Soft catch vs. hard catch video analysis

Jake Jones wrote:

I rewatched the vid, and you're right.  However, as I understand the term "impact force" it is exactly as you state.  The force felt by the climber.  If this is the case, then wouldn't the point at which the "bounce" as they refer to it, i.e. the lowest point in the model, be impact force?  

Creators of this video are concerned with forces that a climber feels when impacting THE WALL. That force goes as ~V*V, V being speed normal to the impacting surface. They use pixels per frame/second since that particular measure is directly proportional to velocity.

Jake Jones wrote:

I rewatched the vid, and you're right.  However, as I understand the term "impact force" it is exactly as you state.  The force felt by the climber.  If this is the case, then wouldn't the point at which the "bounce" as they refer to it, i.e. the lowest point in the model, be impact force?  Even if it's not outright stated or measured specifically, that point is definitely inferred.  Not trying to be a pedantic ass, I'm genuinely asking.  My physics can accurately be described as "rusty" or rudimentary, so I'll concede if for no other reason than you seem to have a superior understanding of it.

What I'm referring to specifically is what I've circled.  Aren't those points essentially "impact force"?

If they were looking for the "impact" flet by the gear (i.e. the tension in the rope), you'd be correct. By taking the derivative of the velocity, you'd be able to determine the acceleration of the climber (and by taking mass into account, you'd have the actual forces involved.) The areas you have circled are the peak accelerations (or rather, decelerations) for those 2 falls, and would be the "impact" experienced by the gear and harness.

However, from the video, it seems that the impact force they're concerned with is the impact with from the pendulum-swing into the wall. They don't have an actual wall here, but they stop their graph at an imaginary vertical line (the dotted line), and display the horizontal velocity at that point. They've done runs with different lateral offsets from the "wall", and with varying amounts of slack in the system. They then expand on this by including the hop for a soft catch. As mentioned upthread, and alluded to in the video, they're not worrying about the actual tension in the rope; it's the velocity with which you hit the wall that does the damage in a pendulum swing.

amarius wrote:

Impact Force - 

F= m*v/t - m is mass, v is speed, t time it takes to go from v to 0.

F = dP/dt. P is impulse, t is time.

Force is not a constant. Force is a momental physical value. It exists (OK, it does not exist, but it does exist in a contex of some physical model) at exact point and exact time.

Pavel Burov wrote:

Obviously, more natural physical value to model climber to wall impact possible outcome is horizontal speed (projection of the speed to the horizontal axis).

I agree that velocity in the direction perpendicular to the wall is the first thing to look at.  However, a large component of the velocity parallel to the wall may result in spin being applied to the climber (who is not a point mass).  That, in turn, may lead to smashed patellae, especially if the wall is hit during the upswing.

brenta wrote:

I agree that velocity in the direction perpendicular to the wall is the first thing to look at.  However, a large component of the velocity parallel to the wall may result in spin being applied to the climber (who is not a point mass).  That, in turn, may lead to smashed patellae, especially if the wall is hit during the upswing.

There is one more parameter to consider. In terms of climber just slept of and taking a fall deeper fall translates to more time to tuck themselves up. To my experience the most danger of being short while falling is everything comes in too fast pace - everything was fine, I was reaching that hold or slightly dynoed to that feature but then all sudden CRUSH! BOOM! BANG! And my heels are already were smashed into the wall. Absolutely uncontrollable.

Hard catches are evil.

brenta wrote:

It's not absurd for a couple of reasons.  First, even in the idealized case, a longer fall due to slack in the system may cause a large fall factor.  Second, for shorter falls the tightening of knots, the squishing of the climber, and similar effects reduce the force on the anchor.

I was confused by the wording of the video, extra slack is not necessary for trad falls but a soft catch from a jump is essential. Shorter falls that lead to knot tightening or climber 'squishing' are indications that your gear is taking a higher load, these things happen much more with high fall factor falls. Spreading the deceleration for as long as possible is the name of the game for having gear hold. 

Jon Rhoderick wrote:

I was confused by the wording of the video, extra slack is not necessary for trad falls but a soft catch from a jump is essential. Shorter falls that lead to knot tightening or climber 'squishing' are indications that your gear is taking a higher load, these things happen much more with high fall factor falls. Spreading the deceleration for as long as possible is the name of the game for having gear hold. 

Is this really true?  I remember a post by rgold saying that with ATC style devices, jumping is not productive, and that the slight slippage of the rope through the device does a lot more to lower the peak force on the gear than jumping can.  I think rgold has spoken multiple times about the phenominon, but this post from way back in 2008 is the only one I can find for now...
https://www.mountainproject.com/v/106211740

rgold wrote:

A number of points:

1. The Petzl simulator (and most theoretical models) use a relatively simple equation that either ignores or only partially accounts for friction through the carabiners and against the rock. The result is that more fall energy has to be absorbed by the rope running and so the predictions of rope running are probably on the high side.

2. The great majority of climbing falls have low fall factors and do not produce very large loads at the belay device anyway.

3. Even when the fall factors are larger, there are usually enough sources of friction in the system dissipating fall energy so that it is rare for most belayers to experience the rope running.

The net effect is that many climbers can climb for years---or even an entire career---and catch many leader falls without ever having the rope run.

On the other hand,

4. Both sophisticated mathematical modeling and extensive practical testing by the Italian Alpine Club (CAI), as well as an extensive historical record of belay testing in the US in the 50's and 60's make it clear that if the belayer is exposed to a high fall-factor situation with few sources of friction, there can be significant amounts of rope running.

5. How much rope runs depends on the friction in the system, the amount of friction supplied by the belay device, and the strength of the belayer's hand. The CAI tests also indicate that there is enormous variation in the performance of experienced individual belayers, so much so that it was hard to get useful data from repeated trials.

I have slightly oversimplified the discussion of rope slippage here, because there are, in reality, two phases of rope slippage, one of which does not produce burns. In the first, or what the CAI calls the inertial phase, the brake hand is pulled up to the belay device. Depending on the position of the brake hand at the moment of impact, this could involve as much as two feet of rope running through the belay device under resistance with no slippage through the brake hand, producing perhaps the energy absorbtion of a Screamer. If the additional energy absorbtion of the inertial phase is not enough to stop the fall, then in the next phase the rope is pulled through the belayer's hand under whatever tension the belayer is able to maintain.

6. As for the friction supplied by the device, the original Reverso was at the bottom of the heap---in my opinion, it was dangerously inadequate for severe falls. Although I have seen no comparative figures for the BD Guide ATC, I am sure that it has a significantly higher braking factor than the original Reverso had.

7. The CAI tests showed that the lifting of the belayer has a relatively minor effect in reducing the peak load to gear, and that only a fraction of the lifting occurs up to the moment of peak load, the majority occuring after the gear has already been maximally stressed. These results are for a non-locking belay device for which other force dissipation factors are involved and these apparently mitigate the contribution from belayer lifting.

8. Another set of tests has shown that, with a Gri-Gri, a properly timed belayer jump can result in a decent load reduction to the top pro. (Note: a Gri-Gri will not slip in any climbing fall caught by pro. There is a chance, depending on the rope and other frictional factors, that a Gri-Gri might slip in a factor 2 fall.) Of course, "properly timed" is critical, and given the very large variation in belayer performance in the CAI tests, one has to wonder whether even an experience jumper can regularly reduce peak loads with a Gri-Gri.

In summary, as with a number of issues in climbing (equalizing belay anchors, backing up rappels, wearing helmets, etc.), there are practices that are effective a large majority of the time but which may not work well or at all in a very small number of exceptional cases. Since the rope doesn't slip in most climbing falls, there is usually no difference in imposed peak load for a Gri-Gri and ATC-type devices. However, in high fall-factor low-friction circumstances, the rope slippage is a potential safety valve that may keep peak loads within the limits sustainable by trad gear, where as the same gear will blow with a Gri-Gri. I say "potential safety valve" because without gloves, it isn't clear that rope slippage will be controllable. If the belayer cannot maintain full strength grip or even loses control, rope slippage becomes not a safety valve but a liability. Moreover, even if the belayer hangs on, the burns sustained can range from annoying to exceptionally severe---as in down to the bone.

Using an ATC-type device rather than a Gri-Gri means (if you were to think logically about it) that you want the potential safety-valve effect that might come into play in exceptional circumstances. Not wearing gloves turns that potential effect into a liability and so contradicts choosing the ATC-type device---at least that's the way it appears to me.

The entire premise to this study is a clean free-fall on a steep overhanging wall.   All well and good to focus on a "soft catch" but in many climbing situations the belayer does not have the luxury to "make it soft".  Aren't ledges more dangerous than slapping the wall sideways?

Russ Keane wrote:

The entire premise to this study is a clean free-fall on a steep overhanging wall.   All well and good to focus on a "soft catch" but in many climbing situations the belayer does not have the luxury to "make it soft".  Aren't ledges more dangerous than slapping the wall sideways?

On behalf of the people who made the video, I apologize. They CLEARLY were meant to make a 5-minute video that covers all situations that one might ever encounter in climbing. How dare they focus on one aspect and try to understand it in depth? Someone somewhere might be hitting a ledge right now...

^^ Woah, ok.....   I guess I'm not allowed to say something that might be relevant.....  Enjoy your thread.  

Frankly, I am really surprised they only have one disclaimer about not climbing trad, and asking not to use their conclusions for trad climbing. They should've included big banner "NOT FOR TRAD CLIMBING" right there on the top of video.

And, seriously, how dare they try and break down sport climbing fall into simple components, and not consider floppy climber model, tightening knot model, the many types of ropes available for climbing model, different belay devices model. And, since they did not talk about hip belay, we should ask YouTube to remove this video.

They put in a ton of work and made a very useful contribution to a ongoing discussion about belaying for sport climbing.  To the extent that their results seem to be justified scientifically, it is reasonable, as is common for all scientific work, to discuss their methods and analyze possible experimental and theoretical flaws.

It was an interesting video, but I'm not able to take any actionable conclusions from it.

A soft catch may reduce the risk of injury, if you do it perfectly, in the right situation?

The squirrel was cute.

Tim Lutz wrote:

Soft catches absolutely reduce the chance of injury.

What I took from the video was that a soft catch's effect on the outcome is not absolute.

amarius wrote:

They should've included big banner "NOT FOR TRAD CLIMBING" right there on the top of video.

I would think the issue is not whether it is sport or trad, the issue is wether the climb is overhanging or not. 98% of all gym climbing is overhanging, so that's where soft catch shines with an "always" rule almost appropriate. 98% of all outdoor climbing is NOT overhanging, so need a little more thinking before blindly applying soft catches and an "almost never" rule is more appropriate (oooh, now I'll get flamed!). Watch for those ledges and slabs.

man, i totally agree.  i think people are trying too hard to find the one silver bullet solution when there isn't one.

My sentiments are close to the previous two posts.

Within the confines the pair had set for their experiments and test runs, they did a credible job.  I certainly learned a few things after watching parts of the video four times.  However, one thing to bear in mind is the lower the fall factor, the greater the rope can absorb the kinetic energy generated by the falling climber.  I doubt that we saw anything greater than a factor 0.40 fall in the video.

What the video does not address is the scenario of lighter belayer, heavier climber which is almost always the case for me.  I keep the extra slack to a minimum but not to the point of short roping the leader until after 5th or 6th clip or about 50 feet of rope out if the climber outweighs me by 15# or more.  As for jumping, sure if the terrain is overhung and there is no danger of ground/ledge fall, I gladly get lifted close to the 1st piece to get in a soft catch.  But in recent memory, I had to make two semi hard catches with the climber between the second and third bolts.  I did not dare to give more slack or jump for fear we would collide mid-air.  In both cases, the climber did not crater nor hit the rock/wall.  Everyone was safe and uninjured.  This is a scenario definitely not addressed in the video but quite real day-to-day.  

Every catch is unique and at least slightly different from the previous one and the next one.  To treat every catch using the same set of simple rules is nor optimum nor realistic.  

The video was interesting and has value.

But....

The entire premiss was to determine "speed" and how it may affect one's "impacting" the wall, as this can cause injury.  But, they commingle speed and velocity in their energy equation inappropriately.  Speed is vague, whereas velocity has direction.  

Any pendulum has a horizontal component and a vertical component.  So, the velocity relative the the wall one is about to impact is what's of importance.  So, if the climber is going to impact a vertical wall, the horizontal "speed" is what matters, not the total velocity and total energy.  Likewise, if the climber is going to impact an overhanging wall, its "speed" perpendicular to the wall is what is most important.  Hence, the use of the energy equation is not implemented appropriately.  

For example, at the instantaneous level, a pendulum, say starting at "3 o'clock" and falling toward "6 o'clock" has a horizontal component of 0% and a vertical component of 100%.  At 6 o'clock, the exact opposite is true.  

Stated another way, there is no horizontal "speed" at 3 o'clock and no vertical speed at 6 o'clock.  

But, the video misses several more important components of reality, yet, makes some blanket conclusions.  If the wall is vertical, the initial horizontal component will be necessarily small.  Hence, final horizontal component will be small (unless the leader pushes off the wall).  But, they test a fictitious distance from the wall and draw conclusions.  On the other hand, if the initial horizontal component is large (very overhanging wall), the final horizontal component will be large.   

Contrary to the video of "top rope" scenarios, any true lead fall where the leader is above the last protection point (excepting horizontal component of a climb out a roof will produce a much greater vertical component than they demonstrate.

Additional slack in the system will always result in a total greater energy.  In some cases, this will result in greater vertical "speed" and vertical energy.  In some cases, this will result in greater horizontal "speed" and horizontal energy, or a combination of both.  

Oh.  And the video confirms what I've been saying for years. Sure, jumping can reduce force, impact, hard catch, etc.  But:

1 The belayer must be watching the leader very closely.

2 The belayer must be in a ready position to jump.

3 The belayer must time the jump within a fraction of  a second.  If not timed correctly, it didn't help.  It may make things worse.