Double Straight Stitch vs. Three Step Zig Zag Stitch

[Lawson Kline]

I am looking for opinions on what you think is a stronger stitch for a lapped seam….

Double Straight Stitch vs. Three Step Zig Zag Stitch

[James D Buch]

The Three Step Zig Zag is often used to give some ability to stretch along the seam axis. This may not be exactly the response that you were seeking.

My sewing machine user guide lists the three step zig-zag as used for "overcasting in elastic, thin or easily frayed fabrics" and"Sewing in terry cloth". As well as "Mending and darning in all fabrics"

The classic two stitching rows of the lap fell seam don't promote easy stretching, and for many tent/tarp seams, that may be more desirable.

As far as strength, usually the best strength is obtained when the fibers run straight in the same direction as the load. For ripping a felled seam apart sideways, there could be some benefit to the three step, but I have never seen it used that way.

[Lawson Kline]

Over the weekend I went to a local sail loft to see if I could get some advice from the sailmaker about construction techniques. He wasn't a camper but always wondered why gear makers used "sailmaker materials but not their methods of construction" His main complaint was the straight stitch…He said sewing a straight stitch creates a "tear here line and the only reason its used is because gear makers don't know what they are doing" He was really really opinionated so I figured hmmm I wonder what gear makers think…

I heard this same comment 5 or so years ago from a sailcloth salesman. After hearing this comment I used the three step zig zag stitch on several tarps and never had a problem…I then upgraded to a industrial straight stitch walking foot machine and sew all my stuff now with a straight stitch. I have since had a few seams give out but I always thought it was just the material… Then this guys comments yesterday made me think, hmmmm maybe hes right…

[Ken Thompson]

Don't you wind up with two rows of parallel stitches either way? I can understand the tear here comment if you were using a short stitch exclusively. Manufacturers have been using the double row stitch technique for a long time now.

[Lawson Kline]

Single Row Three Step Zig Zag Stitch

Double Row Three Step Zig Zag Stitch

[Ken Thompson]

So with a double row of zig zag stitches you wind up with four rows of holes. More to seal. Not an issue with sails. Still don't see an advantage for backpacking as you will increase weight with more material being uses in the seams. Also not a big issue with sails. I use to live in Stuart back in the day!

[Lawson Kline]

>>"So with a double row of zig zag stitches you wind up with four rows of holes."

True but do you think its stronger?

>>"More to seal."

True but a seam is a seam and I always seal the entire seam, even places without holes…

>>"Still don't see an advantage for backpacking as you will increase weight with more material being uses in the seams."

True but were only talking a few grams… Is a few grams worth the "added strenght"

I have no clue whats stronger but I will say that sail makers do know what they are doing. Keep in mind they designed the materials we all love and use… Every sail is designed on CAD to optimize strenght and cut out via CNC machine. The workers that assemble the sails make really good money and eat and breath their job. They are true professionals that know ALOT… If they built cars they would be Formula One Cars…If we built cars they would be Nascars hahaha :)

[Roger Caffin]

Depends on the material. If the material has low stretch then a straight stitch is fine. If the material is seriously stretchy then you really should use the 3-step zigzag, or risk damage.

Remember that the 3-step zigzag uses more thread and has more holes per metre. In some cases that helps; in other cases it doesn't.

For a lapped seam on most ordinary woven fabrics I would use straight stitching. I use that on my 4-season silnylon tunnel tents, and they take a hammering (as you know).

Cheers

[Ken Thompson]

Is a few grams worth the "added strength"
Depends who you ask around here :)

[Lawson Kline]

Good Point!

[James D Buch]

Once upon a time, I read that for outdoor gear, about 10 to 12 stitches per inch was the "optimal" stitch for strength.

I have no longer any idea of where I read this. It was in the 1970's, and I was briefly assigned to a project on NASA Flexible Reusable Shuttle Insulation because I knew this — the insulation was sewn bats of a fiberglass type of material.

I did a google search for "stitches per inch stitch strength" and discovered the link below which describes an equation between stitches per inch,thread strength and seam strength. There are a number of examples of common practices of sewn goods included.

http://www.wireworld.com/amefird/selectingspi.pdf
http://www.wireworld.com/amefird/spi.html

There is also a finding in "Google Books" which describes 8-11 stitches per length for parachutes.

http://books.google.com/books?id=BKTuTXrXQu0C&pg=PA474&lpg=PA474&dq=stitches+per++inch+stitch+strength&source=bl&ots=n_thCp8N1r&sig=77AFW2ZovsRJvvk4KaN4f5SklHU&hl=en&ei=0BS-S5-HE5TWML_PjNIK&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBAQ6AEwAg

The book is by Dan Poynter called "The Parachute Manual:…" from 1984.

The following link is similar to the first.

http://www.syntheticthread.com/seam_engineering.htm

For parachute fabrics, see:

http://www.google.com/url?sa=t&source=web&ct=res&cd=23&ved=0CAwQFjACOBQ&url=http%3A%2F%2Fwww.pilotoutlook.com%2Fparachute_rigger%2Fjoint_efficiency&rct=j&q=stitches+per++inch+stitch+strength&ei=WBm-S5CzBI70NcSi4LcK&usg=AFQjCNF7hGFWP6aBNmUuOsEmT7f0MoILSg

"Stitches per inch—This has a direct correlation to the size of the thread used and the stitch type. There is a fine balance between the security of the seam and overstitching. Too many stitches per inch will dramatically affect the strength of the seam by perforating the material. The number of rows of stitching also affects this. While more rows generally increase the strength of the seam, too many perforate the material as well."

[James D Buch]

From:
http://www.whitemountain.com.au/backpack_construction/material_faqs.html

Quote:

What Stitching is used in backpack manufacturing?

You'll find most backpacks are sewn at 6 to 10 stitches per inch. The general consensus here is that, if the stitching is increased much beyond 10 stitches per inch the strength of the fabric begins to degrade. Anything below 6 stitches per inch begins to become suspect in terms of strength. Be aware that backpacks are made of deniers much higher than you'll find in a tent or sleeping bag, and close stitching can actually damage the threads used in the backpack fabric itself. Another factor is the twin stitching method, where all stitching is doubled stitched for added strength, including the zippers attachment to the backpack. Constant zipper use on less expensive backpacks with single stitching can cause the zippers to be pulled from the backpack, an all too common problem.

All White Mountain™ backpacks utilise high quality YBS zippers and all backpacks are twin stitched at 8 stitches per inch. YBS zippers are manufactured in Korea and are reputably equal to YKK zippers manufactured in Japan.

[David Olsen]

You will notice a lot of variance in stitch patterns on
climbing and parachute web gear too. Also stitch length,
size and material of thread. If the thread has too little give
for the material you can have early
failure as the force isn't spread out enough. For example,
a stitch length which works well with nylon thread will not
be as ideal for the same strength thread made of lower stretch
polyester.

If your sail is made using a polyester thread (more UV resistant, and lower stretch) a zigzag stitch may make more
sense than if the thread were nylon.

In the end, you have to test things and see if they hold up
as they should. Make a few small samples and try them out in
relation to other parts of the system.

[Vick Hines]

1. On long seams, such as a tent or tarp: if the thread has lower stretch than the fabric, i.e. polyester thread on nylon, a zigzag stitch will keep the thread from snapping when the stronger nylon fabric stretches. And it will prevent the thread from sawing against the fabric until one or the other fails. The zigsag does not have to be deep, just enough to give some slack when the long seams stretch.
2. Sailmakers use a zigzag – which is also called the "sailmakers' stitch" partly because sails are subject to a lot of wind shock causing the seams of bias cut fabric to stretch, and partly because it is easy to rip out zigzag stitches when a seam needs adjustment during manufacture as it often does.
3. If the thread has the same stretch as the fabric or more streth than the fabric, a straight stitch is stronger.
4. Consider whether the seams are square or on the bias. Even polyester sailcloth strethes when pulled on the bias. For bias seams, the stretch of nylon thread makes the seam stronger.
5. Tests reported on this site have shown that parallel lines of straight stitches are stronger for reinforcements in ultralight fabric than are zigzags or bar stitches.
6. Gear manufacturers (including sail makers) usually just learn from experience like everyone else and apply rules of thumb. In general outdoor gear, the tendency has been to use long stitches with strong, stretchy thread. It works and long stitches are easier to remove when (not 'if') a mistake is made. General purpose polyester thread is strong enough for almost all DIY projects except perhaps sails. Lightweight nylon upholstery thread is good for DIY sailmaking and when sewing heavier fabrics such as 10 oz or heavier Cordura.
7. Things get tricky with climbing gear because of shock loading versus static loading. Improper over-stitching can weaken webbing, for example. You just have to know. And test. Carefully.

[Lawson Kline]

I can break 300lb test mono fishing line with my bare hands. Most of you guys are thinking im full of it but its true…You ask how…I wrap the line around my hands in a certain fashion and with a couple quick tugs the line actually cuts itself. Why does this matter? When the right size thread is used it cannot cut the fabric… Most people use to short of a stitch and to big of a thread which actually cuts the fabric in tension and makes the seam weaker…

[Roger Caffin]

Yeah, thread selection is important.
On the non-critical parts of my tents I use Rasant 120 thread (very fine) at about 12 stitches/inch.
For the critical seams I use a backing or reinforcing layer of silnylon on each side and Rasant 75 thread, at about 10/".
For packs I use a heavy bonded nylon thread and a #100 needle at about 8/" – in an old black singer which can sew anything!

Sewing machine companies usually recommend quite coarse needles – to avoid getting lots of customer complaints. I find a #60 needle (the finest available) to be suitable with both of the above Rasant threads. You just have to be a little more careful with the #60. Packs – a different matter!

Cheers

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