Bicycle Mechanics - Rear derealure.

Hi.
When I came back from a ride in some rain today, I discovered that the Guide pulley was basically seatting on the rear cogs. I tried adjusting the screw, but I messed up the thread (going to get new screw tomorrow). Although it seemed to work.
Anyway what could have caused it. I don't think the screw got loose. It was practically welded in to place, when I tried adjusting it.
Thx.

There are 3 screws on most rear derailieurs which one are you referring to?

The B one. Suppose to be limiting screw for how far the pulley is from a cog. http://www.parktool.com/images/TTOL/der_parts.gif <-- taken from linky (http://www.parktool.com/repair_help/FAQrindx.shtml)

If you were in the small-small gear combo, it may still be OK. Shouldn't use that gear. If the rear drops are horizontal, the wheel may have shifted forward a bit.

I am having the same problem but I can't seem to fix it. I am new to the sport and for the first time I switched Tires on my bike. When I put the rear wheel back on, I noticed that when I shift to the the largest rear cog, the Guide Pulley is resting on the COgs and making a grinding noise. I tried adjusting the B-Screw to create space between the two, but it maxed out and didn't work at all. Another thing I noticed was that if I go on the small-small combo, the Tension Pulley becomes horizontal to the Guidepulley, and the chain actually rubs against itself at the guide pulley.

Did I do somthing wrong reattaching the back the wheel? I checked and the wheel is completely seated in properly, and the chain is on correctly.

Any help would be appreciated...

DO NOT USE THE SMALL SMALL OR BIG BIG COMBINATIONS!!! We go over this again and again (bangs head against desk). These combos will eat up your drivetrain due to the severe angle the chain must take to comply with your shifting demand. If your derailleur was working fine BEFORE you took the wheel off, and isn't working right now, maybe the problem isn't the derailleur. I'm guessing that you reattached the wheel incorrectly.

I always setup my shifting so that i have the capability to go thje cross chaining blah. Even though i never use those combinations, it makes me feel better than i can :)

DO NOT USE THE SMALL SMALL OR BIG BIG COMBINATIONS!!! We go over this again and again (bangs head against desk). These combos will eat up your drivetrain due to the severe angle the chain must take to comply with your shifting demand. If your derailleur was working fine BEFORE you took the wheel off, and isn't working right now, maybe the problem isn't the derailleur. I'm guessing that you reattached the wheel incorrectly.

Its not just the Small Small or Big Big Combinations. When I am on the large rear cog, regardless of which of the three front gears I chose, the pulley rubs on the Cog... and it might have been like this before I took the wheel off, I don't know. I just purchased the bike about 3 weeks ago, and have ridden it twice with no problems before I switched the tires yesterday. I hvaen't tried to ride it yet, just felt that it was kind of weird that the COg is touching the pulley. I guess I am going to have to contact my LBS that I bought the bike from and have them help me with it.

But I would like more information on the Small Small or Big Big Combo's... I shouldn't use them at all?? Basically that would be gear 1 and 24, correct?? If there is a thread that someone could point me too, discussing this, it would be appreciated. thanks

Both of my bikes function ok without obstruction in the small/small combo. Granted i don't run in this combination or any crosschain combinations. It creates a very poor chain line and accelerated wear. I suggest you both go here (http://www.parktool.com/repair_help/FAQrindx.shtml) to learn about rear deraillers.

Adjusting a rear derailler is really quite simple. Just take some time and study the info before posing questions here. Once you have it down, then come back and ask questions. Also make sure that your rear wheel is properly inserted into the dropouts. Sit the bike on the ground, loosen the Quick Release lever (if that is what you have) then retighten.

Its not just the Small Small or Big Big Combinations. When I am on the large rear cog, regardless of which of the three front gears I chose, the pulley rubs on the Cog... and it might have been like this before I took the wheel off, I don't know. I just purchased the bike about 3 weeks ago, and have ridden it twice with no problems before I switched the tires yesterday. I hvaen't tried to ride it yet, just felt that it was kind of weird that the COg is touching the pulley. I guess I am going to have to contact my LBS that I bought the bike from and have them help me with it.

But I would like more information on the Small Small or Big Big Combo's... I shouldn't use them at all?? Basically that would be gear 1 and 24, correct?? If there is a thread that someone could point me too, discussing this, it would be appreciated. thanks

The gears of a bike are not numbered like the gears of a car. Depending on the geometry of your particular bike, there are usually several combinations that cause undue sideways stress on the chain and excess wear on the rest of the drivetrain. For instance, on my Bianchi, I try not to use the largest 2 cogs in back with the large chainring or the smallest 2 cogs with the small chainring. There are enough other suitable combinations that approximate these ratios without stressing the drivetrain. That is how I got almost a decade of use out of my original cassette and chainrings, along with keeping everything clean and well lubed. As for your original question about the alignment of your derailleur cage, a new bike needs to be taken back to the shop you bought it from for adjusment after breaking in. The general rule of thumb is 30 days, or a couple hundred miles, unless something seems amiss.

Cross-chain combos cause excessive lateral strain and wear on the chain. You dont need to use these combos, similar gear ratios are replicated with a straighter chainline.
You measure gears in the traditional manner using "gear inches" not car-style gear numbers:
http://www.sheldonbrown.com/gloss_g.html#gearinch

see also
http://www.sheldonbrown.com/gloss_ba-n.html#btension
and
http://www.parktool.com/repair_help/clg2.shtml