Steve Jackson Games Forums Bows:From the Ground Up
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 01-12-2009, 08:32 PM #1 Nymdok     Join Date: Dec 2006 Location: Houston Bows:From the Ground Up This will be the first post. I will declare the reults of my research into bows and provide the generating equations. I will follow it up with Logic and Resources (How I came to this conclusion), Assumptions (all the chicken as a sphere foolishness), Modifications for Crossbows(Tension)and Examples. Note that these are consistent with Basic Set except for the weight of the shortbow. Draw Strength of a Bow is BL x 4 Draw Strength/(Specifc Gravity of Material /.67) = Volume of the wood to be used. VolumeOfWood/Length = Cross sectional area of the bow. For composite bows, use the weighted average of the specific gravities. Using these equations you can make a str 18 balsa wood short bow for your halfling, a ST 25 Iron bow for a giant and anything in between. Nymdok EDIT: And so its done. Thanks to all who responded. The Final Product can be found here http://sites.google.com/site/nymdoksgurpsaddons/ Last edited by Nymdok; 01-18-2009 at 03:09 AM. Reason: For the end
 01-12-2009, 08:42 PM #2 Nymdok     Join Date: Dec 2006 Location: Houston Re: Bows:From the Ground Up Assumptions: All bows taper as they come to their ends. Energy absrption is goverened dominately by the density of the material. Energy is Distributed evenly throughout the bow.Thinner parts flex more, thicker parts have more material.(remeber its force * distance for energy) Range and Damage are goverened by the strength of the bow, which is essentially the energy stored in the bow before loosed, no further fiddling is needed. The Mary Rose relics and Otzi's bow are reliable representations of longbows. The Draw length is assumed to be about 30" from braced. Draw Strength is a 2 handed effort for 2 seconds. Eyeballing B353 we have BLx4. Recurving bow does not have an appreciable (less than 10%) energy storage increase from a straight bow. The force required for a full draw remains unchanged. Draw Length of a bow is equal to about half its height. Draw SPan for a Humanoid creature is 1/2 their height.(Length of arm to center of chest) Last edited by Nymdok; 01-21-2009 at 03:47 PM.
 01-12-2009, 08:53 PM #3 Nymdok     Join Date: Dec 2006 Location: Houston Re: Bows:From the Ground Up Logic and resources: The resources I used were: The Mary Rose Relics Data. Otzi's Bow http://www.archive.org/details/chapt...eevol017966mbp http://www.bio.vu.nl/thb/users/kooi/kobe97.pdf http://en.wikipedia.org/wiki/Longbow lists the world record draw for a bow at 200lbs/900N. This is ST 16 for 4xBL. Nymdok Nymdok
 01-12-2009, 09:08 PM #4 Nymdok     Join Date: Dec 2006 Location: Houston Re: Bows:From the Ground Up Modifications for Crossbows and other Draw Lengths: The following covers only tension powered crossbows, not torsion. To modify these for Crossbows, we need to pay attention to the energy invested. The Bows as listed above assume 30" draws which means that the energy under that curve is approximately: 30 x (BL x 4) Now we need to get that same energy in a draw of about 15" for a crossbow bolt, but as we have our FULL basic lift at our disposal (8XBL) its not that tough. As you can see, we get the same energy 15 x (BL x 8) = 30 x (BL x 4) So the trick here is to get the Crossbow prod length, which is somewhere between 24-36" depending on how you choose it. Then use the above equations. EASY! Nymdok Last edited by Nymdok; 01-12-2009 at 09:24 PM.
01-13-2009, 06:13 AM   #6
nerdvana

Join Date: Nov 2006
Re: Bows:From the Ground Up

Quote:
 Originally Posted by Nymdok The ideal shape is rectangular as used by the Native North Americans.
I'm not familiar with this style bow, could you profide an informational link? Perhaps one with pictures also...

 01-13-2009, 06:26 AM #7 Nymdok     Join Date: Dec 2006 Location: Houston Re: Bows:From the Ground Up http://en.wikipedia.org/wiki/Flatbow Remember that Flatbow/Rectangular refers to the cross sectional shape. Nymdok
 01-13-2009, 09:16 AM #8 Nymdok     Join Date: Dec 2006 Location: Houston Re: Bows:From the Ground Up Finally, we have an Example. First an Easy one, Red Maple with a specific gravity of .54. We want a bow of light strength and short stature for a theif so we'll make it a ST 8 bow. ST 8 =>Bow pull 51 Pounds. We want a short bow.....really short....40 inches short. So its 48/(.54/.67) = or 60 cubic inches of this material. Divide the Cubic inches by the height to get the cross sectional area 60/40 = 1.5 square inches in the cross section. For smallish man sized hands 2 inches wide is plenty, so it has a thickness of .75 inches. Note that these dimensions are the average of the bow, its a little bulkier at the grip and thinner at the tips. Since we used specific gravity up top, all we have to do to Find the carrying weight of the bow is multiply its volume, times the Density of water (.036 lbs/cubic inch) or in this case 60 * .036 which gives us a 2 pound bow. There you have it! Steps to a bow accurately desribed in detail for your players, of the ST YOU chose and the material YOU choose and the length YOU choose! So to sum up ST: 8 Material:Red Maple Length: 40" Width: 2" Thickness:.75" ST Required for ROF 1(1) = 12 (pull = 115) Ill do one of a more complex example later right now my boss is looking at me funny :) Nymdok
 01-13-2009, 10:04 AM #9 DouglasCole Doctor of GURPS Ballistics     Join Date: Sep 2004 Location: Burnsville, MN Re: Bows:From the Ground Up The only issue I have here is that having a 50lb bow be ST8 is a bit harsh. In a different thread, we'd come to the conclusion that a person could draw a bow of BLx2 lbs, with StrongBow adding +0.5BL at DX and another at DX+2; Special Exercises (Arm ST) could be applied in two levels for the same bonus. So a specialized archer with good skill and special exercises could draw the bow with 4xBL of pull. Otherwise, I worry that the damage of these bows will be extremely unrealistic. Using the firearms scaling style, I figured a 180lb warbow would likely only do 1d+2 damage... __________________ Gaming Ballistic, LLC
 01-13-2009, 10:16 AM #10 SuedodeuS   Join Date: Sep 2008 Re: Bows:From the Ground Up I like it. You might want to consider editing your initial post to include units, however. Also, I'm curious as to why you used 48 instead of 51 in your calculation. Finally, what is your justification for using the .67 conversion factor? I seem to recall you justifying it, but can't find that anymore. 50# does seem rather high for ST 8. I personally struggle with a 50# compound bow, and my ST is probably around that level. I can do it, but (in GURPS terms) it costs FP for each shot. Were it not for the let-off of compounds, however, I probably wouldn't be able to manage at all. __________________ Quos deus vult perdere, prius dementat. Latin: Those whom a god wishes to destroy, he first drives mad.

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