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HELP!! Need some brain power here!!

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HELP!! Need some brain power here!!

Old 07-14-2006, 09:25 AM
  #1  
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Default HELP!! Need some brain power here!!

I recently joined a post on the Gear Review forum about a member looking at the Leupold RXII rangefinder which supposedly has a ballistic compensator that compensates for the shot angle and give you the actual distance to the targetrather than the "line of sight" distance. I'm in the market for a new rangefinder as well and would like somebody to help me with what I might be missing here because I just can't get the numbers to work out. Now I'm no mathematician so in advance I admit I may be completely calculating things wrong, that's why I'm asking for help.

I'm trying to determine the difference between actual distance vs. line of sight distance from a treestand. My own calculations are that you should be able to use the Pythagorean theorum we all learned at one point or another in Geometry class which states a2 x b2 = c2. You should be able to calculate the distance of any of the three sides of the triangle created (height of rangefinder above ground, distance from base of tree to target, line of sight distance from hunter to target) by applying that formula.

In applying the formula Ifigure that if you're 21 feet up a tree on level ground (7 yards) and your rangefinder reads 30.0 yards the actual distance to the target is 29.2 yards. That's 30 squared minus 7 squared equals the distance squared or 900-49=851 the square root of which is approx 29.2 yards.

A more extreme example, say you are40 feet up a tree. Now that's high! And that's a pretty steep shot angle. You range the animal at 30.0 yards. The actual horizontal distance calculated to26.9 yards.

Ok so lets look at the advertisements in the catalog. The manufacturer shows that a hunter ranging a target at 40 yards would get a true range of 34 yards. That'sa difference of 6 yards or almost 20% of the actual distance, definitely significant. Using the formula however,I would calculate that hunter to be 21 yards above the target level or 63 feet up a tree. Wow, now that's nosebleed! Considering most hunters don't go much above 20 feet that seems unlikely.

Myconclusions seem to be supported by another member who tried the rangefinder in the field and reported in that same thread that he tried ranging bow ranges as steep as he would ever take a shot and didn't get more than a yard or two difference.

So I'm hoping that somebody out there with a lot better understanding of math and ballistics can help explain this to me and anybody else out there who might be interested. Thanks.

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Old 07-14-2006, 09:33 AM
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Default RE: HELP!! Need some brain power here!!

(Ill do the math, later).....but I just practice from a tree.

Jeff

Gone to lunch.....math, later.
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Old 07-14-2006, 09:45 AM
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Default RE: HELP!! Need some brain power here!!

This is a common thing in field archery as well because you are commonly shooting at up and down hill targets in wooded settings. They are notorious for setting up on extreme up and down hill shots. Doug Williams with Copper John/Stan is probably one of the best field archers in the country right now (behind cuz), and he discusses this a bit over on archerylive.com. Here is his response in quotes:

Cutting yardage isn’t quite as simple as the trigonometry explanation but the formula will get you very close on 90% of the situations. It is proven that shooting up hill the arrow will slowdown faster (slower down range speed) than on flat ground and slowdown slower (faster down range speed) on a downhill shot. On a 80yd uphill shot at a 5 degree incline you will have to add a bit(less than a ¼ yd) to be dead center but on a 5 degree downhill you will have to cut 3/4yd almost a full yard difference between the 2 but on a 10 degree uphill shot you will have to cut ½ yd and on a down hill 10 degree slope you have to cut 2 full yds. The reason for this is the change in trajectory in relationship to the line of sight on an uphill 80yd shot 6 degrees is where the two will balance out between the velocity change and the change in trajectory so you will have to shoot it for a straight 80. After more than 6 degrees the change in trajectory will always be greater than the change in velocity so you will have to cut yardage. On downhill shots the two will never balance out and that is why you will always have to cut more on downhill shots in comparison to an uphill shot at the same slope.

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You can download ontarget2's free version and can actually print your own cut chart on this stuff. Below is an example of a angle cut chart from ontarget2. It is the graph on the lower right hand side. I have one of these printed for both my 3D and field bow and these charts are dead nuts accurate every time.

Edit: Pic didn't post....here it is.

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Old 07-14-2006, 09:45 AM
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Default RE: HELP!! Need some brain power here!!

your math is right.... the hunter could be up a hill from the target not just up in a tree.

Its adynamics problem... the trajetory is changed due to the forces acting on the arrow. When you shot horizontal the bow gives the arrow a horizontal vector force and gravity gives it a downward vector force, which gives you the traditional arch. If your shooting at a down angle part of the bows vector will be resolved into a downward force. This actually causes part of the gravity force to make it decelerate at a slower rate. visa versa if you are shooting up hill.

that said i would be leary of that range finder if it shows that the range is actually further than actual. because if you know the actual distance and shoot for that your likely to shoot high anyway due to the lessend effect of gravity. So if its telling you to aim higher cause the distance is futher you will shoot even higher..

Its been a few years since i had a dynamics class so i might be wrong. but i think i still understand the theory.
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Old 07-14-2006, 09:54 AM
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Default RE: HELP!! Need some brain power here!!

Remember...You are reading their ads (marketing) info...Most companies tend to either exagerate the situation to their advantage...I have checked ranges with my 660 Nikon while bowhunting and do see a difference when ranging trees at the same height that I'm setup as well as 3 feet off the ground and at the base of the tree...It is not as extreme as their example....With a high powered rifle its not enough to worry about (when in a tree)...

If we bought in to all the bs thats handed to us on deer hunting we would need a wheel barrow to carry everything thats needed!!!
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Old 07-14-2006, 10:17 AM
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Default RE: HELP!! Need some brain power here!!

Those new rangefinders are exaggerated in their adds for a bowhunting circumstance, rarely if ever will more than a yard or two difference and that is only if you hunt very high on a very steep hill.

A friend of mine wrote an article and tried to have it published, he included a chart from 18 and 21 feet, I'll show the chart for 21 feet in the tree.

Ranged Distance from Stand Actual Distance to Target

20 yards 18.72 yards
25 yards 24 yards
30 yards 29.17 yards
35 yards 34.29 yards
40 yards 39.38 yards

As you can see from his findings unless you hunt 60 feet in a tree and have an animal downhill 8 yards away you won't see a difference really. Those graphs they show you for advertising are entirely incorrect.

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Old 07-14-2006, 10:39 AM
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Default RE: HELP!! Need some brain power here!!

Rob, those numbers prettymuch agree with what I'm coming up with.

I just don't see how you you could have a 6 yard difference in a target at only 40 yards line of sight from a typical treestand set-up. The angle would have to be extreme enough to start questioning the shot angle and making a clean kill. By the way the marketing example shows a shot angle of 40 degrees whis really isn't super steep. I'll attach a copy of the marketing graphic below.

I think there are going to be some diappointed consumers if they buy this product based on the marketing example then get far different results from their treestands.


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Old 07-14-2006, 10:58 AM
  #8  
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Default RE: HELP!! Need some brain power here!!

That ad is what caught my attention before. I've had my rangefinder for years and after shooting 3D for more years than I can remember I immediately knew if was full of it...false advertising and a scare tactic...I was actually appauld at their example.
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Old 07-14-2006, 11:16 AM
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Default RE: HELP!! Need some brain power here!!

I don't think it's going to make a difference for the average deer hunter but where it would definitely be handy is hunting mountain goats where the angles can be extreme (60* up/downhill) and the ranges long (30-40 yards). It may sometimes take me 10-15 minutes to sit there looking at a goat before I can convince myself of an accurate distance to shoot him for, especially in the broken, uneven terrain. I'd never seen that add from Leupold before now, man that must be one tall tree!!! I wonder how many climber steps it took him to get up there?
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Old 07-14-2006, 11:28 AM
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Default RE: HELP!! Need some brain power here!!

I aways aim a tad low to compensate for the difference. I think this is why many shoot over the animals back from a tree stand. I put it on the briskit and it hits just above it for a good clean kill.
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