# Chrono Head Scratcher....



## Metropolitantrout (Feb 15, 2012)

_This is part 2 (originally titled "Band build help" fro 3/26) _

Today I had an hour of good daylight so I put a new battery in the chrono, shortened my bands to get about 5X the stretch and re-ran the FPS. I also tested the chronograph against 2 bows (3 arrows each). Both bows were *dead on* from measurements taken from other chronos and when I shot a slightly lighter arrow I saw +10 FPS. Maybe the chrony works fine after all.









Unfortunately the slingshots did not deliver as expected.







I tried 1/2 steel soaked in vinegar to eliminate shine but that made no difference from standard shot. Below are the results. 5 shots each about six feet away. I have a 30.5" draw pulled it back even further a few times (could not break 190FPS).

6" TBG 25mm (no taper) 165FPS (181 using a glass marble) 
6.5" Tex Shooters Express double latex 170FPS 
6.5" TBG 25/20 taper, double bands 170FPS (210 FPS with 3/8" steel)

8" TBG 25/20 taper (a real easy to shoot plinker) 140 FPS (157 FPS using a glass marble)

The last test was a real head scratcher. The 8" banded ss has half the pull weight of the others but only shoots 25-30 FPS less?????

The chrony is a new model designed to measure everything from bb guns to rifles but clearly something is a miss.







Right? The lighting was ideal, partly cloudy with good even brightness (no glare). I'll probably send it back for a refund.

Any guesses how much it's off (if at all)? -MT


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## Charles (Aug 26, 2010)

One question I have is the temperature. Guys getting really high velocities are usually shooting at pretty warm temperatures.

Also, your draw length is pretty short. It takes a while for the bands to accelerate the pouch and ammo from a dead start up to a reasonable velocity. The analogy is your car. If you have only 50 yards in which to accelerate, you will not be able to reach as high a speed as you would if you had 100 yards to accelerate.

I am not conviced there is anything wrong with your Chrony. What one really needs is for someone with the same bands and set-up to shoot over a different Chrony. If you do not have access to another Chrony, you could set up an acoustic chrony using Audacity software (free download). You set up two targets a measured distance apart, say 10 feet. Then fire a shot. The software will record the sound of the ammo as it passes through the paper. Then by looking at the wave pattern you can measure how long it took to go the measured distance between the two targets, and that will give you the speed.

Sorry, I have not used the acoustic method myself, but hopefully someone on the list has done so and will chime in. You might do a search on this forum for "Audacity" and see if you can turn up a past discussion.

Cheers ......... Charles


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## Bill Hays (Aug 9, 2010)

Can you do a video?
There is certainly something amiss... temperature related is a possibility, and I also had the thought that the TBG might be one of the latex free types or something... but you're also shooting the Tex bands pretty slow too. You're sure that it's a 30.5" draw?
Makes me want to run a test with the same setup just to check.... the only other thing it could be that wasn't mentioned is the pouch... like how big/heavy is that?


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## bullseyeben! (Apr 24, 2011)

Here's a vid of mine showing a few diff bands and cuts, 9mm lead that seems the mimic glass marbles in velocity or 3/8 steel close enough.. scratching me head now! I use rather small pouches but 100 fps is a big difference!


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## Metropolitantrout (Feb 15, 2012)

I'll try to put together a video Bill. The temp today was 67F. The TBG I have is the real deal. I use two types of pouches. One is Tex Shooters and the other is just as thin but a bit smaller. Both very light. My draw is a pretty solid 30+" My bow draw is 27" to my front tooth but for slingshots I anchor at my ear. I pulled a few shots back as far as I could before the 6" band hit a wall and the chrony read 190.

My theory is the 5x stretch rule is not linear. If you stretch an 8" band to 40" it will be far more powerful than 6" stretched to 30". Another possibility but not likely is the shot hasn't fully accelerated to it's max FPS from 6' back of the chrony. Based on other videos I doubt this though.

The velocity and power I'm getting seems more than adequate to take small game but I'd like to be sure and keep shots humane.

I wish paid more attention in HS physics......


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## pop shot (Sep 29, 2011)

It's a factor of draw length and stretch %. But you're right around 200, just run with it


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## Charles (Aug 26, 2010)

Metropolitantrout said:


> My theory is the 5x stretch rule is not linear. If you stretch an 8" band to 40" it will be far more powerful than 6" stretched to 30". Another possibility but not likely is the shot hasn't fully accelerated to it's max FPS from 6' back of the chrony. Based on other videos I doubt this though.


You are absolutely right. In general, the longer your acceleration distance, the faster your final velocity. The more time those bands have to bring the ammo up to speed, the faster it will go. To take your two examples, if you stretch an 8" band to 40", you have an active length of 40-8 = 32". If you stretch a 6" band to 30", you will have an active length of 30-6 = 24". Assuming the same rate of acceleration, a 32" acceleration path will yield a higher velocity than a 24" acceleration path. The easiest way to get higher velocities is to increase your draw length.

Again, I invite you to think of my automobile example. On a straightaway, starting from a dead stop, stomp your foot on the gas all the way to the floor. Your speed 50 feet down the line will be quite a bit less than at 100 feet down the line.

Cheers ...... Charles


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## Charles (Aug 26, 2010)

bullseyeben! said:


> Here's a vid of mine showing a few diff bands and cuts, 9mm lead that seems the mimic glass marbles in velocity or 3/8 steel close enough.. scratching me head now! I use rather small pouches but 100 fps is a big difference!


Hey Ben,

Looking at your video, there are two obvious things going on.

First, you are out in shorts, bare feet, and singlet. So what difference do your clothes make??? Well, a lot actually. Your clothes are an indication of the temperature ... and I'll bet the temperature was a lot higher than 20 C = 68 F.

Second, if you look at your draw length, you are shooting at about 3/4 butterfly all the time. So your draw length is quite a bit more that 30 inches. Longer draw length yields higher velocities.

Cheers ....... Charles


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## bullseyeben! (Apr 24, 2011)

Charles said:


> Here's a vid of mine showing a few diff bands and cuts, 9mm lead that seems the mimic glass marbles in velocity or 3/8 steel close enough.. scratching me head now! I use rather small pouches but 100 fps is a big difference!


Hey Ben,

Looking at your video, there are two obvious things going on.

First, you are out in shorts, bare feet, and singlet. So what difference do your clothes make??? Well, a lot actually. Your clothes are an indication of the temperature ... and I'll bet the temperature was a lot higher than 20 C = 68 F.

Second, if you look at your draw length, you are shooting at about 3/4 butterfly all the time. So your draw length is quite a bit more that 30 inches. Longer draw length yields higher velocities.

Cheers ....... Charles
[/quote]
Thanks once again Charles for stating the obvious, yes as i suggested in pt1 of this topic named band build help, i suggested extending the draw length, as you have noticed I aint a short draw shooter.. secondly no it wasnt 20c degrees, prob more around 30c when i filmed this.. regardless of either, my point was that a 100fps difference seems rather a lot even given these variables..


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## Metropolitantrout (Feb 15, 2012)

OK Bill, here's a video. Let me know if you see anything amiss. Jerry


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## pop shot (Sep 29, 2011)

what ammo are you shooting? sounds heavy. 1/2" steel? lead?
the only thing i can see (i don't think it would affect your speed) is that you have a serious twitch with your frame hand when you release.


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## Metropolitantrout (Feb 15, 2012)

1/2" steel. For this video I was more concerned with velocity than accuracy but I'll pay more attention to my frame hand.


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## pop shot (Sep 29, 2011)

I was thinking you might be jerking back


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## Tex-Shooter (Dec 17, 2009)

You might be shooting a little faster than you think. I have a friend that has the same chronograph that you have and it consistently records about 10 feet per second slower than my "Chrony brand" at slow speeds. At bullet speeds they record about the same. I have had my Chrony checked for accuracy at slow speeds. I won't claim that it is perfect, but it is close. From the video that is about all I can come up with. -- Tex


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## newconvert (Dec 12, 2011)

Metropolitantrout said:


> OK Bill, here's a video. Let me know if you see anything amiss. Jerry


since so much of the craft depends on repetitive motion and a good stance i notice that your draw seems closed off, what i mean is that you are shooting like you are overly concerned with a line of sight with your eyes, shoulders rounded, kinda hunched into the shot. if you were to square your shoulders, stretch out those arms, find your anchor point, i think by your size you might just find that your draw might increase by i am guessing 4"? at least


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## Bill Hays (Aug 9, 2010)

Wow, that looks pretty definitive... I'll need to run a few tests to see what we can come up with... but first, what is the maximum draw weight you feel comfortable with at that draw length?


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## timdix (Oct 1, 2010)

Bill,I'd love to see you run your tapered 1745's through the chrony using your long draw. These little tubes always surprise me.


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## Metropolitantrout (Feb 15, 2012)

Bill Hays said:


> Wow, that looks pretty definitive... I'll need to run a few tests to see what we can come up with... but first, what is the maximum draw weight you feel comfortable with at that draw length?


I appreciate the offer Bill but I hope you don't feel obligated to do anything outside your own interest. 
That being said...Right now I'm comfortable and pretty accurate with 13-14lb pull as an everyday target ss but I'm working up to getting comfortable with 16lb and could probably go a little beyond that. I'm only looking to see an extra 30 FPS so I think it's realistic for a 30-31" draw.
I understand the simplest way to get more speed is learn to draw further but my current style compliments my archery form and I've noticed my arrows are grouping tighter since I started practicing ss shooting. That's why I'm reluctant to increase my draw length.


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## Bill Hays (Aug 9, 2010)

Yes I understand that. I've got a lot of customers who would probably be interested in achieving more speed with heavier ammo and a short (yet standard archery draw) length.... So I'll run some tests pretty soon using .44 lead and 30" draw. I'll use a slingrifle type of setup so the draw will be precisely the same each time.


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## Northerner (Nov 9, 2010)

Hi MT,

I watched your video and have a couple of ideas. First, I think you should do an actual measurement of your draw length while actually drawing for a shot (use a friend). You might be surprised. Second, it looks like you have a soft release (dead release). You might be losing back tension. You appear to be relaxing at the point of release. After the release your hand does not move back at all. You seem to move your drawing hand forward a bit after the shot. This will kill your power/velocity. Try an active release. Bill Hays should have a video that shows an active release. Bill's hand moves backwards slightly after the shot. He doesn't stop and relax. He doesn't lose tension during the release. Bill keeps fully stretched out during the follow-through. Draw to your anchor and then pull back another half inch and release. Your hand is under tension and should move back slightly after the shot.

Cheers,
Northerner


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