Rod bend vs rod unload

[Paul Arden]

Hi Graeme,

I’m not sure why the distance changes but I’m pleased we agree

Cheers, Paul

[Walter]

Hi Paul,

As I say, I glossed over what is actually meant by loss. There is a similar real life case in the spring analogy and that would be the dropped spring. The one where you let the spring stretch and then drop it. The spring seems to be suspended in mid air briefly before falling naturally. But that’s only if you look at the bottom of the spring. The top end is moving far faster than an object under the influence of gravity only. At some point the spring fully collapses and then it falls as expected.

In the falling spring work was initially done on the spring by gravity to give it its initial stretch. But when the whole spring hits the ground where did the elastic energy in the spring go? If we find the center of mass of the spring and drop a ball from that point at the same time as we release the top of the spring both objects will hit the ground at the same time. That work that was done to initially stretch the spring is lost in that it did nothing to make the spring fall faster. It went into contracting the spring.

I know there will be responses about conservation of energy and and kinetic vs potential energy and elastic energy in the rod and how all of that has to be conserved. All true.

I’m just concentrating on whether the spring stretch and the work done to stretch the spring did anything to make the fly end of the spring hit the ground with more speed than a dropped weight from the same height. The top of the spring will hit the ground in less time than a ball dropped from that height BUT when the top of the spring hits the ground it will only be moving at the speed of a ball dropped from the Center of mass of the spring. The falling ball from the higher height will take longer to hit the ground but it will be moving faster when it hits the ground than the top of the spring did.

Many scenarios going through my head at the moment so it’s a bit rambling. The thing is that you did work to bend the rod but now it takes work to unbend the rod so how much does that affect the line speed. There is no hard figure like 15 to 20% to write that down to because it’s on a cast by cast basis.

There is no such thing as a rod that puts out more energy than you put in. We can only hope to make rods that put out approach 100% efficiency. That goes back to the original post.

The question I think for you is if there is a rod that lets you put more than 100% of the other rod so that at similar efficiencies you get more out than you would with the other rod. That’s where I say it’s finely tuned and can only happen on a cast by cast basis because the efficiencies will not be the same.

[Paul Arden]

“ There is no such thing as a rod that puts out more energy than you put in.”

But this is really the point I think because a bendy rod does allow you to put in more energy into the cast and we can feel the difference when we cast when compared to a rigid one.

The difference is considerable. I remember being quite shocked when trying to cast the broomstick because despite being heavy it felt lighter to cast.

Cheers, Paul

[Walter]

Yes but, the next part was about whether putting more in meant more out. In a purely rigid system that would be the case. Use a lever lift a weight, push a bigger lever a bigger weight. Straightforward.

But this isn’t rigid (I know - that’s your point but there is a bit more). Now push a bendy lever. I do work before the weight even moves. I do no work on the weight but I’ve stored elastic potential energy in the lever and I’ve done zero work on the weight. If I stop there I’ve done a lot of work but accomplished nothing. I’m sure we’ve all done this but the next step is jumping up and down on the lever. I can get a much deeper bend in that lever by jumping up and down on it. The fact that it’s bending let’s me store even more elastic energy in it. But if that weight doesn’t move what happens? I get tossed into the air as the lever unloads. Lots of work happening everywhere except where I really want that work to happen. If I’m persistent enough I’ll get that weight to start moving. But maybe it only moves a bit and in the meantime I’ve bent the other end all the way to the ground and can go no farther. The weight just kinda lifts off the ground. Tremendous amount of input and very little output (sorry if that sounds sarcastic. It’s not meant to be, I just have a lot of scenarios running through my head). Maybe the weight to load ratio is such that when I’ve bent that lever halfway to the ground the weight starts to move. If I made that lever bend by jumping on it I may still be doing a lot of work and continuing to load the lever at my end while it’s already starting to unload at the other (Servers self unloading rod concept). This could start to cause oscillations where the rod is continuously partially unloading, partially reloading and then reloading again. That sounds far fetched but I actually did a model years ago that showed exactly that would happen. And, final scenario, you hit the sweet spot - Server’s actual unloading rod concept. The lever fully unloads just as my end hits the ground. That did show up in my model but it was extremely sensitive to hit. I also found that softer the rod the less the return you get. Apologies for not having the model anymore.

[Paul Arden]

Hi Walter,

I agree then that putting more in doesn’t always mean you’ll get more out but it doesn’t mean that you won’t either! If instead of lifting a rock we just looked at the speeds obtained by the other end of the lever (the rod tip) I think that would be much closer to what we have in the fly cast, since the line is only be a few dozen grammes (and rods cast without lines have a remarkably similar bend during the Casting Stroke).

In the case of the 9’ broomstick it was very hard to throw 100’ casts. We measured some but i can’t recall the numbers. Tip path was certainly and issue, as was stopping the rod. What I don’t have any numbers on was line speed. That would be an interesting experiment and if I travel to Spain this summer then maybe I’ll get a chance to make some measurements.

Cheers, Paul

[Walter]

Hi Walter,

I agree then that putting more in doesn’t always mean you’ll get more out but it doesn’t mean that you won’t either!

I’m 100% in agreement there.

I’m sure some of the modeling people can work out the specifics. I may dig out my old model but it was the spring and marble approach. This needs to be more sophisticated and include tip path or many people won’t accept it. I already don’t as a matter of fact.

[Lasse Karlsson]

Aren't we back at the biggest point of the bendy lever, is that our input gets transfered to a more straight path at the tip than with a rigid lever? They both works best when we add a haul that directly accelerates the line at the right point in time. And the bendy one doesn't hurt your elbow as much?
And when we add the haul, the 15% goes out the window anyway. Those % was also for a short cast, there was never an estimate for a longer cast, and since we know we release well before RSP on those, that number might even be much lower..

Cheers
Lasse

[Walter]

Lasse,

Sometimes my haul even adds negative amounts of energy.

[Paul Arden]

One way to test that would be to measure line speeds produced (or tip speed) of a rigid and a bendy lever.

[Walter]

One way to test that would be to measure line speeds produced (or tip speed) of a rigid and a bendy lever.

In a sense you’ve already done that.

If the tip path is reasonably straight then the line speed and distance are directly proportional.

I know you want to eliminate all the other variables though. You could do that by measuring speed specifically or use statistical analysis. One way you take video of a few casts, measure speed and do a statistical analysis because there will still be some variance, or the other way is to measure distance for a bunch of casts and apply a statistical analyses. Some work required either way.

People have asked how many casts are enough. I’ve always been partial to a chebyshev analysis for that. You could plug numbers in as you take them and keep going until you are satisfied with the confidence interval it generates but I expect 10 casts per rod would be enough. Perhaps only 6.