Help! Simple mechanical problem

Seems like your cable got loose. Since the rear derailer has a spring in it it will gradually work its way up the cog as you found out. So the thing to do is put your shifter in the postion were you would normally be if you were on the big cog. There should be abjuster nut where the cable meets the derailer. This allows for adjustment of the derailer. I like this to be somewhere in the middle to three quarters down. Check the cable and make sure it is laying properly and not out of position or any guides. Grip the end of the cable with a pair of pliers and pull it tight while tightening with your allen wrench. You may want to clean the cable end first since any oil or dirt may cause it to slip again. Once it is secure put the bike in a position so you can turn the pedals and shift the gears a bike stand would be nice but you maybe able to just turn your bike upside down. Shift to a middle cog an adjust the nut I mentioned earlier until it is smoothly running on that cog. Than shift through the cogs and make an additional changes you think is needed to the adjustment nut. I believe the two screws you mentioned would be the limiting adjustment screws. They limit the travel of the drailer. They keep you from shifting into your spokes or onto your axel. You only need to adjust these if thats a problem. With indexed shifting you rarely need to look at these. It they were not there however when your cable slipped you may have shifted into your spokes if they were not adjusted properly. Hope that helps.

Quoted post said:

Originally posted by Carrera
Hi,
I was out on a ride today and my big cog failed to release. The gear clicked as I pressed the button but the chain stayed on the large cog. I had to pull it off by hand.
When I got home I noticed the cable was very slack. Obviously I need to tighten the cable and for that I'll have to but an allen key.
Here is where I need some advice. What cogs should my chain be on when I attempt to tighten the cable? At present the chain is fixed on the two biggest cogs (the largest on the rear wheel and the largest on the crank).
I suppose that once I'm sure about that I'll just have to pull the cable with pliars till it's tight.
Secondly there are 2 screws that effect cable pressure for the big cog. Which way do I turn them so my chain will adjust properly from the big cog (pulling outwards)? Which is the relevant screw?
I know I shouldn't be asking such basic questions but mechanics has always caused me a headache and I need to learn.
I know that my other gears simply require turning the screw to the left in order to facilitate gear change in cases where gears stick. My problem now is to learn to adjust my big cog. Any advice appreciated.

www.parktool.com repair section.

Quoted post said:

Originally posted by Carrera
Hi,
I was out on a ride today and my big cog failed to release. The gear clicked as I pressed the button but the chain stayed on the large cog. I had to pull it off by hand.
When I got home I noticed the cable was very slack. Obviously I need to tighten the cable and for that I'll have to but an allen key.
Here is where I need some advice. What cogs should my chain be on when I attempt to tighten the cable? At present the chain is fixed on the two biggest cogs (the largest on the rear wheel and the largest on the crank).
I suppose that once I'm sure about that I'll just have to pull the cable with pliars till it's tight.
Secondly there are 2 screws that effect cable pressure for the big cog. Which way do I turn them so my chain will adjust properly from the big cog (pulling outwards)? Which is the relevant screw?
I know I shouldn't be asking such basic questions but mechanics has always caused me a headache and I need to learn.
I know that my other gears simply require turning the screw to the left in order to facilitate gear change in cases where gears stick. My problem now is to learn to adjust my big cog. Any advice appreciated.

I assume you're talking about the rear derailleur.
The spring on the rear derailleur pushes the cage so that the chain moves down to smaller sprockets. In order to move to larger cogs the cable must be used to pull the cage back.
So to tighten the cable the gear selected should be the highest gear possible (the largest chainring and the smallest cog on the cluster). Once in this gear you can undo the cable fixing bolt and then pull the slack out of the cable before retightening.

If you can't acheive the smallest cog then you need to adjust the high range limit screw only (again on the rear derailleur - one of the screws you talked about). Once ajusted correctly this will allow the cage to move no further than the smallest cag. Once this is done then you retension the cable as above.

Once you have retightened the cable fixing bolt you cna then run the bike through all gears to make sure that shifting performance is as it should be. Use the barrel adjuster on the derailleur to make fine adjustments to the tension. Slacker cables make for fast shifts to smaller cogs but slow shifts to bigger ones and tighter cables make for fast shifts to larger cogs and slower ones to smaller cogs.

Cheers

Quoted post said:

Originally posted by Carrera
Hi,
I was out on a ride today and my big cog failed to release. The gear clicked as I pressed the button but the chain stayed on the large cog. I had to pull it off by hand.
When I got home I noticed the cable was very slack. Obviously I need to tighten the cable and for that I'll have to but an allen key.
Here is where I need some advice. What cogs should my chain be on when I attempt to tighten the cable? At present the chain is fixed on the two biggest cogs (the largest on the rear wheel and the largest on the crank).
I suppose that once I'm sure about that I'll just have to pull the cable with pliars till it's tight.
Secondly there are 2 screws that effect cable pressure for the big cog. Which way do I turn them so my chain will adjust properly from the big cog (pulling outwards)? Which is the relevant screw?
I know I shouldn't be asking such basic questions but mechanics has always caused me a headache and I need to learn.
I know that my other gears simply require turning the screw to the left in order to facilitate gear change in cases where gears stick. My problem now is to learn to adjust my big cog. Any advice appreciated.

http://sheldonbrown.com/derailer-adjustment.html

Quoted post said:

Originally posted by tafi
I assume you're talking about the rear derailleur.
The spring on the rear derailleur pushes the cage so that the chain moves down to smaller sprockets.

Not if it's 'rapid rise' (aka low normal type)

Thanks, everybody, for the advice. I tightened the cable today but it didn't do the trick so I wheeled down to the bike shop and the mechanic told me to soak everything in oil. He told me there was no apparent release of the deraileur but a day or so soaking in oil would help matters.
I'm glad to be informed where my chain should be for tightening the cable (on the largest cog at the crank and the smallest on the wheel if I'm not mistaken).
I'm O.K. fixing computer problems but I don't seem to have a mechanical mind so please excuse such basic questions that I occasionally ask. Cheers.

Quoted post said:

Originally posted by tafi
I assume you're talking about the rear derailleur.
The spring on the rear derailleur pushes the cage so that the chain moves down to smaller sprockets. In order to move to larger cogs the cable must be used to pull the cage back.
So to tighten the cable the gear selected should be the highest gear possible (the largest chainring and the smallest cog on the cluster). Once in this gear you can undo the cable fixing bolt and then pull the slack out of the cable before retightening.

If you can't acheive the smallest cog then you need to adjust the high range limit screw only (again on the rear derailleur - one of the screws you talked about). Once ajusted correctly this will allow the cage to move no further than the smallest cag. Once this is done then you retension the cable as above.

Once you have retightened the cable fixing bolt you cna then run the bike through all gears to make sure that shifting performance is as it should be. Use the barrel adjuster on the derailleur to make fine adjustments to the tension. Slacker cables make for fast shifts to smaller cogs but slow shifts to bigger ones and tighter cables make for fast shifts to larger cogs and slower ones to smaller cogs.

Cheers