Originally Posted by
markk900
True, but the OP's picture shows a "fixed" spacer - ie non-adjustable - so effectively its just a shorter horizontal dropout in his case (and in this case not for the derailleur claw since you can see the dropout has an integrated hanger).
I supposed one could move the spacer forward and tighten, but most of them seem designed to fit right up against the closed end of the dropout. And what if the optimal position for the RD is further back than the spacers allows (I know, take it out!)
Not to say never, but I have never seen the case where having the axle at the back of the dropout makes for better shifting. It has always been the case shifting is better in the vertical dropout position.
I imagine the horizontal dropout is a legacy to when derailleurs were uncommon. There was no derailleur to pick up chain slack and the horizontal dropout allowed to pick up slack less than a half of a chain link. Try making a fixed gear bike with vertical dropouts. It is a dance between the chain, front ring and rear cog that usually doesn't work out well.