In reply to cuppatea:
Well I know that BD specifically said that the ATC was designed with slippage in mind, to reduce fall forces. and I'm pretty sure that I've read something from Petzl that says the Gri gri is unsuitable for trad because it doesn't slip.
On a whim I Googled belay device slippage and came up with this.....
http://www.geir.com/mythbuster.html
Myth 7: GriGris are the best choice for multipitch trad climbing belays
Use of a GriGri for traditional climbing makes it more likely that marginal placements / low rated pieces will fail.
The reason for this is the static nature of the device. With a GriGri, a fall is stopped abruptly when compared to a plate device such as an ATC. The reason for this is the very limited amount of slipping that occurs when a GriGri arrests a fall. An ATC, however, allows the rope to slip through the device during a fall, braking more slowly. This is similar to stopping a car: when pressing the brake slowly, the car stops gradually. When pushing the brakes more, the car jolts to a stop.
Beal did a nice comparison between belay plates (ATC) and autolock devices (GriGri) and has it posted at [broken link - bealplanet.com/sport/anglais/facteurdechute.php.] Breifly, depending on the rope's impact force and friction in the system, they found that the force exerted on the top piece was consistantly higher when using a GriGri versus an ATC.
Jeff and I decided to do a test of our own to get some real-world measurements. After climbing up 40 feet on a sport climb with a heavy, clunky industial dynamometer strapped to my back, I took repeated 10-foot falls and measured the maximum force on the bolt with each fall. After five falls were caught on the GriGri and five falls were caught using an ATC, we repeated the test, this time falling from a bolt closer to the ground (20 feet).
In both cases, the force exerted on the top piece was higher when using a GriGri than with an ATC. When the 10 foot falls were arrested in the first test (falling from 40 feet), the average maximum force on the bolt was 820 pounds with the GriGri as opposed to 435 pounds with the ATC. With less rope out (falling from 20 feet), the force exerted on the top piece was higher, although interestingly, the difference between the two devices was smaller: the average for the GriGri was 1030 pounds while the average for the ATC was 900 pounds. Note that these differences are less pronounced than what the Beal website found, but the differences are still present.
The significance of this for traditional climbing is that the gear used is weaker than a correctly placed bolt. Moreover, the strength of a traditional placement also depends on the nature of the rock it is in, the shape of the feature the climber is trying to protect, the type of gear that is placed in the feature, and a host of subtleties that are dependant on the skill and experience of the leader. So while a traditional placement may be as strong as the gear is rated (generally 5 to 16 kN, or more simply put 1100-3600 pounds), in practice most placements can be weaker. In practice I have found that I have at times needed to rely on sketchy placements sometimes, and have pulled gear multiple times when falling on such gear.
So the question that we need to ask ourselves is how good the gear is on the climb we are planning to do. If you are climbing on bomber gear in bomber rock, the difference between a GriGri and a plate is probably not that big of a deal. If you are climbing on sketchy gear in poor rock, then you'll probably benefit from the softer catch a belay plate provides.
I've recieved a number of comments on this piece. A lot of people think that they can provide a soft enough catch with a GriGri with a well-timed jump. Keep in mind this will be difficult in a hanging belay situation or when tied in close to the anchor. I haven't seen research with actual measurements on soft catches with a GriGri vs an ATC but if you are aware of some please send me a link.