I'm not disparaging anybody, because you are all both correct and helpful. Some of you guys really know your stuff. (And, I love you all, and I love this site). However, I am POSITIVE our friend DC a has been around long enough to already know a lot of what was said. He's just a guy who likes to learn through questions.
"I never consider thickness, but tiller to the weight I'm aiming for." Agreed, but you gotta start somewhere. I lose more bows "roughing out" than I do anywhere else.
"You do have to consider the stability of the very narrow tips(lever)." Indeed, but that's implicit to what he's asking. "If I narrow by this much HERE, how much thickness do I have to add so it bends the same (bad idea)? And how much do I need to add to make it bend less? How much to make it NOT bend? How much narrowing is too much?, etc.
"This only works if the bending stresses do not exceed the proportional limit of your materials however." Indeed. The main, constant, and universal consideration when making bows of wood. Bowyery 101.
"Just wondering and wandering, neither limb length nor limb width enter into the equation." Correct, but that's ok because the rule applies to any length or width. As a rule of thumb, with some experience, it can be applied to the proportions of any given bow.
"Knowing this will not help you make a better bow with natural materials because the engineering properties of organic materials are highly variable." I agree that making wooden bows by formula doesn't work well, but I think it DOES help me make better bows (along with a slew of other design principles).
It has helped me a lot. It helps make sure you have too much, rather than too little, material to work with. It also makes no sense to do work with scrapers and sandpaper that could be done with a plane or rasp, nor to leave dead weight where it isn't needed. It helps ME to realize what I can get away with, and when I really ought to stop.
Marc says: "Two divided by eight." I agree with this. The cubed root calculation is technically CLOSER, more accurate, but I generally use 25%, because, like Pat said, you still have to tiler the bow. It gives me plenty to work with later, but not so much that I get bored to death scraping.
This rule of thumb applies best to starting points, and helping one keep an eye on one's work as tiller progresses.
For instance, say I'm making a 66" elm Mollie. If I add 25% of the bending limb thickness to the outer limb, it will be stiffer, even though half as wide. If I fudge that up to 30%, I'm really in the clear, esp. since I know I will be messing some with the cross section of the lever. So, now I know I can fade-IN from thin to thick over the 2", have totally stiff levers during tillering, and arrive at tiller by working the thickness of the bending limb by TILLER, not measurement. Thinning there only increases the ratio, so when I'm done there, I still have enough material in the levers to tweak them down narrow, work the straight, and maybe reverse trap them a bit as I finish off, all while keeping the
"I may have started this thread but I sure wouldn't use this information for other than the wildest ballpark estimates." I think that's how that rule of thumb is best applied.
