DIY zipline questions

I'm planning yo make a 40m zipline for my kids (and myself) in a park and trying to figure out a few things:

What is the maximum force for tensioning a static caving 23KN rope? Is 400kg too much? (I'm not sure how much tension will increase with addition of a 70kg rider)

I'm using a petzl stop for tension progress capture (something similar to grigri). Is it ok to put such forces on it?

which knot is best (in terms of rope strength) to make a loop on a middle of the rope, for tensioning. It is pulled in the same direction as the rope itself (I noticed that butterfly creates really sharp bend in this application)

Is it OK to use a simple resque pulley to ride it (24mm with ball bearings)?

If the zipline is close to the ground and not too fast - is it OK to use it without harness and helmet?

If you are pulling using human power I wouldn’t worry about damaging your rope. We would tension rope bridges with a team of 5 in the army. I recommend using an alpine butterfly knot and you’ll want an actual zip line trolley if you want any speed. Harness and helmet recommended, but bigger kids might get away with a circular seat style swing or a simple bar over safe terrain. Have fun and be safe! The stop at the end can be dramatic if you have any slope. 

michael v wrote:

I'm planning yo make a 40m zipline for my kids (and myself) in a park and trying to figure out a few things:

What is the maximum force for tensioning a static caving 23KN rope? Is 400kg too much? (I'm not sure how much tension will increase with addition of a 70kg rider)

I'm using a petzl stop for tension progress capture (something similar to grigri). Is it ok to put such forces on it?

which knot is best (in terms of rope strength) to make a loop on a middle of the rope, for tensioning. It is pulled in the same direction as the rope itself (I noticed that butterfly creates really sharp bend in this application)

Is it OK to use a simple resque pulley to ride it (24mm with ball bearings)?

If the zipline is close to the ground and not too fast - is it OK to use it without harness and helmet?

Industrial rope access tension line use works on the basis of working (single person) and rescue loading (two person) scenarios and a 70kg rider is not an issue.

Petzl stops work for rigging tension lines and horizontal protection lines but they slip at low loads unless the rope is locked off over the body of the stop. We don't use stops for primary descenders anymore because they have no anti-panic function not least the slippage. 

It's far easier to rig tension lines with Petzl rigs or IDs if you have access to one of those devices because stops have a double bobbin design that introduces friction into the pulley system linked into the dead end of the rope. A grigri would be easier but they slip too at low loads.

For the pulley system, far better to use a prussik on the rope than introduce a butterfly knot.

A single pulley will not run straight, instead it wobbles sideways. Use a tandem, double pulley unit.

Definitely be very careful about braking...using a trail line into a friction brake to stop the rider is strongly recommended.

Rig and use at your own risk!

Francis Haden wrote:

Industrial rope access tension line use works on the basis of working (single person) and rescue loading (two person) scenarios and a 70kg rider is not an issue.

Petzl stops work for rigging tension lines and horizontal protection lines but they slip at low loads unless the rope is locked off over the body of the stop. We don't use stops for primary descenders anymore because they have no anti-panic function not least the slippage. 

It's far easier to rig tension lines with Petzl rigs or IDs if you have access to one of those devices because stops have a double bobbin design that introduces friction into the pulley system linked into the dead end of the rope. A grigri would be easier but they slip too at low loads.

For the pulley system, far better to use a prussik on the rope than introduce a butterfly knot.

A single pulley will not run straight, instead it wobbles sideways. Use a tandem, double pulley unit.

Definitely be very careful about braking...using a trail line into a friction brake to stop the rider is strongly recommended.

Rig and use at your own risk!

Regarding petzl stop - it is what I have (I'm an amateur caver and its the standard descender for cavers). 

The fact it slips under heavy loads is an advantage for me since it won't let me to overtension the rope. I do lock it off after tensioning the line of course. I was wondering if the stop can get damanged after a bouncing person load is applied, and the initial 3KN can easily turn to 5-6

Thanks for advice on the prussik and the knots. I think I'll use tensionless anchor at one end (few loops around the tree with a carabiner around the main line) , petzl stop on the other, and a pair of prussiks for the 5:1 mechanical advantage system (and as a backup to stop) - this way I'll have a knotless system. 

Caveman Y wrote:

If you are pulling using human power I wouldn’t worry about damaging your rope. We would tension rope bridges with a team of 5 in the army. 

I'm using a 5:1 mechanical advantage system which is redirected downwards to I can use my weight for tensioning. Its realistic ratio is 3:1 and a single person tensions the rope to 2-2.5KN. If 2 people do it - it will be too much ...