whole system is torqued and will pull the mech/chain towards the angle of the bend if there is one. Any play in the mech hanger bolt as this may have the same effect under load.


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