Power stroke from release to launch.

geoffretired

Supporter
Supporter
Heehee yumi yes! It is difficult to know what is the limb and what is the riser on those.
Certainly the part above and below the grip is upright at the raise stage and leaning top forwards by 30 + degrees at the full draw stage with very little bend in it,. The bendy parts are difficult to establish just watching but the most obvious is just at the top of the straight part that I would describe as a sort of riser section. It almost looks as if the bow bends at the top of the riser section, with slight bends in the other parts, That really bend bit is about half way up the bow despite being held about a third of the way up the bow. The top limb could be seen as that bit that starts above the "riser".
I would guess that the same principle is at work. Parts are weakened/ strengthened to give the desired results.
The top limb does move a lot to get back to the start. The bottom part of the bow, from bottom end to top of riser moves a lot too, but movement seems to be a little bit of straightening and a good deal of pivoting. Pivoting about the top of the riser, not the grip.
 

chuffalump

Well-known member
Just to add a little more conundrum. Once the string is released it is essentially slack. The only thing linking the limb tips to each other is the weight of the string and the arrow. Once the limbs hit brace height the string becomes a major player but before then they have much more independence.

So my three-legged race analogy is flawed.
 
D

Deleted member 7654

Guest
Just to add a little more conundrum. Once the string is released it is essentially slack. The only thing linking the limb tips to each other is the weight of the string and the arrow. Once the limbs hit brace height the string becomes a major player but before then they have much more independence.

So my three-legged race analogy is flawed.
Ah... but no, if you do a back of an envelope calculation (other writing media are also available) you'll find that even with an ELB the acceleration is about 300g .... so multiply the weight of the arrow by 300 and you'll see that it has a lot of inertia and thus presents a significant load of the string.
Del
 

geoffretired

Supporter
Supporter
I agree that the string is sort of slack once released. It is kept under some tension, though by the inertia of the arrow holding back at the nocking point. However, I feel there is more independence here for the limbs to do their own things. Not total independence; enough for them to work with more freedom. Whatever they do compared to one another will affect what the riser does and what the nocking point does as regards its line of travel.
It isn't a three legged race. Perhaps it could be seen as two sprinters racing to the line , but both are being subjected to a head wind. The wind resistance felt by each is determined by the speed of each of the runners.
 

geoffretired

Supporter
Supporter
Del, I think you have highlighted tow things now that are important. Riser rotation and the effect of arrow inertia, dragging on the string; changing its progress.
In theory, the stiffer limb travels faster, but perhaps the rotation of the riser does something to slow that down( every action has an equal and opposite reaction. So the bottom limb working harder ,pulls harder on the lower end of the riser.)
 

geoffretired

Supporter
Supporter
A good analogy should represent the truth, in a way that lets us understand something we would otherwise struggle with. The two sprinters, where one is slower but given a head start, so both end up at the line together is a good one( I think). It was not my idea, but I have accepted it as it seems to fit the reality.... but not totally.
I still think the analogy of the sprinters works, and could be made to fit the truth more closely once the missing detail is discovered and added.
If the faster limb reaches the at rest position with the same energy as the slower one, it seems it must have lost more energy on its travels. Perhaps it was lost by going into moving the riser, causing it to rotate.
 
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