Braking Carabineer

Why (What it does):

They are useful and necessary for some types of descenders (spool type) but not necessary for others (brake racks). Some people have difficulty using Braking carabiners with Petzl Stops or other auto-locking descenders. Sometimes you do not need to use one on that particular pitch (Fat rope, light weight etc). They allow the operator to regulate the friction going into their descending device, for better control travelling down the rope. Braking carabiners also assist with tying-off some descenders.

Material properties:
Aluminum or Titanium is lighter than steel, and also softer. In this role the best material is steel, due to its durability. One steel braking carabiner will last years, where aluminum or titanium carabiners wear quickly and need replacing often.This may change though with the new alloys manufactures are developing.

Recommendations:
A steel Braking carabiner, or the Raumer Handy.

Raumer Handy
Weight: 135g   Made from: 304 Stainless Steel   Rated: 12kN
Contains a steel wire (5mm) parallel twist to allow it to fit to you harness master attachment point (MAP). A Handy can be used on all types of ropes. They do not ruin the rope. Weighs and cost as much as 2 AL locking carabiners, or 3 non-locking carabiners. However it  will last much longer as it wears at a much slower rate. Some people file the inside “groove” out so the Handy fits directly on to the harnesses MAP  without the wire, saving some weight etc. The Handy can be “snatchy” on thick or old ropes  especially when the Handy is new. Ideal for 8-9mm cord, heavy loads (acts as a dream). It  does not have the potential to jam your descender into it, like a regular carabineer (a potential  safety issue).
Costs around $22 (must order from Europe)

Stainless Steel Carabiner (non-locking, and "notchless")

Weight: around 190g   Made from: 304 Stainless Steel  Rated: up to 40kN
Some styles can be used as a wrench for the MAP (Maillon). Steel carabineers are much easier  to purchase than a Handy. They are easier to learn to operate than the Handy as well. Best used  on 10+mm ropes. They can be fast on smaller diameter ropes.
Costs around $15

Aluminum Carabiner (non-locking)

Weight: Some as light as 23g Made from: Aluminum alloy  Rated: 20kN and up

Aluminum braking carabiners have worn out in just 6 days of caving.
Some advantages are that they are cheap, readily available and one of the lightest options.
Cost $7 and up