Overstuff Variables

We’ve already established the definition of overstuff as density added to a base fill amount. Base fill amount is enough to just fill a volume. Density added to that is overstuff.

However, one thing we should keep in mind is that this density is not in a rigid box. If that were the case, then the container volume would be constant and we would always have a consistent density. The reality is that fabric chambers have a variable volume, such that finish density is a moving target.

How? Well, first, as you stuff density into a cuboid chamber, it starts to press against the sides, making them arc out. Eventually, with enough density, this shape becomes a cylinder. We know that, as you change a cuboid into a cylinder, the volume increases, which means density decreases if the fill amount is constant. So, really, you might calculate 160% overstuff, based on the cuboid shape, but if that were to fully puff out to a cylinder shape, you have increased the chamber volume by around 20% and lost 20% of fill density.

Realistically, you would never be pushing a chamber into a full cylinder. That would require a lot of density to do. Most of the time, we’re probably dealing with 10% or less of a drop, but that is variable. Consider that two of the sides of the cuboid are the baffles. These baffles are being pressed, equally, from both sides, and mostly remain vertical. Not pressed into an arc. Furthermore, the chamber width determines the length of the external sides that do puff out. Wider spacing will tend to allow those sides to puff out more. This results in a larger increase in volume / decrease in density, but it come with a huge benefit of allowing much higher loft.

Another factor varying volume and density is a differential cut. The intended purpose of a differential cut is to allow the fill to maintain its loft, even when the system is wrapped around a body, with pressure applied to the shells. It is supposed to send the load of that pressure into one shell, while allowing the other to remain loose for down loft. These calculations are made assuming the system is positioned in a circle, wrapping around a person. However, realistically, the system is often in varying positions and many of those positions allow the excessgabric of the diff cut to release some density. The obvious example of this would be you took a diff cut system and laid it flat. All that excess fabric, built in for loft while wrapped in a circle, is now loose and scrunched up when laid flat. This releases density off of the down, in this example.

So, what is my point? Well, you often hear people throwing out recommendations for ideal overstuff specifications, but with a lack of understanding of how it works. It doesn’t always mean the same thing. These are anecdotal recommendations. Someone might have had a system with an advertised 150% overstuff that worked well for them. Without education, they’re likely just to conclude that 150% is a blanket sweet spot. It doesn’t work like that. Like most specs, it is interactive with all the other specs. Our 160% density might be considered very high, but we use wide, high lofting chambers and a huge differential cut. Part of the reason it is high is to counteract those desirable aspects and maintain control. A system with no diff cut and narrow chambers might get away with 50%.

Culturally, we’re always on the lookout for a quick and easy spec to tell all. Most of the time we find it to be an inaccurate oversimplification and we really just need to be smarter.