Help adjusting deraillieurs

[ZeroBike]

I have a Thorn Sherpa and have just noticed that I can't drop down to the smallest cog on the rear cassette.  Well I can if I move up about 3 gears and then drop down to the bottom but I can't change to it from the cog next to it.  I really don't know how to adjust or maintain deraillieurs so would be really grateful if someone could talk me through it.

Best wishes

[jags]

here you go http://www.youtube.com/watch?v=SkzvfCaIbyQ that should get you out of any trouble.

[ZeroBike]

Thanks Jags its working now.  

That's an awesome video.  

The weird thing was that it was the top adjuster screw as when  I unclamped the derailer cable it wasn't naturally dropping onto the smallest cog.  However when I got the bike guys at thorn said that I should probably never need to adjust those screws as they simply control the range that the mechanism moves through.  So I guess what I'm wondering is why has the range changed?  Surely that should be the same as the day I got it.  

Any thoughts on that, in my head I'm trying to build a picture of how these things work.

[jags]

ah you know yourself if it's man made it can break,mind you it is unusual for the adjuster screws to move the front mech  usually give more hassl;e .
anyway now that you fixed it you need never to worry again

[JimK]

One possibility: if e.g. the dropout or maybe the cage with the jockey and tension wheels... if that thing gets bent a little, then the alignment with the sprockets will change.

I wonder too... maybe as the chain gets worn... the jockey wheel pushes and pulls on the chain laterally. That lateral movement has to get carried up the chain to where it's feeding on to the sprocket. If the chain has more lateral play, then it will be harder to pull off one sprocket. That actually fits what you're seeing. Once that chain gets moving, it can land in the right place. But if the movement is smaller, i.e. just one sprocket, then the chain doesn't want to respond.

Hey, two ideas for the price of one!

[mickeg]

If you had trouble getting onto the smallest cog and if the reason for that is that the derailleur or derailleur hanger are bent, that could mean that the derailleur could allow the chain to go too far inwards, the chain could go between the cassette and spokes.  That obviously would be very bad, you may want to double check the other adjustment screw to make sure that does not occur. 

[Danneaux]

Just to add another, future possibility, though it apparently wasn't the case here:

Gummy or worn cable housing can result in enough friction on the cable to overcome spring tension and make it hard to engage the higher (smaller) cassette cogs when using a high-normal rear derailleur.

Another source of shifting friction can be a bit surprising: Sugary energy drinks can leak from your water bottle's spout and run down the cables to collect where the cables go through the under-BB cable guide.

Again, neither of these were in play here, but something to keep in mind for the future if other, more obvious causes and solutions fail to work.

Best,

Dan.

[No]

Someone may have fiddled with the screw for whatever reason.