Weight reduction

[tradhunter88]

I have a 47# at 28” Bear Montana that I would like to shave 4-5 lbs from. Is it possible to accomplish that by narrowing the limbs or would I need to trap them?

[Shredd]

Post pics at brace and 28" draw....

[mmattockx]

I have a 47# at 28” Bear Montana that I would like to shave 4-5 lbs from. Is it possible to accomplish that by narrowing the limbs or would I need to trap them?

I'm curious to see where this goes. On a structural basis, if you want to drop weight by 10% you need to narrow the working portion of the limb by 10%. It is a straight 1:1 ratio. So, if the limbs are 1.5" wide you would need to take 0.15" off the width. That is dead simple to calculate and pretty easy to do, but I want to see what other considerations the experienced guys have in mind.


Mark

[tradhunter88]

[Mad Max]

I use BJansen - Lam Stack Calculator pretty often, I asked him about reducing from 1-3/4" to 1-1/2" and this is what he said.
1/4" narrower limbs = 14% less pounds
47# x 14% = 6.58---47 - 6.58=40.42#
I did this on a recurve and it worked

1/4" =14% so divide 14% by 4 X 3 =10.5 for 3/16"

3/16" narrow should be 10.5% less
47# x 10.5%=4.935----47 - 4.935=42.065#
The hard thing IF you are not a Bowyer is doing it right

      

[Mad Max]

I have a 47# at 28” Bear Montana that I would like to shave 4-5 lbs from. Is it possible to accomplish that by narrowing the limbs or would I need to trap them?

I'm curious to see where this goes. On a structural basis, if you want to drop weight by 10% you need to narrow the working portion of the limb by 10%. It is a straight 1:1 ratio. So, if the limbs are 1.5" wide you would need to take 0.15" off the width. That is dead simple to calculate and pretty easy to do, but I want to see what other considerations the experienced guys have in mind.


Mark

Does not sound correct to me
On a Selfbow thickness is 8 times more than width--I don't think I said that right?
if you take a 1/16" off thickness it will be 8 times more # that taking 1/16" off  the width

[Mad Max]

Tradhunter88
This may be easer to do
If you could make the limbs 1" longer each  (remove tip overlays and make longer overlays you should get 4/6 #less)
Read down at the bottom--Piking

[bjansen]

Mattock said it correctly. If you are reducing weight by reducing width, it is a 1:1 ratio of size reduction to weight reduction.  Therefore, if you reduce the width by 10%, the poundage should reduce 10%.

That being said, what I would do with this bow is trap the back of it. Put a layer of tape on the back and sand off the corners so it is just covering the glass.  Then take your scribe and scribe a 1/16 line all the way down the limbs and then use a file to work the material off to that line (connecting your scribed line to the edge of the belly glass). You will be creating a trapezoidal limb section. Then sand the sides with 220 and round off the sharp edges you made.  String it up and check tiller and weight and if you need to reduce the weight more you can likely achieve it just with rounding the corners more.

Once you get close to your weight finish sand everything and seal / finish it. 

Good luck - take it slow and it will be fun

[mmattockx]

I use BJansen - Lam Stack Calculator pretty often, I asked him about reducing from 1-3/4" to 1-1/2" and this is what he said.
1/4" narrower limbs = 14% less pounds
47# x 14% = 6.58---47 - 6.58=40.42#
I did this on a recurve and it worked

This is exactly the same as what I said. 14% reduction in limb width = 14% reduction in draw weight.

Does not sound correct to me
On a Selfbow thickness is 8 times more than width--I don't think I said that right?
if you take a 1/16" off thickness it will be 8 times more # that taking 1/16" off  the width

It is correct. Stiffness of the limbs (which is what gives us the draw weight) is proportional to the cube of thickness and linear with width. Take a bow that draws 50# and cut that bow in half, width-wise. It will now be half as wide and will draw 25#.

Weight change based on thickness is not always 8x the width, it is the percentage change in thickness cubed. Weight change based on width will be proportional to the percentage width change.


Mark

[Mad Max]

I use BJansen - Lam Stack Calculator pretty often, I asked him about reducing from 1-3/4" to 1-1/2" and this is what he said.
1/4" narrower limbs = 14% less pounds
47# x 14% = 6.58---47 - 6.58=40.42#
I did this on a recurve and it worked

This is exactly the same as what I said. 14% reduction in limb width = 14% reduction in draw weight.

Does not sound correct to me
On a Selfbow thickness is 8 times more than width--I don't think I said that right?
if you take a 1/16" off thickness it will be 8 times more # that taking 1/16" off  the width

It is correct. Stiffness of the limbs (which is what gives us the draw weight) is proportional to the cube of thickness and linear with width. Take a bow that draws 50# and cut that bow in half, width-wise. It will now be half as wide and will draw 25#.

Weight change based on thickness is not always 8x the width, it is the percentage change in thickness cubed. Weight change based on width will be proportional to the percentage width change.


Mark

I got it now, My bad

[Shredd]

  After looking at your bow, I think you got some room to work with and you may even improve the performance...  The full draw pic is not the best and looks canted but I think it tells enough...  It is bending a bit more at the base of the limbs so I would stay about 6 to 7" away from the fades when you are sanding... you can always sand there later if you need to...  You will have to blend your sanding in and make a smooth transition when you get near that area...
   I would do what those guys said... Trap it...  If you like how it shoots now measure the tiller and check it as you go along to keep the same tiller... You want to keep the last 3 to 4" dead stiff... If it starts flexing there, don't sand that area any more...

  Good Luck...

[mmattockx]

I got it now, My bad

No worries, I was in the middle of stuff and may have come across more aggressively than I meant to.

  After looking at your bow, I think you got some room to work with and you may even improve the performance... 

I like it, might as well take the opportunity to re-tiller a bit while adjusting the weight.


Mark

[Mad Max]

All good

[tradhunter88]

I’ve built a couple selfbows but have never messed with any glass bows. I would prefer to go slow and conservative on this one. Could I do this with a sanding block or would a file be more practical?

[bjansen]

Sanding block will work great.  I’d wrap 60ish grit on a hard block and go from there.

[Shredd]

I am curious as to what the bow looks like before and after... It would be great if you took another full draw pic with a plain wall background and the bow not canted and 90 degrees to the camera so we can compare pics fairly...

  60 grit works great... When you are a pound or two away from the weight you want start going up to 80 grit and so forth... Final sanding and rounding the corners can take off a pound or two...

[Flem]

You look like a young guy, it might be easier to get a pair of dumb bells and do some curls

[jess stuart]

Could you just sand some glass off the back of the limbs and leave the width as it is?

[Stagmitis]

Dont know about other bowyers designs but for me I cringe at the thought of reducing weight from my original design.  I beleive my builds are optimized and should not be tampered with after the fact. Whenever I did It never ended well with regards to performance and stability.

[Mad Max]

Sanding block will work great.  I’d wrap 60ish grit on a hard block and go from there.

Sand paper wears out fast on glass so cut plenty