Zipline : how to anchor on two trees, one trunk.

[bigpig]

Please see the two trees. Both emerge from one trunk or rootstock. Unsure of the lingo. The leftmost tree is approx 12" diameter, the rightmost is approx 10" diameter. I am running a 200' zipline at 6-7' from the ground downhill (10ft drop) to another tree (huge Oak). I am planning to anchor the zipline to the rightmost tree in the picture at the top of the slope, and wondered whether for additional safety I could / should somehow 'tie-in' the rightmost tree. Any advice appreciated.

 

[Reach]

My thought would be yes, do attach the trunks together; 200’ of zipline will have 2000 lbs or more of tension, and that’s a lot to support with only a 10” trunk! Which direction does the zipline come off the tree? I.e. towards the camera, to the right of the picture, at an odd angle? That will make a difference in how to anchor the trunks together.

 

[bigpig]

Thanks @Reach. Below is a new picture that is facing the actual path. The red loop around an example piece of string to illustrate direction.

 

[Reach]

Thank you for the picture. My first thought would be to find a bigger tree if possible, as that one is a little small for that length zipline. I believe it should work safely though, if you anchor the trunks together. Personally, I would run one long bolt (a length of all-thread with washers and nuts on both ends) through both trunks. Other option would be to put an eye bolt through each trunk and run a loop of cable between the trunks; it’s debatable which option would be better, in my mind. Maybe someone else out there has some opinions?

 

[bigpig]

With the equipment to hand, I was going to ratchet strap left tree to right tree, then wrap zipline 1/4" steel cable round rightmost tree (protecting tree with wood blocks).

 

[Reach]

That may work short-term, but ratchet straps are not rated or designed for long term use in a situation like that. I would be concerned about it failing suddenly, and if it does not fail, the long term effects of the strap girdling both trees will likely kill the trees in time.

Personally, I do not like wrapping the cable around a tree either, even though that is how the manufacturers recommend installation. I prefer to through-bolt the tree and attach the cable to an eye bolt, with a thimble of course to protect the cable.

 

[climbstihl]

Our preferred method in adventure parks is girth hitching crane slings around the tree, and the also having a steel cable backup loosely around the tree, because slings are not certified for life support. As long as there is some slack in the zipline (which there should be), the sling will open up while the tree is growing. This whole system is semi-permanent, you do have to make sure the sling doesn't max out or the zipline becomes to tight after a couple of years. It's also debatable whether or not this is better for the tree than a through bolt, I am going to stay out of that, because I don't know much about that.

 

[Anawan]

Our preferred method in adventure parks is girth hitching crane slings around the tree, and the also having a steel cable backup loosely around the tree, because slings are not certified for life support. As long as there is some slack in the zipline (which there should be), the sling will open up while the tree is growing. This whole system is semi-permanent, you do have to make sure the sling doesn't max out or the zipline becomes to tight after a couple of years. It's also debatable whether or not this is better for the tree than a through bolt, I am going to stay out of that, because I don't know much about that.

Girthing any kind of gear around the tree is going to choke the vascular tissue for a majority of the circumference of the tree, whereas a through-bolt or other hardware will only affect a much smaller percentage... girthing is probably only suitable for a short term temporary setup.

 

[Rob Stafari]

I don't think I would trust the cherry trees around here that size. What's the tree I think I see hiding hiding behind it in the second, inline picture. If it is suitable, I'd drill a hole through the front tree and run the cable through and attach to the tree behind it to share the load. If that's wasn't a viable option that I would consider running a few guy cables to stabilize the tree in question if there are suitable anchor trees. Or of course, find another more suitable tree to begin with.

 

[bigpig]

I don't think I would trust the cherry trees around here that size. What's the tree I think I see hiding hiding behind it in the second, inline picture. If it is suitable, I'd drill a hole through the front tree and run the cable through and attach to the tree behind it to share the load. If that's wasn't a viable option that I would consider running a few guy cables to stabilize the tree in question if there are suitable anchor trees. Or of course, find another more suitable tree to begin with.

Yeah that's kind of the plan, although rather than drill a hole I'm wrapping the cable around the front tree (protecting with wooden blocks), and then ratchet strapping the front tree to the rear tree just above the cable height.

 

[Knight Garrard]

First step. Cut the poison ivy vine!!

 

[southsoundtree]

Thank you for the picture. My first thought would be to find a bigger tree if possible, as that one is a little small for that length zipline. I believe it should work safely though, if you anchor the trunks together. Personally, I would run one long bolt (a length of all-thread with washers and nuts on both ends) through both trunks. Other option would be to put an eye bolt through each trunk and run a loop of cable between the trunks; it’s debatable which option would be better, in my mind. Maybe someone else out there has some opinions?

Connecting both trunks together with a rod sounds ok. Must be away from the cable for the zip line, if through-bolted.

Wooden blocks may give pressure sores, rather than a tourniquet, unless padded as needed.

Using what is on hand rather than the right gear will get you a lesser end-product.