On my first try to produce a composite part a came across this unexpected issue .I 3d printed a fork core for a bike frame i wrapped the core to CF and then vacuum bag it.But when i measured the dried part the fork legs were closer(25mm-1') they previously were !!!
here my photos 

why this happened ?How can i over come it ?Should i alter my design in order to compensate for the shrinkage ?Should i build a support to hold the fork legs in the vacuum bag ?Thanks in advance!

If you close the print, the're is no force in the core itself when you pull vacuum. The pressure outside the bag is 1 atmosphere, as is the pressure in the core. The pressure between core and bag is zero, and this is why the bag is forced against the core, and the core against the bag with equal strength. An open structure however has to be able to take the forces of the vacuum without collapsing.

But this is not you problem. Your part did shrink to this, but this should be immeasurable small for you. Your problem is that you placed the bag flat over the part. The way across your part, following it's contours is longer than the way across the part without following the contours at all. And as the bag was evacuated, and came to a point where it had to move inward to fill in the space, it was already to tight, to slide over your part and compensate the additional length. So the bag had to stretch to get into the corners. And by doing so, it created an equal force, which pulled the arms of your wishbone inwards.
And this is why you should never put the bag on without wrinkles. Theoretically not even on a flat sheet.