Mass Principle question

[AndyMaan]

I am new to bow-making and really enjoy browsing the many useful threads on this site. I am currently absorbing The Traditional Bowyer’s Bible, Volume IV.  I've got a question about Steve Gardner’s Mass Table on page 92. Why does mass increase with increased draw-length?  For a given bow-length and draw-weight, it seems that the mass should decrease as the draw-length gets longer. What am I missing?

[okie64]

Longer draw length means the limbs have to bend farther so they are put under more strain. This is compensated for by making the limbs longer and/or flatter and wider which is adding more mass to the overall bow. Hope that makes sense, if not someone else will be along to explain it better.

[k-hat]

Think you nailed it Okie!  So my response would be... "what he said" 

[crooketarrow]

 
   Mass is over all weight of the bow most mass comes from the lenth of limbs.
  Differnt woods have different mass to start with.
  You only have trouble with mass when your bow gets behond 68's. You can make bows longer at lower weights. Your removeing more wood. LESS MASS

[AndyMaan]

Thank you all for the helpful comments; I'm pretty sure I follow.
I'm working on a long (74") Red Oak board-bow right now that is currently 25 ounces.
It is backed with a single layer of fiberglass dry-wall tape applied with waterproof woodglue.
I'm still on the long string and the pic is at 50 lbs. Hoping for 50-55# @ 28".
Any comments would be greatly appreciated.

[mikekeswick]

 
   Mass is over all weight of the bow most mass comes from the lenth of limbs.
  Differnt woods have different mass to start with.
  You only have trouble with mass when your bow gets behond 68's. You can make bows longer at lower weights. Your removeing more wood. LESS MASS

Most excess weight on bows that are overbuilt is nothing to do with the length of the limbs but more that the limbs are too wide and thin eg. the wood isn't stressed to it's optimal level.
Bow making is all about stress and how it's distributed along the limbs.
Rule of thumb is the thinner the limb the further it can safely bend.

[AndyMaan]

Forgot the pic, ooops!
The pic is tilted and the posterboard is warped, but the lines are in the right place.

[sharpend60]

I wouldnt worry about the mass at this piont.
Just get a shooting bow.

Tiller comes before mass.
And tillering is the essence of bow building.
These guys that post the mass weight of their bows, have built many bows and are just trying to top off the skill set.
The chapter in TBB4 is assuming you can tiller a bow already WITHOUT having to deal with an other factor.

Plus the backing you put on there is gonna throw off your mass numbers. Also fiberglass is a no-no here. Plenty of other backings that are cheap and easy. Im mean like .05 cents or less per bow

[SLIMBOB]

  Agree with Mike.  Longer limbs need a higher draw weight to justify the extra mass, not a lower weight.  So...The longer draw length will need a wider thinner limb to keep from taking set or breaking outright....This equals more mass.