Carabiner keepers on quickdraws? What do you use?

Good going Fortuna. From the book On Rope the strongest pattern found by the authors was a zig zag that goes down the webbing as you have done with the straight passes.

With doubled up thread and just straight passes you're probably correct about double the thread strength. If using lock stitches,from my unscientific testing, you lose about a quarter of the doubled strength.

A few people in the arborist community are trying their hand at sewing friction hitch cords as well as climbing ropes with some success. I have done several that I sent in to be tested. The sewn splices held together but the cords broke at about half their tensile strength where the stitching started, somewhat how knots break.

As someone posted earlier, test them with a vehicle and a couple of biners you don't mind breaking. The biners should break before the webbing and stitches.

The forum user "20 kn" says that he will test my gear, but it maybe 1-2 months before he can get around to it.
In the meantime I have some old biners that I don't mind breaking and if I can demonstrate that the biners break before the slings and cords do that will be useful. I have a tree and a car :)
Everything I've seen has sewn slings and cords breaking where the stitching starts with a 10-20% decrease in strength of the webbing or cord alone.
This applies to bar tack as well.

Fortuna, are you talking about machine or hand sewn in the above post?

Both. My cords are hand sewn, and my webbing is machine sewn.

Thanks I was reading without my glasses and missed the hand sewn part. The place I sent mine to charges 15 dollars per test and they send you a graph readout of your test. Took about two weeks also for 5dollars more they will send your stuff back to you.

If you are really interested I am a mechanical engineering student with access to an instron that can pull 30kn and I can get you the failure load and graphs. Fyi thought I'm not uiaa certifying your stuff