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.:eek:

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.

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.

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:

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.

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.

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!!!:D

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.

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.

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.

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?

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.

RE: HELP!! Need some brain power here!!
 

There is margin distance difference from the average height of a treestand, but I agree with Kodiak, for those hunting out west from perhaps extreme angles, it would be helpful. I'm a gadget person, unfortunately, and bought one. I like it, but for most of my hunting it shouldn't be much of an aid. So, if you primarily hunt deer from stand and want to be penny wise, there are plenty of cheaper less complicated models out there.

The formula for calclulating the distance is a simple geometry problem a-sqared + b-sqared = c-squared, where a if the height and b is the distance from the base of the tree or stand.Convert the height to yards, square it then add the square of the 20 yards from the base then take the sqare root of the sum. That confused even me, so may it's not that simple...lol Rob's calculations are correct in the above post. Good luck

Kelly

RE: HELP!! Need some brain power here!!
 

Distance is not the issue.

The issue is the difference in angle between the arrows flight path and gravity. On level ground, gravity is acting perpendicularto the arrows flight path. From an elevated postion, this angle becomesless and reduces gravity's effect.

For a demo, hold a limber fishing rod straight level and note the bend caused by gravity. Hold the rod downward (or upward)at any appreciableangle and the bend will be less.

This is why you must hold under on uphill as well as downhill shots. Pythagoras will get you close buthe won't help you undestand what's happening to your arrow in flight and this clever Greek'serror grows larger at steeper angles.

RE: HELP!! Need some brain power here!!
 

That is absolutely, positively FALSE! I've shot competitive archery for more years than I can remember, once you have the distance you aim dead on, never high or low or you will go high or low. Let's say you have a 3D target at 20 yards, if you bend at the waist you should put your 20 yard pin dead center of the 10 ring and if you execute a good shot, you will nail it dead center.

Same uphill....I've done it too many years.

Remember, the fraction it takes an arrow to travel said distances, gravity has very little effect on the arrow comparing shooting level, uphill or downhill. Distance is distance is fractions of a second.

The only reason to hold low on uphill shots is path of the arrow through the animal, same downhill, aim for the exit hole.

RE: HELP!! Need some brain power here!!
 

First off let me thank you for giving me something to do this afternoon. The engineer in me wouldn't leave this one alone and I have spent the last two hours with a MS Excel combating this problem.

All of the calculations discussed above are correct. However, from what I got out of their literature, they are displaying "ballistic" range and not the levelized range. The ballistics calculationsare done based upon a user setup. The archery calculation can use one of three pre-programmed values with the worst case being an initial arrow speed of less than 215 feet per second and the fastest being over 250 feet per second. The Leupold exampleuses the "TBR A" setting which is thebelow 215 ft/s calculations.

I think we all agree that a 40 yard target from 20 feet high is closer to 39 yards levelizedbut still not a problem. On a level 40 yard (120 ft) shot with an arrow that travels 215 ft/s it would take 0.56 seconds to reach the target. In that length of time the arrow would drop about5 ft (x = 1/2 *32*t^2). Now that’s a pretty big drop. However, we do not see that since we arch the arrow to the target.My calcs show a little over 2 degrees is the required arch. Because of the arch, we must look atour arrowspeed using the same Pythagoreans theorem as above such that part of the speed is in the levelized direction and part is away from the ground.Gravity will overcome this and start pulling the arrow toward the ground such that when we reach the target we are at the levelized point. Now lets shoot at a downward angle (40 yds from 20 ft high or 9.6 degrees). The distance to the target has been reduced by 2 feet but gravity is now assisting the arrow in its movement towards the target. This combined effect with the slow bow results in an impact that is1.2 inches high. As arrow speed increases, this difference of course decreases.

Now for their example, 40 yards line of sight at 40 degrees. This places the hunter at an elevation of 77.2 feet (ouch) above the target with a levelized distance of 92 feet. Using our 40 yard pin with its 2 degree arch will result in an arrow that is 1.6 feet high. Using the levelized distance method we would use a pin set at 92 feet (~31 yds). This results in an arrow impact that is 1.6 inches high. Adjusting to a pin set at 34 yards (as suggested by Leupold) results in an arrow that is almost 6 inches high. However, if you add in some wind resistance to slow the arrow down, which I am sure they do, the 34 yard pin will probably come in real close.

Thus, I would have to say that their example is probably correct. However, I do not think that it is realistic. If you use a faster bow or hunt from a reasonable height, any range finder will get the job done.

RE: HELP!! Need some brain power here!!
 

Hey War...where were you before I dropped $ 300 on this thing!!!

LOL...gotta love engineers. Just couldn't let the hype pull you in..had to go and spoil it for me.

Kelly

RE: HELP!! Need some brain power here!!
 

Well this is a good topic. I wonder what the company would say?