# Recurve center of gravity?



## Greg Bouras (Nov 17, 2006)

mfriedma said:


> What's a good position for the center of gravity on a recurve bow? I am asking this to help 'tune' my stabilizer set-up. I have a simple set of cartel stabilizers that includes 30" long rod, v-bar, 3" extension, and 12" side rods. The center of gravity(as measured by hanging from two distinct points and finding the intersection of a plumb line from each point) currently falls just at the back side of the bow in line with the throat of the grip. Although it does fall forward, it just doesn't really jump out of my hand and I was wondering if this had anything to do with the balance of the bow. I think I need to go with a longer extension at least. Shorter side rods? etc...
> 
> Thanks,
> Mark


Hello Mark: Joe Tapley's site has some very good information on the give and take issue of Center of Gravity, Moments of Inertia and riser rotation.

http--www.tap46home.plus.com-mechanics-


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## Joe T (Apr 5, 2003)

Bottom line here is the mysterious topic of Dynamic Tiller. Because the arrow and the nock point are above the (OR centrally located) grip during the power stroke a torque is generated on the riser in the direction of the riser top going backwards. General solution is to generate a torque on the riser in the opposite direction to counteract this

With a longbow solution was to make the bottom limb shorter/stronger. Limbs on an OR bow are same strength and geometry so different solution required. There are two methods - positive static tiller and moving the bow balance point forward of the grip. They both work but IMO moving the balance point forward is the more forgiving option. 

Positive tiller is like standing on the top of a mountain any sideways movement and down you go so very critical to archer variations in bow/draw hand etc.

COG forward of grip is like standing on the edge of a cliff. The further the COG in front of the grip the further away from the edge your standing so you can accommodate sideways movement (archer variation) and stay on the level bit.

So no "best position" for COG only being far enough forward. Symptoms of dynamic tiller problem is seeing riser move back/long rod end lift after shot or a lot of post shot limb vibration. Easiest solution is just add more weight to the end of the long rod.


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## Vittorio (Jul 17, 2003)

Just to add to JoeT very clear explantion, side rods that go to the back of the vertical of the riser are moving the COG backword, so for sure a 3" extender and 12" side rods are a very wrong solutions until the weight on top of ththe long rod becomes really huge.


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## mfriedma (Sep 28, 2009)

I agree that the combination of side rod and extension are not giving me the affect I intended. It is kind of a pain to have to ship back stuff for exchanges, but I'm not sure if simply going to a 5" extension is appropriate. Or should I also exchange the side rods for say 10" rods? Making incremental changes would be better, but not having a bow shop around here makes it difficult. Thus the reason for my seeking advice on this forum, albeit a little late. 

Mark


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## mfriedma (Sep 28, 2009)

*Tiller*

Joe,

Thanks for your input. I assume that is your website. I think I am beginning to understand tiller, but maybe you can correct me if I am wrong.

The typical recommendation is to increase the stiffness of the lower limb by a small amount. At this setting, if one were to have a perfect pivot point grip, the bow would tip down slightly during the draw. This assumes a level draw of the nock point. The string length, and ones grasp of the nock point and the act of drawing it on the level are constraints on the system. The tips of the bow limbs would have to move back an equal amount with the draw because of these constraints. The only way to achieve this, given a stiffer lower limb, is for the riser(bow) to pivot slightly downwards, to make up for the lesser amount that the lower limb is willing to flex. Thus the tips move back equally. A less than perfect pivot point at the grip obviously induces some torque on the riser. No static tiller, no such torque.

This is however the limit of what I think I understand. Several questions come to mind. What happens when I remove one of the constraints when I let go of the string with my theoretical perfect grip and a stiffer lower limb? Which way does the nocking point go as it travels forward due to tiller(Ignoring the affect of COG)? A weight forward COG obviously tends to move it upward. Is this beneficial to my not so perfect pivot grip which is inducing at least some upward(nock down) torque? I'm sure there are more questions, but I need to go to work, to pay for my new hobby(driving myself crazy trying to perfect the imperfectable)

Mark


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## Shinigami3 (Oct 7, 2009)

If you look at the FITA world cup and world championship matches on archery.tv, you see nearly all finalists have pretty much the same setups- medium length V-bars set straight (no down angle, mostly) and 2-3 front weights on 26-30" fronts, with 4-5" extensions, a couple weights on each v-bar, and a top rod with a little bit of weight.

Sure there's some variation, like a couple of people using no top rod, but mostly the highest scores are shot with that sort of setup.

Whether that is a self-perpetuating phenom is an other question


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## Joe T (Apr 5, 2003)

mfriedma said:


> Joe,
> 
> Thanks for your input. I assume that is your website. I think I am beginning to understand tiller, but maybe you can correct me if I am wrong.
> 
> ...


You can't change the stiffness of a recurve limb (fixed by the manufacturer) only the angle the limb comes out of the riser. This changes how the limb bends for "static" draw (hence the limb stored energy) and affects the magnitude and direction of the limb reaction force on each limb pocket dynamically.

Not much relationship between static draw and dynamic behaviour. e.g. if you hook a finger round the nock point and pull the riser rotates bring the push and pull in line. The torque and friction of the tab on the string keeps the riser vertical - but no tab dynamically. The arrow has no effect on the static draw but is significant dynamically.

As for nock travel dynamically haven't got a clue 
Dynamic behaviour of an asymmetrical OR bow AFAIK is an Everest yet to be climbed. How the arrow behaves on the bow on a practical level is irrelevant, so triple somersaults - no problem. It's how the arrow leaves the bow that matters.


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## Vittorio (Jul 17, 2003)

Shinigami3 said:


> If you look at the FITA world cup and world championship matches on archery.tv, you see nearly all finalists have pretty much the same setups- medium length V-bars set straight (no down angle, mostly) and 2-3 front weights on 26-30" fronts, with 4-5" extensions, a couple weights on each v-bar, and a top rod with a little bit of weight.
> 
> Sure there's some variation, like a couple of people using no top rod, but mostly the highest scores are shot with that sort of setup.
> 
> Whether that is a self-perpetuating phenom is an other question


In Ulsan the solution was used by 99% of the archers., not by those in the TV matches, only. This set up has shown to be the best one since 20 years ago, when it started spreading around. It means probably 90% of the World and Olympic medals have been got with this set up since then. 
Try to find a better one is just a waste of time, IMHO.

Of course, same is also applicable to other solutions, like Spin Wing vanes, X10 arrows (from 1996), Carbon core limbs and Dyneema based strings.

Agree that whether these are a self-perpetuating phenoms is an other question


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## TheAncientOne (Feb 14, 2007)

Link to an article on "Controlling Bow Behaviour with Stabilizers"

http://www.centenaryarchers.gil.com.au/images/Bow Stabilisation.pdf

TAO


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## OldSchoolNEO (May 11, 2009)

*Cog?*

I wonder what the COG position was with this set-up?


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