Chain guides have come a long way since the days of home-made devices. File off the teeth of an outer ring, run with a cable-less front derailleur was an early "system." Some cyclocrossers used to use two filed down chainrings. Today, those outer rings are replaced with bash guards -- purpose-built chainrings without teeth--and the inside is protected by way more metal than even the stoutest derailleur can offer.

The e.Thireteen DRS (Dual Ring Security) keeps the chain on and protected for freeriding. The polycarbonate bash ring replaces the outer ring, and a lower roller keeps the chain from bouncing off on rough stuff or backpedaling. The roller, at the bottom, is a "flip-flop" design. That is, there are two separate diameters on the roller, one to meet under the chain when it is coming off the big ring, and the other when it is rolling off the small ring. The upper roller is replaced by the front derailleur, either the kind that mounts on some sort of seat tube or the e-type, which mounts on the E-Thirteen plate attached to the bottom bracket shell. It also comes in a version meeting specifications for the International Standard Chain Guide (ISCG). The DRS is available for four-arm 104mm bolt circle diameter cranks and five-arm 100mm BCD cranks. Two sizes, for 36t big ring and 40t big ring.