Self tensioners - long v short - an experiment!

**Martyn** · 03-28-2015, 20:29

Hi guys, I'm new to the forum and this is my first post.

Great forum, super informative. I've been lurking for ages and do recognise a few names from other forums I think.

Anyway, to the point.

I've been looking at self-tensioners on guy lines for a while. I switched to dynema on my tarp and the lack of stretch can make tensioning tricky. Some time back I was building slingshots with theraband green tubes and switched to gold flatbands, which left me with a surplus of green tubing. When I saw folks were using it to make self-tensioners, I thought I've got to get me some of that.

I made a few, the first were made with roughly 6" of tube, stretching out to about 12" plus loops - probably 16" overall when fully stretched. They work fine and do the job, but the length of em is a bit of an irritation. They are not really heavy, but they are bulky and they do limit how close to the ground you can pitch. So in an effort to streamline them, I made some shorter ones, about 3.6 inches of rubber stretching to around 7 inches - and I integrated them into the whole guyline. I wasn't sure these shorter ones would have enough 'twang' power, but it was worth an experiment. Just for the sake of absurdity really, I also made one using about 1.75 inches of rubber, stretching to about 3.5 inches. I was pretty confident this would be useless, but it looked cute. A quick pic....

You get the idea, all using theraband green tube, all built on 2.2mm dynema (Stein Skyline). They all seemed pretty strong to be honest, even the little one, which got me wondering....

If it takes - lets say 10lbs of effort - to pull a 6 inch tube out to 12 inches, how much effort does it take to pull a 2 inch tube out to 4 inches?

I remembered I had one of those cheap digital fishing scales, I bought to weigh luggage and realised I could used this to measure the actual pull required to take my self tensioners to full stretch. Now I know there are a bunch of clever people on this forum who are probably already doing the physics, but the answer genuinely surprised me.

First up, the 6" rubber, stretched out to 12 inches....

That's 6.50lbs.

Next, 3.5 inches of rubber, stretched out to about 7 inches or so....

That's 6.20lbs.

Next, 1.75 inches of rubber, stretched out to about 3 inches...

That's 6.30lbs.

Pretty much all the same really. It's not an exact test, the lengths are all a bit off as I made em up using mark I eyeball, but I'd say that the differences are just from minor differences in the percentage of stretch applied. But they are all roughly 200% stretched and they all require the same amount of effort to achieve it. Sounds obvious when I type it out, but I really wasn't expecting that. I honestly thought the longer tubes would be 'stronger'. Basically the draw weight is the same regardless of 6 inches or 6 foot, the only difference is the draw length. The longer tubes take less effort per inch. The shorter tubes though, are less bulky, weigh less, are cheaper to make, while they exert/absorb the same poundage of tension.

I would imagine the physics holds true regardless of the type of rubber used. The question is, is there anything I'm missing here? Before I dive in and make a bunch of mini-tensioners, is there any advantage to using longer rubber you can think of?

Sorry if all this is old news or has been covered before.

Cheers.

**craige** · 03-28-2015, 22:00

Hey and welcome to HF from Scotland!
That was a nice experiment. I did already think that was the case, however it is nice to have it confirmed

the effectiveness of the tensioner will depend on the amount of stretch in your tarp... if your tarp stretches 6" when wet and under tension then your tensioner should have a little over 6" of stretch. Ime theraband will actually stretch to well more than 400% before it will be anywhere close to snapping and it gets more difficult to stretch the further you... well, stretch it. So, you could quite easily get away with a much smaller tensioner that will work with more tension tham what is normally used, you just have to have a larger % of stretch.
So far I have had no need of tensioners though, I just crank the tarp down really tight and it's still pretty tight in the morning. YMMV.

**Martyn** · 03-28-2015, 22:17

Originally Posted by craige

Hey and welcome to HF from Scotland!

Hi and thanks for the welcome.

That was a nice experiment. I did already think that was the case, however it is nice to have it confirmed

If nothing else, I've learned that theraband green tubing takes 6.5lbs of tension to stretch it 200%, regardless of length. It's like a cosmological constant.

...the effectiveness of the tensioner will depend on the amount of stretch in your tarp... if your tarp stretches 6" when wet and under tension then your tensioner should have a little over 6" of stretch.

Yeah, that's what I was thinking too. I dont think the tarp stretches much at all really, the problem is that dynema is so static that all the tension comes from the stretch in the tarp - so even a 1/4 inch of stretch will take the tension out. Didn't used to be a problem in the old days with super-stretchy paracord. I like using dynema for other reasons, but I feel it benefits from having a bit of controlled elasticity put back into the system, so you are not reliant on the stretch in the tarp material for tension. I know it's not essential, but well, y'know how it is....

If I can get away with tensioners under 2" long, it ads very little to the overall weight, but has the benefit of set it and forget it. Like you say, I think it all boils down to tarp stretch, which I suppose is different for every tarp. I think 2" on each tie out should do it though.

**prussik** · 03-29-2015, 01:23

I was wondering this myself. It is good to know that it is constant

**TPole** · 03-29-2015, 15:22

The only advantage to using the longer rubber would be the amount of "take up" that it will provide. The smaller tensioners are much stiffer than the longer ones and as such require less deflection to achieve the same force. Only problem is that smaller variances in the tubing stretch will cause much larger changes to the force applied. If you lose an inch due to tarp stretch on the 1.75" long tubing you will lose 80% (1"/1.25") of your force. On the 6" long tubing you only lose 16% (1"/6") of the force. The thing to determine is how much you think the tarp will stretch and size the length of the tubing accordingly.

Nice work on the testing BTW, good to see people doing their homework and sharing!

**Martyn** · 03-29-2015, 16:42

Thanks TPole. I'm on the same page.

There are other factors that affect the decision though. For example tarps dont just get slack, sometimes they get tight too. When a wet tarp dries out, it can add unwelcome tension into the setup. So over say a 24 hour period of sun, rain, dark, high humidity, low humidity, dew etc, the tarp can move in all sorts of ways. The tensioners mitigate this movement in both directions - so local climate plays a role. In the UK it's common for us to get 4 seasons in one day. The other thing is what is the intention of the tensioners? Is it to completely remove the need for intervention, or just reduce it? It might be that while very short tensioners might not be long enough to completely compensate for the full range of stretch in the tarp, they compensate for the majority of it, reducing the need for intervention rather than eliminating it. That could be an acceptable trade-off?

**Baby Huey** · 03-29-2015, 19:45

Also weigh in the other factors that can benefit from having tensioner system in play. Wind and accidental contact with the tarp. I have seen people kick out stakes and wind put some pressure on tarp setups. These are just a couple more situations that can be mitigated by a tensioning system. In the case of people tripping over guy lines or accidentally falling into the tarp the longer tensioners would shine a bit more I would think.

**Martyn** · 03-29-2015, 20:43

Yeah, I think if you were erring on the side of belt & braces, then longer tensioners definitely cover more of the unknowns. But I think like so many things, it's a question of diminishing returns. It's not black and white, it's where to draw the line? I've had noticeable tarp stretch before, but cant be certain much of it wasn't due to my poor pitching in the first place and I'm not sure I can say I've ever seen several inches of stretch. I'm seriously tempted to make up 4x super short ones and see how I get on with em. I think a couple of inches of elasticity on each tie out offers not all, but a lot of the benefits of full size tensioners and is a ton better than nothing at all. I guess the only way to know if they will work for me is to make some up and give em a go. I've got a set of 4x Dutch Fleaz on the way and combining a really short tensioner with a flea spliced on the end, could be a really neat solution.

**Martyn** · 03-30-2015, 06:56

The Dutch fleaz arrived today, so I thought I'd make one of these mini tensioners up spliced onto a flea. Pretty cute little thing....

It measures about 5.5 inches total length (it would be a bit shorter when girth-hitched onto a tie out) and the rubber bit is just about 2 inches with no load....

As before, roughly 200% stretch takes 6.36lbs of load, making the whole thing just under 8 inches when fully loaded....

Weight for the tensioner is right on 5g each including tarp flea...

The 6 inch tensioners I made were 9g each, not inc tarp flea, so 10g overall. Not a massive weight saving, but definitely a clutter/bulk saving. I can certainly live with 5g each/20g for a set of 4. The dutch flea on it's own is 1g plus collar, so really not adding much than about 15g to the whole setup for a full set of auto-tensioners. They also dont add much length - you'd have to have some kind of loop to attach the fleaz to the tarp anyway. The overall length of 8 inches still allows for a pretty close to the ground pitch I think. I'm definitely going to 'field trial' these.

I like em, the only reservation I have is that I think 6.5lbs is a bit heavy. You have to load these fully to get the 2 inches of stretch, which is needed. So I have ordered a yard of surgical tubing which I think is less strong than the theraband, so should give the same stretch with a lower load.