# Thread: Rule of thumb for the differential cut??

1. I didn't get too scientific about it as I am sure movement in the hammock, pull of the cords, etc. would throw off my calculations anyway.

So, I added twice the loft of my insulation to the outer shell with darts at the corners and along the side. The thinking is that the outer shell will start away from the inner shell at the edges and the distance will remain pretty constant across the UQ. In my case this was an additional 7" (3.5" loft) like Dutch said.

2. I drew a cross section of the quilt on a piece of contractor's paper using an 18" radius (for my body), then measured the sizes. May be overkill though the 1" per baffle would probably be fine.

3. This is what I was talking about.

Since I assumed a half circle as the approximation, it led the equation I posted earlier. This doesn't make any adjustment for the other axises. I'm only trying to calculate the width of the fabric, ie the end view.

To correct the shape in an overhead view, you need the darts. To correct for the side view, the length differences in the fabric will be a smaller multiple since it isn't 180 degrees of the arc, ie less than pi. Maybe 2*baffle size?

My recommendation? Either you hang in the hammock and let someone measure for you, or you measure with someone else in the hammock.

I just liked your original question and thought I could come up with an equation to calculate the width difference.

Make sure you post pics of your quilt when you finish.

4. Please pardon my dependency on mathematics, but it's just easier for me.
You see, I can't help it because I'm an engineer ...

Where:
BH = baffle height
W1 = width of layer next to hammock (no seam allow)
W2 = width of layer away from hammock (no seam allow)

W2 = (W1*360/170/pi+2*BH)*pi*170/360+2*BH

Finally, decide how many chambers you want and the baffle spacing will be:

BS = baffle spacing
CH = number of chambers
BH = baffle height
W1 or W2 = width of layer (from above)

BS = (W1-(2*BH))/CH

Remember that the outside chamber widths need to equal BS + BH to allow those baffles to come to full height.

Please pardon my dependency on mathematics, but it's just easier for me.
You see, I can't help it because I'm an engineer ...

Where:
BH = baffle height
W1 = width of layer next to hammock (no seam allow)
W2 = width of layer away from hammock (no seam allow)

W2 = (W1*360/170/pi+2*BH)*pi*170/360+2*BH

I used the circumference equation C = pi * 2R
I assumed a half circle, so my equations are (see diagram):
W1 = 1/2*pi*2*R
W2 = 1/2*pi*2*(R+B)

The halves and 2's cancel leading to:
R = W1/pi
Substituting gives us
W2 = pi*((W1/pi)+B) now distribute pi and get
W2 = W1 + B*pi

I'm not sure how you got your equation. Sure looks more complex than mine

Is my math wrong? Can you explain yours? I'm not saying yours is wrong, I just don't understand it. Where is 170 coming from? Are you assuming less than the semi-circle I'm using?

6. I took pictures of my under quilt on my hammock while someone was inside and it didn't quite come around to a half circle (180*). So I eyeballed it at 170*.
Therefore, it makes up 170/360 fraction of a circle.

Basically, I'm using the same method as you are.
I also assumed that the layer next to the hammock is tight against it and therefore does not contribute to the thickness at the outside edges. So all of the height at those edges must come from the layer away from the hammock. For example, if the baffles are 2" high, then the outside chambers need to be 2" wider than the inside chambers (only in the layer away from the hammock) so that the quilt edge is 2" thick.

I took pictures of my under quilt on my hammock while someone was inside and it didn't quite come around to a half circle (180*). So I eyeballed it at 170*.
Therefore, it makes up 170/360 fraction of a circle.

Basically, I'm using the same method as you are.
I also assumed that the layer next to the hammock is tight against it and therefore does not contribute to the thickness at the outside edges. So all of the height at those edges must come from the layer away from the hammock. For example, if the baffles are 2" high, then the outside chambers need to be 2" wider than the inside chambers (only in the layer away from the hammock) so that the quilt edge is 2" thick.
Thanks MAD777, that's what I was thinking you were doing. I didn't add anything for seam allowances or that gab (B) at the top. I thought it would be obvious to add those amount. My goal was an equation you could plug in values to see the difference between the two pieces of fabric.

Most likely the difference between our methods won't be much, and probably wouldn't matter too much in the finished product. I'll use mine cause it's easier. :P

8. Originally Posted by hikelite
Most likely the difference between our methods won't be much, and probably wouldn't matter too much in the finished product. I'll use mine cause it's easier. :P
I absolutely agree! These underquilts are curved in all sorts of directions when on the hammock, trying to conform to our various body shapes, even as we switch from back to side to fetal position.

When I started out, I didn't have a clue how much differential to account for, so I gravitated to my calculator because that's what I understand and am comfortable at.

I haven't seen a down underquilt yet that didn't get the job done in a scrumptiously comfortable manner

I haven't seen a down underquilt yet that didn't get the job done in a scrumptiously comfortable manner
I haven't even seen one in person yet :P

Probably won't even make one until I get back from the PCT next fall.

10. Originally Posted by Bomber
Do you mean spacing between baffles here
Yes. I should have made that clearer.