Prod Efficiency

Hey all,

After a few weeks of browsing the forum learning everything I can from all of the knowledgeable folks here and drooling over everyone's incredible projects, I thought it was the right time to register and introduce myself. I also have a question that I have not yet found an answer to, and was wondering if anyone here could point me in the right direction.

I recently built a very basic Central European-style bow using a cheap fiberglass "replacement" prod from a manufacturer who sells cheap commercial bows. It was advertised at 150#, but I estimate its actual draw weight at about 120-130# at about eleven inches draw from about three inches brace. Now, having been inspired by the projects on display here, I'm looking to build a bow with a steel prod that will look/feel a bit more authentic. I'm trying to find a steel prod with similar performance to the fiberglass one from my previous bow, and am trying to figure out what draw weight will be required. I've heard that steel prods are less efficient than fiberglass ones because of their weight, etc., but I'd like to get a better idea of just what this means. Does anyone have any advice or educated guesses on approximate differences in efficiency? I'm thinking about putting in an order at Alchem soon so that the prod will be ready by the time I am able to begin work in late August.

Thanks, everyone, and it's been a pleasure learning from you so far!

Wilhelm,

I don't think it will be possible to make a steel prod which will replace your fiberglass one without making other design concessions. Fiberglas has higher allowable values for elongation and compression then steel. If you made a steel prod to the same dimensions as the fiberglass one, it could not be drawn as far.

Fiberglass behaves more like horn and sinew composites. It can start from a reflex, and be drawn a great distance. Steel bows are typically deflexed, and are are drawn only a short distance. They make up for short draw lengths with high draw weights.

I have a "feel" for these things, but do not have the engineering and math to demonstrate them. Perhaps someone else can put some numbers on this.

Mac

Welcome to the forum Wilhelm.
Sounds like you have fun project ahead of you.
As for your question: I am not sure of the difference in effency between the two styles of prod in question but I think if you could go with a steel prod in the 150 lb range than it is likely to give similiar results. I have a feeling that the cheap compsite prods are not the paragons of effency.

But Mac does bring up a good point about about draw lengths. You may have already checked this out but I was looking at the recommend draw lengths for the Alchem prods and they have two classes. One was a 8 in power stroke with 3.5 in brace height (11.5 inches total draw length) and the other was 10.25 in power stroke with a 4.5 brace height (14.75 inches total draw length) The group with longer power stroke were less powerful (as far as I could tell the Alchem website isn't the easiest to understand). That may help with what you want to do.

It seems like you could design a steel bow to have a longer draw length if you just made it a bit longer and narrower than the heavier short draw bows. It's just a matter of the right dimensions...right?

Hey all,

Thank you for the very helpful responses! My plan so far is to redesign the new bow from the ground up with a the proper power stroke for whichever steel prod I end up using. At the advice of others above, I've looked at the different power strokes for prods from Alchem and have taken a look at the dimensions of the products available there in a way that can help me simplify my question. The more basic question I am trying to ask is, for a given amount of "work" put into spanning a steel bow (force x distance), is the output power significantly different than for a fiberglass crossbow with the same work input? In other words, taking power stroke, draw weight, etc. into account, does steel output power at a significantly lower efficiency that I would have to up the "work" in to get similar performance out? I'm not necessarily attached to concrete numbers, but has anyone noticed detectable differences in the performance of these materials? I imagine that steel has at least some difference in efficiency (work out/work in) from fiberglass, but is it enough that I have to order, say, a 200# prod to get the same performance?

Thanks again for all the advice!

WIlhelm: You asked about relative prod efficiency. I can't give you any hard figures for efficiency, but it works like this. A bow's performance is based on springiness and mass. Yew wood, f'rinstance, is very springy AND very low-mass, so it makes an excellent spring, subject to so degradation in hot weather. Hickory is also a very springy wood, but it's much more massive, so it wastes some of that spring-factor just moving its lard-ass, so to speak.
Steel bows are quite springy, but they're heavy. Fiberglass bows, when properly made, are also quite springy, but they have less mass, so they should perform better, all other things being equal.
So given two prods, braced to the same height, drawn the same distance, released with the same sort of lock and the same weight of bolt, the glass prod should throw a little faster and further. How much faster and further depends on a lotta factors. If you work on the assumption that a good quality 125 lb. fiberglass prod will throw about as well as a good quality 150 lb. steel prod, you'll be pretty close to right.
Of course there are other prods to choose from. You could make a yew or hickory prod, or you could buy one of the nice laminated-wood prods that people have been buying from Elkridge archery. Those shoot very well indeed.
So why are there so many to choose from? Well, each type has its advantages. Steel prods are nearly indestructable. They shoot well in all kinds of weather and require minimal maintenance. Glass prods are good shooters and often quite inexpensive, but they will eventually delaminate over time, and should be protected from nicks and scratches. The best prods I have seen for performance are the laminated prods mentioned above. They look great and shoot beautifully, but they're expensive and may be subject to failure if abused.
Name your poison and proceed from here. For my money, you can't really go wrong with a steel prod, but if you want best performance, you'll have to move on to something that requires more Tender Loving Care.
The important thing is to choose one and get out to the shop! Geezer.. happy building.

I second everything Geezer has posted. I was going to say something similiar but Geezer covered it in much better detail.

I am not sure of anyone who has done a detailed study of different prod efficiency so it has a lot to do with experiance and some basic physics. But as Geezer noted this is with all things being equal which rarely happens.
Personally I would rank prod efficiency a bit down on the list of things to worry about when it comes to getting more speed out of the crossbow. First would be draw length, either trying to get more draw length or get more power for the given draw length, Second would be string drag and other sources of friction, then I would look at prod efficiency .

But you most likely have already thought about the first two issues and are now wondering about efficiency.

I have some experience with different prods. Here are some of my results:

Alchem prod- 205# @ 8" powerstroke= 207 fps with 428 grain bolt
(steel, 27" wide)

Elk Ridge Archery prod- 157# @ 12" powerstroke=243 fps with 428 grain bolt
(laminated wood/fiberglass, 34" wide)

Despite being nearly 50 pounds heavier the steel prod could not even come close to the performance of the laminated prod with it's longer powerstroke. Not exactly apples to apples but then things rarely are.

Todd

And there you have it in a nutshell. So why would anybody use a steel prod in preference to the far-superior laminated prod? Well, the steel prod costs half as much and it is nearly maintenance free. It's also half a foot narrower and has a shorter power-stroke. That works out to a much more compact (though heavier) crossbow to carry in the field. So Todd's got it right another way. It's apples to oranges, but even so his example pretty much defines the choices. So choose one and get to work! Geezer

Excellent opinions, everyone! I've never really heard a full pro/con discussion between different prod materials, so this is extremely helpful.

A quick look at the numbers from Todd yields some interesting results. The work input for the steel bow he mentioned is actually lower than the laminated bow (8" x 205# = 1640 vs. 12" x 157# = 1884). The input power of the laminated bow at 12" and 157# is 14.9% greater than the steel prod at 8" and 205#, but the performance (in fps) is only 17.4% greater. People better with numbers than me can probably do much more with that than I can, but this suggests to me that the difference in the efficiency of the material (with a given amount of input power) is pretty mild, and I shouldn't have major performance differences between the two materials. All of this corroborates what others have said above, and makes a lot of sense by intuition as well.

It sounds like my project should be just fine with a steel prod that has a decent poundage - I'll shoot a little north of equivalent power stroke x draw weight as Geezer suggested.

Looking forward to starting a new project, and thanks for the help!

And just out of curiosity, how long does Alchem usually take to ship prods? I've heard that it sometimes takes a while for them to ship, and I want to time it to my arrival back at my shop if possible. Does anyone have experience dealing with Alchem and some insight into what kind of wait I can expect?

Geezer here, regarding Alchem's steel prods.
I have been dealing Alchem for years and generally find their product sturdy and reliable.
Delivery? Aye, there's the rub. It's always a good idea to call or write Alchem and find out what they have in stock and when they expect their next supply of prods. Otherwise, delivery times can vary from days to months. Yes, I mean months.
On the positive side, they have prods available in a large number of sizes and they'll stand behind their work. On the negative side, if the prod you want isn't actually IN STOCK, you may have to wait a while.
If you just can't wait, I recommend getting in touch with Darkwood Armory. They have only one standard steel prod, @ 150-160 lb, the same size as Alchem's narrower prods (28 inches) The price is about the same, BUT Darkwood almost always has prods in stock. So if the 150 is what you want anyway, you might give them a call.

Chiming in here:

The fiberglass prods weigh less, so their dry fire speed is much faster than that of steel prods. When using a lighter bolt, this is seen very clearly. So while two similar steel and fiberglass prods may hold the same energy, the steel prod's dry fire speed is so slow that it can't cast the light bolt fast, no matter what. But if you were to increase the bolt mass, the heavier you made it, the gap of FPS would narrow.

It's like composite recurves versus longbows. I have a 65# Turkish recurve and a 70# English longbow (both drawn to 31"). The Turkish has a dry fire speed WAY higher than the ELB. So when I use a light (say 400 grain) arrow, I get about 240 FPS out of my Turkish but only 200 out of my longbow. However, keeping all factors the same, but using a 760 grain arrow, I get 190 out of the Turkish and 175 out of my longbow. The reason being that the Turkish is MUCH more efficient with lighter arrows than the longbow, but when arrow mass becomes quite large, the speed of the limbs at dry fire no longer matters, because both bows are operating at around 90% effiency.

I hope I've explained it in a sort-of simple way. I'm not sure if the steel prods/composite/fiberglass prods follow this same principle, but I'm almost positive that they do.

It would be great for someone to test a steel vs. other prod at similar draw weights and lengths for bolts ranging from 400 to 1200 grains to see how pronounced the efficiency discrepancies are.

Sorry for the ramble.

Brian

Can anyone tell me what the optimal power stroke would be for the ubiquitous 150# fiberglass prod? I have had a couple lying around for a while now that I'd like to use, and I'd like to take full advantage of the available power and speed without breaking them to find out their limitations.

Gnome

I believe the total draw length for those guys is 13 - 13.5 inches. The brace height is 3".

First time I built with one of those FG prods I make the draw length 14", but it really stacks at that draw and I would have to jerk it at the end to load the string into the latch...one of my first mishaps since I was building for my little bro and that last bit really pissed him come forth or fifth time to load it. I'm building him a new one now, watching that poor fella struggling with my gift just breaks my heart.

Anyway, back to the topic...the powerstroke on them is around 10". But I've read that these prods can be re-tillered slightly for better performance...well not so much re-tillered, but unnecessary material removed off the tips to make them lighter for a snappier shot. If I'm not mistaken Middleton's book "Man powered bullets..." explains the adjustment of that exact bit.

Hope someone can be of better help than me here, I underpowered my prod last time I touched it on the sanding belt.

Ivo

Todd the archer wrote:I have some experience with different prods. Here are some of my results:

Alchem prod- 205# @ 8" powerstroke= 207 fps with 428 grain bolt
(steel, 27" wide)

Elk Ridge Archery prod- 157# @ 12" powerstroke=243 fps with 428 grain bolt
(laminated wood/fiberglass, 34" wide)

Despite being nearly 50 pounds heavier the steel prod could not even come close to the performance of the laminated prod with it's longer powerstroke. Not exactly apples to apples but then things rarely are.

Todd

--------------------------------

I’ll give the usual list of disclaimers about ‘not being a ballistics expert’...

As I understand it, glass/metal aside, and all other things being equal, a heavier draw weight will not shoot the a bolt proportionally faster. What it will do is shoot a heaver bolt with greater kinetic energy.