Dry Fire Speed

[adb]

I understand the concept of DFS, but can someone explain how to determine it?

[bubbles]

I seem to remember reading something about shooting lighter and lighter arrows through an chrony. As you approach the DFS for your bow, you will stop getting gains in FPS by dropping arrow weight.   

[Badger]

    I think dry fire speed is just another word for efficiency. I have never shot anything under 100 grain from a bow but from what I have found lighter arrows keep getting faster. I don't see where the term has a useful application even though a few years ago we used this term quite a bit. Inefficient bows will show less gains as arrow weights go down, very efficient bows tend to show large gains. Kvillo recently posted a bow showing various speeds with various weight arrows. It gave a clear illustration of how lighter arrows will gain a lot of speed with small drops in arrow weight.


193 grain arrow: 299 fps
236 grain arrow: 252 fps
323 grain arrow: 216 fps
545 grain arrow: 174 fps

[Kviljo]

I think you are right, Steve. I never thought about it this way, but when I plotted a graph of the same numbers, and made a hypothetical line of what the dry loose speed of a slow bow would look like in theory, it does not seem to make much sense. It would be interesting to test a slower bow and see what the graph looks like. I think there would be a flattening out at some point, or that's what I have thought before. Perhaps there is not, and that lighter arrows beyond the DLS fool us with their low capacity to overcome the air resistance.

Perhaps we have been fooled by the fact that light arrows will not go far because they take with them too little energy.

In any case, with normal target bows there is nothing to gain in distance by using lighter arrows below a certain point.

[bubbles]

I think this is what I was remembering TBB4 pg 162:
1) Maximum dry fire arrow weight : the point where arrow velocity gains from reducing the arrows weight drop off to where they are no longer of benefit.
- I guess I just assumed that guys were testing out their bows to find this point.

[Badger]

  Bubbles when you put it in that context it makes sense. I don't see where dry fire speed would actually fit that though. I am kind of looking for that sweet spot right now but it seems to have more to do with the amount of energy in the arrow being able to overcme the drag in the arrow sufficiently to gain more distance. The more refined the arrow gets the lower the weight needed and the faster it will come out of the bow. Somehwere around 230 grains right now seems to give me my best distances but not consistently. I feel like with a very refined arrow I should be able to get that below 200 grains and get more benefit from added speed.

[JW_Halverson]

Badger, you remind me of that movie about the Aussie that worked his butt off building faster and faster motorcycles with plans to go to the Bonneville Salt Flats to race for a record.  In one scene he is placing yet another busted piston on a shelf where it had been crudely written, "Sacrifices to the God of Speed".

You got a shelf of those arrows? 

And a side note, in the history of flight shooting on the salt flats, are there any Robin Hooded arrows???

[Badger]

  JW, I am not aware of any Robin Hooded arrows but several years ago I placed a coke can on an arrow shft as a marker 225 yards down the line and on the very first shot put a broadhead right through it.

   I know the movie you are talking about " the fastes Indian" my all time favorite movie. You would laugh if I told you of all the paralells him and I have had in flight shooting and racing. In one scene he woke up and went outside to piss on his peach tree. I like to fell on the floor cause I do the same thing with my plum tree. In another scene he broke down going to the flight shoots and while he was getting his vehicle running he met a cool lady, same thing happened to me. Love that movie.

[adb]

Burt Munro was played by Anthony Hopkins in the movie "The Worlds Fastest Indian." Burt was not an Aussie, but a Kiwi. Just an aside, and yes, a great movie... one of my favs as well. And true.

Thanks for the replies gentlemen. Very helpful. I have been contemplating getting a chronograph, but it looks now like perhaps it's not necessary. Helpful, but not essential. Arrow velocity (as I suspected) is not the be-all and end-all of flight shooting.

[Badger]

  ADB, I think once you get up around 80# the speed realaly starts to count because you can make your arrows a bit heavier and stiffer. A 3 grain per pound at 80# is 240 grains, at 50# it would only be about 150 grains. We are limited to 1" X1/4" high fletches, at 250 grains you can overcome the drag from the fletches pretty well. Joseph for Hungary did extremly well at out shoot and I am guessing his arrows were around 350 grains or so, he was shooting them from 100# plus bows.

[Kviljo]

A chronograph is ok to have, but I really don't use mine that often. They are usually a bit frustrating to use, as getting readings is not always easy, and then there is the danger of putting an arrow through the whole thing...  Going out to a suitable field and loosing some arrows is a much better test of the whole setup.

Is there a restriction on the size of fletchings in some of the classes? 1x1/4"?