sight mark V arrow weight

[Bird_2112]

OK... some of you have broken one or two 10gn sections off point shafts. What sort of height difference does losing 10grns make at say 90 or 70m. I'm actually interested in the opposite ie how much an arrow will drop when 10 or 20grns are added but most people would have had experience from lightening points. Thinking along the lines of navigators or ACE's.

G.

[joetapley]

3 grains per ring at 70m is a reasonable rule of thumb. Less if you have supersonic arrows of course

So 20 grains = about half a target.

[Schme1440]

Originally Posted by joetapley

3 grains per ring at 70m is a reasonable rule of thumb. Less if you have supersonic arrows of course

So 20 grains = about half a target.

Does that also not depend on poundage as well? Surely a 20# bow would be more affected by 20 grains than a bow with 50#?

[joetapley]

Does that also not depend on poundage as well? Surely a 20# bow would be more affected by 20 grains than a bow with 50#?

Of course.
Drop depends on arrow speed. But a lighter poundage bow would use a lighter arrow . 3 grains per ring is only a ball park number. Don't need to the nearest cm.

[Schme1440]

Originally Posted by joetapley

Of course.
Drop depends on arrow speed. But a lighter poundage bow would use a lighter arrow . 3 grains per ring is only a ball park number. Don't need to the nearest cm.

Fair enough. This is another one of those topics that are difficult to prove with out extensive and very complicated tests.

[Robin Astra]

My own experience was that 10 grains equated to about three rings at 100 yds/90m.
Arrows: 820 Cartel X-pert, 27 and half inch
Bow: 44 lb peak, round wheels.

Robin

[SLOWHAND]

I was going to say about six inches per 10 grns but so much depends on your bow, the arrows, the draw weight and you.

[Bird_2112]

I may as well post my reason for asking. Shooting 311/4" ACE 430's at the moment which are too stiff as I have become lazy and dropped poundage to around 38lb on winnex limbs. I'm also down to 8. Thinking of getting navigators and going to sell my 400 cartel triples (any interest ?) which seem stiffer than the ACE's. Keeping the ACE's for field and they currently have 120gn points fitted to flex them up a bit.

Anyway, I based the maths on a 31" shaft and the recommended point weights for 470 ACE's (95gns) and 480 Navs (110gns)

Current 430's with 120gn points total 340gns
470's with 95gn points 308.9gns
Nav 480s with 110gn 358gns
Current Cartels approx 353.35gn

So it looks like if I went for Navs my arrows will drop to the black if I leave the points at 110gns. Think that will mean pulling the sight in as with my new Shib ultima the bar is shorter than my old sight. I have also had to move the vertical bar up as far as possible to get a 20yd sight mark ( Pin almost at the top of the bar) meaning more fiddling changing distances. Getting 470 ACE's will push my arrows just over the top. Another draw back then is my 30m sight mark may creep up towards my 20yd mark and I'll have difficulty seeing the sight pin due to the curve at the top of the winact sight window. Can not get a sightmark for less than 18m now as the pin is hidden and no more upward movement... very annoying for field bunnies.

Dont know now whether to just pay the extra for the better sightmark the 470 ACE's will give and stick with what I know (would be 4th set of ACE's) or get the Navs and see how they go.

Anyone shooting just below 40lb with a 31 -31.5" Nav/ACE.. what spine are you using ???

G.

[joetapley]

The ACE 470 would give you a marginally better FOC (mid 13s). At 70m around a metre height over the Nav and at 90M around 1.75 metres over the Nav (computer estimate so pinch of salt). Big difference in price though.

[jadlem]

Interesting question on weight, I've been doing some calculations in the last week as I've been too ill to practise.

The first thing I did was calculate the amount of energy stored in my bow.

Produced a force draw curve using a set of digital scales with a tape measure attached.

This gave me a wonderful curve which when input into autocad and the area underneath it calculated, produced a value of around 88Joules.

Next thing you need to do is weigh your arrows, I weighed my 470 aces in at 302grains or 0.0196kg (need to use SI units!) Measured them using my new chronograph at 275fps or 83.8meters per second.

This then told me (using me old a level physics): distance fallen = half gravity times time in seconds squared.

83.8mps equates to the arrow taking 1.07 seconds to reach 90m

the distance it falls over this distance equates to 5.6m! I was quite shocked by this! I didn't think I was aiming up by that amount at 90m.

Any how back to the point at hand, I repeated the same process with some similar arrows weighing in at 420grains or 0.0271kg, the same bow shot these arrows at 243fps or 74mps. This means they drop 7.25m at 90m!

The weight difference and drop difference works out as being 0.014m per grain or 1.4cm per grain! This works out as mentioned above to close to the 6inches per 10grains rule, however there is far more to read into this:

The ratio of the two distances fallen was about 0.78 compared to the ratio of the weight difference being 0.72. For the above statement to hold true these two ratios must be the same!

One critical factor in explaining this difference is the efficiency of the entire setup, draw length, draw weight and arrow mass do not produce a directly proportional graph. The lightweight arrows on my setup are only 78% efficient. (Only 78 percent of the available stored energy in the bow is transferred into the arrow) The heavier arrows are 84% efficient. This tells me that for every archer there is a sweet spot when it comes to arrow mass. Being armed with a force draw curve for your bow can tell you its potential stored energy, then simply weighing and chronographing different arrows will let you calculate how much of the energy in the bow they can cope with. There will then come a point where reducing arrow weight will stop affecting velocity. An easier change would be to adjust the bow weight to match your arrows.

Coming from someone who shoots compound, the affects of dynamic spine have less of an impact than the mass of the arrow. Recurve archers will always have to balance this factor with that of arrow spine and bow tuning although I’m fairly sure there is substantial evidence to prove that point weight does more to absorb discrepancies in consistency than to affect the spine of an arrow.
I’d be interested to hear from anyone with a grounding in physics who can argue the above as I didn’t pay too much attention in class. I’m fairly sure I may have overlooked something. Let me know your thoughts.