Post by: tradhunter88 on December 22, 2020, 01:51:16 PM
Post by: Shredd on December 22, 2020, 02:42:29 PM
Post by: mmattockx on December 22, 2020, 03:17:10 PM
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
Post by: tradhunter88 on December 22, 2020, 04:57:40 PM
Post by: Mad Max on December 22, 2020, 05:16:40 PM
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 :dunno:
Post by: Mad Max on December 22, 2020, 05:36:34 PM
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 :dunno:
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
Post by: Mad Max on December 22, 2020, 05:46:30 PM
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
(https://i.imgur.com/pEdFIoq.jpg)
Post by: bjansen on December 22, 2020, 07:08:30 PM
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
Post by: mmattockx on December 22, 2020, 07:09:24 PM
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 :dunno:
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
Post by: Mad Max on December 22, 2020, 07:24:35 PM
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 :dunno:
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 ;)
Post by: Shredd on December 22, 2020, 08:04:45 PM
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...
Post by: mmattockx on December 22, 2020, 08:11:01 PM
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
Post by: Mad Max on December 22, 2020, 08:32:38 PM
Post by: tradhunter88 on December 23, 2020, 08:34:21 AM
Post by: bjansen on December 23, 2020, 09:13:14 AM
Post by: Shredd on December 23, 2020, 10:42:28 AM
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...
Post by: Flem on December 23, 2020, 11:00:04 AM
Post by: jess stuart on December 23, 2020, 02:25:44 PM
Post by: Stagmitis on December 23, 2020, 06:08:16 PM
Post by: Mad Max on December 23, 2020, 06:20:04 PM
Sanding block will work great. I’d wrap 60ish grit on a hard block and go from there.:thumbsup:
Sand paper wears out fast on glass so cut plenty :thumbsup:
Post by: Tim Finley on December 23, 2020, 07:32:49 PM
Post by: Crooked Stic on December 23, 2020, 08:48:41 PM
Post by: Roy from Pa on December 24, 2020, 07:38:09 AM
Click it and save to your pictures.
[ You are not allowed to view attachments ]
Post by: Mad Max on December 24, 2020, 07:46:47 AM
PM me your email address
Post by: Roy from Pa on December 24, 2020, 08:09:20 AM
Looks like an envelope:)
Post by: kennym on December 24, 2020, 10:09:17 AM
Post by: Roy from Pa on December 24, 2020, 10:14:46 AM
Post by: mmattockx on December 24, 2020, 10:26:45 AM
Okay where do I go to download the Bjanson calculator. Lost it when I got my new PC.
I will be gone all day but I will try to send it to you tonight.
PM me your email address
I can email it as well if you can't wait for MM to get home tonight.
Mark
Post by: Longcruise on December 24, 2020, 11:58:18 AM
Post by: Crooked Stic on December 24, 2020, 11:00:22 PM
Post by: Mad Max on December 25, 2020, 07:22:21 AM
Post by: Roy from Pa on December 25, 2020, 08:36:21 AM