Tent poles - technical questions

[Orion]

I really like that little blue tent you built. I'm toying around with making a similar design.

I'm not sure which "blue tent" you mean. I've made two, one silnylon single pole tent and one cuben pyramid tent. The silnylon one was patterned after the Tarptent Moment (and many other similar tents). I almost bought a Moment but it had certain features that I didn't like so I made my own with some design changes. Some of my ideas worked out very well, some I learned not to do again.

Not counting tarps and tents that use trekking poles, that single hoop pole design could be the lightest weight solo design, but the struts on the end are a source of annoyance. In most cases they are embedded and you fold up the tent with these short rigid pieces and that determines the packing size/shape of the tent. They also place a limit on the end height of the tent. With The Moment they solved that problem by making the tent fly longer, thus increasing the interior end height. That's one of the things I didn't like: The Moment has a larger footprint than my two-person summer tent.

So I chose longer struts and a shorter overall tent, but with similar interior space. That meant that the struts had to be removable, which makes packing easier but also means the struts have to be installed and removed (and not dropped or lost). It also compromised ventilation somewhat although that hasn't been an issue where I've used it.

I'd love to have a Nallo 1 tent. I know it would be heavier than a one pole tent like the Moment but I believe that design results in a better mountain tent. The thing is, I'd need poles that turned a very tight radius...

[Chezza]

With a Nallo 1 you'd also end up with a tent that is very tall relative to its width. Might not be very stable in side winds without side guylines deployed. Lightwave make some inner-first pitching tents like that.

Feel free to drop me a PM if you have any more questions along these lines.

[Moondog55]

Here I was thinking you wanted an Akto but with hooped ends to take a snow load better
It is those corner struts and all the flat area that didn't put this tent on my list despite all its other good points

[Orion]

I looked seriously at the Akto and the Enan (the three season version). But when I saw the latter in person I realized very quickly that it was not for me. I pictured myself wet, cold, with a big pack full of wet gear trying to unfurl my sleeping bag in that tiny space. Despite the Hilleberg claim that it is "roomy" it seemed like a death coffin to me. I was prepared to spend a lot of money on the high quality of Hilleberg but it was just too small, for me at least.

One could size the design up some. The Tarptent Scarp 1 is an example, but it's a bit more of a three season tent.

[Moondog55]

Honestly I don't think the Nallo has enough poles. It's like the difference between the WE First arrow and Second Arrow
I trust my Minaret under most conditions except snow load and I hate getting up in the middle of the night

[Orion]

With a Nallo 1 you'd also end up with a tent that is very tall relative to its width. Might not be very stable in side winds without side guylines deployed. Lightwave make some inner-first pitching tents like that.

I would guy it out in heavy side winds. I guy our Nallo 3 when it's windy. I guy my homemade one pole tent when it's windy.

Here's my little tent next to an old version of the Nallo 2 from earlier this year. It was a super windy night and the windward guyline was critical. Although my tent is not designed for snow (mesh inside) it worked well with snow piled around the edges on the windward side. My friends in the Nallo didn't sleep very well as it is prone to being noisy in the wind.




The Lightwave Raid is pretty close to the size and shape I was imagining. At 107cm tall (external) it has almost exactly the same peak height as the Nallo 3 tent. And while it's true that the Nallo can be pretty flappity in the wind it seems to hold up well enough, provided it is properly guyed. I'd love to have a three pole design in windy weather but the tradeoff is weight. The Nallo 3 is a joy to carry.

I noticed that the Raid has an elbow in each of the poles. Hmmm.
A pity they went with inner first pitching though.

[Chezza]

Here I was thinking you wanted an Akto but with hooped ends to take a snow load better
It is those corner struts and all the flat area that didn't put this tent on my list despite all its other good points

I thought I could come up with something like that, but I gave up. By the time you put hooped ends on the Akto you end up with something so big and heavy you may as well just carry a traditional 2p tunnel.

One could size the design up some. The Tarptent Scarp 1 is an example, but it's a bit more of a three season tent.

Not sure why you say that. The main pole is now 9mm, same as the Akto, and the fly now comes down to the ground. The fabric patterning is better than the Akto. And there are always the crossing poles, and stronger pitchlock corners. The only reasons to pick the Akto over the Scarp, in my opinion, are the larger vestibule and the improved stitching quality.

Here's my little tent next to an old version of the Nallo 2 from earlier this year. It was a super windy night and the windward guyline was critical. Although my tent is not designed for snow (mesh inside) it worked well with snow piled around the edges on the windward side. My friends in the Nallo didn't sleep very well as it is prone to being noisy in the wind.

You did a nice job with the fabric pattern, considering you did this as a one-off. Would love to see pictures inside and how you connected the mesh to the body.

[Orion]

One could size the design up some. The Tarptent Scarp 1 is an example, but it's a bit more of a three season tent.

Not sure why you say that. The main pole is now 9mm, same as the Akto, and the fly now comes down to the ground. The fabric patterning is better than the Akto. And there are always the crossing poles, and stronger pitchlock corners. The only reasons to pick the Akto over the Scarp, in my opinion, are the larger vestibule and the improved stitching quality.

The reason I say it's "a bit more of a three season tent" than the Akto is because the fly doesn't quite go all the way to the ground and the solid inner has mesh sections that cannot be zipped closed. Apparently the fly goes lower than it used to but all the photos I've seen show a gap that is absent in the Akto.

What does the Scarp 1 weigh anyway? The Tarptent website is lacking in certain details which makes it hard to calculate and do comparisons

You did a nice job with the fabric pattern, considering you did this as a one-off. Would love to see pictures inside and how you connected the mesh to the body.

I glued it to the tent, in a big hurry as I was going to be leaving for a trip in 48 hours. It was a really sloppy job. I didn't fit it in there very well and I used too much glue. I had the idea to glue it in the first place as a way to maintain the integrity of the fly. But if I had to do it over again I'd sew it in and then seal the stitching.

Here are a couple of photos. You can see the wrinkled band of silnylon which is sewn to the mosquito netting and then glued to the inside of the fly. There's some more just visible in the back of the tent in the second photo. At the toe there's a zippered vent with fixed netting. I used some glue down there to connect the floor to the tent. It's pretty sealed up on the ends. I've used it in the Sierra Nevada where high humidity is rare. I've wondered how bad condensation might be in a wetter climate.

And as small as it is, it's still bigger inside than the Akto/Enan.

[Moondog55]

Here I was thinking you wanted an Akto but with hooped ends to take a snow load better
It is those corner struts and all the flat area that didn't put this tent on my list despite all its other good points

I thought I could come up with something like that, but I gave up. By the time you put hooped ends on the Akto you end up with something so big and heavy you may as well just carry a traditional 2p tunnel.

Then a couple of CF struts in an A perhaps or a single CF centre strut?
Solo tents are actually quite hard because a 2P tent isn't much heavier, the weight savings are small but not insignificant when saving every gram counts and most commercial 2P tents are what I consider to be a solo tent in winter anyway

[Franco]

"What does the Scarp 1 weigh anyway? The Tarptent website is lacking in certain details which makes it hard to calculate and do comparisons"

What exact details were you after ?
From the Scarp 1 product page :

Sleeps 1-2

Weight (oz/kg) 52/ 1.47 (std kit with solid interior)
Floor Width (in/cm) 32-52/
Floor Length (in/cm) 86/ 218
Floor Area (ft2/m2) 25.1 (max)/2.3 (max)
Interior Height (in/cm) 39/ 99
Stakes (included)
Packed Size (in/cm) 18 x 4 / 46 x 10
Price $359

sc1-1.jpg (82.24 KiB) Viewed 36031 times

one of 10 photos plus a est up video and 2 #D inside look video and this :

sc1_dimensions.png (38.04 KiB) Viewed 36031 times

The weight of the 6 stakes is under Extras (14g each)

[Orion]

What exact details were you after ?

Hi Franco. There are several things that I couldn't quite figure out.

1. Which inner tent is assumed in the specified weight?
You just gave me that. Thank you. If it's on the Tarptent website I missed it.

2. What is the weight of the other inner(s)?

3. The stakes
Thanks again. I found the stakes in "extras" but didn't drill down to the actual page where the weight is listed. I often take few or no stakes on trips so in my view they aren't part of the minimum weight.

4. The stuff sacks
Probably not much. But, like stakes, a stuff sack is optional.

5. Seam sealing
I found a BPL thread just now where it was said that it should only be about 1oz, if done correctly. If all tents required this then it wouldn't matter, but some don't.

6. Floor deslippification
Silnylon unfortunately has low friction with inflatable pads. When I painted the floor of our summer tent with diluted silicone it added significant weight (a few ounces). I guess I overdid it but it didn't seem like it as I was doing it. Perhaps an even higher dilution ratio was needed (I used 4:1). A decent PU coated floor might only be marginally heavier than a deslippified silnylon floor.

[Franco]

What exact details were you after ?

Hi Franco. There are several things that I couldn't quite figure out.

1. Which inner tent is assumed in the specified weight?
You just gave me that. Thank you. If it's on the Tarptent website I missed it.

2. What is the weight of the other inner(s)?
The mesh inner is about 1 oz lighter

3. The stakes
Thanks again. I found the stakes in "extras" but didn't drill down to the actual page where the weight is listed. I often take few or no stakes on trips so in my view they aren't part of the minimum weight.

4. The stuff sacks
Probably not much. But, like stakes, a stuff sack is optional.
About 3/4 oz

5. Seam sealing
I found a BPL thread just now where it was said that it should only be about 1oz, if done correctly. If all tents required this then it wouldn't matter, but some don't.
Depends on how much you dilute the silicone. higher dilution will allow you to put less , the idea is to penetrate the stitching not to coat it.
In silicone around 1 oz for the Scarp will do it . (the mineral spirit/white gas will evaporate)

6. Floor deslippification
Silnylon unfortunately has low friction with inflatable pads. When I painted the floor of our summer tent with diluted silicone it added significant weight (a few ounces). I guess I overdid it but it didn't seem like it as I was doing it. Perhaps an even higher dilution ratio was needed (I used 4:1). A decent PU coated floor might only be marginally heavier than a deslippified silnylon floor.

If you paint more than 1/2 oz you put far too much.
You only need a VERY thin layer doing dots or wide stripes. I use a cloth for that (dip the cloth wrapped around a finger and then paint) The thicker the coating the easier it will peel off.

[Orion]

Thanks, Franco.

[rcaffin]

First of all, the 'natural' curve for a tent pole is not a semi-circle. It is usually close to being a section of an ellipse, since there is more bending moment in the middle compared to the ends.
That said, by the time you have attached any side guys (which you SHOULD), the pole won't be that shape either.

Yes, I do use carbon fibre poles on my tunnel tents. For a 7.5 mm OD 6.25 mm ID I allow an 1,800 mm radius of curvature. This usually means something like 30 degree elbows for a 2-man tunnel.

The calculations for my tents involve solving elliptic equations. You can't solve them - and they are used in elliptic curve cryptography for that reason! Al, but I can do a numerical approximation solution which gets me to well under 1 mm error. A 1 mm error in a storm is not really significant ...

You can't pre-bend CF tubing (hence the elbows), but you can pre-bend Al tubing. You should do this if you can as it reduces any long-term work-hardening. The Easton 7075 T9 poles are good, but their foray in to 7087 alloy was a failure. The stuff was too brittle and work-hardened and ... well, it broke. On the last day of a traverse of the Pyrenees in Europe. However, pre-bending the 7075 tubing is an art and defies common sense in a way.

Flexibility: well, you probably know what you can do with Al poles. CF tubing is another matter, as there are several sorts. There is simple pultruded tubing, which splits with surprising ease. Do NOT use it. There is wrapped fabric tubing, which has improved over the years. It used to be a bit floppy but some vendors have got better at that. The good stuff is probably OK today.

Then there is 2D-wrapped tubing. This is complex stuff needing a very big machine to make it. I only know of one such machine in Asia, but there may be more today. It uses a layer of parallel fibres, then a layer of fibres wound tightly around that, then another layer of parallel fibres, another layer of wound fibres, and another layer of parallel fibres: 5 layers in all. The highest quality arrow shafts use this as it is nearly indestructible. Nearly! This is what I use, with machine stainless steel elbows I make.

Cheers
Roger
PS: biased opinion: a 2-pole tunnel is fine for a sheltered site, but not for exposed positions. I have made some and tested them. A 3-pole tunnel is not bad - I have also made them and tested them, and the Macpac Olympus is good. These days I use a 4-pole tunnel: bombproof and tested on the Main Range on snow under 100 kph winds all night. We slept well.

[Moondog55]

Yes Roger as I understand your posts it is all about keeping the supported panels to 900mm or less, so a 3 pole Minaret of a 4 pole Olympus would naturally be stronger but more expensive to make and therefore much more expensive at retail and the general traffic will only take so much
Given that; even Hilleberg tents are badly compromised for the worst conditions

[Orion]

Flexibility: well, you probably know what you can do with Al poles.

I don't know. That's why I asked:

...how much can you bend an aluminum tent pole? What is the minimum safe radius of curvature? Obviously that will depend on the specific pole, but are there general guidelines for aluminium poles?

[Chezza]

Try this, Orion:

http://www.geocities.jp/toshimi_taki/st ... m/beam.htm

There’s an Excel spreadsheet part way down. I haven’t actually played with it, but the author claims that it replicates the analytical solution. The advantage of this method over the analytical approach is that with a bit of tweaking it can handle pre-bends, elbows and changes in materials, assuming Excel’s Solver add-in can still perform the energy minimisation as the shapes get more complex.

You’ve got me thinking about something a bit more portable and computationally efficient than a commercial FEA package, that can handle all sorts of tent pole designs.

EDIT: I've have played around with this a bit and it seems to get lost in local minima, as I feared.

[Chezza]

However, pre-bending the 7075 tubing is an art and defies common sense in a way.

Roger, is there any chance you can share what you know about how the pre-bends are actually made?

[Chezza]

Hi Orion,

I have a solution for you. I've modified Toshimi Taki's Excel spreadsheet and it is much easier to use now. By enforcing symmetry and adding a few constraints I was able to improve convergence to the real solution and r3educe computation time. I did then expand it to 50 finite elements to provide more resolution, so it will take a while to solve each time. I'd recommend using this on a computer with a relatively new i5 processor or better.

The spreadsheet can analyse two cases - straight poles, and straight poles with a circular curved section in the middle.

I have verified the results of the spreadsheet in two ways. I looked up the analytical solution and wrote some Python code to crunch the numbers on straight poles. Agreement between the Taki method and the analytical method for straight buckled poles appears to be << 1mm displacement, and < 10 MPa in peak stress for several cases akin to real tunnel tent poles, which is nothing less than remarkable. I also verified the curved pole case that you'll see in the spreadsheet with a nonlinear ANSYS simulation and achieved results within 1mm displacement and 10 MPa stress of each other. I've only verified the one case with FEA so far. I'll do more FEA cases when I have more time, but I have a high degree of confidence in this tool for the intended use.

While this method could notionally be extended to poles geometries involving elbow joiners, in practice I think they are best left to more sophisticated software. However, with a bit of modification, you could actually analyse wind and snow loads with this spreadsheet tool.

Both the spreadsheet and the (poorly documented) Python code for the analytical solution can be found here:

https://drive.google.com/open?id=19IgZte4nFOEZYANqz8ZCI5fS2Y1A4mMX

[rcaffin]

how the pre-bends are actually made?
It's not that simple. 30 years ago, Jack Fox of Eastwood camping used to pre-bend replacement poles by running a new straight pole over a rounded table edge once, fairly fast. That worked, but he had had a LOT of practice.
I use a 3-roll bender to get a precisely controlled bend. But I had to make it myself.

8597.jpg (41.63 KiB) Viewed 35536 times

The thing is, that 7075 alloy is WEIRD. You can bend it once (or twice) to the curvature you want, and it will take that bend and hold it, but do NOT try to undo the bend! It will snap. Equally, do NOT try to bend it unsupported. It will collapse and break.

This is what you can do to an Easton arrow shaft. You may break a few along the way though.

On the other hand, if you repeatedly flex the pole to a bit less, it will eventually work-harden and snap as well.

Cheers
Roger

[Chezza]

Thanks for that Roger. I like the cut of your jig.

I might be wrong, but I think what you're seeing is the fact that a lot of these Al alloys have very low strain hardening exponents, and the crystal structures gather dislocations quickly.

http://www.eng-tips.com/viewthread.cfm?qid=272022

Most of these poles are cold worked during manufacture (that's the difference between T6 and T9 temper), so that probably makes the difference in plastic behaviour between these alloys and your average annealed steel even more glaring.

I imagine it takes a bit of testing to determine the minimum safe cold bend radius for any given pole material. I found a paper that recommends limiting 7075-T6 to 4% strain, but to use that you'd need to know strains already imparted during tube manufacture.

[Orion]

Hi Orion,

I have a solution for you. I've modified Toshimi Taki's Excel spreadsheet and it is much easier to use now....

...Both the spreadsheet and the (poorly documented) Python code for the analytical solution can be found here:

https://drive.google.com/open?id=19IgZte4nFOEZYANqz8ZCI5fS2Y1A4mMX

Wow! Thanks, Chezza. That's way cool. Excellent work.

I asked about this partly out of curiosity and partly because I was thinking about making a tent. Time constraints finally forced me to buy one off the shelf even though it wasn't quite what I was looking for. I think it will work for some, but not all, of the conditions I'm interested in. I've already spent some time making small modifications to it and I'm not quite done. So the motivation to make my own tent is still very much there. I suspect it's the only way to get what I want.

[Chezza]

No worries. I found it a useful exercise. I've been working on a tunnel design similar to Roger's with much stiffer poles, and the FEA for those poles is pretty straightforward. It was instructive to have a go at flexible aluminium poles, which are harder to analyse.

Now, to the second part of your question - what's the safe static stress in the pole? If your Nallo 3 is a recent model, I suggest some reverse engineering.

- Measure the radius of the pre-bent section(s) and treat this as the maximum safe pre-bend curvature; confirm TPT agree this is safe if using Easton poles.
- Use the spreadsheet to input the values of your Nallo 3 poles, and reproduce the height & width. Some differences due to ferrule stiffness are to be expected, get as close as you can.
- Treat the peak stress that you get out of the spreadsheet as the maximum safe static stress, and design your own tent poles to be near or under this value of static stress, with a pre-bend curvature no greater than what's in your Nallo.
- Do me a favour and post the Nallo 3 values here I've sold all my Hillebergs.

Off the top of my head, I'd say the peak stress should be in the 200-300 MPa range, but a formal set of calculations would need to assume numbers of wind load cycles, and snow load cycles, and your results then are only as good as your assumptions. Better to try and reverse engineer the Nallo, which (if it is a recent model), carries in its geometry decades of experience at Hilleberg & DAC.

Oh, just compared the results the spreadsheet against the ANSYS FEA along the whole length of the pole. Peak deviation in position was 2.5mm half way up the side (only 1mm in height, with width & starting geometry set to be the same), which is still more than precise enough for the needs of MYOG tent design.

[Orion]

I don't understand your approach. Why would you treat the pre-bent radius as the maxium safe pre-bend curvature? Wouldn't it be more likely that Hilleberg was more conservative than that?

Also, what does "TPT" stand for?

[Orion]

Our Nallo 3 is vintage 2008. I don't believe the design has changed significantly since then. And I'm fairly certain the poles are the same as well (9mm DAC Featherlite NSL).

I measured both the poles from our Nallo. The front pole has 3 segments that were pre-bent and I calculated a radius of curvature of 202cm. The shorter rear pole has two curved segments with a 201cm radius of curvature, essentially the same given my crude measurements.

Compare this to the 9mm Easton poles which allow 100cm radius pre-bends.

[Moondog55]

[quote="Orion"

Also, what does "TPT" stand for?[/quote]
\Common shorthand for Tent Pole Technologies

http://tentpoletechnologies.com/

[Orion]

Doh! That's the second time in this thread that you've had to tell me that.

Do you know where on their website you can find technical information? All I see is pricing.

[Moondog55]

All technical information has been removed from the site. Ditto all the links that used to be there

[Orion]

That's too bad. I tried to dig up old versions of the website on archive.org but going back as far as they have archives (2012) the site seemed virtually identical to how it is now. Either the technical stuff wasn't directly linked from the homepage or they tagged it as non-archivable.

I also tried searching using the "site:" feature of google and I got a bunch of really weird results, pages that had nothing to do with tent poles -- lawyers, automobile parts, all kinds of things. And clicking on any of the links just redirected me to odd destinations. Funny stuff going on with that domain.

[Chezza]

I don't understand your approach. Why would you treat the pre-bent radius as the maxium safe pre-bend curvature? Wouldn't it be more likely that Hilleberg was more conservative than that?
...

The front pole has 3 segments that were pre-bent and I calculated a radius of curvature of 202cm. The shorter rear pole has two curved segments with a 201cm radius of curvature, essentially the same given my crude measurements.

Compare this to the 9mm Easton poles which allow 100cm radius pre-bends.

Generally speaking, the more pre-bend you put into the pole, the lower the stresses are going to be. However, as Roger indicated, there is a trade-off and you can pre-bend a pole to such an extent that it might crack under the first or second large dump of snow you see, as the external forces attempt to reverse that bend. So there's a tradeoff here.

I don't know what assumptions the Easton recommendation is based on, so I would ignore that. The pole dimensions that Hilleberg have settled on represent the result of decades of experience (I pick Hilleberg because they're family owned, and hence likely to retain lessons learned, and because their tents see lots of winter use; then again DAC might just be designing the poles for them, and the same logic can be applied to DAC). The achievable range of tradeoffs between pre-bend, static stress and external loads will be quite different for different pole widths.

If I wanted to be really conservative, I'd do the following. Pick my desired pole base width, and find the Hille with the nearest dimensions in the pole diameter I intend to use. Ask somebody who has the tent to measure the width. Then buy a set of spare poles from Hilleberg and build my tent around it. Use the spreadsheet to give me a curve I can plug into SketchUp or whatever I'm using.

TL;DR, just use suitable Hilleberg poles and design your tent around them. Use my spreadsheet to give you a shape to import into CAD.