glass thickness

[Sherm65]

What is the performance difference between .04 and .05 fiberglass?  If  I am building a 60lbs. bow what is the advantage of one over the other?  Can you use a combination of the two?

[Jason Scott]

You can use a combination of the two. You basically want the fiberglass to be between 20% and 25% of the total stack. This is so the draw is smooth and you still have good cast. More Glass and your draw will be smoother but cast will drag. Less and the bow will draw a little harsh. There are a lot of opinions about tension and compression but you basically can't go wrong if you stay around these %s. I use .04 on the back and .05 on the belly for my last medium r/d bow with a total stack of .370 and it feels good and crisp but I havn't weighed the bow or chrono'd it. It depends on the limb design and taper if you want to get really into it and tweak your design here and there. You could build the same weight bow with .08, .09 or .1 glass by adjusting the wood stack and one of them may perform better than the others. For instance, on my form, if I want a 60# 64" rd longbow I can choose from the following stacks:
(.1 glass in a total stack of .380 / .09 glass in a total stack of .390 / .08 glass in a total stack of .400). For this 64" 60# bow I would use .04 on the back and .05 on the belly and end up around 22.75% of a .395 total stack which is just right. I have learned to go a little bit thicker on my wood stack and just narrow the limbs a little untill I hit my weight. I hate comming in under weight more than anything.

How long is your bow going to be?

[Jason Scott]

Generally, I like .08 glass for anything .360 total stack and less/ .09 glass for anything between .370 to .420 total stack / .1 glass for .430 total stack and up. But for 60 " and less bows it's mainly .08 glass and for bows over 66" it's just about all .1 glass. I know that's a lot of info but I have it in spreadsheet if you or anyone would like to have it.

[Bob Barnes]

I'm interested...I have just bought several pieces of .05 glass and was planning on using it mainly to build 58-62" bows in the 50-55# range...you make it sound like i should have bought .04 instead...?  Does it make a difference if I use it for a narrow/deep limb rather than a wide/thin limb?
thanks.
Bob
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[DanB]

That's a very good explanation of the balance between glass and core. The best I have seen. Thanks! Could you send me your XL file please?

Dan

[Swissbow]

Thanks a lot for that XLS-file Jason. It took me just a few minutes to figure out the stack for my next bow.

That's what I like about this site, there is always somebody that can and will help answer your questions. GREAT !!!!

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Andy

[Sherm65]

Thanks for the great answer! I am so impressed with the advise I see and now get on this site.

[Jason Scott]

Bob, you will be just fine using the .05 on any design or stack. The narrow deep core will be better suited for the .01 glass but the results are marginal at best. We may not be able to tell the difference in a blind shooting test, who knows. Most of this archery stuff is opinion and preference anyway. My preference changes all the time.

[coulter]

thats a great reply jason i would love a copy of your file. i have just started building my first lam bow, so it would be a great help    thanks.      noel

[Jason Scott]

Bob, if you are grinding your own lams there is no reason why you can't run the .05 glass through the sander too rough side up and take .01 off it to get a .04 glass. If you have to buy bulk glass then .05 is the best thickness to buy for this reason. A lot of the time when you order .04 glass it's not exactly .04 over the length of it anyway and sanding it down may usually be necessary, the top bowyers out there mic every piece they get and sand if they have too. A lot of them spine test glass too by letting the glass hang off the table by several inches and putting a weight on the end and group the glass by spine weights. They say there is quite a variation in what you get. I haven't become that involved yet.

[DanB]

Got the XLS file.  Thanks again. My new RD form has a little more deflex in the riser so still trying to get the recipes refined. I had been using three .0015" tapers with a 20" riser. My stacks had been around .350",including .040" glass back and belly. Ended up with about a 60# bow at 60". Limb profile is 1.5" at the fades to .5" at the limb tips.  Trying four .001" tapers this time around with .040" glass for total stack of .360" and an 18" riser. Should be interesting!  Probaly will make the length of this bow 62".

[Jason Scott]

For your finished bow, it seems to fit into the chart just about right.

On that chart, since there was not a designation for .350 stack for a .08 glass option, I split the difference between .340 and .360 for a 60" bow and it says 67.5#. Remember I am splitting when you look at it. I will take away 2# for the additional 2.5 thousanths taper and 7# for the additional 2" in riser length and that results to 58.5#. Pretty close.

For the new bow. On the cart, .360 stack with .08 glass option @ 62" lists 60#. Subtract 2# from that for the additional .02 taper results in 58#.

You may come out very close in weight to the other bow unless you have to narrow it any to get good glue lines like I usually do because the lams slip and twist any. I bet the draw will feel a little easier than the 20" riser 60" bow even if they make the same weight. I bet that 60" bow smokes an arrow.

[coulter]

thanks a lot for the file jason.     noel

[Bob Barnes]

thanks...I'll figure it out...some day.     I buy lams and I bought 20 pieces of .05 glass...I'll use what I have and then worry about it...  
thanks again.
Bob

[Jason Scott]

DanB, I was off on the calcs for the bow you have finished already. The chart says it should be 72.5# intstead of 60# so I would say that it's not very close if it's that far off. I took 2# away from the listed weight but should have added 7# for the 2" longer riser instead of subtracting. 67.5# - 2# + 7# = 72.5#

[DanB]

Jason, it probably is the design of my limb.  I believe that I have the recurve begining early and the curve continuing through the limb tip. As soon as I figure out how to post pics, I will show the design.

Dan

[longbow maddy]

Jason, Hands down the best glass to core ratio explanation I've seen. If could you please send me your file it would  be greatly appreciated.Thanks Allen

[DanB]

Jason, the bow I gave the stats for earlier was a Hybrid longbow. It has quite a bit of reflex and deflex.  I was looking back over my notes from a mild RD longbow from 2003 and the wood stack was .340" and .040" glass back and belly. The 66" bow weighed out at 53# @ 28".  That is just about where it would fall on the chart you have.  Thanks again.

Dan

[skeaterbait]

Hey Jason,


I sent you a PM, I would love to see your chart as well.

[SteveD]

Hi Jason I sent you a PM also about the chart.
Thank you for the info