Air dried wood is better then kiln dried wood but every batch of wood acts different and I found that it is a matter of trial and error .
Straight grain along the edge and face ,moisture content ,temperature ,strap tension etc all play into this .
On this bend I go slowly and back off a couple of times rather than force the wood around the form and also release some pressure on the strap as I get to the end when I clamp on the plywood block to form the reverse bend .
I have been told to put fabric softener in the water to soften the wood but have not tried that .
Ash is a good wood for bending but walnut in this case works just fine .
I get the white aspen from a small mill that makes high quality products from local woods .A great place and I like to visit there and get the good and interesting pieces that they set aside for me .
Here is a link http://www.forestnet.com/archives/Feb_05/small_sawmilling.htm