Fire By Friction: The Joy of Short Fire Bows

I tried not to over think it, but being the mathematically inclined engineering professor that I am in real life, you know I had to work out a formula to estimate the “optimum” length for my short fire bows. I started by calculating the circumference of a spindle, then multiplying by six. To that I added a half inch on either end to allow me to recess the holes I run my bow cord through. I added another half inch on either end to give me room enough not to “bottom out” my spindles on full bow strokes. Then I added another couple of inches to allow me to get a good grip on the bow. The crude formula for “optimum” bow length was then:

Bow Length = 6*(pi)*D + ~4, where D is the spindle diameter.

My largest wildflower spindles are about 7/16” in diameter. Plugging that in for D gave me an “optimum” bow length of 12.25”, which I rounded down to one foot, or 12”.

My largest hand carved wood spindles (and some Mullein and Ironweed ones too) are about 5/8” in diameter. Plugging that in for D yields an “optimum” bow length of 15.8”, which I rounded up to 16”. Good enough!

Now, an astute observer might remember that I said only 1-2 spindle rotations are often necessary for ember formation when fast passing down hand drill spindles. Why not, then, cut my bows so that I only get two spindle rotations per stroke, instead of six? Well, it’s because it’s the AVERAGE bow speed that matters, not the peek speed. Speed wins every time and the faster you can bow the better. However, your average speed decreases each time you push/pull the bow forward/backward, stop and then reverse direction. That is, stopping, starting and reversing direction takes time. A short bow that’s still long enough that the time required to reverse direction is a small fraction of the bow stroke cycle will allow a high average bow speed to be maintained. (This is actually difficult to achieve when hand drilling and is one of the factors that makes hand drilling a challenge.)

For me, bows that allow about 6 total rotations have proven to be plenty long enough for quick ember formation, but that may be a pure coincidence. An alternate method for determining optimum bow length might be for a firemaker to record a video of him/herself bowing as quickly as they can sustain for a minute or so. From there an estimate how much of the bow is actually being used could be made, with future bow lengths chosen accordingly.

The bottom line is that speed wins! You don’t need a bow longer than you can manipulate as quickly as possible for a minute or so. Most bows are just way too long and most folks would agree that they don’t use anywhere near the full bow length when bowing quickly. By contrast, short, skinny, lightly curved and light weight bows can make fire by friction so much easier. If you haven’t already, I really encourage you to give a short(er) fire bow a try. I think you’ll love it!

Ahi! Sua! Qul!

[Jadeeboy]

I’m going to give it try since you were so eloquently able to explain the science and sense behind it. Then I’m going to try to build an altoid can size break down one for the fun of it. Thanks for helping me rekindle my love of fire crafting.

[telldpablo]

P.S. Thanks for sharing. Just found your channel and am really enjoying your videos!

[Razor.Edge.Reviews]

What wood are you using for the bottom board ?
Thank you