MYOG Trailstar – radiating fabric… Overdone cat cut?

[James N]

This here is a Clone of the MLD Trailstar in silnylon that we made. I can’t for the life of me figure out why there are significant rolls of fabric radiating out equaly from each corner when under tension – its quite clear in the image. Is it an overdone cat curve at the base of each triangle? Is it the circular corner reinforments distributing the load – would it be better with triangular reinforced corners?

Any ideas from you guys on this would be much appreciated, so we can improve this next time.

(BTW, those mitten hooks on the outside are a consequence of the wife and I poorly communicating. It will be fixed!)

[Matthew / BPL]

Just guessing and I have no real experience with fabric but I’m guessing the fabric is aligned to the mid panel seams vertically so the main diagonal seams you are tensioning are getting lots of bias stretch.

Which way is the fabric oriented?

[Jerry Adams]

I don’t think you have too much cat cut – the peak gets really pointy if there’s too much, but with the trailstar 5 sided pitch maybe it’s different?

You can pinch out more cat cut with your hand and see if it makes any difference.

You have folds radiating out of the corner.  If there’s not enough cat cut then there’s a fold that goes parallel to the ridge and the ridge gets floppy.

MLD web page has those same folds, except only a little bit

Maybe just use it and don’t worry about it?

I wonder what would happen if you had a stake mid panel, pulling down at the middle.

Is each of the 5 panels one piece of fabric?  I don’t see any flat felled seams piecing together pieces of fabric.  How wide and tall are each side panel and how does that fit into a 60 inch fabric width?

 

[James N]

This is something that has been bothering me as a poss cause – the bias of the fabric. As it is the fabric is cut “Straight bias” (I think I am correct?)  so it runs straight up from the base of the triangle to the top.

Here is another shot when the fabric is dry – its not as bad as the intial pic, but those wrinnkles are definitely there by virtue of a design cock up  of some kind!

[James N]

@retiredjerry … The triangles are indeed one piece each. Measurements are:

140 cm is about 55″.

I tightened up mid point guys to see the effect – if anything it got worse, though it could have been my imagination, it certainly didnt get better with mid points tightened.

 

[Matthew / BPL]

Again, I should be quiet because of my lack of experience in sewing but I think the topic is interesting so I’m participating. Please take what I saw with several grains of salt…

The grain of the fabric is vertical giving you diagonal bias stretch along the main seams that you are applying tension to. I think the wrinkles are caused by a diagonal shear in the fabric from bias stretch along the seams.

[James N]

@matthewphx I think you are right on the money. Its the vertical bias cut, giving stretch on the bias radiating outward at 45 degrees. Assuming this is the case… It would be great if anyone knows how to cut the triangles correctly without wasting a ton of fabric… I have dug around the interwebs at length and I didnt find anything about how to apply fabric bias  properly in such a situation to ensure even tensioning throughout the fabric.

[Jerry Adams]

I tried laying out a pyramid on the bias.  It has it’s own problems.  I think laying out normal is just as good. https://backpackinglight.com/forums/topic/tent-on-bias/

In the second picture above the folds are less, getting closer to what’s on the MLD site https://mountainlaureldesigns.com/product/trailstar/.  I think you should just use your tent and not worry about it : )

The middle of the sides is up in the air leaving a gap.  That is a consequence of how the tent is laid out with the bias.  If you just make the side maybe 8 inches wider in the middle you wouldn’t have a gap.  Rather than being a triangle, it would be a weird 4 sided trapezoid.  This isn’t so much an issue with trailstar maybe, because there are large air gaps by design.  Better, if you wanted a pyramid with all sides close to ground to maximize rain and wind protection.

 

[Ivo Vanmontfort]

My last trailstar clone has no curve at the base,
2,5 cm for the slope sites
https://ivovanmontfort.blogspot.com/2015/05/de-ultimat-trailstarcopy.html?showComment=1476483533375#c9031237224490457488

[Jerry Adams]

I looked at that link Ivo, nice job, that looks real good.  I see Roger’s cat curve spreadsheet : )

There is a gap between the ground and the middle of the sides like James, maybe a little less than James

I assume that’s for the same reason, the fabric stretches more on the ridges because they’re on the bias.

Some people intentionally cut a cat curve on the edges, maybe it makes it flap in the wind less or lets in more air flow.  I’ve found that it lets in more wind with dirt and sand if it’s windy and possibly water if it’s rainy so I try to minimize the gap.

 

[Paul McLaughlin]

I would say, as others have, that the bias cut on the ridges is the culprit. I am convinced from previous pyramid experience that the way to cut  a pyramid is fabric grain aligned with the ridges. that way, you set the thing up,with the ridges taut, and the stretch on the bias up the center of the panel ( and along its bottom edge as well) allows the center pullout of each panel to go out beyond a straight line between the two corners and tauten everything up. With the bias on the ridges, you run into the issue that when (or IF) you get the ridges taut, those center bottoms are up in the air and you can’t bring them down. It may be less efficient as far as fabric use, and it requires a  seam down the center of the panel unless your pattern is a 90 degree corner at the peak and 45 degree corners at the outside bottoms – which I suppose a trailstar clone might have, though a 4-sided pyramid would not.

[Jerry Adams]

I tried laying out with grain on ridges https://backpackinglight.com/forums/topic/tent-on-bias/

It’s only a little bit inefficient with fabric use.

There’s a weird bend in the ridge, towards the bottom, where it lines up along the grain with the center bottom:

That makes the bottom angle out at a low slope so the outer edge of the floor isn’t as usable because there isn’t as much headroom.

Mostly this is just aesthetic though.  Either laying out on bias or laying out normal will work fine.

If you lay out normal, to fix the center edge gap in the middle, lay out the pieces about 8 inches longer in the middle:

At least that’s my theory.  It turns a square pyramid into sort of an octagon.

[Paul McLaughlin]

Jerry, I think you are right that the longer center of panel would help deal with the stretch issues if you have bias running down the ridges, in that it compensates for the difference in stretch. Ideally, when set up taut it would look like you don’t have that extra length, as the ridges would stretch just the right mount so that the bottom edge from corner to corner becomes a straight line. But hitting that ideal would sure be tricky, and I haven’t experimented with that, just thought about it a lot.

And one other factor to consider here is stitching and thread stretch/strength and how that affects this. If a seam does not stretch like the fabric does, it’s hard to get the panels taut. And if a thread and seam combination has different enough stretch characteristics from the fabric there is the potential for the thread breaking as the seam stretches. how much of an issue that is in real life I don’t know.

What I do know is that back in 1984 I had one of the first Megamids – mine was under the Chouinard brand, prior to black diamond being spun off of Chouinard Equipment – and it was made with a very stretchy light ripstop, with ridge seams on the bias, and was nearly impossible to pitch taut. With the supplied pole you could not get the peak high enough to do it, and if you extended the pole by setting it a a pile of rocks or something you had to get the centers of the panels more than a foot off the ground before the ridges were taut, and no way could you eliminate wrinkles in the panels. that was a shelter and a fabric that would definitely have been better off with ridge seams following the grain of the fabric.

[Jerry Adams]

If you make a triangle panel too long at the middle no problem, it just turns into an octagon like shape.  Based on just pulling out the sides a little, it doesn’t change tautness or add wrinkles.  I have to make another mid this way some time.

Interesting idea about seam not stretching as easy as the rest of the fabric.  The only time I’ve had threads break is on the ridge seam, near the corner, on a mid laid out normally.  But it’s a not a catastrophic failure, there isn’t a lot of side ways force there.  I just went back at the end of the trip and added a zigzag seam so the seam stretches even though the thread doesn’t very much.

How the tent behaves as a whole, as effected by stretch along grain vs on bias is non intuitive.

And if the 4 corners and bottom of the pole are not all on the same plane that makes it more complicated.

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