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If I fletch my arrows straight rather than offset will I leave a bit more energy in the arrow? Seems like there must be some sort of trade-off. Am I missing something?
QWhile manufacturers put effort into making high quality arrows that are straight, they are not ‘perfectly straight’. Careful testing with my shooting machine shows that if we ensure that the arrows spin about their longitudinal axis the groups will be tighter and we will score more. We can do this by placing the fletches at a small angle to the shaft – 1 or 2 degrees is quite sufficient and in any case is about as much as you can get on a small diameter shaft such as a Protour.
It does take a small amount of energy to get the arrows spinning, and that energy comes by slightly decreasing the arrow’s speed. However
Athe decrease is actually very small – only 1 or 2 ft/sec typically. So, is there a trade-off? Yes, you get smaller groups and a higher score at the cost of a very small amount of arrow speed. Is it worth it? Absolutely.
One misconception I regularly hear is that with the vanes at an angle the drag will be higher. However that is not the case. Once the arrow is up to its stable spin rate (which happens within about 10 m from the bow) the vanes are effectively edge on to the air flow and their drag is no different to that for an arrow without angled vanes which is not spinning.
Another frequent misconception is that vanes stabilise the arrow from their
THE LARGEST CONTRIBUTION COMES FROM AERODYNAMIC LIFT, NOT DRAG.
drag. While they do contribute drag and it does help stabilise the arrow, by far the largest contribution comes from the vane’s aerodynamic lift, not drag. The lever arm from the arrow’s centre of mass for the drag is quite small while that for the lift is large.
Interestingly, the fletches contribute both friction drag from their sides and profile drag from their projected frontal area. That is, for a given fletch area, the drag will be lower for a low profile fletch.
Ideally, I like to see archers using low profile vanes set at a small angle (1 to 2 degrees) to the longitudinal axis of the arrow. Both my modelling and measurements has convinced me that will give the best groups and the highest scores. Of course, for a spinwing or similar fletch, the angle is built into the fletch, so we should place them with their straight edge aligned with the shaft’s axis.
DR. JAMES PARK
If I back off the limb bolts to lower my draw weight will it affect the accuracy of my compound bow? I heard that the limbs should be wound all the way in to work at their best.
QThe majority of compound bows have an adjustable draw weight that can be lowered by 10lbs from the peak weight; by winding out the limb bolts you can adjust your bow to suit your strength, shooting discipline or to fine tune your arrows.
Years ago most manufacturers recommended that their bows worked at their best when the limbs were fully wound in at their maximum poundage, but this was largely because of limb design. In the past the majority of compound limbs were forward facing in profile, similar to a recurve bow, in fact some compound bows even had limbs with a slight reflex in them. Because of this design there was a lot less tension and flex in the limbs at brace height; winding them out to achieve a lower peak weight reduced this tension even more. This meant they could start to feel
Aslack if you backed the weight off too much with some models loosing almost all the tension in their limbs. The drop in tension lowered the bows efficiency considerably and it often had a negative effect on the downrange accuracy of the arrow, as there was much less energy being transferred from the bow. The majority of bows these days have a parallel limb design meaning there is a lot more tension and flex in the limb at rest, so winding them out has much less of an effect on their efficiency, making the bow much more usable across its draw weight range.
While winding the limbs out will not affect the accuracy or efficiency of the modern bow it will slightly alter the specs; the bows brace height will increase by a small amount; this will in turn increase your draw length by the same amount, so you may need to correct it in order to get the bow to fit you the same.
Lowering your bows peak weight will also lower your holding weight as your let-off amount remains unchanged. For example a 60lb bow with a typical 65% let off with have a holding weight of 21lbs, a 5lb decrease in peak weight to 55lbs will lower the holding weight by 1.75lbs.
A decrease in holding weight can affect how the bow feels to shoot as well as how well it holds on the target, so you may need to either alter the bows let off (if this is something your bow model allows) or change the stabilisation slightly in order to counteract this. You also need take any drop in holding weight into account if you are using a resistance style release aid, since it will now take extra pressure to activate it.
Finally, lowering your poundage will affect the way your arrows tune; a lower peak weight will make your arrows stiffer which can impact how they group. Although altering your draw weight by a small amount is a great way to fine tune your arrows to your bow, you may need to consider a different spine arrow if you alter your draw weight considerably.
As modern compound bows are designed to work efficiently and accurately anywhere in their draw weight range, your bow will be just as accurate as it was at its peak weight. Just remember to wind both the limbs out by equal amounts and for safety reasons do not wind the bolts out further than the manufacturer recommends, as this can be dangerous.
DUNCAN BUSBY
Is there a 'best' testing distance for bareshafts, where you are trying to make the bareshafts land amongst the fletched arrows? Does it depend on your standard? Different guides seem to say different things.
QBare shaft tuning is a really good starting point for the initial tuning of a bow, and involves shooting a mixture of arrows that have been fletched and arrows that have had the fletchings removed. The purpose of fletchings is to encourage the arrow to spin in order to stabilise the flight and straighten the rear end of the arrow once it leaves the bow. A bare shaft does not have this stabilising influence, and will continue in whatever direction it left the bow in. The “ideal” tune is when a bare shaft and a fletched shaft lands in
Athe same place on the target, which implies that the fletchings are only working to spin the arrow and do not need to correct the trajectory as it leaves the bow.
When you consider that a bare shaft will continue along its intended trajectory, you need to have an idea of how “close” you think the arrow spine will be before you shoot the arrow. If you are shooting an arrow that is a spine group too stiff, you won’t want to begin tuning at 70m unless you have a metal detector to hand. But if you know your arrow is the correct spine for your set up, the group size will depend on the consistency of your shooting. Any deviation of the bare shafts from the fletched shafts will only be amplified by an increase in distance. These deviations may be caused by the initial bow set up, or due to the ability of the archer to perform a consistent enough shot. Simon Needham and Rick Mckinney suggest in their books that you should bare shaft tune your arrows at a distance between 15 and 20 metres. However George Tekmitchov, one of the senior engineers at Easton and other companies, suggests that you choose the maximum distance that you are able to consistently shoot a group with fletched shafts within the gold on the appropriate target for that distance. If you are capable of this, you will expect your bare shafts to land among the fletched shafts when tuned correctly.
For most archers this is 30m, and Tekmitchov states that this is far enough to tell you everything you need to know in a basic bare shaft tuning test, and archers should not push to attempt to shoot bare shafts at 70m if they are not capable of consistently grouping fletched shafts in the gold at this distance.
REBEKAH TIPPING