Question on short reflex deflex bows

[Badger]

  You do an amazing job of holding the reflex you hold. Are you familiar with establishing virtual mass on your bows? I think you would find it helpful in your testing.

[DC]

If you're talking to me I'm not familiar with any of the math but I'm thinking I have to learn.

[Badger]

  Yes, I was talking to you LOL. It is a good way to identify hysteresis in your bows. A glass bow has very little hysteresis so once the virtual mass is established it will remain fairly constant regardless of arrow weight even though efficiency will continue to drop with arrow weight. A wood bow that has taken set will display more hysteresis and because set is time sensitive it increases rapidly as arrow weights go down. The math is very basic and I even have a cheat I use to speed things up a bit on taking force draw curves. I will get back to you in a few minutes with the details or I can e email them to you if you prefer.

[bownarra]

To make this sort of bow it makes sense to use some billets. Reflex and heat treat them then splice in deflex. You should aim to end up with around 1 - 2" overall reflex once spliced.
This design is perfect for a short draw/short bow. Very stable to shoot and fast.

[DC]

  Yes, I was talking to you LOL. It is a good way to identify hysteresis in your bows. A glass bow has very little hysteresis so once the virtual mass is established it will remain fairly constant regardless of arrow weight even though efficiency will continue to drop with arrow weight. A wood bow that has taken set will display more hysteresis and because set is time sensitive it increases rapidly as arrow weights go down. The math is very basic and I even have a cheat I use to speed things up a bit on taking force draw curves. I will get back to you in a few minutes with the details or I can e email them to you if you prefer.

You might as well put them here, others may be interested. I will save them

[willie]

You might as well put them here, others may be interested.

I would be interested also Steve.

 

A wood bow that has taken set will display more hysteresis and because set is time sensitive it increases rapidly as arrow weights go down.

very well spoken.

[Badger]

  I will try and explain the best I can. Virtual mass is the amount of weight difference between a tested arrow and an arrow shot at 100% efficiency to reach the same speed as the test arrow.

  Example: DC shoots a 500 grain arrow from a 50# bow and hits 200 fps. This imparts 44 ft# of Ke into the arrow. DC has done a force draw curve on the bow and found that he is storing 55 ft# of energy. Using a KE calculator he starts adding virtual weight to his arrow until it reaches the full full 55# of stored energy at the same speed as the 500 grain arrow. In this case 620 grains of arrow traveling at 200 fps would store 55 ft# of KE. So the virtual mass of his bow would be about 120 grains.

   The interesting thing about a wood bow is that if the bow has no hysteresis the virtual mass will remain relatively constant regardless of arrow speed and arrow weight. Being that hysteresis is speed sensitive a bow that has taken set will exhibit more hysteresis, your virtual mass will rise as arrow weights decrease and it will go down as arrow weights increase.

  one of the cheats I mentioned was to use an extremely heavy test arrow to determine your stored energy instead of measuring a force draw curve. A 2000 grain arrow should give you about 95% efficiency. You may have to play with it a little to nail it down but I believe 90% is pretty close. This will vary with different bow types so if you are working with the same style bow it should get you pretty close once you nail it down.

  Lets say your 2000 grain arrow shoots at 106 fps and gives you about 50# KE add 10% to that and you most likely have about 55# stored energy. You are looking for a consistent number on your virtual mass with a variety of arrow weights. Ideally it will only rise slightly as arrow weights decrease.

[DC]

So what do I use virtual mass for?

 I found a KE calculator here.  https://www.calculatorsoup.com/calculators/physics/kinetic.php

Is adding up all the squares and part squares more accurate than a heavy arrow?

[Badger]

So what do I use virtual mass for?

 I found a KE calculator here.  https://www.calculatorsoup.com/calculators/physics/kinetic.php

Is adding up all the squares and part squares more accurate than a heavy arrow?

  I like the heavy arrow method but you do have to get dialed in on it by using the long method first. Once you figure out what percentage to use you should be good to go. Virtual mass is a good way to predict speeds of various weight arrows.

[sleek]

Yes it's Boo backed Yew. I've had difficulty getting selfbows to hold enough reflex for the speeds that I'm after.

Dc, what speeds are you after?

[DC]

As fast as I can

[DC]

Virtual mass is a good way to predict speeds of various weight arrows.

Isn't that what I bought two chronos for?

[willie]

thanks Steve

you compare an arrow to an ideal arrow and the mass difference is attributed to the bow?

example:  my new bow has a Virtual Mass of 123grains. or do you need to say: my new bow has a Virtual Mass of 123grains @ XX grains per pound?

if bows loose efficiency as arrows get lighter, then where is the benchmark in arrow weight?

Virtual mass is a good way to predict speeds of various weight arrows.

once you determine the VM of the bow, how do you figure speeds for other weight arrows?

[Badger]

thanks Steve

you compare an arrow to an ideal arrow and the mass difference is attributed to the bow?

example:  my new bow has a Virtual Mass of 123grains. or do you need to say: my new bow has a Virtual Mass of 123grains @ XX grains per pound?

if bows loose efficiency as arrows get lighter, then where is the benchmark in arrow weight?

Virtual mass is a good way to predict speeds of various weight arrows.

once you determine the VM of the bow, how do you figure speeds for other weight arrows?

  Knowing virtual mass is a good way to identify where your losses are coming from. All bows loose efficiency with lighter arrows but the virtual mass will remain fairly constant if losses are not from hysteresis. The only thing that causes hysteresis in wood bows is set or high moisture.

[DC]

I feel that I'm missing something here. Is a higher VM better or worse? Could I use this number to compare two bows to see if one took more invisible set than the other?
Sorry for hijacking your thread Eric. That wasn't the intent.

PS Does the "Technical Archery" site do a decent job of explaining this
https://sites.google.com/site/technicalarchery/home

I read the "Virtual Mass" section and saw a glimmer of understanding.