In this photo taken looking down the muzzle,  the starboard limb  (the one on the left) shows itself to be tracking a tad above the plane of the bolt groove.

Corrections can be made by inducing more twist into the top of the bundle which tends to drive the limb tip downwards.   I will tighten the top of the bundle by 7 1/2 degrees and loosen the bottom by 7  1/2 degrees.  This should keep the overall balance between the two bundles the same.  More tomorrow…..

……It’s now “tomorrow” and the adjustments mentioned above have been performed.

It’s kind of hard to see the difference between these two photos, I know.  However, the two shots I tried this morning appear to be flying true and with minimal wobble in the tail.  The velocity on “Little Walloper” climbed significantly after performing this limb adjustment.  No surprise there.  Leaving the  string should never be unduly traumatic.  Bolts are such sensitive little creatures.

Shot #16 was with a new bolt that weighed in at 6,651 grains.  It had a velocity of 287 feet per second and was packing  1216 foot pounds of energy.  Flight of bolt 95% of perfect.

Shot #17 was done with “little Walloper” at 8966 grains.  It had a velocity of 245 fps and generated 1,184 fpe.  Bolt flight 85% of perfect.  So much better.

Both of these shots were done with a mere 36″ of draw length and 3700 lbs of draw weight.  I intend to rack up a good deal more shots at this particular power setting as it does not appear unduly stressful on the either the machine or myself.  More later.

2 Responses to “The value of asymetry.”

  1. Randi Richert says:

    Excellent observation about the asymetry and solving it by trading top twist for bottom while keeping the same relative tension. For my part, I don’t know why everyone is so hung up about having the tip/string/axis of rotation on the same plane as the rail. Wilkins says that it is “Axiomatic” that the aperture is in the dead center of the springframe, but there is no such requirement in the texts. In Marsden’s drawings of the Gastraphetes it shows that the bowstring pitches upward to the slider, why not on ballistae as well? A little downwards presure takes some tension off the block/trigger and keeps the block and string firmly planted on the rails. If nothing else it would keep the string from wanting to override the bolt and might reduce the string’s side-to-side movement, thereby reducing tail wobble.

  2. Nick Watts says:

    At the moment Firefly has her string lifted off the rail by almost 3/8″ in the at rest position. There was no deliberate attempt to make it turn out like that. It is just that all of a sudden she started shooting those amazingly tight groups. Naturally I decided just to thank the CG’s and leave her the hell alone.

    It does seem that having the limbs track in matching planes is vital for accuracy.

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