Riser length and other things that effect performence

[arachnid]

Hello gang.

I watched a video the other day and the guy was shooting a 1965 Ben Pearson recurve. That thing has a VERY long riser section. And I wonder, is there any benefit for a such a long riser (besides making the working limbs shorter, thus storing more energy)?

And while on that topic- Since I`m still reaserchnig on maknig glass lam bows (while waiting for the shipment of materials to arrive), and since there`s so much info online and on this forum, I`d like to concentrate it on one thread (for my benefit and other as well). So, if you have anything to add, ANYTHING, about designing a better performing bow- please do.

Here`s what I know for now (currect me if I`m wrong):
a) Longer riser =shorter working limb =more stored energy.
b) Narrow tips =less mass thrown forward= faster limb recovery =faster bow.
c) Lighter limb materials (carbon, bamboo) =less mass =faster bow.

Please add as much as you know and willing to share... your knowledge is very valueble.

Thanks in advance.
Dor

[LittleBen]

All other things being equal -

A) no
B) yes
C) yes

[mikkekeswick]

Making a bow longer overall stores more energy due to string angles getting smaller. Not when you just increase riser length.
Stiffer outer limbs store more energy for the same reason.

[Swissbow]

Maximum stored energy is not all that there is to build a fine bow. For instance you can increase the preload of the limbs by adding reflex. But you can't just  add as much reflex as possible because it does not work like that. You need to find a good balance between performance and handling. The best was is to take a proven design and try to optimize it in small steps...
__________
Andy

[mikkekeswick]

Preload of the limbs simply raises the intial forces at the start of the draw - thus storing more energy.
As mentioned perhaps more important than simply storing loads of energy is how efficent the bow is.
One factor rarely talked about with modern bows is that a thicker limb has a high frequency of vibration. Thus thicker = quicker. Think of tuning forks.
Also glass 'indestructibility' allows you to almost forget 'tiller logic'. Eg. pyramid - arc of circle tiller, parrallel width limbs only tapering in width near the tips - elliptical tiller. This is very important to having an efficent bow.

[arachnid]

Thanks for the info guys. Keep it coming.

Originally posted by mikkekeswick:
Preload of the limbs simply raises the intial forces at the start of the draw - thus storing more energy.
As mentioned perhaps more important than simply storing loads of energy is how efficent the bow is.
One factor rarely talked about with modern bows is that a thicker limb has a high frequency of vibration. Thus thicker = quicker. Think of tuning forks.
Also glass 'indestructibility' allows you to almost forget 'tiller logic'. Eg. pyramid - arc of circle tiller, parrallel width limbs only tapering in width near the tips - elliptical tiller. This is very important to having an efficent bow.

I agree that thicker will return faster...  But I would think thicker will stack faster also...

[passion for knowledge]

Thickness has a much bigger effect on the stiffness of a bow than width - Cube rule for thickness - 1:1 for width, but what you are asking the materials in your bow to do are a hugely complex mix of forces and materials that there isn't the 'perfect bow'

If you are using wood, you you just added in a whole other bag of variables.

If it puts an arrow where you want it to, it's a good bow.

If you enjoyed making it as much as you enjoy shooting it, it's a great bow.

If it looks good too, you're golden.

At least that's my approach to it.

I am less than an expert when it comes to making bows though.

[mikkekeswick]

Originally posted by Shredd:
 

Originally posted by mikkekeswick:
Preload of the limbs simply raises the intial forces at the start of the draw - thus storing more energy.
As mentioned perhaps more important than simply storing loads of energy is how efficent the bow is.
One factor rarely talked about with modern bows is that a thicker limb has a high frequency of vibration. Thus thicker = quicker. Think of tuning forks.

Stack is simply to do with string angle and applied 'leverage'. Nothing to do with how thick the limb is. Short bow will stack before a longer bow. That is (one off) the reason for recurves.
Also glass 'indestructibility' allows you to almost forget 'tiller logic'. Eg. pyramid - arc of circle tiller, parrallel width limbs only tapering in width near the tips - elliptical tiller. This is very important to having an efficent bow.

I agree that thicker will return faster...  But I would think thicker will stack faster also... [/b]

[arachnid]

I don't understand why thicker limbs will return faster.
And if so, why do recurves have thinner and wider limbs then hybrid longbows? Is it because of the amount of reflex/recurve?

[monterey]

Thicker limbs would be lighter per pound of stored energy so have less of their own mass to move forward.

You guys are getting into something interesting here... Yes...  I know and agree with string angle and stacking...  But it seems to me that thickness would also be a factor to consider when it comes to stacking...  I am not just talking at the end of the draw, what is the true meaning of stacking. I am talking about the adding on of lbs. through the whole draw cycle... I guess I used the wrong terminology... This is gonna give me something to wrap my head around for the next few days or do some experiments... I believe that finding out the true mechanics of this is a major factor in performance...  Another words what's gonna give you your best preload to full draw weight ratio...

[forestdweller]

Correct me if I'm wrong but a narrower and thicker limb would also be more stable so to speak as well.

It's much more difficult to torque something narrow and thick vs wide and not thick.

[mikkekeswick]

Arachnid - look at tuning forks. A thicker material bent to the same degree as a thinner material  will return faster when released this is a fact. This can be applied to bow design but very few ever talk about it. Make some slats of wood up to test it.
Forest - yup!
Shred - I think what you are saying is related to the rule of stiffness - 1/8th thicker limb = double the dra weight. Stiffness rises to the cube.
Recurves are made with wide thin limbs because a wider/thinner limb will be more inclined to bend only in the direction you want it to bend in. A recurve (in general) is faster than a longbow because they store more energy during the draw (recurves = lower string angle, all things being equal you are bending a thicker limb than for the same force and no recurve advantage). Recurves don't have to be made that way (wide/thin) but it makes the job of making an untwisted bow easier.
Look at the best (arguably!) recurves ever made from natural materials - Turkish hornbows they have narrow thick limbs. The beauty of a true hornbow is that you can use heat to elliminate any twist that shows up during tillering.

[die_dunkelheit]

I'm interested to see some testing on thickness to stacking ratios, but from all of my findings stacking is a result of poor string angle.

And yeah, longer limbs = more stored energy, so the only reason I can see to make the riser longer, as arachnid originally asked, is to reduce string angle while using a stock limb design. From the business' perspective this makes sense because there's less development involved.

[arachnid]

Great info all around guys. I'm learning a lot and I'm sure others are too.

I have no other specific question, I just want to know the "best" design elements that will make a well balanced and good performing bow (a hybrid kongbow that is..)

[Sixby]

Food for thought. Todays hot compounds have very long risers and very short limbs. Yes they are mechanical but then so is a recurve or a longbow. The principles work similarly. if the riser is super long and stiff it could become a quite heavy bow. However I have been playing with this concept for quite a long time.
The best factory performance bows, like the Martin Hunter , have long risers and very short working limbs in comparison to many other bows. There are a lot of benefits to this as the longer the bow and the longer the riser the less distance the limbs travel from full draw back to the brace height. Its not string angle but the string angle shows what the entire bow is doing.
Just saying,.

God bless, Steve

[die_dunkelheit]

Sixby, I have to disagree on the relevance of compound bows to this issue. Compound bow limbs are actually of extremely high weights which is reduced through mechanical advantage of the pulleys (cams) and of the string angle which in this case is measured where the string breaks contact with the cam. The reason they must be of such huge weights is to make up for the loss of stored energy in their short length, in this way they are more akin to a medieval crossbow. Yeah, a 100lb draw ELB stores more energy in it's long limbs than a crossbow of the period could in it's very short limbs until it's draw weight were over huge numbers as they were, see the same logic applied here?

String angle wins that argument in the math, it's very easily demonstrated in T=mg/2sin a. With relatively simple math we can also see that the more working material involved the energy stored goes up. Math always wins, or at least tells us what wins.

I think the answer is multifaceted like everything else. Shorter limb travel is a good observation here and ultimately contributes to stability. Low string angle also contributes to stability and makes more efficient use of the energy that is stored in those short limbs. There's also the point I made before about them using the same limb design as another of their bows, ever notice how many commercial bows (from a given manufacturer) have exactly the same limb profile? Want to make a longer bow? Make the riser longer and also gain lower string angle and some stability to boot.

[arachnid]

What about the balance between limb mass and length? You obviously can't just make a long limb. At spme point the performance will start to drop due to high mass. Currect?

So, is there some kind of formula to calculate the optimum limb length to a given mass/thickness ?
It seems there are proven lengths that everyone uses, say for longbow they come at 62"-70". Do I just guess the thickness/mass to a given length?

[die_dunkelheit]

That's another good point, absolutely you'd have negative returns after some point. To answer your question I'd want to make up some test limbs and try it.