Pyramid tent build

[Orion]

Maybe 'Snaps'?

I thought of that and guessed they'd be heavier. I don't have one handy to weigh at the moment. Someone on BPL quoted 0.4g for size 14, whatever that is.

What I was thinking earlier Orion was that you could actually build the shelter just that way (ie use the cuben itself for the mock up)
- blue painters tape holds cuben, comes off easily. Carefully get it set up then pinch/measure the catenary then lay it out and cut (not that you need to now). Lots of ways to skin (or measure) a cat.

Yes, that makes sense. That seems preferable to changing it after it's glued together.
But I was really hoping that a shape as simple as a pyramid wouldn't require these sorts of shenanigans.

[Redback]

I'm now on to constructing the vents. I am planing to to do nanoseeum inside. Will use some whipper snipper cord to stiffen the edges.

Still deciding on the shape of the vents? and how many.

[Mark F]

Many tents only have a top vent on one side but I prefer to have two, one on each side of the tent on the steepest panels to minimise the chance of rain blowing in. Whether you need low level vents depends on the tent design.

[Redback]

Many tents only have a top vent on one side but I prefer to have two, one on each side of the tent on the steepest panels to minimise the chance of rain blowing in. Whether you need low level vents depends on the tent design.

Yeah, I think I will go with two. I like how the HMG Ultamid does theirs.

[Mark F]

Revisiting the catenary curve issue:
Cuben tarp with 3" over 9' so a bit over 25mm per metre http://www.lytw8.com/uploads/LytW8_Cuben_Tarp_Design.pdf

This is a link to a short technical article by Roger Caffin on catenary curves. It may be behind the bpl pay wall. http://www.backpackinglight.com/cgi-bin/backpackinglight/myog_tech_note_catenary_curves.html#.UmsRIdIbBRw

[Orion]

Thanks Mark, but I've seen both of those before. I wasn't sure -- does a tarp with a single ridgeline compare well to a pyramid tent?

Roger's spreadsheet is handy for generating a scaled catenary but it doesn't decide anything for you.
He suggests doing more or less what Nuts said. In Roger's own words:

"I suggest you experiment a bit before cutting out the curve on the ridge line of your fabric. Cut the fabric out with a straight ridge line - no curvature. Then mark out the line for a small sag, pin it together and string it up. Now you can see what that looks like in reality. If there isn't enough sag, take it down, increase the sag parameter (b) a little, and try again. If you approach this from the 'not-enough-sag' side, rather than overestimating and having to reduce the sag, the pin holes will always be on the bit you cut off."

[Mark F]

Orion - I started walking using a nylon fly sheet (flat tarp) around NSW in the late 60's. Most of my early walking was in NSW and tended to be places where one camped in river valleys and relatively sheltered spots. Much more recently I bought a MLD Grace Duo in cuben (catenary cut flat tarp) which I used on the AAWT and other trips but my preferred walking is now in higher and more exposed areas and I decided that for this I preferred an enclosed tarp. I started with a SMD Gatewood Cape (silnylon) and now have a Hexamid Plus with beak and I am quite happy with it but will try to build something a bit more wind resistant over the Christmas holidays.

My gut feeling is that with awning type tarps where the ridge is horizontal and quite long (2.4-3 metres) a larger catenary is beneficial. On enclosed tarps like the Hexamid I believe that about 10-20mm per metre all that is required and only where there is a significant angle between the panels - just enough to take out any irregularities. When one examines the Hexamid you realise that it is really two pieces of cuben with some extensions where required to extend the width of the fabric so there are no (apparent) catenary joins at all. For silnylon I would increase the catenary to minimise issues with stretch in the fabric. A catenary is also useful on long edges to minimise flap such as where the fabric reaches the ground.

When you examine the photos of pyramids on the MDL site you can discern some catenary but nothing like the 9" mentioned earlier in this thread.

[Nuts]

It's not really a case of right or wrong, perhaps mathematically speaking it's measurable but.. go to far and the penalty is decreasing inside space. Though, to a point, the material gets tighter. For me that point was a smidge over 1" per ft for the hem. It may have been less for the upright seams (i'll measure it again some time) and much more (eventually) for the ridgeline- so i'd agree.. the curve needed changes depending on where the seam is. I expect this is a function of where (and how much) fabric weight is 'hanging'.

The point was definite and perhaps unique to my design. The figure came from Lawson who was building similar ridgeline cuben shelters... and it worked. Measuring something similar (simply with a string line and ruler) to what you want to build makes sense as a guideline. I'd be interested in what it is in the cuben Duomid if someone has one.. and does the duomid have a straight hem/seam?

[andrewa]

Isn't the "tautness"/ catenary cut issue most practically related to wind and/ or snow and rain deflection?

I was surprised as to how much my Hilleberg Nallo 3 GT flapped around in the very strong winds on Bogong a few weekends ago. Whilst I appreciate that it survived the night, my underlying feeling is that if it had been designed a bit tighter ( ie deeper catenarIes) we would have had a more comfortable night. I know this sounds harsh , as the general Hilleberg feedback is positive, and, mine would also be same, but its just an extension of thought...

I have recollections of an Early Winters tunnel tent back in the 80s which pitched "as taut as" on Feathertop in high winds. My Nallo is a lot more flappy than that was......but it still stood up to 70-90kmh winds. At the end of the day, I'd rather have a "taut as" tent that doesn't keep me up at night...

My feelings are that if you are planning on pitching in windy areas, and requiring high snow shedding ability , go the deeper catenary cut. If you only need rain deflection, go less catenary.

Nuts, have you got some piccies of your tent in use? I don't think I ever saw the finished product, but the design looked fantastic.

Andrew A

[Mark F]

Doing the trig and some a quick scaling on the Duomid photo on the MLD web site each corner seam is 2.05 metres long and the catenary is about 100mm.

[Nuts]

Iv'e only used it on a few overnight trips Andrew and nowhere near rough enough to trust everything but all in good time. The backyard photo's (on here) are probably the best. I camped with the shell out near Guardians with a few gusts and with the mesh inner on a few nights in the Tyndalls, one night of heavy rain.. I'll have to remember to photograph the tent (and pitch it properly before grabbing the camera) the *&%$#! scenery gets in the way

[andrewa]

Nuts, spectacular place. Irritating how the forums seem to cut off the right side of photos. ...I spent a fair chunk of time trying to post some piccies on another forum yesterday, and had same problem. Do you know why it happens? Anyway, back to the MYO tent brigade.....

[Nuts]

Can you see a tent andrew , the tent is very close to the right border? Perhaps send a screenshot if it's bothering you.
(sorry, the photo's not that useful.. I'm sure i have others- my photo laptop took a drink of coffee.. it's dead : ( if anyone has any suggestions for retrieval ..)

Doing the trig and some a quick scaling on the Duomid photo on the MLD web site each corner seam is 2.05 metres long and the catenary is about 100mm.

Was looking at my first photos. I think what you guys were saying about having 'some' curve tightening the seam is right. It doesn't take a lot to take up the stretch with this material. I recall it was more the small wrinkles surrounding the peg points that took extra catenary to pull out. Maybe a bit fussy, I did have a lot of interior space to work with so that wasn't a problem.

[Redback]

It's like the Where's Wally of finding tents in a picture. Wouldn't make a bad coffee table book.

[icefest]

Nuts, spectacular place. Irritating how the forums seem to cut off the right side of photos. ...I spent a fair chunk of time trying to post some piccies on another forum yesterday, and had same problem. Do you know why it happens? Anyway, back to the MYO tent brigade.....

The forums are a set amount of pixels wide. This number increases or decreases with display size and how much you are zoomed in.

You can see the whole picture by right clicking and viewing in a new page.

[Redback]

Nuts, where abouts are the Guardians? That is an amazing view.

[Nuts]

The Guardians are a southern spur off the Ducane Range in the Cradle Mt. Lake St. Clair National Park. This photo is not in the Guardians.
There's a track almost too this spot but I feel a perhaps odd tinge of 'wrong' about labeling it. Someone else can if they like, it may already be described on here.. Anyhow.. I'll send you a message.

[Redback]

The Guardians are a southern spur off the Ducane Range in the Cradle Mt. Lake St. Clair National Park. This photo is not in the Guardians.
There's a track almost too this spot but I feel a perhaps odd tinge of 'wrong' about labeling it. Someone else can if they like, it may already be described on here.. Anyhow.. I'll send you a message.

Thanks. It is a picture that inspires some more hiking.

[Redback]

I have now cannibalized a small kids chair for the top hat fabric. I'm running out of tent building jobs until my tape arrives.

[Orion]

On enclosed tarps like the Hexamid I believe that about 10-20mm per metre all that is required and only where there is a significant angle between the panels - just enough to take out any irregularities.

Doing the trig and some a quick scaling on the Duomid photo on the MLD web site each corner seam is 2.05 metres long and the catenary is about 100mm.

That is considerably more than what you believe is sufficient, which leaves me wondering.... Why would they use more curvature than is necessary?

By the way, when you say 10-20mm per metre what exactly does that mean? I think I understand this and then I'm not sure. Is it the ratio of maximum deviation from a straight line at the midpoint of the ridge versus the end to midpoint length? Or is it versus the total ridge line length? For example, a corner ridge on the Dumoid is about 2 meters long and the maximum deviation is about 100mm. Does that translate into 50 mm/meter or 100 mm/m? Furthermore, when talking about the cut of the panel is this number a reference to the curvature on the panel or the final curvature of the seam?

[Mark F]

Orion - Sorry not to comment earlier - the joys of making a living.

The image I used is of a silnylon version so more catenary is needed due to the greater stretch of the material. I also think that as seams become longer so the required catenary per metre of seam increases. I took a very rough measure from a photo that is not quite end on and I erred a little towards the high side as I was wanting to comment on the 25mm per 300m figure.

My measurements are are per metre so 25mm per metre of seam over a 2 metre long seam would have a total deviation of 50mm. If you have a 90 degree angle and view the seam with one panel parallel to the line of sight, the viewed deviation should be the same as the catenary cut into each panel - assuming minimal stretch. Happy to told otherwise but I am pretty sure that is what you will see.

[Orion]

Isn't the "tautness"/ catenary cut issue most practically related to wind and/ or snow and rain deflection?

Wouldn't a saggy tent also have less interior space?

I was surprised as to how much my Hilleberg Nallo 3 GT flapped around in the very strong winds on Bogong a few weekends ago. Whilst I appreciate that it survived the night, my underlying feeling is that if it had been designed a bit tighter ( ie deeper catenarIes) we would have had a more comfortable night. I know this sounds harsh , as the general Hilleberg feedback is positive, and, mine would also be same, but its just an extension of thought...

That's an interesting observation about the Nallo. I've imagined that the flapping is because so much fabric spans between the poles and that a third pole would fix it, at a cost of extra weight of course. My BD Megamid, with it's long seams and large panels, is pretty flappy in the wind even though it has a serious catenary cut. I've never considered pyramid tents a great choice for windy weather unless dug into the snow.

[Orion]

The image I used is of a silnylon version so more catenary is needed due to the greater stretch of the material.

Oh, wrong tent. If you get the chance can you have a look at the Cuben version? I've tried to measure catenary from photos but it's tricky. Phil has a bunch of shots of his here:
http://bushwalk.com/forum/viewtopic.php?f=15&t=11177&hilit=duomid&start=270

I also think that as seams become longer so the required catenary per metre of seam increases.

Interesting. I don't understand this either!

My measurements are are per metre so 25mm per metre of seam over a 2 metre long seam would have a total deviation of 50mm. If you have a 90 degree angle and view the seam with one panel parallel to the line of sight, the viewed deviation should be the same as the catenary cut into each panel - assuming minimal stretch. Happy to told otherwise but I am pretty sure that is what you will see.

I see. Correct me if I still haven't gotten this right:
It's max deviation divided by total seam length for the panel edge in 2-D, not the seam in 3-D.

[ULWalkingPhil]

The image I used is of a silnylon version so more catenary is needed due to the greater stretch of the material.

Oh, wrong tent. If you get the chance can you have a look at the Cuben version? I've tried to measure catenary from photos but it's tricky. Phil has a bunch of shots of his here:
http://bushwalk.com/forum/viewtopic.php?f=15&t=11177&hilit=duomid&start=270

I also think that as seams become longer so the required catenary per metre of seam increases.

Interesting. I don't understand this either!

My measurements are are per metre so 25mm per metre of seam over a 2 metre long seam would have a total deviation of 50mm. If you have a 90 degree angle and view the seam with one panel parallel to the line of sight, the viewed deviation should be the same as the catenary cut into each panel - assuming minimal stretch. Happy to told otherwise but I am pretty sure that is what you will see.

I see. Correct me if I still haven't gotten this right:
It's max deviation divided by total seam length for the panel edge in 2-D, not the seam in 3-D.

Page 13, 14 and 15 you will find images of my cuben Duomid in the Cuben Fiber Tent topic.

[Mark F]

I would always talk about catenary in 2D as this is what you are cutting into the fabric. If you try to talk about it in 3D you have to define the angle between the two sheets of fabric. If the 2 panels join at 90 deg a 2D catenary of 10cm in both panels translates into a maximum deflection of 14 cm from the straight seam location.

Having a look at Phils photos I estimate the 2D catenary is about 9cm assuming a seam about 2.1 metres long.

[Redback]

Just wondering if someone can tell me the theory as to why the vents take up the entire 1/4 side face at the top. Duomid and Ultamid both use the whole face side. I'll probably do the same, but wonder why it's done that way.

[Mark F]

I suspect there are two issues at play.

1. to make the vent as large as possible
2. building it into the seams is easier and I expect a bit more stable

[Mark F]

A thread on the size of catenary cuts just opened up on bpl.
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=84070

[Orion]

I would always talk about catenary in 2D as this is what you are cutting into the fabric. If you try to talk about it in 3D you have to define the angle between the two sheets of fabric. If the 2 panels join at 90 deg a 2D catenary of 10cm in both panels translates into a maximum deflection of 14 cm from the straight seam location.

Okay, thanks again for the clarification on what you mean, and I hope what other people mean too. Of course the panels in a rectangular pyramid will never join at 90 degrees; the angle will be somewhat larger. The other ingredient is whether the 2D catenary is the same for different sized panels. If the curve is there to account for cutting/joining errors and unequal stretch then I think it makes sense to size the deflection proportional to the panel width, although this may not matter much in practice.

Having a look at Phils photos I estimate the 2D catenary is about 9cm assuming a seam about 2.1 metres long.

So now that we're looking at a Cuben Duomid estimate I'll ask my question again.

Why would MLD use more curvature than is necessary?
Might it merely have been easier for them to copy the silnylon pattern?

[Orion]

A thread on the size of catenary cuts just opened up on bpl.
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=84070

Thanks, I'll keep an eye on that.