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. 

New Flight Arrows

There is much more to flight shooting than simply driving out to a dry lake bed, drawing back a heavy bow, and magically breaking a new record. Shooting a bow can't happen without arrows and it can't be done with the kind of arrows produced commercially.  Even the tiniest available target or hunting arrows are too large and heavy with too much aerodynamic drag to be competitive for flight shooting.

Due to the trackless design of my bow, my new arrows will need to be a minimum of 14" in length through 17" long. I'm using 4mm diameter high modulus carbon fiber solid rod for the arrow shafts and stainless steel for the points and nocks. Based on past experiences, I have found that the highly alkali salt at Bonneville eats aluminum and attacks the zinc in brass in little time, so I made the switch to stainless steel.

My machining capabilities are pretty limited, but I will try to make up for that with a little extra patience. I had planned to build as many as 36 new foot bow arrows before the US Nationals in August, but quality trumps quantity in this area. I'd be happier with three great arrows than have a hundred pretty good ones. 

Here's some pictures of the process:

Marking Carbon Rod for Cutting

Cutting the Carbon Rod Lengths Using a Dremel Cutoff Blade

Tapering the Ends of the Rods to Receive the Points and Nocks

Making Nocks - Drilling 1/8" Dia Stainless Tube

Roughed Out Nocks

Drilling an Inverse Cone Into 1/8" Solid Rod to Form Points

Simple Jig to Guide and Center the Bit to Drill the End of the Stainless Rod

Showing Drilled Point to Match the Conical Carbon Shaft End

A bunch of Arrow Shafts and Nocks Ready to be Glued in place

Long draw, short arrow

It is easy to concentrate on arrow speed  but put off figuring out how to get a consistent clean shot.  It is hard to do and takes time. The ideal projectile is short, stiff, and not forgiving of errors.  The ideal draw length is much longer than the arrow, so it is no surprise that a little misalignment would send an arrow out of the bow tumbling end over end.

George Alavekiu struggled with getting clean arrow flight and wrote about it often in his letters. In one of the letters is a statement that these bows could shoot over 2500 yards if the arrow would just come out clean. In these letters is lots of interesting correspondence with target crossbow shooter Gil Frey.  Gil recommended a trackless design with a D-loop on the string to help solve the issue.  For whatever reason, George did not follow through with this advice and kept with the standard crossbow arrow tracks and release claw.  I believe it was driven by his need to shoot very short arrows made of steel.  George tried putting arrows under the arrow tracks in an effort to keep them from jumping out but these approaches only resulted in modest improvements.  A few years back, I decided against arrow tracks when I tried test shooting George's old bows and found it was probably only an occasional lucky shot that gave him his best distances.  I am sure the majority of the shots resulted in sideways arrows and relatively poor distances.  I feel the same is true for Harry Drake's foot bow record.

I don't live anywhere near a place where I can actually flight shoot, so the next best thing is to test shoot into a soft target to prevent damage to the arrows.  I use a form of paper tuning because I can't actually see the arrow leave the bow.  The piece of paper sits out on a long stabilizer-like support and I analyze the size and shape of the holes left by the arrow to determine if the arrow is leaving the bow perfectly straight, or if it is point up or down, or side to side.  If the bow was shooting perfect, I would expect to see a single circular hole left in the paper with each successive arrow going through the same hole.  

For the test arrows, I am using very short steel and composite arrows that are as little as 7-1/4 inches long.  The shorter the arrow and longer the draw, the less forgiving the bow will be of errors in the tuning and set-up.   I figure if I can get consistent flight with these short arrows, then it should be much easier to get clean flight with the 14-16 inch long arrows that I plan to use at the Salt Flats this August.  To complicate the issue even further, my test arrows vary in geometry and weight.  I rarely have build my flight arrows completely uniform.  Some are heavier than others.  Some are a little stiffer than others.  The competition limits the amount of arrows we can shoot, and there is not a lot of time to figure out which arrow design is the perfect match for the bow that I will be shooting.

The test bow limbs that I am using are much lower in draw weight than the bow limbs that I will be using in August.  I am often test shooting this contraption in my house, and I want to limit the possible damage to the bow, arrow and living room if something goes wrong at over 700 feet per second!  The test bow limbs limit the arrow speeds between 300 and 400 feet per second, which is much more manageable.  So far, I have several hundred test shots on a single set of bow limbs without any sign of failure. The arrow rest and release are both mounted to a sliding "carriage" so that I can draw the short arrow back farther than the length of the arrow.  This also reduces the chances that the arrow pops off the rest or off the string as the bow is drawn.  My goal is to be able to achieve consistent and clean flight with an arrow drawn nearly twice as far as the arrow is long!

Beginning a couple months ago, I made this bow test into a sort of a game.  I set it up and try to achieve perfect holes in the paper with a random set of arrows and draw lengths.  At first, I got pretty crazy results.  I even put a couple holes in the bow riser (see below) and this wasn't even close to using a draw length double the arrow length. 

Chaotic paper tears indicating a tuning issue

Bow Shown with arrow being drawn back

I was also getting frustrated with the durability of my arrow rests (see photo below).  They were made from thick mylar and would start to come apart after only a couple of shots. I switched to a much thinner mylar rest that is used for overhead projector transparencies and doubled them up if I needed more support.

I was able to progress to the point where I could achieve several clean holes, but this would be followed by an occasional wild flying arrow.  It took awhile, but I realized that the very short D-loop I was using was probably the main contributor.  I assume that the release loop would snag itself on the arrow nock and cause all sorts of issues.  Some of the test arrows even shows evidence of this when I would notice one ear of the nock would be bent after an erratic shot.  When I replaced the short D-loop with a longer one, the issue went away.

Mylar Arrow Rest Damage After Two Shots

Much better, three different arrows, three different draw lengths, all pretty clean holes in the paper.

Another view of the test setup with arrow nocked.

I started using post-it notes to minimize changing the paper.

I am applying a little lip-stick to the arrow point which leaves a mark in the paper so I can tell which part of the paper tear is from the arrow point, and which part of the tear is from the nock end.

Comparison of arrow length to Draw Length.

I am now able to consistently draw one of these seven inch long arrows 13 or more inches without issue.  This is a big deal given my bow does not make use of crossbow-like arrow tracks.  I'm getting more excited than ever to see what this bow will do in August!

Limb Set #28 Repair Complete

I'll continue with the status of the repairs to limb set #28. So far, the limbs had been reshaped, reglued, and the new limb tip overlays are in place. All that remains is to string them up to check the tiller and rebalance if necessary.  

Last year, I used 10-strands of Brownell Rhino for the string and I never had a failure.  10 strands plus a thin serving left little material in the 1/8" diameter arrow nocks. The new strings I made consist of only 8 strands of Rhino which I hope should hold up for at least a half dozen shots.

In the 1970's, George Alavekiu and Harry Drake were lucky if their Kevlar strings held up for two shots before blowing apart. The new more durable string materials save considerable time and add to the longevity of the limbs. 

I was a little nervous stringing and bracing the freshly repaired bow limbs.  I expected to hear a crack as the jack screws swung the limbs forward, but the only sound came from the string as the tension climbed higher and higher. I rejoiced as it reached the intended brace height. A few quick measurements revealed that the limbs were bending evenly and in perfect unison. 

Next, I carefully began exercising the limbs by drawing them back, letting down, then drawing a little farther.  I continued this process until I achieved full draw. I didn't find even a hint of uneven balance to the bend in the limbs or unusual noise.  The job is complete, successful, and I look forward to shooting these limbs again.  They may have lost as much as two pounds of draw weight, but this is less than a 1% change. In addition, they lost about a half ounce of outer limb mass during the process, which should make them easier on the strings and maybe a little quicker. 

I won't know for sure how the durability is affected until I shoot them.  At the least, the bow limbs are capable of one more shot than they would have had without the repair.