Push pull lams and glass

[Watsonjay]

I have been told there there are two different trains of thought on this. Lams if heard you want to do 60-40 back and belly lams. I have heard thing like its better to have thicker glass on belly because it is easier to push than pull. I’ve heard you get more snap with thicker core and thicker glass on back. I was thinking about using 050 and 040 on a bow , which is why I am asking. Always looking to learn from people with more experience. Thanks

[Crooked Stic]

A well designed bow rules. A quicker bow may have thinner belly glass but not durable as 40/40. Or same thickness belly back.
Narrow deep core good on longbows.

[Kirkll]

I have been told there there are two different trains of thought on this. Lams if heard you want to do 60-40 back and belly lams. I have heard thing like its better to have thicker glass on belly because it is easier to push than pull. I’ve heard you get more snap with thicker core and thicker glass on back. I was thinking about using 050 and 040 on a bow , which is why I am asking. Always looking to learn from people with more experience. Thanks

Regardless of what you have heard on the internet, or read in the tabloids, i think you will find very little difference just staying with 40/40 glass belly and back, or even 50/50 glass.... I did a lot of very precise testing years ago with engineers arguing about neutral plane in the cors and glass thickness. and i determined through experience first hand and testing that your limb shape and design has a hell of a lot more impact on a bows performance. 

Length of your limbs, where they are bending, length of working section that stores the energy, and amount of preload on the limbs are all part of a limb design too.  Once you have that design dialed in you'll find very little difference even using carbon backing.

My advise is don't waste your time messing with different backing thickness. 9 times out of 10 you need to sand glass to balance the limbs out anyway.     Kirk

[Watsonjay]

Okay guys. Thanks for keeping me from overthinking it.

[mmattockx]

I have been told there there are two different trains of thought on this. Lams if heard you want to do 60-40 back and belly lams. I have heard thing like its better to have thicker glass on belly because it is easier to push than pull. I’ve heard you get more snap with thicker core and thicker glass on back. I was thinking about using 050 and 040 on a bow , which is why I am asking. Always looking to learn from people with more experience. Thanks

From a structural standpoint thinner glass on the back makes the most sense (I can explain fully if you care why), but I would agree with Kurt that other factors have a much bigger influence than slightly different glass thicknesses.


Mark

[Stagmitis]

Mark I would love to hear your take on this specifically addressed to a narrow deep core design like a hill bow.

[Watsonjay]

I accidentally made a bow heavy once(okay more than once) so I had to really narrow the limbs to get to poundage and it was fast and vibe free but it sure was sketchy stringing it. Very unstable.

[Watsonjay]

Ive also heard glass is better under tension than compression as well, not sure if that is true but that would make me believe it better with .050 on belly and .040 on back??

[onetone]

“From a structural standpoint thinner glass on the back makes the most sense (I can explain fully if you care why), but I would agree with Kurt that other factors have a much bigger influence than slightly different glass thicknesses.”

Mark, I too would like to see your explanation re thinner glass on the back of a bow. I always appreciate the engineer’s perspective on all things archery! 

[Watsonjay]

A bunch of little differences add up to a big difference. I am here to learn. I would like to make the best bow possible. I will be happy to learn the why’s and be very appreciative as well. 

[mmattockx]

Due to popular request, etc...

There are a number of reasons to put thinner glass on the back.

1) Tension is an inherently stable condition, compression is not. This means higher stresses on the back do not cause delaminations and set.

2) In trad bows we don't get near to using all the strength available from the FG lams, so running the back lam at a higher stress makes for a more efficient use of the material with little risk of a failure and we get lower limb weight out of the deal because FG is heavy compared to wood.

3) By using a heavier lam on the belly than the back the neutral axis is shifted towards the belly. This lowers belly stresses and reduces the chances of the core wood next to the FG taking set, which increases performance.

As I noted, these are all pretty small effects overall and amount to working on the last bit of performance. Kirk's list of variables have a much bigger effect on the overall performance and should be sorted first, but you won't hurt any of them by using a thinner lam on the back in the meantime.


Mark

[Watsonjay]

Thank you

[onetone]

Likewise, thanks

[mmattockx]

You're welcome, gentlemen.

One thing I forgot to add is you can also increase stress in the back lam and cut some limb weight by trapping the back of the limb instead of using a thinner lam. This gives a more robust lam and more leeway for sanding to adjust tiller if you need it. You can also tweak tiller a bit as you trap the limb, by varying how narrow you make the back. It is a lot more work than just substituting a thinner lam during the glue up, though.

Mad Max has posted pictures of some beautiful trapped bows here in the past if anyone is looking for an example.


Mark

[Kirkll]

That's the fun part about this bow building trade. You can pick all kinds of rabbit holes to go down, and dive in as deep as you want to into all the intricate details.

But ......unless you have a very accurate way to test these bows including accurate scales, a shooting machine, a good chrono with a light kit, a high speed video camera, and all the time it takes to get accurate data accumulated.... All these little tweaks with different glass thickness, trapping to reduce limb mass, are  going to add up, but won't really make a noticeable difference in performance just shooting them.

If you want a higher performance long bow using the same form you have now. Simply reduce the core to glass ratio. Deeper core narrow limbs, with thinner glass will give you a noticeable difference. This will be very noticeable on Hill style bows where the preload is too low to stop the limbs clean. The mass weight reduction helps.

Have fun whatever ya do...    Kirk

[Crooked Stic]

They got doppler radar chronos now that need no light kits. Just turn them on and shoot past them. But they are$$$$$

[onetone]

Ballpark, Stic?

[Crooked Stic]

$600

[Kirkll]

Wow! better run right down and buy a couple of those dudes.

[onetone]

Kinda like a club investment …