The ultimate draw weight miss.

[Kirkll]

I thought i'd share a little adventure i'm having trying to hit draw weight on a carbon backed hybrid long bow. I think think is a record for me. after 18 years of building these things i've had my share of missed draw weights on custom bows, but its rare that i miss one 15# low... My biggest boo boo's are usually coming in way to heavy on glass limbs. i've learned i can deal with about a 12# reduction Max on glass backed limbs depending on draw weight required, by narrowing the width profile, trapping, and sanding glass.   But carbon backed limbs you only have width profile, trapping, and sanding belly glass. ( i don't ever use carbon on the belly side of my limbs.)

Well this particular long bow has a 19" riser with long carbon backed limbs at 64" length, and i have very few entries in my bowyers log for carbon backed limbs at the  40 @ 30" draw weight range. and the entries i do have was for a matrix carbon i no longer stock So i was pretty much guessing at my stack height and i failed miserably coming in at 25 @ 30" at rough profile  tiller check. I always leave a wee bit of extra width in my rough profile for weight and balancing adjustments. On this one i used the .015 Twill carbon over stable core for my backing. I had excellent results with light draw weight RC limbs doing this, and get excellent torsional stability. So i thought i'd give it a go on my hybrid limbs.  WRONG!  Way to light... 

Considering i always check my draw weight before doing my tip overlays, i decided to put the limbs back in the form and add another layer of carbon. it's a tricky procedure, but i've pulled it off before.

After getting that done i got her back on the tiller tree and am now 18# heavy at
 58 @ 30".        go figure. So now i'm eyeballing these limbs thinking i have got a serious carbon matrix backing on this baby, and am going to narrow this set of limbs skinny as a whip and see if i can shave 18# off.    

I know...i know... This sounds crazy. But if i can pull this off i might have a serious set of hot rod limbs here. I might have to blow the dust off the chronograph and see if i can ring the bell with these limbs....   

I thought you guys might find this story entertaining.... i'll come back and let you know how it comes out.... Time to do a little thrill seeking here. 

[Kirkll]

Just a follow up here.... I did in fact get er done. Not quite an 18# reduction, but i got 16# less and came in at 42 @30". I would have never even tried this if it wasn't for the serious carbon backing. These limbs came out really narrow, with an aggressive belly trap ta boot with no stability issues.

A new limb reduction record was broken, but I wouldn't recommend it.   

[Jackpine Boyz]

Intersesting, been messing with all wood bows and a couple wood backed bows. The glass and carbon is sure durable and versatile but all these details overwhelm me. 
Curious making 64" ntn bow for 30 draw with 19" riser leaves 22.5" limbs, guessing 18" working limb max.
Is there a particular rule you use for bow limb to riser ratio?  for a long draw like that is the tall riser helping destress limbs?  Would a 14" riser with 25" limbs perform better due to more working limb?  Or is riser just preference? Sorry for the off topic question.  A glass bow is something I am due to tackle pretty soon I think. 

[Kirkll]

Glass and carbon backed bows have an advantage of being able to store much more energy in a shorter working limb length with much less limb travel. This allows the bowyer to manipulate the actual working portion of the limbs location and length using different geometry shapes to the limb, taper rates, and different wedge configurations.

It's fascinating how many different characteristics can be manipulated just by changing the limb pad angles and string length which changes the pre load string tension at brace.

Different wedge configurations make a huge difference too. I'm talking about both the butt wedges and tip wedges. Many bowyers do not mess with tip wedges. Especially guys building long bows saying they are not really necessary. But I disagree with that philosophy. I believe using a tip wedge, and making those tips static, forces more energy to be stored in a shorter section of the limb, and helps increase the preload. And there is limb travel distance to think about too.

You are probably wondering what on earth all this talk about pre load and energy storage is all about.  Well the actual efficiency of a bow can be confused with actual performance in many aspects because performance can be judged differently by different archers. 

 A good example of that is the highly popular ASL designs, more commonly known as Howard Hill style D shaped long bows. This design that starts out as a straight shape without any reflex and has a very low preload on the limbs. I'm talking about actual string tension at brace height. This particular bow design has a totally different feel to the draw cycle. The first few inches of the draw are effortless with only 2-3 pounds per inch build up and it slowly increases as the draw lengthens. While this may sound very desirable, it has a huge effect on the bows efficiency. After Elvis has left the building, (the arrow is gone) , the string tension at brace typically isn't high enough to stop the forward limb travel dead, and does not transfer as much of the stored energy in the limbs to the arrow, and produces a lot of hand shock too. The only case where this will be mitigated in this design is higher draw weight bows above 70#.  (Which increases the pre load)  The std most popular draw weights from 40-50# kick like a mule and are not very high in performance at all.

Higher preload = much higher draw weight in the first few inches of the draw, but pays big dividends after Elvis has left the building. The higher string tension stops the limbs dead, and transfers much more stored energy into the shaft, and has much less, if any hand shock in the riser.

I could go on and on about how you can change the draw cycle characteristics just shifting the wedges vs lengthening a limb. But I think you got the idea now....

 That's about.10 cents worth.

[Watsonjay]

So, did you chisel the dust off the chrono then???? Inquiring minds want to know!!!