IBO bow speed – do you choose one bow over another because it rates faster IBO? Why not? It’s so easy to compare bows with IBO speed – if one number is bigger than the other, it’s got to be better, right? The IBO game is rigged, so maybe not…
First off, IBO speed is measured when a compound bow draw is set at a 30″. Actually that’s not 100% correct – bowmakers can lean on that measurement and nudge it close to 31″ and still achieve a legal IBO measure. IBO speed is measured at a 70# draw weight – not 100% true either – bowmakers can winkle their bow up to 72# and achieve a legal IBO measure too. Hmmm. At least the IBO arrow weight standard of 350gr is a rock solid constant. A 350gr arrow is little light for a hunting arrow though…
IBO speed is a bowmakers marketing arm wrestle. It’s the gym bunny’s 6-pack, rack and here’s my… and as we oggle the oh so pretty, there’s not much air left to ask “But what’s between your ears”. This sleight of hand starts with us thinking bow speed is the same as our arrow speed. But if a manufacturer makes test runs in a controlled environment, tweaking this and that, to give hundreds of setups and then eventually pick the fastest arrow speed – where does the fact stop and the fiction start?
None of us are likely to ever achieve the advertised IBO bow speed. I mean there’s not that many archers who shoot 70#, have a 30″ plus draw and shoot 350gr arrows. For every inch shorter than 30″, the rule of thumb is you subtract 10fps. Ouch. You subtract 1fps for every 3grs of overall arrow weight over 5gr per pound of draw weight. Got more than a nocking point on your bowstring? A peep, a couple of brass nok sets, string silencers? – for every 3gr, subtract 1fps…
Granted, light arrows will give you tighter sight pin gaps. Yup, you’ll get more kinetic energy with a high speed bow and the blazing arrow speed you’ll get with light arrows – it will reduce arrow drop (out to 50m) and sighting problems too. Cranking up the kinetic energy maths… more speed and less mass… that’s great (and recommended) if you only shoot to score. If you bowhunt though, or want to do a bit of both, you need momentum on your side. The kinetic energy maths must now skew to more arrow weight. Now watch your arrow speed fall away. Arrow mass does not evaporate as it starts penetrating game – only arrow speed does. This is the key to arrow penetration for the ethical bowhunter. And hey, light arrows make your bow noisier too!
IBO speed is not the only thing to think about when you choose your new bow – not by a long shot. Judging bow quietness, build quality, draw smoothness, dealer support etc – it’d be pretty cool to have easy numbers to check these.
