The simple reason behind using one or the other is that as wood gets thinner it can bend further without taking set.
A flatbow with parallel width limbs for most of it's length has to have some taper somewhere. So the taper is in the thickness.
So the thick bit near the handle can't bend much but as you move along the limb towards the tips the limb is getting thinnerall the time....therefore it can (and should) be bending further as it thins. eg. elliptical tiller. The bit you can adjust with this design is how much of the limbs are the same width. If there is only say 4 inches with the same width out of the fades then that bow should have that first bit of the limb moving less than the straight taper out to the tips (this bit should be circular - no thickness taper). As you extend that parallel section out further then the tiller should become progressively more elliptical.
A pyramid tapered bow however has the majority of it's taper happening with the width. This means that there is very little thickness taper (usually!). So if the limb is staying a constant thickness for it's length then of course each bit of the limb should be bending the same amount eg. circular tiller
Messing about with and understanding this principle is the key to making good bows in my eyes.
Adb - have you noticed that you get less set,handshock and better performance with an elliptical tiller on your longbows. I certainly have. Only once the weight is up over about 80# do I start deviating from this tiller and start to get the handle working more. If you think about it elb's have a large amount of thickness taper going on.
Well said.
mike... no, I don't notice an appreciable difference with a more elliptical tiller, as far as set goes. If I do my job well, any bow I tiller will have minimal set, regardless of design. I do notice better performance with stiffer tips, however. Performance to me being a combination of arrow speed and shootability (ie minimal handshock, etc.).
I think ELBs have basically nothing but thickness taper. That being said, I tiller my warbows with a more elliptical design than I used to. They perform better, but I have to be careful they don't take more set, being that the grip and tips are a bit stiff. If I can pull this off with a heavy draw weight warbow, it has amazing cast.
Something I learned long ago: even tapers (either width or thickness) make even bends. If you can keep your tapers smooth and regular and even, the wood will bend evenly for the most part, and final tiller will only require small bits of tweaking.
Just remember... for best results (low set, good performance) the tiller shape should match the design.
