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I had installed my front derailleur and was adjusting the high/low when I noticed my rear derailleur was hitting the spokes of my wheel. Suddenly, I saw a screw pop out and then my derailleur popped into a very contorted position and I had to take it off.

The below photo shows the underside of the derailleur ( Sram x-3) to the right of the place where it connects to the frame is the broken plastic where the screw came from.

enter image description here

What happened, and what does this screw do? Exactly how did I break my bike.

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You said it -- the derailer hit the spokes, an instant death scenario. This is one reason why spoke guards, while considered too low-brow for bike snobs, are often a good idea. Hopefully your derailer hanger was not damaged, so you only need to replace the derailer. –  Daniel R Hicks Jul 4 at 22:20
    
So what does the screw do? –  Clint Eastwood Jul 4 at 23:00
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It's hard to tell from your picture, but my best guess is that it was a limit adjustment. –  Daniel R Hicks Jul 4 at 23:35
    
@DanielRHicks: It sounds like maybe it hit the spokes while he was adjusting it, not riding? –  whatsisname Jul 5 at 4:40
    
@ClintEastwood: did you bump the spokes while riding or while the bike was stationary and you were just spinning the wheel by hand? –  whatsisname Jul 5 at 4:41

1 Answer 1

The screw that you are missing there is the B Tension screw. This screw rests on your derailleur hanger on your frame and sets the angle of the derailleur body.

Usually, when you put a new derailleur on a bike you use this screw to adjust the distance of the upper pulley (Jockey pulley) on your derailleur up or down from the teeth on your cassette. You want the top pulley to be 5-10mm away from your cassette teeth when then chain is on the largest / easiest gear. This keeps shifting nice and tight, because the derailleur is as close to the cassette as possible, without riding the pulleys on the cassette teeth.

Screwing this in will move the derailleur body and pulleys downwards, away from the cassette, while screwing it out (or completely out in your case) will cause the derailleur body to be pulled up by it's spring into the contorted position you mention.

Read more about it here (scroll halfway down the page) http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

Also, if it wasn't obvious already— you've now learned why you shouldn't spin the rear wheel while you adjust the High Limit screw...

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I was adjusting the high/low of the front derailer. Does your last warning apply to that? –  Clint Eastwood Jul 17 at 0:36
    
With the front derailleur (FD) there isn't a risk that you'll push something into the spokes like there is with the rear. Adjusting the high limit screw too much will mean you can't shift up, or the FD will push the chain off the top ring. Setting the low limit screw on the FD too far will either result in being unable to shift down to the small chainring, or the chain dropping off the inside. This is bad as it could get stuck between the chainrings and the chainstay, damaging the paintwork/frame and increasing the risk of an "unscheduled dismount". –  adey_888 Sep 15 at 1:55

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