Why are climbing ropes rated so low?

Hi all,
I have just had this question in mind recently. I've tried looking but can't really find an answer. Why are climbing ropes only rated for an impact force of 8kn, if gear is rated to 22 (typically)?
Thanks!

Impact force and gear ratings are two different things. 

Impact force is not a measure of the strength of a rope.

If you generate enough force in a fall to break a rope you will wish you were dead.  

Kevin Mokracek wrote: If you generate enough force in a fall to break a rope you will be dead before you hit the ground.  

Dynamic climbing ropes are shock absorbers. Their purpose is to stretch and absorbe energy, which results in a friendly low "impact force".  When they tested how strong paratrooper harnesses had to be, they figured that anything above 15kN (or was it 12?) would break the soldiers basin, so when they wrote the specs for climbing gear they decided that we should strive to have ropes that keep impact forced well below this threshold.

The rating on the rope is the max it can give
The rating on gear is the max it can get

Ahh, I see, thanks everyone!

 At the risk of repeating what has already been said, you want your climbing rope to have a low UIAA impact rating, because that's a cap on the load transferred to your body and the gear during a fall no matter how severe.  You might instead ask why it the impact rating is so high (bungie cords have far lower impact ratings).  The answer is that the manufacturers have to balance low impact ratings with percentage stretch; a climbing rope that is too stretchy would be dangerous, as the faller would be colliding with things without having had much fall energy absorbed.  Climbing ropes already stretch about 30% when the maximum load is approached, which is really quite a lot already.

rgold wrote: ...Climbing ropes already stretch about 30% when the maximum load is approached, which is really quite a lot already.

A 200 ft rope can stretch 60 ft? 

Kristian Solem wrote:

A 200 ft rope can stretch 60 ft? 

Yes but not under normal fall circumstances.  

rgold wrote:   At the risk of repeating what has already been said, you want your climbing rope to have a low UIAA impact rating, because that's a cap on the load transferred to your body and the gear during a fall no matter how severe.  

So then if the max force distributed to gear is the impact force (~8kn), then why do biners,slings, etc. get built to 22kn strength?

Terrible Climber wrote:

So then if the max force distributed to gear is the impact force (~8kn), then why do biners,slings, etc. get built to 22kn strength?

The rope rates the max force on the climber.   Due to pulley effects, the pro/anchor can see almost double (not quite though, due to friction, etc)

Terrible Climber wrote:

So then if the max force distributed to gear is the impact force (~8kn), then why do biners,slings, etc. get built to 22kn strength?

Impact force can be as high as 12 kn I believe. Pulley effect on top piece of gear could be 20 kn.

Terrible Climber wrote:

So then if the max force distributed to gear is the impact force (~8kn), then why do biners,slings, etc. get built to 22kn strength?

And most everything in climbing is “overbuilt”. It sometimes takes into the account user error, freak shit and the “guarantee” for gear not to fail, as failure in this sport is injury/death.  I prefer overbuilt gear for the 90% of the climbing I do, and too heavy of gear for me to want to carry for the other 10% of the climbing. 

Gunkiemike wrote:

Impact force can be as high as 12 kn I believe. Pulley effect on top piece of gear could be 20 kn.

Correct.

Kristian Solem wrote:

A 200 ft rope can stretch 60 ft? 

Yes, if you take a 400 foot fall on it.

Terrible Climber wrote:

So then if the max force distributed to gear is the impact force (~8kn), then why do biners,slings, etc. get built to 22kn strength?

In the case of slings, knots used to limit extension can cut the strength of the sling in half, so a substantial margin is called for.  In the case of leader falls, the pulley effect will approximately double the load to the top piece in a fall (with possible reductions from rope friction).  The UIAA standard for maximum rope tension is 12 kN, so everything is designed around that number.  And note that ropes stiffen as they age, so your brand-new temperature and humidity controlled laboratory 8 kN reading is going to rise (does anyone have good data on how much?) over the lifetime of the rope.  Finally, gear is designed to handle forces that might not occur with the mitigating intervention of the rope.  Someone who tethers in with a dyneema sling and then manages to fall from above the anchor can impose considerably higher loads than those obtained from rope connections.  Even with ratings in the 22 kN range, it is possible to break gear, for example by clipping direct to a piece with a draw and then falling from above the anchor, which can produce fall factors in excess of the usual maximum of 2.

rgold wrote:

... And note that ropes stiffen as they age, so your brand-new temperature and humidity controlled laboratory 8 kN reading is going to rise (does anyone have good data on how much?) over the lifetime of the rope.  ...

Not only do the rope stiffens (giving higher impact force) with age, but also with several consecutive falls. I think (but not 100% sure) that the UIAA rope impact rating is for the very first fall on that rope.

Similarly, the 22-24kN rating of slings is for a brand new sling without any wear-and-tear. No need to consider these numbers as hugely overdesigned as these breaking strengths certainly will drop with sun exposure and a few years of normal wear.

So, the real question is why is protection rated so low, right? If my old rope now limits forces of a 1.77 factor fall to 9kn, and we use Petzl's 2/3s number for the pulley reduction due to friction, we get 5/3 * 9kN = 15kN on that top piece. But my nice new 1" cam is rated to 14kN.

Of course we are floppy bags of mostly water, and the belayer moves, and 14kN is the mean failure minus 3 sigma. But what if I can only get a .3 cam (rated 8kn) for my jesus nut? The actual protection part of the system sure doesn't seem over-designed. It's always seemed scarily close to catastrophic failure if we are only surviving because of our squishiness. But maybe I'm doing some arithmetic wrong?

Kristian Solem wrote:

A 200 ft rope can stretch 60 ft?