Why does a bow take set?

[4giveme]

hi guys,
why does a bow take set? i know why the bowyers bible says it does, but things are always a little different in the real world. so i am looking for advice on how to avoid set in my next bow. or i should say more then i want, i usually build in 2 or 3inches of deflex on my short bows. so what causes it and how to avoid it is what i am asking.
4giveme

[Pappy]

Sometimes moisture,sometimes over stressing the wood as you tiller,sometimes to short for a specific draw and sometimes just the wood selection. Being sure the wood is dry/never pulling the bow more than an inch at a time as you adjust the tiller/never pull over intending draw weight/make sure the bow is long enough for the draw length[especially self bows] I use twice the draw plus the non bending area as a general rule. Make sure the bow is nice and floor tillered and ready before first brace and get it on a short string as soon as you can, all can come into play. I also give mine a break as I am tillering,I will get them floor tillered and ready to brace and give it a break,then out to 15/16 inches and give a break then on out to intended draw- 1 inch.Then rest then shoot it in slowly going on out to full draw. Seems to work for me.I am sure others will chim in.
  Pappy
   

[MWirwicki]

Basically, the stress put on an all wood bow (even with sinew) causes the wood to take on set.  Because the wood always gives in a bit, starting out with reflex helps in ending up with a straight standing bow. 

How much set depends can depend on a few factors.  Moisture content is one of them.  If your bow wood is high in moisture, or hasn't been seasoned enough it will likely take on considerably more set than if adequately dry.

Also, minimizing the stress during the build process helps.  Never exceed your intended draw weight while tillering.  While on the bow scale during the tillering process, if you're bow reads your intended weight at 18" of draw stop there, identify any stiff spots, remove wood there.  If it's bending in a nice arc, remove wood evenly on the limbs then put back on the wall (scale).  Keep strategically removing wood on the belly until you are able to pull the bow to your draw length at the intended draw weight.

[MWirwicki]

I see Pappy got his comments in as I was typing.  Good to see we think alike.   

[Del the cat]

Overstressed...
Next!
Del

[George Tsoukalas]

I think you meant you put in reflex. With enough reflex you may no get string follow but you'll still get set. Some set is unavoidable. 2 in is acceptable. Set happens.  Jawge

[bushboy]

If you over stress your inner limbs it magnifies the set @ the tips! Check out tim bakers mantra. Am @ work so I don't know the exact wording.

[DuBois]

So I am tillering a short 49" buckthorn with sinew and I get it to what I think is close to a good tiller for a low brace. I brace it and go from 2.75" reflex to about 1 and 7/8". The mid to outer limb areas of both limbs were bending too much so I unbraced it quickly.
Did I screw it up permanently? I got it tillered to draw at 35#@24". Man I am always coming in low.
I ask cause I am considering a fourth thick layer of sinew.

[Gordon]

For me rule #1 when tillering is don't put more stress on the bow than the limbs can handle without taking a set. When I see that the bow is beginning to take a set, I back off and lighten the poundage.

[mikekeswick]

Every wood has a set of properties. Get to know them before trying to make a bow from it.
The ratio of width to thickness is the key in my opinion.