Straight longbow design considerations?

[Apex Predator]

I have built two fiberglass laminated longbows.  They are both 66" between nocks.  These are the differences:

Bow #1- This one draws 50@28.  She is 1 5/16" wide narrowing to 9/16" tips.  The bottom limb is 2" shorter than the top.  The arrow pass is 1" above center of the bow.  Construction is of three parallel lams and one .002 per inch tapered lam.  It has 3/16" positive tiller, but I shoot three under.  This one has maple core lams and red cedar lams on the faces.  The red cedar on the bow's back is the tapered lam.  I used .050 clear glass and smooth on for epoxy. I feel that this one has tip overlays that are a little too thick, and could stand some narrower tips.

Bow #2- This one draws 49@28.  She is 1 3/8" wide narrowing to 7/16" tips.  The bottom limb is 1" shorter than the top.  The arrow pass is 2" above center of the bow.  Construction is of two parallel lams and two .002 per inch tapered lams.  It has 3/16" positive tiller, but I shoot three under.  This one has two maple .002 tapered core lams and parallel walnut lams on the faces.  I used .050 clear glass and smooth on for epoxy.

Between the two you can see that the #1 bow bends slightly more out side the fades and less at the tips, compared to bow #2.  Bow #1 is in the first photo.

 

 

I've shot both bows quite a bit with 10 grain per pound arrows and 13 gpp (which I prefer for hunting).  With the heavier arrows the difference in handshock and noise was not noticible to me.  When shooting the lighter arrows, bow #2 had noticeably less shock and noise.  Brace height on both is the same at 6 1/2".  They both have the best manners at this brace.  My nock set is similarly 3/4" above sqaure.

Here you can see some side by side photos.

 

 

 

In that last photo of both bottom limbs, you can see that bow #1 is bending more at the fades and less at the tips.  I don't own a chronograph, but believe that bow #1 is a better performer, and expected that.  

What other considerations should I be giving to my design?  Should I be tillering them even for three under?  I'm very pleased with both bows, and neither has more shock than the Hills that I have owned.

[RC]

I think you should build one 55@ 28 and send it to me. I`ll give it the RC swamp treatment,Rides in the boat,walks across ridges and purty much a joyful life. It`ll probably get some blood on it but that builds character.....


You building some fine bows and I`m sure they`ll get better.I would try one with all boo and carbon.The thinner the glass the smoother the bow in my experience but I am no expert.RC

[Apex Predator]

Thanks Robert.  I plan to start using .040 glass, as it seems to be better all around than the .050.  I wanted a little more margin for error for my first few.

[Lewis Brookshire III]

Marty why are you building them with a shorter bottom limb?

[RC]

Can`t speak for Marty but I believe that puts the center of the bow where it should be.If I`m not mistaken Harrison - Fisk longbows are built like that. I do know I`ve only shot one of them and it was a very accurate bow that felt good.RC

[Apex Predator]

From what I understand, getting the arrow pass as close to center is desirable.  Of course there are trade-offs.  The bottom limb must be weaker, or it will be way to stiff and you will have like 1/2" or more positive tiller.  My Hills have been tillered for split finger and they have the bottom limb 1 3/4" shorter and 1/8" pos tiller.  Don't quote me on the 1 3/4", because I don't have my notes in front of me.  I think that's what I remember.  I shoot the Hills just fine three under.

[Apex Predator]

My latest bow with the center being 1" below the shelf, balances about 2" below the shelf.  That puts the balance point right in the meaty portion of my hand, where most of my grip pressure is.  It balances really well in my hand, and I think I'm getting close to where the balance should be in my future bows.

[Lewis Brookshire III]

Hmm, I am familiar with that on wood bows but didnt know they did that on glass bows. It must be a hill style bow design, none of the hybrid longbows I have owned or made were built that way. If you measure from center of the riser to each limb tip, everyone I have had and built is even and you adjust the tiller  measuring from end of the fades out to the string to adjust for split/3 under tiller. But I have never made nor shot a HH style bow.

[Orion]

Apex:  It's not uncommon for folks to use all tapered laminations in their longbows.  The greater use of tapered lams in your second bow makes for more bend toward the tip, an almost perfect arc, compared to bow number one, which is just a hair straighter/more static toward the tips as you note.  I would expect one to be faster and two to be smoother, and I would expect you could accentuate that difference by using more or fewer tapered lams.

Lewis, almost all bows have shorter lower limbs.  It's dependent on where the bowyer chooses to put the arrow rest, but most who build glass laminated bows (recurves and longbows) put it 1 1/4" or so above the center of the bow.  Thus, 2 3/4" of the grip area is below the center of the bow, in effect shortening the effective working area of the lower limb.

The pivot point then, where the bow's riser meets the shooter's hand in the web at the base of the thumb and index finger usually ends up about 3/4 inch below the rest and a half-inch or so above the center of the bow.  This effectively puts most of the bow hand pressure in the center of the bow when shot.

Some builders of glass laminated longbows and a lot of self-bow makers put the arrow shelf two inches above the center of the bow, which places a 4-inch grip area in the center of the bow and equalizes the top and bottom limb length.  Self-bow makers do it because they can let the bow decide which will become the top and bottom limb during the tillering process and just flip it end for end to get the somewhat softer limb on top.    

Nice looking bows, Apex.

[ChristopherO]

AP,
Why did you suspect bow #1 would be the best performer?  Both look very good.  I would have thought the thinner and lighter bow at that poundage would have cast the arrow a bit zippier.

[Orion]

Lewis:  Measure again from the center of the grip area, which usually is not in the center of the riser.

[Lewis Brookshire III]

Ok, thats where the difference is. I am speaking of measuring from the center of the riser, or actually measuring from end of fade to tip on both ends. I have not taken into account the height difference of putting the shelf above center line of riser.

Measuring the way you speak of is acually stating that the bottom half of the bow is shorter not the actual limb, am I understanding that right?

[Orion]

Actually, I misspoke a little.  The center of the grip area is usually close to the center of the riser.  It's the entire riser that's not in the center of the bow.  More of it is below the center of the bow than above the center of the bow.  Thus, if you measure from the end of the fadeout to the string groove, the lower limb will, in fact be shorter than the upper limb.  I've been taking these measurements on bows for about 10 years.  The only bow I currently own in which the top and bottom limbs are the same when measured from the end of the fade out to the string groove is a Dwyer, which has the riser located exactly in the center of the bow and the arrow shelf cut 2 inches above center.  My Hills, Great Northerns, Robertsons, all mild r/d bows, all have the arrow shelves less than two inches above the center of the bow and they all have shorter lower limbs than upper limbs when measured from the fadeouts to the string grooves.  Don't have any now, but that was also true of my highly reflex/deflex Morrisons.  Not on the takedowns of course.  All of those limbs are made the same.

[Apex Predator]

ChristopherO, The stiffer tips will perform better in most cases from what I understand.  I think if I narrowed the tips some and thinned the overlays, I would gain more performance.  I may still do this, since this was my first and will never get rid of it.

[Lewis Brookshire III]

Hmm, I will have to go measure again and see what I was doing wrong. I only have one that I didnt build right now I will see what it measures at. I built my one piece hybrid even and my takedown is obviously even.

Learn something new every day!

[DCM]

http://www.goarchers.org.uk/mechanics/index.htm

I drag this tired old link out too frequently perhaps, but it's the only one I've found.  Tapley does a good job putting the dry science (Kooi predominantly) into terms we can understand.  The website format has changed recently.  Find the menu on [Bow Mechanic], which I could not see until I positioned the mouse pointer over it, left of [Arrow Vibration].

In particular the [Tiller] menu (not the Static v Dynamic] submenu, helped me to understand the physics behind, the purpose of "positive tiller."  I have concluded, perhaps or even probably incorrectly, one uses some combination of either arrow pass at center (commonly mislabeled "shorter" lower limb) or positive tiller.  This is to say the more offset above dimensional center the arrow pass, but more appropriately the nock point or where the arrow bisects the string, the more positive tiller is required to balance this inherent asymetry.

Very interesting discussions and theoretical findings about stiffer outer limbs, and it's benefit related to limiting vibration or distortion of the limb on the power stroke recently in the flight shooter area.  Re-enforces what we already know about string angle, stiff outer limbs is generally good.  On the other hand, the ever present trade off, is stiff outer limbs are relatively heavy, contributing to vibration in handshock and lost efficiency from KE left in the limbs.  Best case scenario is stiff and light (generally meaning narrow) outer limbs is better.

String material also makes a huge difference in combating distortion of the limb on the power stroke, generally the less elastic the better in terms of taming vibration, ie. handshock, and performance.  Also in terms of total string mass, less being better, and again info we already knew from direct experience.

Generally one can use less stack height, less limb mass, by enducing some reflex, essentially just taking better advantage of the extra capacity fiberglass materials provide for us.

Just some thoughts to keep the conversation alive.  Very interesting so far.

[Orion]

Good stuff, David.  The chapter titled "Design and Performance Revisited," by Tim Baker in issue four of Traditional Bowyers Bible, has an excellent discussion of the effect of stiff, but very thin outer limbs on self-bows.  Apparently the highest performance design yet developed, for self-bows at least.  It's a pretty radical departure from the "little more stiff" outer limb that Apex has in bow number 1.  It takes that feature to the extreme.  Of course, we've  known for quite a while that static recurves (stiff, unbendable or slight bend tips) are quicker, though perhaps not as smooth, than non-static recurves.

[Jerry Gille]

Are you putting any reflex into these bows?  If so, how much and where does the reflex begin?  If not, how much set are they taking off of the form?

What are you using to finish the bows and how do you apply it?

I've built a few longbows off of a reflexed form I made a couple years ago that was reflexed evenly from about an inch into the fades all the way out to the limb tips.  It had about 1 1/2 inches of reflex at the tips compared to the handle.  I never really was satisfied with those bows as they seemed to take some set right in the tips bringing them in line, or even behind in one case, the mid limb.  I narrow my tips down to 9/16 just below the nocks and have only used all tapered lams. The bows I've made off of this form all seemed to be a little to whippy at the ends for my taste.  That's probably the all tapered lams and the narrow limb tips combined I'm guessing.

I didn't really like that design so I built a new form not long ago where the reflex starts pretty much mid-handle and goes out real mildly until about the last 14 or so inches of the limb.  There it starts to reflex more up to the tips.  I think I'm going to like this design better but got distracted building some d/r bows off of another form I made and haven't given the reflexed bow a go just yet.

The few adult bows I've built (and there haven't been many) have even limbs with the shelves cut 1 1/4 above the centerline of the bow, just as has been said here.

I'm interested in learning all I can about building straight or slightly reflexed glass longbows.

[Jerry Gille]

Oh, and nice job on the bows!

[DCM]

Jerry,

Yes, I have heard a lot about the Chapter in TBB IV, some of it before it was written.  I reckon I should buy the book.  I think a recurve works on the draw force side, storing more energy on the draw versus the new hysteresis theory which focuses on the power stroke side or what comes out.