I decided to bite the bullet. I spent the money and built myself an endless loop string jig, and have material on order.

Here is my question. If I have a string the length I want, can I just place that string on the jig to set my length?  Or will this result in a longer or shorter string?
Or is this something I will have to figure out thru trial and error, as everyone will be a little different based on how tight you pull the strands?

Yes you can just shorten it a little bit to account for stretching. How much depends on your material.

On an endless loop, you want the string to be a little shorter. It's inevitable that their will be some trial and error to learn the stretch rate of different materials and how much you lose. That's just part of the journey.

Depends on the material.  For instance, with B-55 I build mine 1/2" short--comes in right when I pre-stretch.  For BCY-X, I add 3/8" or so, to allow for a few twists (even endless needs some twist).

OUCH! That hurt!    

Yes, any string building, material or style, like building bows, trial and error.

The suggestions given are good to start with. A lot of help here!

Thanks for the replies. Guess I should have mentioned that I am  going to be using b-50.
How much weight/pressure do you apply to pre stretch?
Sorry frank didn't think you'd  feel it thru the web. Lol.

For me, 250# or so for 12 or more strands, then let it "rest".  You can actually see it contract.  AMO specs say it's to be measured under a 100# load.

You can't just place your string on the jig to set your length.  Your bundles will need to be about 7-8 inches longer on either end, i'e; about 14-16 inches longer than your string to account for twisting the loops.  

After you've twisted one loop, and counter twisted the string about 12-15 turns and stretched the string, can then set your existing string along side it(matching the two bottom loops, for example), and start your second/top loop about where the loop ends on your existing string.  

This is where the trial and error comes in.  Will probably need to adjust the number of counter turns as well as the second loop starting point a little to accommodate your own particular technique. Good luck.

Orion, I think he is building an endless string.

Chad's 1/2" shorter, is a good start for dacron.

Thanks again guys for the replies. And yes, I'm building endless.  Chad, your answers and FAQ page were very helpful.

I understand strand count will effect strength, noise, vibration, stretch, etc......How many strands of b-50 do you recommend for bows in the 45-50# range(mostly productions bows from late 60s and 70s)?

I don't shoot competitive, just pleasure and hunting.

For 45-50#, I like 12 strands.  It's a good compromise, and not usually hard to get a good nock fit.  If stretch/creep becomes a problem, move up to 14 strands.  For 30# and under, I use 10.

Chad, on a 12 strand 8190 string what size serving would you recommend to fit Bohning Classic nocks?

I haven't made a 12 strand with 8190 (I normally use 18-20 with that).  .032 Power Grip might be large enough, but most likely you will need to double-serve.  Maybe .018 or .021 over .014.  I have a lot of Power Grip serving for sale right now if you need some.

Chad

Woohoo!! Box of goodies just arrived. Time to give it a whirl.
One last question..... How long do you leave it under tension for prestretching??

I put the string on the bow and lay the front of the riser across my lap and push down on both limbs forcefully. You will get the bulk of the stretch out. Your string will still elongate in the summertime when the wax gets warm and oozes out. The displacement allows for elongation.

Sorry Chad-I should have read the previous post closer, just realized it was B-50  
Thought that was going to be mighty thin.

No problem.  I think I use .024 Halo or .025 62XS with 12 strands of B-55.  Haven't made one lately--I keep various nocks on hand to double-check to be sure.

I usually only pre-stretch for a few minutes--longer won't hurt, but it won't help a lot either.  With enough load, it will get the slack and excess wax out pretty quick.

Pretty much all materials will creep a little under heat--how much depends on lots of variables.  Polyester (B-55)is worst, 100% HMPE (Dyneema)is next, HMPE/Vectran blends (452X, BCY-X) have the least.

Well, I got my first couple strings under my belt(actually second isn't complete).
I misinformed you before. I'm actually using B-55(forgot what I ordered).
First string came out a little long, still had to add more twists than I like to get to BH.  It was better than the string I had, but warranted another try. Shortened the jig up about 3/8" and second came out nice.
Bad part is I need to get some smaller braided material for center serving. Had gotten .025 braided, but it is too big. Even the .021 twisted I used for end serving is a little large. Guess I'll have to find some .018 or .019.
Thanks again to all who replied and helped out!!

For end loop serving, I like 3D or .019 (or smaller) Halo.  What type nocks are you using on your arrows?  I probably have something that will work, and at a bargain.

Chad

Oops.  I need to read more carefully.  There it is in your first sentence, "endless loop" string.  I apparently blew right by it.  Of course, my advice applied to a Flemish twist string.  Looks like everything's been straightened out already.  Sorry to create the confusion.