Troubleshooting

Good afternoon all,

A few years ago I built a crossbow after stalking this forum and much research. I fired it a few times, but then moved several times and so was never able to fine-tune it. I came across it in my garage recently and would like to finish it properly. 

1. The first issue was the string's penchant to jump over the bolt. This I believe is caused by the string riding up the roller nut, most likely due to it being constructed of mild steel. I believe I tried to counteract this by giving the roller nut curved fingers and relieving weight by drilling holes in the nut. My thoughts at the time were that this would counter the rising string so it would release later.  The prod is Alchems 1732, I think it was a #225 lb draw. I never thought a Delrin nut would be strong enough for this application, am I wrong in that thought?
   
2. The second issue I remember was when the bolts I made did fire, the back end made a large circle all the way to the target (10-15yd) without stabilizing. Now the "fletching" was ductape I believe, with a very minor twist. Taking that into consideration, am I right in thinking that the bolts were too light? Or am I way off base?
   

Any advice is greatly appreciated! 

-Chad

Are you pushing the tail end of the bolt in between the fingers of the nut ?

I do not have the bolts anymore, but yes, the bots were .5" with the back end relieved to fit between the fingers of the nut.

Most of the crossbows I am familiar with have flat bolt tables or bolt grooves for the string to slide along. Is it possible the string is dropping below the bolt so the bolt is not being accelerated in a straight line? Or is the “slap” instantaneous and the string path is irrelevant? Some pellet bows had clip-on bolt tables, maybe you could fabricate something just to see if it makes a difference??

I built the tiller off of a pre-existing pattern that I believe I got from alchem when I had ordered the prod, and the string does not follow the dip (that I can tell). The string immediately jumped the bolt up on firing, even when making sure the bolt was solidly set against the string. It fired one-third of the time once removing the bolt retainer (I forget the correct term). In my mind, this means that without the retainer in place,  the butt of the bolt can ride up with the string allowing the occasional successful shot. With the retainer in place, the bolt is kept in place, so the string hops over. The retainer is not putting pressure on the butt of the bolt, it just keeps it from moving out of the trough.

Ok, seems to me Geezer once recommended replacing the bolt clip with a string snubber where the clip sits down between the fingers and is strong enough to prevent the string rising.  If your clip is strong enough Then it’s just a few minutes work to reshape it

Ah, that is an idea. I will have to look into that. I will have to see whether I made mine from steel or spring bronze.

Don’t forget it will need to move out of the way to allow for pulling the string over the nut.  I just used one screw to hold the clip which let it pivot

Nice looking build!

1. String rising and passing over bolt shaft could also be caused by having too acute prod mounting angle. At full draw the natural string line of travel could be above the bolt. A latch cover that prevents any string rise... or slightly concave nock face (half moon nock) could prevent this. I'd certainly include a half moon nock fired from a "trackless" crossbow of sorts like yours anyway.

2. Your bows trackless design doesn't lend itself to accurate string latching when hand drawing. Because there is no track to guide your fingers, there is an increased chance of string misalignment at the latch. This can cause spiraling, unstable bolt flight.
Also, longer bolts can be more stable.
And, duct tape is too soft and flexible for good bolt flight IMO.

I recently found a video of some test shots, and thankfully I did not go the duct tape route. The fletching looks to have been replacement fletching for modern compound bows I had probably pilfered from my father. As for the prod mounting, I used alchem's tiller pattern, and used a mill to cut the prod mount. I will have to check it for accuracy, I am pretty sure it was square and angled correctly, but Jet drill mills aren't exactly known to be the peak of reliability. I need to order a replacement string from alchem and source some heads as I can no longer forge my own. I would like to take this hunting at least once, are there any recommendations for heads meeting that need?

Sign into Crossbow Nation. A member call sign ‘Moon’ posted a thread on hunting medieval style. He is very active there and will be very interested in your build

So in some good news I found my old string as well as one bolt. After concaving the butt end slightly and making sure the string was centered when drawn it fired just fine, going through 3/4 ply by at least four inches. Until I shot high and it disappeared into the ether. Guess I still need to make more. The nut had a sharp spot, which damaged the serving, so I still need to acquire a new string as well.

I have another question for you all. I took the crossbow and my recurve out to shoot some yesterday afternoon. The center serving was cut due to a rough edge on the roller nut. After cleaning the rough spot , I but some electrical tape over the serving until I can re-serve it or aquire a new string. After a few additional shots, this left marks on the tiller a good 1.75 inches past where the string sits at rest shown In the picture. Is this expected? Or, does this mean there is excess energy left over, damaging the prod?

Also, it seems to be shooting quite high, I am assuming this could be due to the string seemingly resting lower than the track at the front end of the bow. If I lower this area, maybe installing a bone rest at the front, could that lower the shots?

Find a slow motion video of a bow being fired, they don’t stop as such, they oscillate, but the limbs go quite a away forward of the final rest point.  This can be a problem for wooden bows where the tips go past the “handle” and over stretch the belly, taking it from compressed, which it is designed to cope with, to tension, which it is not designed to cope with.

Crossbows do throw their bolts on a rising trajectory, if it is shooting consistently then it’s more a question of changing your aim point than redesigning the bow.

Stretchy string could do that, I'd say half of what you have would be normal. I only use hemp so I wouldn't really know how synthetic behaves at common prod poundages. If there is excess energy left I believe the stretch would actually relieve the stress (and there is always excess energy unless you shoot massive bolts), while less stretch increases efficiency. A lot of stretch is very uncomfortable on hand bows as it makes for a brutal wrist slap and the issue is worse the longer your bow gets because there is more string to stretch while brace height remains around constant. Increase the brace height and you shorten the power stroke. Now your longbow doesn't shoot as far; the hundred years war mythos kicks in and you rather swallow your tears for those extra few yards. Longbow archers will never admit to this.
The stretch can be reduced either by using less stretchy materials or by bulking up the string (or both). The latter is unacceptable to most archers because of increased string mass and their arrow nocks wouldn't fit anymore. But on crossbows I'd say it's pretty historical. To be honest it's historical for hand bows as well but most people don't have the nerve to go full bulbous nock which was common until the late middle ages in Europe and beyond in other places.

First time I've seen a "discontinuous" bolt deck with the hollow between the prod and the nut.  The fact that your design "forces' the string to change direction and flip up may be causing some of this issue.  As mentioned, strings don't STOP -- they whip back and forth at the end of the stroke before settling to rest.

I built this following the design blueprints offered on the Alchem website here. I have been over the designs today, and I built it to exact dimensions as far as the location and angle for the prod and deck. The idea with the discontinuous deck, I suppose, was to minimize friction. The string is resting below the front deck area, though, so I thought removing the front portion of the deck and installing a bone or Delrin insert up front as the only point of contact would help. I have been debating remaking the tiller entirely; we shall see, I do not have much free time nowadays. 

The string was also from alchem, consisting of B50, The specs of the prod state that it should have a brace of 3.5 inches, but it is resting at 2.75", and the string is .75" longer than it was originally. If it is supposed to have an 8" draw, it currently has 8.75", which accounts for why it rests in the valley between the front and back deck.

Thanks for the insight guys, I appreciate being able to pick the brains of those with all of the experience and knowledge. I enjoy experimenting, learning, and creating, so this is great. I had completely forgotten about many of the details on how these should function and why. 

On another note, is there a rule of thumb for guestimating the proper bolt weights for the draw weight of prod? This prod is estimated to be around 225 lbs; the bolt that I currently have is about 550 grains. I managed to find a thread here on the forum where Geezer mentioned that bolt weight should be about 1/60th of the actual prod weight. Assuming my math is correct, this comes out to 228.6 grain, based upon a 1.96lb prod. This just seems, to my inexperienced mind, really rather light.

That does seem light. I asked this question on Crossbow Nation and the answer was play around with bolts of different sizes and weights until you find something that suits your style/bow/purposes.  Bolts for target are different from 3-d which are different from hunting

Cscott wrote: After a few additional shots, this left marks on the tiller a good 1.75 inches past where the string sits at rest shown In the picture. Is this expected?

Yes. The string has inertia.
The faster and heavier the string, the further it slaps past the resting position. This is not a problem, it helps to brake the bow limbs and absorb left over energy.

Cscott wrote:
If I lower this area, maybe installing a bone rest at the front, could that lower the shots?

The prod seems to be more vertical than it needs to be.
Tilting the bow forward a bit would make it shoot cleaner and reduce friction.

Bolt weight is more about overall energy stored in the prod and limb mass than about final poundage. Some composite crossbowmen shoot 2.5-4 grains per pound, while steel crossbowman might shoot as light as 1-1.5 grain per pound to get decent performance. I'd say shoot as heavy as you can while still achieving 150 yards distance.

A general shooting tip: when experimentally shooting for distance it is much more trouble-free to shoot several bolts of the same weight at the same elevation because later you have to spot only one and the others will be nearby. They have to be grouped within 10-15 yards at most, often much less. On the other hand, trying to spot a single bolt in a large plain that was shot to an unknown distance can be very frustrating. Especially if it hit a hard spot and is lying flat on the ground.

hullutiedemies wrote:The prod seems to be more vertical than it needs to be.
Tilting the bow forward a bit would make it shoot cleaner and reduce friction.

The prod is angled 5-8°

Hey there! I haven't checked on this forum in a while, but this thread sparked my interest - mainly because it sounds so familiar. My first build was based on the Alchem plans and the products, using the same prod and irons. That tickler looks great, by the way. Is that the Alchem product that you forged and twisted?
Anyhow, my first build had issues that helped me to learn a lot. A couple observations:

I definitely suspect that steel roller nut is the primary suspect in your misfires. Even after lightening it, it's still just too heavy, too slow to move out of the way of where the string wants to go quick enough so the string finds the path of least resistance, jumping up and over. Yes, Delrin will work with that prod, I've done it successfully with two bows and no issues. You do want to make sure you're using real Delrin, that is Acetal Monopolymer, and not the more common Acetal Copolymer.

Regarding the Alchem tiller plan, I did not take that curve in the deck area to be a contour line, but a rebate line, an area you would carve away at an angle, leaving the bolt deck straight but thinner and in that way reducing drag. I think you'd be much better off with a flat bolt deck. I wouldn't at all like my string smacking into that bump at the front.

A little bit of friction isn't a bad thing, remember you want as much energy from the release of that big blade spring you're playing with a few inches from your face to be going into the projectile you're forcing downrange as possible, and not damaging the components of your contraption. When in doubt, go heavy with bolts until you've worked out all the kinks in your build.
Anyway, best of luck!
Gnome

Gnome wrote:

Regarding the Alchem tiller plan, I did not take that curve in the deck area to be a contour line, but a rebate line, an area you would carve away at an angle, leaving the bolt deck straight but thinner 
Gnome

Yes of course. Reckon you're on the money there Gnome.