There was a discussion today on Archery Talk that inspired me to write this post. This discussion began about people theorizing which company would be the first to put out a 400 fps bow to IBO specifications. It soon morphed into a discussion about whether or not 400 fps could even be obtained. If you know me at all, you would know that I cannot resist a discussion about archery and physics. Something in my blood makes this literally impossible for me to avoid.
Rather than reading and enjoying the thread, or even just posting a few passing comments, I was moved to break out Mathcad. Here are the results of the calculations showing what the theoretical speed limit is for a perfect bow:
Now to explain all the assumptions. This simple calculation shows that a perfect bow in a perfect world set at 30″, 70 lbs, a 5″ brace height and shooting a 350 grain arrow could reach 418fps. However, this assumes that there is no friction, no vibration, no sound, no heat, no anything produced at the shot except 100% energy transference from the bow (PE) into the arrow (KE). It also assumes that the bow starts to draw immediately at 70 lbs, remains at 70 lbs the entire draw length and has zero let-off. Doesn’t sound like much of a fun bow to draw back! The flip side is that after the arrow leaves the string, the shooter would feel absolutely nothing except the weight of the bow in their hand.
Of course we all know that this scenario is not only unlikely, but also impossible. Add in a bit of a ramp to get the bow to 70 lbs peak weight, let-off to make it possible to hold and aim, vibration/sound/heat/etc. throughout the bow on the shot and the speed drops right down to where we are today, around the 360 fps mark.
Is it possible to increases today’s speed within the IBO specifications and if so how? Yes, it is very possible and the “how” can happen either of two ways: make the act of drawing the bow store more energy (quicker ramp to peak draw weight, shorter valley, less let-off) or increase the efficiency of the bow. Many of today’s bows are pushing and even surpassing 85% efficiency; more gains can be made in reducing bearing friction in the cams, more efficient cam design, taking vibration out of the shot and putting that energy in arrow, etc. Manufacturers will continue to make strides this way and continue to see some speed increase.
But will archers submit to putting more energy into the bow with their muscles and put up with a “harsher” draw? How stiff of a draw will be usable and practical? Some “speed freaks” will put up with anything if it gets more fps on the chronograph while other archers want a “smooth” draw that is easy to pull and forgiving to shoot. It will be up to the bow designers to find the designs that people are willing to spend their hard earned dollars on.
Other posts you may enjoy:
- Bow Efficiency and Why You Should Care
- 2011 Elite Bows – Pulse, Pure, Hunter and Tour
- Arrow Kinetic Energy and Momentum: what they mean to the archer
- Kinetic Energy, Momentum and Arrows: a Simplified Approach
- How to: Estimate Bow/Arrow Speed