Humans are curvy things, and it only seems natural that the chairs that they rest in should reflect this. I've played a lot with curves in the chairs that I've made over the past few years, and they (the curves) are starting to migrate away from the expected places, where the flesh meets the wood, and into the undercarriage, where strength and aesthetics take precedent.
It began with my desire to have an uncluttered looking undercarraige. I've never been happy with the H stretcher on rockers (actually I've never made one!). The side stretcher seem to compete visually with the rockers and structurally, they are redundant, the rockers act as stretchers. Add to this that the center stretcher seems too far away to support the highly stressed back legs, and I'm firmly in the box stretcher camp. The only advantage that I see in the H stretcher is the ability of the sitter to put their legs comfortably under the chair, which is a nice feature.
So, as I've been focusing on my rockers lately, it dawned on me that the curved stretcher might satisfy all of my requirements, it's out of the way of the sitter, doesn't interfere with the rockers visually and provides plenty of strength right where it's needed.
Below is the basic layout that I used to make the stretchers on my latest rocking chair. I began with the basic idea that the tenons (the only straight part of the stretchers) would point directly at the opposite leg on the rear of the chair. So I began by running string from the front legs to the opposite rear leg at the height of the first large "node" in the bamboo turnings. Then by running a string across the two front legs, I was able to distill the triangle that would be the basis for the bend.
Here is the drawing that I made to make my bending form. You can see the triangle in place. In this instance, I simple drew a freehand curve that I found pleasing for the center axis of the bend. You can see that the tenons run right in line with the initial triange, the rest can diverge as desired.
Next, I drew the shape of the turning around the center axis, establishing the locations of the details that looked balanced to my eye.
Then, as you see below, I cut out the perimeter of the turning so that I'd be left with the pattern for the bending form.
Finally, I used a flexible rule to figure the length that the turning would have to be to match the desired bend. (yes this one is for a different curve, the rear stretcher, but you get the idea)
And here's the resulting bend. It worked out nicely, but took a few extra clamps to tame it!
I'll show how to go about drilling the mortises in part 2, but you'll have to forgive any delay, my goats need a winter shelter, so for now I'm a carpenter, which is not my strong suit, but I'm sure they'll let it slide.