Different Thickness FG Lams On Belly and Back?

[mmattockx]

From an engineering standpoint there is no reason to use the same thickness of FG lams on both the belly and the back. The tension side is inherently stable and thinner glass can be used to utilize more of its potential. Yet almost everyone seems to use the same thickness of lam on both belly and back.

Why is that and what am I missing?


Thanks,
Mark

[Shredd]

  all my glass is .040 so I just use that...  less sanding... i'm not sure that taking off .005 of glass on the back is going to do a whole lot... if was going for all out speed I might take it off and possibly gain a couple of fps... on bows 36# and under i will do a .035 belly and a .030 back...

[kennym]

I've done a couple like that but no bow with same specs to test against, so no help...

[Flem]

Hey you found the can of worms I have been looking for! Not everybody uses the same thickness back and front. I believe your premise is sound and in fact I would take it a step further and say thickness is not as critical as proportion of glass to resin depending on where the laminate is located. More glass to resin on back and the opposite on the belly.

[Crooked Stic]

Flem are you saying thinner belly glass than front glass?  Always heard that was a key to a hot rod bow but not durability.

[Flem]

Not necessarily thinner, just a higher proportion of glass fibers on the back compared to the belly.
For instance Bearpaw's  glass to resin ratio is higher than Gordons, so to optimize the properties of each, Bearpaw on the back, Gordons on the belly. It's same premise as to why some bows are backed with Bamboo because of its high proportion of longitudinal fibers.

[mmattockx]

  all my glass is .040 so I just use that...  less sanding... i'm not sure that taking off .005 of glass on the back is going to do a whole lot...

I was thinking 0.030" on the back and 0.040" on the belly.

Your 'less sanding' comment brings to mind another question - how many of us routinely thickness sand their FG lams before use? What grit of paper do you use on your thickness sander when reducing the FG thickness?

Hey you found the can of worms I have been looking for!

<snip>

More glass to resin on back and the opposite on the belly.

  Glad I could help.

Why would you want more resin on the belly side? It does little to carry the load in either tension or compression compared to the glass fibres. I do find the idea of mixing brands/types of lams to optimize the bow interesting. Another can of worms to crack open!


Mark

[Mark R]

Well I just got done cooking a bow with bear paw .040 on the back and gordons .050 on the belly because that's what I had left. should be fine but i don't have anything similar to compare it to except a bow I did with identical core lay up but with bearpaw .040 glass back and belly which I sold to a friend so I'll check with him when I'm done with this one. There should be a difference but not sure yet how much.

[mmattockx]

Well I just got done cooking a bow with bear paw .040 on the back and gordons .050 on the belly because that's what I had left.

I'll be curious to hear how it turns out, please post an update here when you have it done.


Mark

[Flem]

Well I just got done cooking a bow with bear paw .040 on the back and gordons .050 on the belly because that's what I had left. should be fine but i don't have anything similar to compare it to except a bow I did with identical core lay up but with bearpaw .040 glass back and belly which I sold to a friend so I'll check with him when I'm done with this one. There should be a difference but not sure yet how much.

Thats some good timing!

[Stagmitis]

Mmattockx I fully agree with you.

Also Flem has a good point with regards to a higher glass to resin ratio on the back.....The stiffer the better and if you can shed weight on top of that its a bonus.

Ill open another can of worms......Woven glass on back and linear on belly....

[Mad Max]

If they made glass  .035, .045, Also, I would use them
I do grind .002/.003 off some times

[Flem]

"Ill open another can of worms......Woven glass on back and linear on belly...."

Don't think woven fiber laminates have any advantage over linear ones. Unless you are making some seriously wide limbs or wings, which I know you are not Charles, there is a moderate percentage of material thats not really contributing to the overall performance of the bow. Dead weight. If you want to really increase the performance of your laminate, use "S" glass fibers instead of the commonly used "E" glass, its 40% stronger. 

[Stagmitis]

I agree Flem and have been using and making laiminates with S2 for quite a while. The problem I have had wih the narrow bows is losing some backset after they are strung and shot even for a short period of time. The woven back which is stronger in tension ( I think?) than linear helps.

[Shredd]

i don't want to be the stinker here but does any one have some facts or are we just discussing theories??

another words...  Has anyone built three or more exact bows and tried different glass thicknesses on each one and done testing for performance on each bow??  it would be cool to hear some solid proof and exact numbers of what the gains are...

i just want to add something that i feel is important when talking about performance and should not be over looked...

 imho...  if you are wanting more performance, limb shape and taper to give you optimal bending should be your top priority for best performance...  that's why some bows shoot 165 fps while others shoot 185 fps... that's a difference of 20 fps...  everything else seems to be secondary but does matter when going for best performance from a limb... 

i basically just want to make the statement that if you take the time to sand your glass to a specific
 thickness and glue it on limbs that are shooting 165 fps for a 2 fps gain, you are hustling backwards... Might as well take the time and invest it in a better limb shape that will get you over 180 fps and then if you choose shave that glass down to get optimal speed from your limbs...

in closing...  So before you go sanding your glass make sure you have optimal performance from your limbs otherwise it's like installing a high performance transmission to an engine with bad plugs and/or a clogged air filter...

[Flem]

You are right Rich. But here's the thing, if you believe something to be true even though you have not quantified it, do you incorporate that knowledge or ignore it?

I agree Flem and have been using and making laiminates with S2 for quite a while. The problem I have had wih the narrow bows is losing some backset after they are strung and shot even for a short period of time. The woven back which is stronger in tension ( I think?) than linear helps.

No help here. I build my bows flat, straight and thick cored so they take 1/4"-1/2" of set. I like to know the wood is doing some work.

[Shredd]

seeing is believing... in this craft for me it's sometimes hard to believe what i am told and through experience sometimes doubt my own beliefs...  i have been led down the wrong path a few times by others expert advice... at the same time i have believed something that seemed obvious and was common sense to me, i had been proven wrong by someone else's suggestion but only after i had physically made the efforts to prove it to myself first...

  as for ignoring or incorporating...  it all gets down to the worms... how many cans do you want to open??  once you reach close to optimal performance you have choices to ignore or incorporate to make small gains...  Just remember, are you willing to change a recipe that took you so long and hard to achieve to gain 2 fps...  it may turn out to be more worms and more time invested if things don't go the way you thought they would...  things get funny when you start maxing out a design and little changes could have adverse effects...  god knows i am not against experimenting but it's a matter if i want to take the time... as you see, you rarely see hard numbers and strict bow comparisons on this site... why?? because it takes a lot of time, dedication and exactness that few can or want to invest...

you often see the guy that says i really don't care how fast a bow shoots...  in the end, once i reach 185 to 190 fps i begin to cop the same attitude and i don't care how fast my bow shoots anymore...   

sorry if i ranted... been known to do that...

[mmattockx]

sorry if i ranted... been known to do that...

Rants are OK with me. You raise valid questions that need to be answered.

On your proposed testing of 3 or 4 similar bows, what variables do you keep constant? Back and side profiles, of course, but what else? Stack thickness? Draw weight? Something else?


Mark

[Mad Max]

If they made glass  .035, .045, Also, I would use them
I do grind .002/.003 off some times

Sanding 2 or 3 thousand off for me is just to get some humps out

[Stagmitis]

One thing i wanted to clarify. There is an initial set after the bow comes off the form and profiled/ tillered. Usually 1/4 to 1/2 Like Flem indicated.  Then there is a secondary set that happens after the bow is strung and shot for a short period of time. This seconday set is momentary. When the bow is unstrung It springs back to the initial set incurred after a few minutes. This is the movement I dont like