Decided to take the scenic route.

This next experiment will involve three, successively shorter, limb lengths. It will be a lot of fiddlin’ around. …..But, no use grumping. The Gods are never appeased by half measures.

What’s a minion supposed to do, anyway? I don’t want my catapult license revoked for failure to turn over all the stones in the garden. … Besides, every ponderous tome needs its fuel.

We start off with a limb length of 26″, and then move on to the 23″ and 20″ lengths. Limb length is measured from the center of the bundle to the center of the bowstring nock on the limb tip. (For our purposes, this is a new way of defining limb length. Previous limb lengths claimed in this blog were meant to reflect manufacturing, and are not comparable to these which speak more to leverage.) I will announce with a new posting when the limb length changes. Limb weight on the Mk IX’s is 8 lbs each.

…And so, off we go with those string-bean hooligans, the 26 inchers.

Shot #1
I decided to try the first shot with the bundles in a state of fair to middlin’ relaxitude. That is, the nylon in the springs is soft and plumpish when probed with a stout finger. About 1″ of lift for the limb away from its stanchion pocket, (when attacked by stern shoulder of fat old white guy). Velocity was, quite naturally, very slow. 150 feet per second with 377 gram bolt. I’m hoping this is actually very, very good. Those bundles are very, very slack. And my shoulder has turned touchingly weak.

Shot #2
Advanced all washers by 15 degrees (or 2 of the minimum rotational units in our vernier based locking system). The limb can still be manually lifted about 1/2″. The bundles appear firmer, but are obviously still mucho slack. The shot flew straight and true above the bolt groove. Nice balance. Velocity: 240 fps.

At this point in the testing I take careful measurement of the air-gap between the inside of the curved stanchion and the outside forward edge of the limb iron. Like so:

The port side limb shows a gap of .213, whie the starboard side is .330. All of the locking pegs occupy the same rotational position in the washer holes. That is, in all four stations, the peg positons match. It is decided to make the next advancement at the MRU (minimum rotational unit) of 7 1/2 degrees, for all locations except the lower washer on the starboard side. That we rotate 15 degrees and new measurements with the pin guages show us a gap of .222″ on the starboard side and .321 on the port side. How this assymetry in the gap moves back and forth as we tune the machine will signal the state of balance in the machine between the two power plants.

Shot # 3

No reading on chrono.

Shot # 4

With the washer rotation as mentioned last, shot # 4 shows a velocity of 285 250 fps. draw weight (3500 lbs -correction-) 3100 lbs, draw length 38″. Bundles are still mushy, 3/8″ of limb lift is still visible.

Shot # 5

The air gaps are measured at .260 starboard, .280 port. This only means that the resting point of the limbs relative to the surrounding steel framework (i.e. field frames & kamarion), has found this much clearance a happy and repeatable phenomenon. …… Yeah! team.

There is another thing going on, though. At 38″ of draw length, the starboard limb was observed to ride maybe 10 degrees above the plane of the flight deck. Ideally it should ride a couple of degrees below it. Damm! the dihedral! The fix is to induce more twist into the half of the bundle that rides above the plane of the limb. With an upward planing limb, the constriction that comes from an increase of torsion in the upper half of the bundle can be used to alter that dihedral into anhedral. Upward inclining into downward inclining.
Currently the starboard bottom washer, has an extra helping of MRU. Corrections as follows: SB zero degrees, ST +7 1/2 degrees, PB +7 1/2″, PT +7 1/2″. For ’ems wot ‘aint seen it, here’s the clobber we use to induce torsion into the bundles.

And we shoot….. Velocity- 268 fps. Draw weight 3500 lbs. Draw length 38″. Bolt weight 377 grams.

In a well tuned torsion spring, all the fibres bear a near equal share of the burden. I have found that as they age, rope torsion springs improve with use. That is to say, they become more internally balanced, as the tight ropes tend to loosen, and the loose ropes tend to tighten. This was true of the manila line we rigged our old Gallwey with, and also all the nylon 3-strand we have used over the years. Firefly is currently rigged with the Mk VI nylon springs, and they have been maturing nicely these last 150 shots. Movements in the stitching that hold the ropes in position indicate that there is a process of slipage over the cross-bars. These rotational movements of the ropes around the cross-bars tend to drive things towards a state of equal strain through out the spring.

A couple of days ago I removed the chaffing gear and could see the nylon spring in good detail. Happy to report, Zero damage from our grand nemesis, the dreaded Chaffing Trolls. Any bare metal rubbing on the spring is all the invitation they need. Eventually they can nibble into the ropes a fair way if the protective gear isn’t in order. Hungry little bastards, that they are.

…But, I digress.

Shot # 6

I took a check of the tautness of the bowstring by using my old subjective standard. One maxed-out Nick power. It’s a bit like a calibated torque wrench, except the data is measured by feel and compared by memory.

I usually record this measurement, not with a photo, but by remembering the apparent level of deflection that appears in the string right about the time a familiar stab of elbow pain occurs. The pain indicates we have arrived at maximum effort for this procedure.
…. Maybe a quick snapshot isn’t such a bad idea afterall. It is the amount of deflection in the string that we are trying to gauge. In any event, in past shooting I have noticed a perfect correlation between the tautness of the bowstring, and a marked improvement in the velocity of the bolt.
The greater leverage of the increased limb length of the Mk IX’s (3″ more than the MK VIII’s) produces less tension in the bowstring. (At least for any given level of strain in the spring.) It will be interesting to see if the aforementioned correlation continues to hold as the limb length, and presumedly string tautness, changes.

While tomes are usually tombs for all interred therein, there is some comfort in an experiment just begun. The illusion of animation it brings can propel us forward when all around sits becalmed in seas of bleak indifference. Cue the tiny violins.

….We proceed, with our tome.

All four washers are advanced 1 MRU, or as it is otherwise known, 7 1/2 degrees.

Velocity 263 fps. Draw weight 4000 lbs. Bolt, as before.

So why did the velocity go down to 263 fps after more tension was introduced into the bundles? I have seen this before. At this early stage of pre-loading the spring, the fibres collectively cycle between giving way, and standing strong. Twist ’em with enough preload — eventually they will all get the message and stand in more equal measure to one another as they bear the load.

That being said, I sense there is more velocity waiting for us as we shorten the limb. (There had better be, because 263 fps is appallingly slow for a 4,000 lb draw weight.) I will try a few more shots at this setting, and then prepare to shorten the limb by installing a new nock placement on the limb. Probably I should make some more bolts before going any further.

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As promised here are a few more shots with this long limb length, 4,000 lb draw weight, 38″ draw length, 377 gram bolt. The additional nock whippings have been added, and from here on in there will be no attempt to improve velocity by increasing washer rotation. This whole experiment is only about assessing the relative effects of limb length. We will get to hot-rodding after that has been figured.

Shot # 7 260 fps

Shot # 8 261 fps

Shot # 9 259 fps

Shot # 10 259 fps

Shot # 11 263 fps

Shot #12 261 fps

Okay, so with the last seven shots it looks like we have a nice stable baseline of around 260 feet per second. I would like to call attention to one obvious defecit with these long limbs, it will never be possible to go much beyond the 38″ draw length because the string angle becomes too acute to allow the use of finned projectiles. The interference issues inevitably leads to erratic bolt flight, what I have always called “waggle tails”. The following photo of the string angle gives some indication of how draw length becomes limited by these lanky 26″ limbs.

It is now time to wind up this stage of long limb testing and move on to the medium length.

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Well, okay. Maybe not just yet, time to wind it up. It is now a different day than the above. (And yes, I know: why didn’t I date those entries? ….. no excuse. Just gitin’ slow)

I was getting ready to switch the string down to the next station for the medium length foo-fer-rah, and discovered a distinct lack of tension in the at-rest bowstring. A shot was made. Velocity had dropped to a piddling 217 fps. Also the starboard limb had risen, dihedral fashion, a good ten degrees above the deck. I applied 7 1/2 degrees rotation to the top half of the starboard bundle. The constriction thus created tends to push upwardly incling limbs, downward towards the flight deck.

Another shot was made (# 13 by this count). Velocity was 277 fps. The limb rise was down to perhaps +2 degrees. I smacked downwards on the at-rest limb tip with a heavily weighted nylon mallet. Probably about 20 good whacks.

And then came, Shot # 14. Velocity was 280 fps. Starboard limb rise nominal @ -2 degrees. The mallet work had successfully reseated the limb in the bind of the spring.

We wipe our brow and shoot on.

Shot # 15. 277 fps.

Shot # 16. 276 fps.

Shot # 17. 277 fps.

Shot # 18. 277 fps.

That leaves a fella fair tuckered out. Final baseline for long limbs established @ 277 fps. Medium length testing imminent. Nap now.

2 Responses to “Mk IX long limb testing underway.”


  1. Randi Richert says:

    Nick,
    Nice to see Firefly back in action. As usual, I am following what you are doing and hopefully learning a thing or two in the process. I’ve been off messing around with gunpowder era stuff. Your posts are a reminder to get back to work on the ancient torsion stuff where twisted relics like us belong.


  2. Nick says:

    “twisted relics” ? Champions of the past ….

    Hmm, me likey! There’s got to be a verse or two about that one.

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