The Hanover Collaboration

I built a zipline the other day. I thought the internet might want to know about it. Other people might want to do something similar.

I went out and bought 125’ of the strongest cable that they had at Home Depot which was 1/4” steel with a plastic coating. The first time I did it, the cable coating melted and plugged up the wood block. This actually was a more gentle stop than it became after I stripped the coating off the stopping region. (NB: Don’t use coated cable!). I would have used 5/16” if they had it but they didn’t, so my “Factor Of Safety” might not be up to aerospace standards. This also meant I couldn’t have as taut a line as I had originally planned¹. As it happens, it turns out that the more slack line is a better ride anyway since it is closer to the ideal brachistochrone.

To attach the cable to the trees I got three u-shaped wire clamps for each end, end loops, a handful of (5/16”) steel quick-links (sometimes called rapides) and 2×6’ of 5/16” chain. I stripped the ends before I put the clamps and loop on.

For stopping, I got 75’ of 3/16” coated cable and 2×75’ of 1/4” and 50’ of 3/8” bungee cord (to be used as 6×25’,2×25’ resp). I put a lot of calculation into getting these lengths right.

1. First of all, you want it so that at maximum elongation, you don’t hit the lower tree.
2. Secondly, you want to keep the stress tolerable in the bungees.
3. Third, you want to make the force on the participant tolerable.
4. Fourth, you want the thing as fun as possible which means stopping as quickly as possible while satisfying one through three.

I decided that (3) would be satisfied by a peak acceleration of about 3.5G. Our target audience weighs about 140lb with gear, this plus (1), (plus that you would like to go nearly all the way t the lower tree) specifies a length and spring constant for the bungees. Then I dictated a maximum weight of 200lb and checked to make sure that 1 is not violated. This design made the top speed about 30-35mph.

This is made much harder by the fact that it is impossible to find specs on the bungees. What I ended up doing was experimenting for half an hour in REI with a yardstick and a quart of water. Based on reading in the internet, I think 2 is not being violated—I would like to have put a little more area of less bouncy bungee in there, but that’s what they sold.

I got one more 50’ length of bungee so that I could have the rest position of the arrest bungees be out of the way. I’ll make a diagram.

Then I got some scrap 2×4 and notched a groove into each so that I could bolt them together onto the wire. The arrest bungees are attached to this block.

I put the thing together almost completely by myself. I had Courtney belay me for setting up an anchor and threading a pull cord one day and that’s it. After that, I just used ascenders. To get the cable up, I used the trucker’s hitch and an auto come-along. I also had some turnbuckles but I didn’t need them. In fact I tried using one, but broke it. You should read (and believe) the spec on the hardware store turnbuckles: they are weaker then they look.

What else? Let’s see, there’s getting up and getting down. Initially I had planned on climbing a knotted rope (with an ascender for self-belay) but between weakness and the difficulty of finding an outdoor-durable, non-slippery rope material, that got scrapped. Instead we have a rope ascent. This is done with an ascender and grigri. Yes a grigri — it is awesome.

Once you get up there, you put your pulley (trolley actually) on the cable, put a backup beiner on there too and take the ride.

Once you stop, there is a rope and a sling attached to the arrest block. You put the rope through your grigri, stand up on the sling and unweight the pulley. Take it (and the beiner) off the cable and then you can lower yourself.

From beginning to end the whole process is about 4min if you’re comfortable with all the gear. Here’s a video. I set the camera up on a branch so the camera-work is on the minimalist side (do you guys know that I’m a cinematographer now?). The beginning and end are sped up, but the ride itself is at regular speed. I’ll try to get some better video later.

¹ You can take a rope and tie one end to something fixed and the other to something heavy. Then by pushing perpendicularly at the midpoint, you can exert a much large force on the heavy object. Clearly this is a simple machine, but it defies easy catagorization as an inclined plane, pulley or lever. Any thoughts?