Bicycle Mechanics - what does this screw do?

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the sloth
04-29-06, 12:40 AM
you're looking at a shimano 600 rear derailleur, upside down. i have no idea what this screw on the bottom of the arm does... tried turning it in both directions with no noticeable change to shifting. my first thought was that it somehow controlled arm tension. any ideas?

http://umbc.edu/~asoont1/der.jpg

HillRider
04-29-06, 06:40 AM
I just looked at 8-speed and 9-speed 105, a 7-speed STX and an 8-speed Sora rear derailleurs and none of them have that screw so it isn't universal. How old and what "speed" is your 600 derailleur?

The only thing I can guess is that it's the anchor for the spring in the parallelogram.
hi565
04-29-06, 07:08 AM
look inside of the derailleur if you can. Do you see the screw touching up against any tabs of any kind? Or any other screw or something of that sort? If not, Then i am just going to guess that it is holding some part of the derailleur together.

Just a guess.

Little Darwin
04-29-06, 07:11 AM
My Shimano 600 RD (dated late 1987) doesn't have this screw either...
cascade168
04-29-06, 08:52 AM
I may be wrong, but I believe that is the reference point (i.e. pivot) the the parallelogram is based. In other words, that is the one point on the four sided parallelogram that does not move. This would be easy to test. Just observe that point when the cable is pulled and see if it moves, or not.
Peek the Geek
04-29-06, 10:01 AM
That screw is a governor, and it controls bike speed. The further in it's screwed, the more it limits how fast your bike is able to go. From the photo, it looks like you would be able to improve your max speed significantly by turning it several revolutions counterclockwise.















...:p
kenhill3
04-29-06, 10:05 AM
Paralellogram tension screw, similar to the old XT's I used to use (RD 737 ?). It doesn't wind up the spring but rather deflects it in order to put a little more kabush into the spring action.
Svr
04-29-06, 10:10 AM
Yep, it controlls parallelogram spring preload.
kenhill3
04-29-06, 02:48 PM
Yep, it controlls parallelogram spring preload.
Aha! PRELOAD was the word I was searching for that would be a little more accurate than KABUSH. Thanks.