Ivo, Great idea to avoid the hassle of pinning. Would you share more information about your diluted silicone mixture (like mixture ratios , etc.) ?

Edited by tr-browsing on 02/27/2013 07:17:37 MST.

I said:
That knock off diagram seam has 5 layers leading to unequal seam allowances .

'Faux' flat felled has 4. Equal seam allowances.

I know which I'll use when measuring panels.

KISS anyone?

Kevin said:
Actually, the reason there are five layers is because the seam allowances are the same for the first seam; the two edges of the fabric are aligned, then the seam is sewn with a large seam allowance, then the seam is flattened out, and the seam allowances are folded in half and tucked under itself. Then you sew two lines of stitching through the five layers.

It looks like the seam allowances are different because the diagram is a much-expanded view; in reality the 'vertical' distances are minimal.

You can use double and single seam allowances to make a double-lapped seam, in which case the single allowance is on the bottom of the stack as shown, and the double allowance is folded under it.

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I think you don't understand what I mean.

If 2 equal sized panels are cut and sewn together (e.g. flat tarp ridgeline):

With the 'faux' flat felled (traditional)seam the centre of the seam (where you might sew a 3rd line of stitches) is the centre of the full tarp. (Because the pieces are first sewn with one edge one seam allowance away from the other)

With the Knock off, the centre of the tarp will be now at one edge of the seam not in its' centre - so an extra seam allowance width of fabric will be necessary measured on one piece to produce equal sized panels. (If position either side of centre of the seam as is usual)

I just made the knock off seam fold several times with 2 equal sized pieces of paper and found this to be the case.

If I'm mistaken here, please show me how! (Scratches head!)

Edited by MoleJ on 02/27/2013 12:53:07 MST.

But if you're making a tarp does it really matter that the left side is 1/2" narrower than the right side?

Edited by cwford on 02/27/2013 16:23:36 MST.

Wade Ford says:
"But if you're making a tarp does it really matter that the left side is 1/2" narrower than the right side?"

Probably not, but it will matter a lot in something with panels that need to fit together properly, as in a tent or pack.

The usual way to make a mock felled seam is not with the edges of the fabric alligned, but with uneven allowances like this[1]:



That's half inch on the bottom, quarter on the top. Then you fold the longer allowance around the shorter one, and stitch. That gives two visible lines of stiching on the side with the added bulk, one on the other.



And third pass or not as fancy strikes you.

You will see, particularly in home sewing patterns, instructions telling you to sew the allowances even, and then trim one of them. That's a pain to do, but it's another option. I don't trust my self to cut the right allowance, either, nor to not cut too much or too little.

[1] that first picture also shows why this is not considered a lapped seam. the two pieces don't over lap, they're superimposed, like a normal plain seam, but with the offset.

Edited by dscheidt on 02/27/2013 17:45:24 MST.

Samuel,
you are right, using undiluted silicone takes away much of the elasticity but with a catenary cut there is no problem.
Undiluted silicone makes the shelter heavier .
My first trailstar copy was about 100 g heavier than needed . There I used undiluted silicone

Now, the only intention was a temporary and no permanent bonding. It makes the silnylon less thin and easier to handle.
If strength is important then you should use undiluted silicone I think.
See: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=51361
I used pure silicone without additives. Dilution is not so important.
Slightly thicker than olive oil. See movie.
Too thin, then it does not stick.
Thick, heavier than necessary and you have to work fast and the solution has a short shelf life

" the only intention was a temporary and no permanent bonding

Didn't get that till now. Thanks.

> I think you don't understand what I mean.

You're right; I didn't. And, as you say, the resulting seam will be off-centre (the initial seam is roughly the centre of the panels). So, if you want the centre of the folded seam to be the panel centre, you will need to adjust the both panel designs to account for this, or, at least, move the initial seam point by half the folded seam width.

Making a four-layer, equal seam allowance, faux lap seam has other difficulties if you want to run two seams through all four layers on both seams. Not insurmountable, but about the same degree of non-KISS as the five-layer seam...