How can a rear derailleur cause chain slip?

[Roasted]

I have a Diamondback hardtail that I've been slowly upgrading ever since I got it. Lately my project has been to upgrade the drivetrain, so I did. I did everything except the rear derailleur. In doing so, I was expecting my chain-slip problem to go away, but it did not.

I replaced:

Shift cable
Cable housing
Cassette
Chain
Chainrings
Crankset

I also had the hanger checked for straightness and it was fine. I also made sure the rear derailleur was adjusted properly, along with the H/L limit screws and the B tension. Everything was good, and the bike shop confirmed the settings were good. Yet still I had chain slip problems. I had zero issues with shifting, it was just regular chain slip. I'd be in my driveway and torque down and sometimes it would skip.

Then I replaced the rear derailleur with a higher quality one. Blam. Problem fixed.

But, HOW. How can a rear derailleur cause chain skip problems? I can understand SHIFTING problems, but how can a stationary rear derailleur on flat ground cause a slip?

[xerxes]

Quote:But, HOW. How can a rear derailleur cause chain skip problems? I can understand SHIFTING problems, but how can a stationary rear derailleur on flat ground cause a slip?

If a derailleur gets really worn and sloppy, the jockey wheel cage can sit at an angle, rather than vertical, when viewed from the rear of the bicycle. This means that the chain has to curve from the angled upper jockey wheel onto the vertical cassette sprocket and can cause it to skip. That's not to say that's definitely what it was, but it's a possibility.