Octamid Help

[Adam Thibault]

I'm designing an octamid similar to the following:
http://hikinghq.net/forum/showthread.php?2815-HomeMade-backpacking-Tipi&p=26466#post26466

Does anyone know how to cut the material to minimize waste yet keep the panels stretch correctly? I could cut the triangular pieces so one of the edges is along the selvage and one along the bias to minimize waste, however would you match selvage/bias or selvage/selvage? Or would it be better to cut the triangular pieces so both edge is along the bias? It would cause much more waste but it may perform a lot better (equal stretch along each one of the seams…

These guys make something very similar as well: http://wyominglostandfound.com/

Then again I could save some headache and just make a big pyramid like this http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=52245

I am drawn to the more conical shape however for aesthetics (I think it may perform a little better as well)

Any advice would be much appreciated!

[Jerry Adams]

How wide do you want the panels along the bottom?

I think you would want the center of the panel parallel to the selvage.

You could cut one panel straighhtforwardly, and then fit the two halfs of the next panel on either side and then sew together down the middle. That would minimize wastage.

[Jerry Adams]

Or, you could just make a square rather than octogon – very little performance difference.

If you want to place several sleeping bags side to side, a square base is actually better.

[Adam Thibault]

Thanks for the insight Jerry, I've been scouring your MYOG posts lately and have been thoroughly enjoying all the information you've put out there!

I think the panels should be about 4'7" along the side for a 12' diameter octamid. I've thought about cutting each one of the panels down the middle like you suggested. It would add more seams but it also would result in a lot less waste. I think you're right about squaring up each panel with the selvage as well.

I've been bouncing back and forth between a pyramid and octamid and the ease of construction of the pyramid might just win out…I'm sure there is very little performance difference between the two. I'm just having a hard time letting go of the conical design because of the appeal of the tipi…

[Adam Thibault]

On second thought I could do the triangular panels like a MLD Supermid and cut the panel horizontally (i.e. trapezoid with a smaller triangle on top. This might mean less seams than splitting each triangular panel vertically…

[Jerry Adams]

Oh yeah, I forgot about that, great idea, fewer seams and little waste.

[Adam Thibault]

Having all vertical seams may be stronger though because most of the stress would be parallel to the seam

Here's my sketchup – 12' in diameter, 6.5' tall

I'm hoping it will fit 3 people and a ti goat stove…

[Dustin Short]

Adam, you'll still have vertical seams on most of the side panels (when you flatten your image the connected trapezoids won't all fit on a standard width roll of fabric). So structurally they vertical seems will still carry most of the forces and horizontal seems will only be loaded in windy conditions.

[Ben Wortman]

When I have sewed tipis in the past like the one you want to make this is what I did. If the edge of your vertical panel is 8' for example, I just started at the corner of the material I had and measured 8'. Then from that point I measured 8' angling down to the other edge of the fabric. This will cause your shortest edge (the bottom when you sew it all together) to be offset a little from the original stating point. If you start with a 60” wide piece of fabric, the bottom edge ends up being 62-65”. When I use sketchup, I just start off by making a polygon with the # of sides I want, and then I make each side equal to the width I can get out of my fabric. If the sides of the tipi panels are 8’, you can get 2 panels out of about 9 feet of fabric.

I usually draw the flat pieces of fabric on sketchup also when I am done with the tipi drawing and lay the panels on the fabric to make sure it all fits before I start cutting.

I have never paid attention to the orientation of the threads between 2 panels in the past.

Good luck.
Ben

[Walter Carrington]

If I were making a 12' diameter tipi/octomid I'd make it taller — 8' or even 9'. It would give you a lot more usable space, especially for standing up.

[Jerry Adams]

If you make it taller it's heavier and blows around in the wind more

Not so good for ultralight backpacking

But a good winter shelter for several people that wanted to hang out protected from the elements

[David Olsen]

I would go with taller if you plan on using a stove inside, or in snow, or with lots of
people. I haven't found a taller 'mid to be much of a hindrance in winds and the weight
difference is made up in the usable space increase you get. I have a friend that teaches
winter survival training to kids, and he gets 16 people sitting inside a shelter of the
dimensions you describe for teaching lessons during foul weather.

[Paul McLaughlin]

You'll get a lot more efficient cutting layout if you have horizontal seams in each panel – you can nest the upper triangular pieces in between the lower trapezoids. Your selvage edges will be bottom and middle, and your ridge seams will be bias/bias. My experience with pyramids is that the absolute tightest, best pitch is with ridge seams that are parallel to the weave of the fabric, but you can only do this if you also run seams up the center of each panel that are bias/bias, which results in a lot of extra seams. So I'd go with the bias/bias ridges and you should do well. I would definitely NOT have seams that are bias/selvage.
If you are intending it for use in the snow(I can't tell from your post) then snow flaps are a great idea at the bottom.

I am with you in the opinion that the 8 sides will do somewhat better in the wind than 4. But, the square floor plan my yield more useable space than the octagon. worth considering.
If I were making a 4-sided pyramid, I would plan for a cutting layout that yielded bias/bias seams up the center of each panel, and selvage/selvage up the ridges. With that combination you should be able to pitch the edges tighter to the ground when that is desired, as the ridges should stretch less than the centers.

[Kevin @ Seek Outside]

The shape, fabric being taught , staking , pole and guyouts are the main things that will determine wind performance.

I spent the night in this the other night

It's 260+ square feet.

Winds were 30 + measured pretty consistently, and then it got a lot windier after dark. I'm estimating several hours, of 40 – 50 from every direction. It held up very well, in fact better than a small trekking pole supported tent that did not have the guyouts etc.

The round shape does stand up better in wind, the square is better for maximizing space.

The amount of cat cut will be vastly different from a square to more round shape.

6.5 feet will not be really stand up / walk around room, but 12 ft circum will be plenty for 3 people, if they don't mind being close. It might not for a stove though, although different groups have different comfort levels on closeness.

If you can go taller, it will feel more spacious, and if it is tight wind performance won't change much, what will change is you will need a stronger / heavier center pole. At 6.5 ft, you are at about the limits of any trekking pole setup.

[David Olsen]

11ft square, 10' tall including 2' Side walls.

[Adam Thibault]

Hey Kevin, I've been closely eyeing the tipis you offer as well. It seems to me that the circular shape (i.e. like a traditional tipi is sewn plus the extensions to the door) would be much easier to sew, however it seems to me that the circular shape needs more stake out points than an octagon…thoughts? Also I've heard the ridges leading up to the center act kind of like guylines or add some structure to the design. However, the circular cut like your tipis are seem pretty sweet.

I came up with 6.5' height because it seemed like that's about the height chosen for other tipis/pyramids about the size of mine from oware, kifaru, ti goat, etc. However, I agree that a little extra height may not be bad…

Also, I'm thinking of not splitting the panels at all (either horizontally or vertically), it means I'll have to order twice as much material (to account for all of the waste) however it will mean a lot less sewing which is worth the extra $50 to me!

[Adam Thibault]

Kevin also said that the amount of cat curves differs between a square pyramid and something like an ocatamid….I would assume that an octamid would need less cat curve because the panels have relatively less surface area? I was thinking 2.5"-3.5" of deflection for my 106" ridgeline, do I need less…more?

I really appreciate all of the feedback, this is helping immensely, thanks!

[Kevin @ Seek Outside]

Hi

You can use a selvage / bias orientation on each seam and stretch will be similar. The thread also comes into play, minimizing stretch in the seam.

Circular vs. Vertical and Hybrid orientation of seams:

It depends, our 6 person and 8 person are a hybrid as is the 3 person (but a different orientation). The BCS and the 12 shown were vertical.
You probably setup with less stakes easier on vertical, but for winds above 25 or 30 I'd think it's best to use them all anyway. On the circular, you don't need to use all stakes, but I usually do.

The ridges (vertical seams) add some structure, but there are other ways to do this, and they also create some challenges. I would consider guyouts just as important.

Height, is mostly a pole problem. FWIW the BCS is very tall to it's footprint and handles wind very well. If you are planning to use trekking poles, stick to about 6 ft.

Regarding cat cut, you probably need a lot less, 3" would be more what you would need if you were going square / mid style. You could take two panels, pin them how you want into the shape and see how much they droop, and then make that your cat cut. My guess, is you will be closer to 1.5 ", depending on fabric.

Kevin

[Kevin @ Seek Outside]

As a point of reference, our BCS which is a vertical seam, mid / tipi hybrid has vastly different cat cut amounts from the mid / corners to the more tipi style front side. In fact the cat might only be 1/3 the other rear section.

[Adam Thibault]

Wow, wouldn't expect there to be that much difference between the cat cuts for a tipi vs mid, wonder why?

[Kevin @ Seek Outside]

Longer runs to the corner of mids require more, and more of a turn in the fabric (like 90 degree) seems to bring some sort of a natural tension against each other rather than a rounding effect. It's bizarre, but build a few and it will be very apparent.

[Jerry Adams]

If you had 8 seams with cat cut (octomid) you'de need half as much cat deflection as if you had 4 seams (pyramid)?

[Kevin @ Seek Outside]

It's where I start usually on a proto, although that assumes all have the same length. If you odd ball shaped then it would factor in as well.

[Jerry Adams]

The other "rule" is like 2.5 inches of deflection for each 8 feet of ridge length. So 3 1/8 inch for a 10 foot long ridge. or 1 7/8 inch for a 6 foot ridge length.

[Kevin @ Seek Outside]

The steeper the less cat cut, you don't need much at 90 degrees :)

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