Originally Posted by AeroGut

"This bicycle has a short-cage rear derailleur, which shifts faster, weighs less, and has more ground (or obstacle) clearance than the usual long-cage rear derailleur.
However, the small price you pay for the above advantages is that, with the chain on the inner chainwheel and the three smallest rear cogs [note: this was out of 7 total cogs], the shorter cage won't take up sufficient slack; the chain rubs on the derailleur cage, and flops around excessively over bumps."