[Ansteorra-archery] corkscrews...

Sun Dec 14 21:21:49 PST 2003

We will assume that the shafts are straight  and the table of the crossbow is flat and smooth, and that the string travel along the entire power stroke is smooth and even (not the string touches the table when cocked and is half-inch above the table when at rest or vice-versa).

We will also assume that a good fletching job was done and the fletches regardless of how may are there are straight, symmetrical, evenly spaced, and don't drag the table of the crossbow excessively.  If you have a tight helical or big offset fletch or if the bolt channel is too deep then the front of the fletch can drag on the table on one side and the rear on the other and that can (not always) throw things off.

We will also assume that if you are using a Jo-Jan jig to fletch them then hopefully you are using a  F-51, F-61, or F-71 nock receiver setup for 4-fletch and turning 2-clicks to set each fletch so that they are 180 degrees apart.  A 3-fletch receiver setup only using 2 fletches would not really be good.

All the above should be pretty easy to inspect for.

Now, define what ya mean by "corkscrew".  

Hopefully they aren't flipping end for end vertically (what most people call cartwheeling) and hopefully they aren't flipping end for end horizontally (what most people call helicoptering).  If they are doing that then they are more than likely WAY off balanced and too heavy on one end.

I say WAY off balance because weight balance is not nearly as important on crossbow bolts as it is on arrows.  Arrows gain lift by "point planing" and for this reason  the balance point on an arrow should be about 9% front of center.  The weight balance on a crossbow bolt is not nearly that important and it can be fore of center or aft (or dead center) as long as it is not WAY off to one end.

If the bolt ends bouncing up and down (but not flipping end over end) then that is usually called porpoising.  If the end of the bolts are wagging side to side then that's typically called fishtailing.  For handbow archers popoising is usually a nocking point that is too high or low and fishtailing is usually a bad release.  For crossbows these can be bad bolt balance, bad fletching, a bad release off the bow, a bow that is pulling unevenly, and a bunch of other things.  

A cork screw where the tail of the arrow is moving in circles is often a bad fletching job or otherwise a combination of a several factors (something making it fishtail and something making is porpoise).  And there is something else it can be, read on...

I said earlier that balance isn't as critical on crossbow bolts, and neither is spine.  Since crossbows are "center shot" and there is no archers paradox where spine is critical.  It's a good thing that spine isn't a real issue for crossbows since shortening a shaft by an inch increases the effective spine by about 3 pounds.  So ya cut 10 inches off a shaft and ya add an effective 30# of spine to it.  Additionally since the bolts is supported by the table of the crossbow and cradles in a groove that adds stiffness.   So if spine where critical then a lot of crossbow shafts would be way over spined.

So if weight balance isn't critical and spine isn't critical then what is?

DRAG!  and the relationship of the drag to the weight balance.

The center of the drag should be slightly behind the center of the balance.  

Without good modeling software taking into account specifics of all your materials it is very hard to determine precisely where the center of drag is.  However you can easily adjust the center of drag and you can adjust the center of balance so that the bolt will fly right.

Since the weight balance is usually forward of center (particularly on crossbow bolts) usually the problem with crossbow bolts is that the center of balance is far forward (from the weight of the point) and the center of drag is far to the rear (from the drag of the fletches).  If those problems get too excessive then the two ends of the bolts are fighting each other for control of the bolt while it is in flight.

Obviously heavier points will move the balance farther forward so try to get the lightest points that you can get to allow the center of balance to come as far back as ya can to center or just forward of center. 

More fletch will move the drag further back.  When I say "more" I mean physically larger fletches or a larger number of fletches (3 instead of 2).  Helical fletching or offset fletching also adds more drag.  Use smaller fletches to move the center of drag forward.  2" is usually the smallest feather fletches you can find but you can always cut your own.  Straight fletching instead of helical or offset will also allow the center of drag to move forward.

You wanna make all these factors come together and make a bolt whose point of balance and center of drag are in harmony and working together for a smooth flight.  Play with the diameter and weight of your shafts, the weight of your points, and the size, number, and angle of fletch.

If you put your points on with a holt melt glue then it will be easy to play with point weight and see if that changes things.  If you originally used a heavy point then take one off and try successively lighter ones and see if it appears to get better or worse.  If you originally used a lighter point then try successively  heavier ones and see if it gets better or worse.

If ya cant fix it with points then you'll need to try refletching.  4" fletches have likely pushed your center of drag way back.  Take the same bolt you have been playing with the points and refletch it using smaller fletches and with a straight fletch and see what happens.  You can take some of your 4" parabolic fletches and VERY easily with a sharp pair of embroidery scissors cut then down and make smaller fletches of the "traditional" shape.

To fix center of drag and balance problems some people will simply use longer shafts.  They learn that using the same points and fletches that they can just use approx X weight shafts cut to X length.  Longer shafts will have a tendency shift the balance point backwards and the center of drag forwards.  While sometimes that is easiest way to correct things, it isn't necessarily the best.  Longer shafts are more likely to break, more likely to bend and not stay straight, and are of course heavier and add to overall drag so that you are going to lose cast as compared to the shorter, lighter, well balanced bolts.  Barrel tapered bolts (very period) will move the balance towards the center of the bolt and decrease drag both while in flight and friction/drag along the face of the crossbow during launch.  Of course I don't know anywhere you can buy barrel tapered crossbows bolt length shafts so you would have to taper your own.

Good luck.  Tinker with your bolts and HAVE FUN.  My experience is that it is harder to get bolts right than it is arrows.  When you get them right if you have taken your time to figure out exactly what was wrong and why they are now right then you will have learned a lot.

Regards,

HL Eadric Anstapa
Ansteorra
eadric at scabrewer.com

WHEN YA TYPE IN ALL CAPS IT SIGNIFIES SHOUTING
  ----- Original Message ----- 
  From: LD.BLACKMOON 
  To: Archery within the Kingdom of Ansteorra 
  Sent: Sunday, December 14, 2003 7:21 PM
  Subject: [Ansteorra-archery] corkscrews...

  greetings

  ok, this is probably going to be a stupid question ...
  but , i just made my first 1/2 dozen crossbow bolts , from old 11 / 32 arrows , using 4" parabolic fletching . 
  and every last one of them has a severe corkscrew every time i fire it ...
  so ...
  a: what did i do wrong ???
  b: how do i fix it ???
  c: what causes it ???

  oh, i'm using a jojan jig, and 2 fletching them .
  thanx
  ld.blackmoon

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