How does your belay device affect impact forces?
Have you heard some version of this?
“You should NEVER use a Grigri for trad climbing, because it puts too much force on the top piece of gear.”
There are plenty of (often very experienced) climbers who trad climb with a Grigri, and they're not ripping gear out of the crack every time they take a fall, so what's the deal? (Actually, using a Grigri for alpine rock climbing has a host of benefits, which we cover at this article.)
I've also heard absolute numbers tossed around, such as, “Using a Grigri increases the force on the top piece of gear by 30%.” Percentage increase in force is not nearly as important as the actual force. For example, if the force on the top piece of gear goes from 2 kN to 3 kN, that's a large percentage increase, but a rather small numerical increase.
Fortunately, the clever gear testing gnomes at Petzl did some testing on this, and the results are pretty interesting. (Please check this link at the button below. There's a nice video of the test climbers catching big whippers, always fun to watch.)
Petzl compared falls of increasing severity, using real live belayers and climbers. They measured the force on the climber, the top piece of gear, and the belayer.
It's important to note that falls involving real people have a much different (usually lower) forces then those typically measured in the harsh environment of a drop tower. This is typically due to belayer displacement, squishy human bodies, and some rope slipping through the device as braking is applied.
(With real people, you can't really test past factor 1 because someone's probably going to get hurt.)
Keep in mind these are not absolute rules for every kind of rope, belay device, etc. From the Petzl web page: “The results are influenced by slippage in the belay device and therefore by the belayer's grip. Variability is thus high: these results cannot be generalized to all situations and allow us only to quantify, in a broad sense, the influence of the belay device.”
If your French is a bit rusty: the gray dot is the top piece of gear “l‘ancrage”, the blue dot is the force on the climber “grimpeur”, and the black dot is the force on the belayer “l’assureur”.
image: https://www.petzl.com/US/en/Sport/Influence-of-the-belay-device?ActivityName=rock-climbing
For a Grigri belay with larger fall factors (0.7 and 1.0) the force on the top piece of gear is about 2 kN higher than a belay with a Reverso. The highest force measured overall was 6 kN (factor 1, with a Grigri, on the top piece of gear). Not catastrophically huge, but potentially concerning, depending on the quality of that top placement.
However, for the more common lower fall factor of 0.3, the Grigri generated only a very minor increase in force on the top piece of gear. That’s good!
Comments . . .
The takeaway: provided you can keep your fall factor relatively low, and you have good placements in good quality rock, using a Grigri for trad climbing is fine.
It's not the Grigri that's solely responsible for increased force, but more the amount of dynamic rope in play to help absorb the force.
Fortunately, high fall factors are rare and usually avoidable. Here are some ways to mitigate them.
If you're on a multi pitch route and looking at marginal gear and hard climbing right off the anchor, it might be best to initially belay with a tube style device or munter hitch. Once the leader has a few good placements and has some dynamic rope in the system, fall forces should remain low, so belaying with a Grigri should be fine. There's nothing in the rulebook that says you have to use one method of belaying for the entire pitch.
A downside to using the Reverso (or similar device) is that by giving a softer catch, your climber might fall farther. This can obviously a problem in if there's a ledge or (ground) below them.
Want to learn more? See these articles that cover similar studies by Petzl.