What causes Verticle Instability...

[Shredd]

I am sure it has been discussed before... Longcruise brought it up in his thread...  I think it's a good subject and something that is important to know in bow making...

  What are your feelings on verticle instabilty??
    What is your answer or theory as to what causes verticle instabilty?? 
      Does it make a bow finicky, inacurate or more sensitive to shoot??
        Do you have any guide lines or rules of thumb in designing a bow that keeps it from being unstabe??

           What say you??

[Shredd]

  On Longcruise's thread he asks how verticle stability manifests... I copied and pasted my answer here... I am not the best on putting words to my thoughts and some may be a little fuzzy so if anyone agrees with some of my thoughts and could define it better that would be much a appreciated...



      I believe Stic says, If you put too much reflex right off the fades it will cause instability... If my memory serves me correct too shallow of an angle coming off the fades with certain limb types can cause instability... Too long of a limb for certain designs can be too unstable... I took a 64" r/d with a decent amount of reflex out to 68" and it was highly unstable...

  My theory for vertical instability is when you have two limbs and they are pulling the string at a shallow angle it is easier for one limb to over power the other because of a mechanical advantage...  A tighter radius in your limb shape (agressive R/D) somewhat adds to a shallow limb angle to the string...

Brace height can be a factor also... Too little of a brace height can cause things to go floppy... But mainly because you have a floppy design to begin with and your limbs could be riding on that semi unstable/very unstable balance point...

   So if you are gonna have reflex in your bows it's probably important to know how much that you want to add and the angle of the limbs...  There is gonna be a balancing point... Tip the scales too much and you have got instability...

   If you are concerned about Performance in relation to Stability and Limb Angle off the Fades...  I have found with certain limb designs that there was no change in performance with changing the angle of your limbs...

[Crooked Stic]

Somebody here will tell you if your limb is floppy move the bolt hole straight back maybe 1 in. at a time until it gets where you want it. Meaning also the string groove moves also. The butt part being less reflex eventually will stiffen things up. That somebody is right.
Not the only way to do it but it works very well.

[Mad Max]

 
I will give a stab at it
I made a new design on a static Recurve and braced it, 1 limb flattened out and the other was bent a lot.
The brace was low so I braced it higher and then it needed a tiller adjustment. After that it was fine.

I think the limb needs to have enough bend (preload) in it at brace to be stable.

[4 point]

A engineer would explain it as when horizontal torque overpowers the torsional stiffness of the limb I think. I’m just a dumb redneck so I just trial and error stuff till it works.

[Shredd]

  4 point........   Well believe it or not those engineers who have it all figured out on paper don't always get it right on the first shot either...   

[Mad Max]

I’m just a dumb redneck so I just trial and error stuff till it works.

That pretty much covers all of us.

[Longcruise]

Well,  in our defense,  the guys who built those holmegards got it done.   Apparently without Supertiller. 

[Kirkll]

Well Shredd …. I don’t think limb pad angle has a dad burn thing to do with vertical stability myself. It’s the geometric shape of the limb that determines the limb thickness of the limb to get a set draw weight, and where the limb is bending makes a difference too.

Let’s do an example here…. A straight unstrung profile bow like an ASL or Hill style long bow requires a stack height at say .400 for a 50# bow. These typically run at at. .003 - .004 forward taper rate, have no power wedges, and the limbs bend from the fades to the tips in a D shape. These require longer lengths of 66”-68” to get a 30-32” draw without finger pinch. But…. There is rarely a vertical or torsional stability issue because the limb thickness is still . .320 in the outer working portion of the limb.

As you start introducing more reflex and an elliptical shape to the form it requires much less stack height to achieve the same draw weight. And width to depth strength ratio run 8:1 .This is where changing taper rates and manipulating WHERE the limb is bending. “The working portion” or working limb for short, comes into play.  It’s the depth and thickness of the limb in the working portion of the limb , and the length of the working portion that effects vertical stability the most. Width can and will have an effect on torsional stability to a certain point, but the true strength comes with limb thickness.

Look closely at that video I did on vertical instability. It’s the geometric shape of the limb that reduces the limb thickness in the outer portion that causes this flip flop limb.

You can apply the same theory to recurve bows in terms of geometric shape and limb thickness. But it’s torsional stability most guys fight with the RC designs. The vertical stability of a recurve is mitigated by the string laying on the back of the hook at brace. Most guys try to stay with wider limb widths to stabilize a recurve, and it does help a bit…. But the thickness of the limb at the base of the curl is the most important factor. If you have good depth at that point, you can build a very narrow recurve with good stability, and be pleasantly surprised at your performance increase.

The only time the limb pad angle comes into play is taking a finished limb and changing the limb pad angle. But this changes everything…. Preload, brace height, the point of lift off from the curve, and potential draw length parameters. 

An adjustable riser is excellent for fine tuning the preload by adjusting limb pad angle on a prototype limb design. But that only goes so far before you need to go back and revisit your limb shape coming out of the form…..

I have a hybrid long bow with a 22 degree limb pad angle on my first generation Sasquatch LB that is rock solid at 50# with an .003 taper rate. With that same form building a 35 pound bow, I drop to an .002 taper rate to maintain stability.

So the question will be asked…. How do you get your geometric shape that will give you decent torsional and vertical stability?  Besides building many many different forms and trail and error I can’t give you a secret formula, and neither can an engineer. But I can tell you one thing that will help a lot. Go easy on the deflex sweep, come off your fade tips with a flat portion or maybe slightly deflexed towards the belly, and push your reflex out towards the tips more.  This will stiffen thing up dramatically and give you more depth to the limb thickness. Then you can adjust location and length of where that limb bends with wedges, (working limb) and THEN fine tune the preload with limb pad angle and string length once you find a winner.

It’s pretty simple…. Right?        I think that was about .05 cents worth there…

Kirk

[Shredd]

  Agreed... Limb geometry is the number one cause of verticle instability... 

     But a mildly unstable bow will become more stable with more angle off the pads with certain limb designs...  Whether that is better or worse for the shootability of the bow, I guess that depends on the bow...  But if you can have a radical design that is stable with more angle off the fades and is a good and fast shooter, I would say it all works out...

   For me personally it's going with the most reflex through the limb possible in balance with the angle coming off the fades that give me good stability and great performance... 

   I am sure that there is more than one way of skinning that cat with a number of variations and everyone has there own recipe how to come about with a stable bow...  The main thing is to keep it all in balance...

[Shredd]

  After reading Kirk's post again...  I agree with coming off the fades easy...  Too tight of a radius will get you in trouble...  But I mentioned this in my first post...  Stic is a big advocate of this...  So between the three of us it is agreed... Come off the fades easy...  Besides, you really don't need a lot of reflex back there... Limb thickness is fine to stiffen things up and give you some performance... At the first third of the limb off the fades that limb is barely moving and the extra weight of a thicker limb in that area will not slow things down... So trade thickness for reflex...

  As for angle of the limbs or limb pad angle... This is when it gets down to fine adjustments and a matter what you think is best...  With a radical reflex in your limb you may not have a choice... It all matters though...  Like I said before it is a balance of all of the above that gets you the best performance and there may be multiple ways of getting there...  But to say, angle of the limbs does not have any effect  on vertical stability...  Well........   That's Kirk's opinion...  What I have experienced and what I have learned from another bowyer is different...     We have disagreed on things before...

   Small adjustments in limb pad angle (2 to 4*) will change bend in limb when strung, draw weight, string tension and possibly vertical stability amongst possibly some other things like shootability or quietness of the bow but I have found with the limbs I experimented with it does not change performance...  The limbs shot the same speed at 10 GPP at different pad angles...  (what I found surprising)...
   


 

[Kirkll]

Maybe I should reword my statement regarding limb angle not having any effect on vertical stability so it’s understood better. I should say  The “Limb pad angle” makes no difference.  I have built a lot of bows ranging from 30 degrees to a 5 degree limb pad angle on the riser.   It’s the angle of the fades going into the limb that makes the difference Shredd is talking about. I agree with this completely….
But it’s still the geometric shape of the limb in the form from the fades to the tip that controls vertical stability. And lighter draw weight bows need a change in taper rate to adjust for limb thickness in the working portion.
 
The  wedge thickness and length to the fade tips controls where the start of the working limb begins. Tip wedges coming down from the top controls the length of the working portion. When you are prototyping limbs and testing them, if you keep track of that distance between the fade tips and tip wedge you’ll find a noticeable difference in your stacking point. An 8” working limb will stack up quicker than a 10”.  You can use that to customize  performance for guys with a shorter draw length too. But…. Building draw length specific limbs has its draw backs too. You build a hot set of limbs for a guy that has a 26” draw that stacks up tight at 29”, and he sells the bow to a guy with a 30-31” draw…. Well…. That doesn’t  work out to well for longevity.

One other thing about wedges that I found very important is that thicker wedges with a faster taper rate going into the fades often creates a hinge point in your limb. Longer thinner wedges help eliminate that hinge.

taking it one step further… if you start a mild reflex shape to your limb shape in the form about 5” off the limb butts and put some reflex in the wedge itself, you will get amazing results. I mill a 12” straight taper wedge and force it into a reflex at the fades in the form…. Not much… but enough so that it loads up when stringing the bow. This helps raise your preload significantly without as much limb bending, and it works as an overload system as you approach the stacking point.

It’s amazing what you can do to a limb just tinkering with wedges and taper rates.

Kirk

[Mad Max]

I was going to ask the other day about your fades being a different angle than the riser pad.
I have a recurve design for a 1 piece, for a T/D I would have to glue up a curve (reflex) in the limb butt fades.

[Kirkll]

I was going to ask the other day about your fades being a different angle than the riser pad.
I have a recurve design for a 1 piece, for a T/D I would have to glue up a curve (reflex) in the limb butt fades.

The key to success is milling really thin wedges. I go about .300 to nothing in 12” in a straight taper both sides, and this allows for bending or reflexing the last 3-4” of the wedge to the fade tips. Typically  I have a slight gap at the end of my riser pad bolting the limbs on the riser that closes up when stringing the bow, and straightens the wedge.  This is putting a load on the wedges at brace that carry   the preload and stiffens up the fades a lot. As the limb is drawn the stiffer fades induced by a reflex shape causes the the limb to bend further out on the limb first and keeps a nice string angle…. As you reach about 28-29” this is where your tip wedge really comes into play. The outer 6” doesn’t bend at all and forces the limb to bend even more in the working section.

As your limb is drawn slowly to 32” the working section maxes out and the fade tips pick up more load like an overload spring works. The energy storage is like a wave…..The swell starts as you string the bow, then through the draw cycle it loads the working portion until it maxes out and breaks over, then it travels back to the fades again where your long thin wedges flex even more instead of stacking…… maybe that is a poor analogy, but that’s how I see it in my head….

With shorter fade tips that do not move, that’s where all that pressure gathers as the bow as the limb stacks up, and you can flush any energy storage at that point down the toilet….    Kirk

[Mad Max]

 

[Kirkll]

Max… I just now noticed you are talking one piece bow. Not takedown….

What I have found very effective on pushing the fades out on a one piece bow is milling a double ended power lam. I use these on all my one piece bows. I milled a wood sled 30” in length and typically use 18” -22” riser block lengths. The thickness of the power lam is about .140 and has a flat section in the middle about 12” in length, and tapers off 9” from .140 to nothing on each end.

Originally I used shorter power lams on the belly ramps to push the fade tips out, but it was very difficult to keep those babies from moving around while laying the bow up. That can really screw up your limb balance  having one shift, and end up with longer fade tips on one end than the other…… don’t ask why I know this.    

There are a lot of bowyers out there that think messing with tip wedges and power lams is a waste of time and materials, and choose to use custom taper sleds with
Multiple taper rates. This does work well, and does save milling time….. but….

 You cannot shift your lams in the form at all without effecting the balance of your limbs. Also the length of working portion of the limb stays the same on all the bows you use these on regardless of limb length. So this changes the dynamics of the limb from one bow length to another.

So I prefer to use wedges on everything so I can manipulate the location and length of the working portion of the limbs to accommodate different draw lengths.
All these little things add up…… Guys say, “Hey Kirk, you are splitting hairs here!”
But if you split enough hairs, sweep em up in a pile, you end up with a lot more gains than you can imagine.

Kirk

 Kirk