First Tri Lam Questions

[Japes4]

Hey all, I have built some successful board bows, backed bows, and one glass bow. I now want to try my hand at a wood tri lam. Just need some help with lam "formula".  With the lumber I have on hand, I was looking at a 1/8" hickory back, cherry core 3/8" tapered to 1/4", 12" bloodwood riser, and 3/16" ipe belly (hopefully it's enough to tiller as needed). I am looking to run the belly over the riser (mainly because 3/16 is the thickest ipe I have).  Will this work for a 62" ntn bow at +/- 50# @ 26"?  And I'm guessing 1 1/4" wide. Form I'm using is reflex deflex post style.  Any help is appreciated.

[macbow]

Can't say for sure but I'm thinking your cherry core may be too thick for the weight your looking for.

If it is you'll tiller away  most of your IPE.

[Japes4]

So, do you think I would be better off with a 1/4" core tapered to 1/8"?

[LittleBen]

My guess is that will be way over 50#.

Here's how I do it: each lam is tapered .002 for a total taper of .006.
With that taper rate, and 1.5" width, 12" riser, I built one which is 62" R/D longbow that draws about 55# @26". The stack thickness is .470" or .480" at be but end of the lams IIRC. That's hickory back, hickory core, Ipe belly. Rectangular limb profile (perfectly flat back and belly)

I'd probably start at about .500" total thickness, so say 1/8" hickory back, 3/16" cherry core tapered to 1/8", 3/16 or maybe 1/4" ipe belly also tapered to 1/8" or 3/16". That's gives you about .005" total taper, and should get you in the ballpark.

I tend to keep mine full width until 12" from tips, then taper to 3/16" nicks, but you can decide the precise width taper of the limbs during tilering.

A lot of people will tell you that you can't build tri-lams like you can glass bows, but my experience is that you can. I keep data on all my bows, and can usually estimate weight withing 5# or so based on stack thickness. Ipe lams from the same board are pretty consistent in density, so you can do a pretty good job of calculating the stack.
 FWIW I use a lot of taper, because my goal is basically to not have to do any tilering on the belly of these bows. Often they get finished with not much more than tapering the sides of the limbs as desired, and sanding for finish. More than a couple still show marks from the jointer on the belly lam from when the lams were cut (board is jointed before each lam is cut for precision) ... My point is taper is your friend, up to a point, more taper means less tilering in the end. I just prefer to tiller with a lam grinder before glue up and not a cabinet scraper after.

[Japes4]

wow, seems I was way off on the core thickness. Thinking more about this, if I go with these dimensions (1/8, 3/16/, 3/16), would it be easier to glue up all three lams then glue on the riser? Sorry for all the questions, just trying to get a solid plan in place before I start gluing.

[Troy D. Breeding]

On my tri-lam I used boo that was a approx. 1/8" in the middle down to approx 1/16" on the tips. The walnut core was 1/4" in the middle tapering down to 1/8" on the tips. The Osage belly was .300" parallel. Once finished at 64" the belly wood is really thin at the tips (maybe 3/32") and I'd say just past the fades of the riser it not over .200". Before I started shooting it the bow pulled 60#@28".

[LittleBen]

I would at least put a power lam (or thin riser) between the backing and the core.mthen when you glue your handle on you won't have to worry about popping it off. Check out Sam Harper's site .... He diagrams this well.

 http://poorfolkbows.com/ipe1.htm

[macbow]

On the handle I glue mine on after the bow comes off the form.
Roy likes to make his,in one piece to match the R/d of the handle spot.
I suck at that so I glue my handles up with 4 to 6 slats about 1/8 or more thick so they will bend and match the blank when glued on.
If you want the belly wood to run up the riser then the handle has to be addressed before glue up.

[mikkekeswick]

Definately make the belly lam ride up the riser. They look better and will never 'pop off'. I've seen it happen even with thin slats glued together. Once thin pieces are glued together they act just the same as a thick piece. I've seen it happened too many times with a friend of mine to know thin pieces for your handle aren't really the answer to this problem, the only other way is to have a plenty thick belly lam...but a powerlam and belly going up the fades is the recipe for success! I'd go with LittleBens lam advice he has quoted almost the exact numbers I would use. I also agree with what he says about profile and glass/wood. Also the performance will be right up there too.

[Japes4]

Thanks all, this is a huge help.  I will get started on this thing and post how it came out.

[Roy from Pa]

1/4" core lam, tapered to 1/8th at tips. 1/4" parallel belly lam. 1 1/4" wide at flares to 1/2" at tips. 62 to 64" long.

[LittleBen]

Just went and calipered the bow I mentioned, the limb is .460 at the fade, so the butt thickness was .496" give or take a few thou since the lam is tapered full length. I also scaled it and it was 55# @ 26".

Don't mean to be a turd, but I believe Roy's recommendation would be more appropriate for Osage belly than Ipe. IIRC Roy also tends to make a slightly rounded belly which will require a bit more stack. If you built the bow to Roy's recommended thickness, you will certainly be able to make 50# But I believe you will scrape through more than half of the belly lam to get there. Now if we're talking Osage, I take his word like the word of God, but we all know what happens if Roy touches Ipe or even looks at it too close ....

The numbers:
Ipe is nearly 40% greater in flexural strength than Osage, and about twice as stiff according to wood database.com so an Ipe bellied limb will be thinner than an Osage bellied limb at the same width and draw weight. I question flexural strength data on wood because it is so dependent on grain direction, but compressive stiffness (compressive modulus) data you can take to the bank.
On the flip side, Osage is far more elastic than Ipe, so your thicker Osage limb will bend just as far as your thinner Ipe limb, and should have about the same mass.

I'm not advocating Ipe over Osage, I'm just saying your stack thickness has to be adjusted according to your belly wood. Just like your stack depends on whether your using fiberglass or carbon in your "glass" bow.

[Japes4]

Unfortunately I don't have any Osage, and I haven't found a local supplier.  I will definitely have to order some though, as I do want to make an Osage bow.  I am using 3/16" Ipe, because that is what I have on hand (leftover from a previous project).  So, my plan is to use the 1/8" backing, 3/16" tapered cherry core, and the 3/16" parallel Ipe belly. Riser will be 12" long and about 1.5-1.75" thick.  We shall see how this comes out as I am gonna start building this week...at the very least I figure I will have a baseline for adjustments on future bow.

[LittleBen]

I like that attitude. Sometimes you just gotta build one, see how it turns out and adjust.

Don't stress on the Osage vs. Ipe. Ipe is incredible stuff, I think if Fred Bear had Ipe, he mig never have gone to glass .... Maybe that's overstating it but you get the idea.

[mikkekeswick]

Agreed but watch out for any hint of a pin knot/grain swirl with ipe.

[Roy from Pa]

Whoops, I don't get along well with ipe. My figures are for Osage.

[Japes4]

No worries Roy, Osage will be my next one and I will definitely be taking your advise!

[canopyboy]

Originally posted by LittleBen:
Ipe is incredible stuff, I think if Fred Bear had Ipe, he mig never have gone to glass .... Maybe that's overstating it but you get the idea.

I'm with Roy on this one. I've watched Ben build some sweet bows with the stuff, but I'm 0 for 3 with it due to small defects in the wood and leery to let that dog bite my hand again. I think good ipe is tough to beat, but Professor Bear would not have been able to run a business with wood that shows hidden defects only after shooting in. Osage has worked well as an alternative for me.

[LittleBen]

Dave, That is the sad truth.

[Nezwin]

I class Ipe as a 'compression strong due to density' wood, much like a lot of the native timbers we have in Australia (Spotted Gum & Ironbark being the obvious).

Some timbers, like Yew, seem to be compression-strong as a result of inherent properties. Osage is dense but seems to display some of this characteristic. Many very dense timbers, like Ipe, seem to be compression strong because they are so inherently dense they cannot move enough internally to fail.

Don't know if that explains it all that well but just my thoughts!