Chrono Test - March 17th

I had to make a special string for limb set #67.  #67 is very stout and has more recurve than any others I've tried up to this point. Braced, the string rang at the highest pitch I've ever heard on a strung bow. It sounded absolutely wicked, but felt like something had to give under all that pressure. 

I considered unstringing the bow and reworking the tip overlays. They seemed like to be weak link in the chain, but decided to try a supposedly "gentle" shot. I set the trigger to only 11 inches of draw, 6 inches short of full draw, drew back and let it loose.  The limb exploded on the shot with pieces of the limb core ricochetting about the shop like shrapnel. The right limb tip was completely sheared through, rendering the limb unrepairable, and leaving me feeling sick. 

My workbench is now littered with busted up bow limbs and I can't see accomplishing much without making some changes.  After a string of setbacks, it is time to stop, take a step back, and come up with a solution that will not only perform, but will be durable enough to withstand the punishment of repeated 600+ foot per second shots.

Alan

Chrono Test - March 15

I tested two more limb pairs with disappointing outcomes for both. First up was set #73. This set does not have the reverse taper in the outer limb that most of the other limb sets have, so the thickness continues to decrease around the recurves, toward the tips.  The outer limbs for this limb set is left wider than those limb sets with the reverse taper otherwise, the tiller would end up very "whip ended".  

Drawn to 11" (6" short of full draw), I shot a heavy and light arrow at 387 fps and 438 fps, respectively, but there was substantial vibration and noise compared the prior set tested.  Dynamic efficiency for the 387 fps shot was 57%, and only 45% for the 438fps shot, which is very poor.  75% or better is expected at these low speeds. As a general rule, I do not shoot below 50% dynamic efficiency, therefore these limbs are far from contention.  Inspection revealed signs of compression around the nock groove so I aborted any further shooting at longer draws. 

Next up, set #61. I had high expectations for this set. The thickness profile controlling the bend is closer to ideal and the outer limb is stiff and narrow. It looked very good strung up. I lightly plucked the string a couple of times. The high pitch revealed good string tension at brace. Then all of a sudden, there was a loud popping sound and large piece of the left outer limb landed a sharp blow to my belly before coming to rest on the floor.  What a disappointment. The test ended without a single speed measurement. 

You can see in the picture that the glue bond at the fades between the belly-side glass and core failed since it pulled away clean in that area. Oh well. I have to remind myself again that it is better for this to happen at home instead of at the salt flats. 

I am sure it is salvageable. It will just add to the growing pile of footbow equipment needing repair. 

Alan

Chrono Test - March 10

After the exploding arrow incident, I dialed back the draw length to 16", loaded a high modulus arrow, and let it fly through the chronograph.

 

As you can see in the video, it didn't go without incident. The anchor holding the bow to the shooting stand broke free and the bow nearly rolled off the stand when it released. The arrow remained intact, punching a perfect hole through the paper. Also, notice that the tip overlay sheared off the left limb tip overlay and went sailing dead center through the paper!

The chrono registered 609 fps, which is not bad, but not good enough either.  Given how straight the arrow left the bow, this arrow may have landed 1600-1800 yards away at Bonneville.  I have no doubt that some speed was lost when the limb tip sheared free, but I will need a system capable of reliably delivering 700+ fps before I feel confident about the chances of exceeding Drake's Unlimited Foodbow record of 2028 yards.

This is getting a little fun.  I think I'll see what some of the other limb sets can do...

Alan

Chrono test March 2nd

I had a little excitement the night of March 3rd, chrono testing the Footbow in the garage.  I think I am finally starting to learn it isn't a good idea to chrono test a bow in the garage no matter how controlled I may think the conditions are. 

I wanted to see if I could get consistent speeds above 700 fps with a 13" long 125 grain arrow made from a standard grade carbon. Unfortunately, the arrow seemed to have buckled from the acceleration and exploded, sending shards in a wide shotgun pattern out the front of the bow. The metal nock mangled the metal frame that holds the paper indicator and still had enough energy down range to put a deep gouge in the thick aluminum light-kit frame on my chrono and penetrate deep into the thick aluminum guide on my table saw!  I still haven't found the arrow point. One of the razor blade fletches put a perfectly clean slot through a metal door panel. 

The good news is that the foot protector worked and the bow survived. 

This picture illustrates what the arrow looked like before and after. I am still finding arrow fragments days later. 

Note the bottom of the paper tune frame is bent. The clean hole toward the lower center is from the arrow point, the more jagged holes are from the disintegrating arrow shaft. 

Hole in the aluminum guide of the table saw left by the tiny metal nock.