Quick top quilt question - differential?

[jwright]

I am just finishing up the plans for a ~10 deg down top quilt.

I guess I am getting a bit tired, because all of the sudden I am wondering if I need to include a differential in the chamber size for top quilts. (example: 5" spacing between the baffles on the inner shell and 5.5" spacing on the outer.) I know on my UQ, this really allowed the chambers to loft out, but I can't for the life of me reason my way through this for a TQ

I am planning on a 3" loft height using a 2.5 inch tall baffle, and would calculate fill as 3" X length X width.

My crude representation below - no differential on left, differential on right. I was planning for no differential, but all of the sudden I am locked in an indecision loop.

differential2.jpg

Thanks,
Jason

[chickenwing]

If you don't use a differential cut...
As the top quilt curves around your body, the inner shell pulls the outer shell closer to your body causing a "flattening" at the bend.

I made this mistake with my first underquilt and (after talking with those in the know) understand this to be the same with top quilts.
hope this helps muddy the water
(c o) (O)

[gmcttr]

An UQ usually has the chambers parallel to the length of the quilt requiring the differential baffle spacing.

A TQ usually has the chambers perpendicular to the length of the quilt and the differential baffle spacing is not needed. A differential cut between the inner and outer shells will still be required to allow lofting.

[jwright]

Arrrrrgh.

gmcttr,
Just to clarify, (1) you are saying keep the chamber width (between the baffles) the same for both the inner and outer shells, e.g. 5.5 inches.
And (2) there should be a side to side (width) differential - 48" wide inner shell width and 50" outer shell width. If this is correct, how much differential is required?

This makes sense, especially after seeing how well a dual differential worked in my UQ, but I don't recall anyone mentioning a differential in the TQ threads I have been referencing. I did some studying of my current TQ before drifting off to sleep and noticed that after the sewn foot box, the quilt flattens out fairly quickly, without much curvature.

Thanks

A TQ usually has the chambers perpendicular to the length of the quilt and the differential baffle spacing is not needed. A differential cut between the inner and outer shells will still be required to allow lofting.

[doogie]

I did some studying of my current TQ before drifting off to sleep and noticed that after the sewn foot box, the quilt flattens out fairly quickly, without much curvature.

Even when ground dwelling (lots of curvature) there is no problem with the loft in a non-differential cut TQ....at least that has been my experience. YMMV

[Cranky Bear]

Yes the TQ should be made with a differential cut to allow for maximum lofting when used. If it is not differential cut then when used it will cause a flattening (think compressing) of the quilt, thus decreasing the loft. Which in turn lowers the overall warmth rating.

Easy way to think about it is the baffle is the no-seeum that is stitched to the top and bottom pieces of fabric (so 1" or 2" or 3" +) then the distance between each baffle on the bottom (side thats going to be closest to your body, or touching you when worn) let's say is 4". The outer fabric spacing between each baffle should be more than the bottom, so lets say 5". This is a differential cut, this allows for a bowing of the outer fabric to let the insulation (down) loft without being compressed.

Hope this helps??

[jwright]

Thanks for the reply everybody.

It looks like a true HF consensus:
jason - confused
chickenwing - + width differential
gmcttr - no baffle spacing differential, + width differential (I think)
doogie - no differential
cranky bear - + baffle spacing differential

Looks like it is back to the old UQ posts to dig up the formula for calculating the required dfferential.
jason

[doogie]

The reason it's so important on an UQ is that the top of the UQ is being held up against the bottom of the hammock/body with a fairly strong force (shock cord), the weight of the down and bottom fabric is the only force pulling down (very little force, so the suspension wins the battle), so if the fabric of the outer layer does not have a larger radius, it will not allow the down to loft out and will greatly reduce the warmth. With a TQ in a hammock the quilt is almost flat (except for the foot box), but even when ground dwelling, the only force trying to deloft the down is gravity...any you can't get away from that. You might have some extra folds inside your quilt when ground dwelling, but I have not noticed any reduction in the loft. Hope this makes sense, but if you want to go differential go for it.

[gmcttr]

The following photo of the head end corner of a TQ produced by one of our "cottage" vendors, has the inner shell stretched tight while the outer differential cut shell wraps around the edge allowing full loft.

IMG_3371 (Medium).JPG

Here's a sketch to further illustrate the principle.

TQ Cross Section.jpg

[Cranky Bear]

The following photo of the head end corner of a TQ produced by one of our "cottage" vendors, has the inner shell stretched tight while the outer differential cut shell wraps around the edge allowing full loft.

IMG_3371 (Medium).JPG

Here's a sketch to further illustrate the principle.

TQ Cross Section.jpg

Exactly.

All of mine are made this way!