Hokay, so. This is the earth...Oh wait, thats been done. So there has been rumors flying around Calgary about slacklines, damage to trees, bylaws, etc...did you know that etc is actually &c ("and sic." or "et sic.")...ANYWAYS, I'm apparently truely tangential today.
So to set some things into internet stone and do some napkin math for the skeptics, here we go.
Slacklining falls under the tree abuse bylaws (23M2002), specifically this page: http://www.calgary.ca/CA/City-Clerks/Documents/Legislative-services/Bylaws/23M2002-TreeProtection.pdf. The debate comes down to section 15 and 16, specifically the use of the word 'secure'.
- No person shall use a Public Tree to secure any object.
- Without restricting the generality of section 15, no person shall secure newspaper vending boxes, bicycle racks, dog chains, clothes lines, guy wires, swings, or tree house to a Public Tree.
In the spirit of the law, the city does not want permanent structures. Temporary structures like slacklines, dog leashes (not chains), etc, seem to be up to the descretion of the bylaw officer based on thier opinion of harm to the trees.
What a wonderful segway into my next topic...I didn't even plan that actually. What harms trees? Well there are 3 types of damage we are worried about slacklining. Two deal with the bark, and the last with uprooting the whole dang tree. There are 2 types of outer bark as far as we are concerned. Soft/smooth/thin bark and gnarly/crusty stuff. The gnarly stuff is not going to show any damage, in fact it's meant to take a whole lot of damage. Smooth/thin bark will cut/abraid easier, and that is what we have to consider.
Outer Bark Damage
The outer bark of a tree is DEAD. It's armor for the rest of the tree, you can't hurt it. Thus our problem is rubbing and scratching the outer bark off, opening it to infection. This is easy to fix however, a simple wrap around the tree like carpet, or even cardboard. Something between the tree and the line kept reasonably tight so things rub it instead of the tree.
A second advantage is that YOUR gear does not get rubbed and frayed by the tree either, extending the life of it too!
Inner Bark Damage
Inner bark (phloem) damage is impossible to identify mainly because it's almost impossible to do, hence easy to use as an excuse to stop slacklining. There is nothing anywhere (books, internet, garden centers) about the amount of pressure needed to bruise a tree, so good luck arguing numbers (wow, I just invalidated the rest of my page...oh well). My friend Christine is an arborist, and she says, and I quote:
"Good luck damaging the phloem without removing the outer bark. Bylaw can go f#*k themselves if they say otherwise."
I might not repeat that to them though. Basically in order to minimize the 'possible' damage you should, again, cushion the tree straps.
If you attach your slack line to a tree that is too small, you'll actually rip/vibrate the roots out of the ground on the opposite side. A good rule of thumb is generally avoid any tree under 8" in diameter.
So what kind of forces are we putting on the trees as we play? I dunno. But we will solve that...wait for it...NOW:
Obviously we have the tension of the line between trees. But without a big ass scale, we don't know what it is. We can however infer based on some triangley math. A couple notes before we start;
- The better your measurements, the better the math...duh
- Partners are useful and good.
- This equation factors in stretch.
Alrighty then, the process:
- Weigh yourself (in kg). I currently weigh 73kg.
- Multiply your weight by 9.81 to get your downward force.
73 * 9.81 = 716N
- Divide this number by 2.
716 / 2 = 358N
- Measure the distance between trees. Mine is about 30 feet. Units don't matter.
- Find the exact center of the line.
- With no weight on the line, measure it's height. Mine is 40 inches.
- Now stand on the line in the exact center and measure the new height (I suggest a partner). My new height is 28 inches.
- Subtract your standing height from the rest hight.
40 - 28 = 12
- Convert your answer to the same unit as step 4. In my case, to feet.
12 inches = 1 foot
- Divide your distance to the center by your sag.
15 / 1 = 15
- Square your answer.
15² = 225
- Add 1.
225 + 1 = 226
- Take the square root of your answer.
√226 = 15.03
- Multiply your answer from step 3 with your answer from step 13.
358 * 15.03 = 5383
- Multiply your final answer by 0.225 to get pounds.
5383 * 0.225 = 1210 lbs
1200 pounds, pretty snappy eh? Check out snap in action -->
BTW 1200lbs is only standing weight, bouncing adds more!
Now then, what kind of pressure are we putting directly on the bark? This is harder, but do-able. First of all, a couple things we know.
- The front half of the tree doesn't do any work, so the useable length of sling is half the circumference of the tree.
- The force pushing into the tree is pretty much equal everywhere it touches, either because of the tension, or because it is squeezing.
Alright. The diameter of our pretend tree is 8 inches (minimum from above). The useable length is therefore:
Useable L = pi * d / 2
Useable L = 3.14 * 8 / 2
Useable L = 12.56 inches
With our common 1 inch tree sling that everyone uses, we get:
Pressure = 1200lbs / 12.5 inches
Pressure = 96psi
Egads! 100psi on the tree trunk! No wonder they complain of damage. But what is 100psi? or 50psi? or even 25 psi? Is 100psi really damaging, or just sounds like it should be an impressive amount?
Here are some really rough comparisons (say +/- 10%)
- ~100psi = A ballet dancer's toes while en pointe.
- ~75psi = the pressure of water coming out of your garden hose (in Calgary, the rest of N.A. uses 30-50psi)
- ~50psi = The ball of your foot while running.
- ~15psi = Your foot while standing on the ball of 1 foot.
- ~3psi = Your feet while standing flat footed on bare feet.
- ~1/3psi = Proper fitting snowshoes on snow.
After writing this all up, and talking to Christine, I've come to the sad conclusion that the city is wrong. So now that I've lost my state of blissful ignorance, how can I get to conscientious bliss?
- Use bigger trees. Twice the diameter = half the pressure.
- Use a wider sling. Twice the width = half the pressure.
Excellent, if we do both we get 1/4 the pressure. But we can do better...I think.
- First of all, I already use larger trees, not quite double, but ~150% bigger...So I've reduced the pressure by about a third. I'm down to ~67psi.
- Secondly I've built a large tree protector out of some old canvas, blue foamy, and some wood slats, it's a full 6 inches wide...I've reduced the pressure by 6x! Now nearer to ~11psi.
- Long term I can use different trees to give them time to 'heal'.
- I could ban fat people, but that's not nice.
- I could lose weight...Actually I could, but 5lbs isn't going to affect things.
Since there are only 2 factors going into the equation, are we surprised there are only 2 ways to fix things?
Respect the trees. Seriously. Your in the city, is an extra wide sling really going to inconvenience you? I would say as a rule, 2 inch tree protectors. If nothing else, it will probably keep bylaw off you back.