Does science help?

[geoffretired]

I think it does. But it sometimes confuses me and that is something I don't like. I don't like to see confused beginners/improvers either.
Can someone please help to clear my mind? (without resorting to drugs; legal or otherwise)
Set out below is my thinking,right or wrong;I'm not sure!
Assume a bare bow for the moment. On release, the arrow, string, limbs move forward. There must be something moving backward, the riser I assume.
If everything was symmetrical and in balance, with the bow hand pressing into the grip on a level with the arrow(impossible but just imagine for now) then the riser would move back quickly with no tendency to tip backwards or forwards.
The bare bow would need a reflex riser to put some mass in front of the hand otherwise there would be a slow backward rotation.
Now move the hand down to the normal level. The bow is balanced around the arrow still but it is held below. The forces now will tend to rotate the bow so the top of the riser moves towards the archer. This will happen very quickly and, in amongst everything else, it will go unnoticed I suspect.
IF my thinking is correct so far, is this the reason why the long rods tend to flick upwards when they are "underweight"?
I have shot with a very light long rod and a laser. The video shows the flick upwards clearly.The laser indications are upwards too. It is very quick so it's not the bow tipping back because it's out of balance(that would start slowly)
I believe it is the result of the reaction to the arrow launch.
Getting the hand and arrow as close to each other as possible would reduce it. I believe some top archers set up like that.
I think archers put weight on the long rod to reduce it too(and for other reasons ) I keep having nagging doubts that there is a better way of reducing this backwards rotation other than mass added to the long rod.
Please help me!!!

[JohnK]

Originally Posted by geoffretired

Assume a bare bow for the moment. On release, the arrow, string, limbs move forward. There must be something moving backward, the riser I assume.

Err, I don't think it works like that (sayeth the Arts graduate).

For every action, there must be an equal and opposite reaction. The action is drawing the bow (pulling the limbs back) and the reaction is what happens on release (the limbs go forward).

That seems logical to me anyway. I shall wait to be blown out of the water by a clever scientist

[joetapley]

Geoff
Your 95% correct. Because the riser recoil "centre of force" is above the pivot point there is a torque tipping the bow (top limb) back. The riser recoil force on the bow hand is fairly complex - over time it's "hill" shaped and peaks somewhere around twice the draw weight.

Increasing bow moment of inertia has no effect on the recoil torque it only reduces the resulting angular acceleration. To get the bow to "hold still" you need to have a torque in the opposite direction to the recoil torque. Pushing the bow cog forward generates a gravity torque about the pivot counteracting the recoil torque. Why a bow feels forward heavy when holding it is because of this gravity torque which is always there.

This is why putting the bow cog at the grip doesn't work. You can only increase bow moment of inertia. You can do nothing to counteract the recoil torque.

I suppose you could install a bungee between the bottom part of the riser and your quiver belt.

The reason barebow archers add weight to the bottom part of the riser is partly to increase bow moment of inertia, but also as you tilt the bow (top limb back) the cog being below the grip moves forwards of the pivot and generates a gravity torque. Practically you can't lower the cog very far as the bow would get so heavy as to need a fork lift

JohnK...........BOOM!

[geoffretired]

Thanks for the input.
JohnK I think your action reaction is more to do with input output. You put in the bend and you get out a straightening effect. An elastic band or spring gives similar.
Joe, I have always thought the recoil torque was very difficult to subdue. I never knew about the term "recoil torque" but I'm still learning(Thank goodness) I like the bungee on the belt. I did toy around with ideas of weights on the string to match the arrow weight, equidistant below the hand.
Slows the bow a bit I suspect. How about half the arrow weight at twice the distance?Or some other combination?
So, as it stands, the recoil torque has to stay but it can be slowed by adding mass anywhere but at the pivot point/grip, and further away the better for that purpose. Adding that mass in front of the grip will also give a gravity torque that will give the added mass two ways of slowing the recoil torque.
Infront and below would seem better than infront and above, as that would make the bow top heavy and inclined to fall over sideways.
I feel better now, Thanks folks.
Geoff

[Deadeye Doc]

There's a website - really for the technically minded - on which things like the effect of tiller (which may cause the long-rod to flick upwards during the execution of the shot) arrow spinning, bracing height, Archer's Paradox etc. are pretty definitively discussed. I don't understand the mathematics as much of this is about engineering and ballistics, but there are some very clear explanations in some parts of the site. It's called Topics on bow mechanics and is regularly updated.
Another possible reason for the long-rod flicking upwards of course might be "heeling" the grip.
Here's the website.
http://homepage.ntlworld.com/joetapley/

[geoffretired]

Thanks for all the input on this, I appreciate your help. My mind is much clearer now.

[elastoplastscavenger]

Originally Posted by JohnK

Err, I don't think it works like that (sayeth the Arts graduate).

For every action, there must be an equal and opposite reaction. The action is drawing the bow (pulling the limbs back) and the reaction is what happens on release (the limbs go forward).

That seems logical to me anyway. I shall wait to be blown out of the water by a clever scientist

action and the equal and opposite reaction happen at the exact same instant. on drawing, the action is pulling the string, the reaction (through the limbs) is an equal force in the opposite direction applied by your other hand to the riser.

on release the action is a force accelerating the arrow forwards down the range so the equal and opposite reaction must be a backwards recoil force towards the archer.

i think you were thinking more along the lines of energy rather than forces

[Marcus26]

One of the things to remember with the stabilizer is that it will vibrate as the bow is released because energy is running through it. This will also cause the tip to kick upwards, even if balanced correctly.

[Deadeye Doc]

Originally Posted by Deadeye Doc

There's a website ....
Here's the website.
http://homepage.ntlworld.com/joetapley/

Sorry Joe

I don't think I noticed your contribution. Indeed it may not have been there when I put my ha'pennyworth in. Great website if sometimes confusing to a non-mathematician/non-engineer.

[Bald Eagle]

We can go round and round in circles on this one, suffice to say a 41" or 45" Beiter is the best all round long rod for compounds as well as recurves! After trying out various combinations of long rod and twins, archers I've coached have bought Beiter 41" or 45". The difference they make to the shot is unbelievable, low frequency, like the tightrope walker with his "long pole" Try it, it works!