Loft Posted June 16, 2015 Report Share Posted June 16, 2015 Moment of inertia Hello all J My experience as a lurebuilder is limited to just started, but I have been following the hobby for some years now, meaning this is theoretical not pragmatic. I was asked to elaborate the basics of moment of inertia (via PM), after mentioning it in another thread, and thought I’d make a thread so I wouldn’t misinform the guy, without someone to draw the BS card, or come with the real world way of doing stuff when it is applied to lurebuilding. Moment of inertia: Is the ability of an object to resist being rotated. (My own way of putting it, I don’t know the correct definition). To understand the meaning of it, imagine a broomstick, place two weights of 5lbs. in each, and the same broomstick with the two weights placed in the middle of the broomstick. They have the same weight, the same center of gravity (CG), but the one with weights in the middle will be much easier to rotate around the CG. The easiest way I can come up with to apply this to lurebuilding. Is if we think about copying a lure. Make the lure the exact same shape, and put the weight so it sinks the same way (the lure has the same: shape (the same center of lift), volume (the same amount of lift), mass (weight) and CG). This way everything seems to be right. However, it might act different. This is where the theory of the imaginary broomstick comes into play. If the lure action is too fast move the weights further apart, and the other way around if the action is too slow. There is no easy way (I know of) to measure moment of inertia. However, if the lure in the example above was a glidebait, a couple of prototypes might get you there. But if it was a multi joint swimbait, then the amount of variables to make the replica swim the same nearly gives me a headache. (remember, I’m lazy and have never tried making a swimbait replica (infact I’ve never made a working wooden lure L)) Finding the center of gravity Lay the lure on the side on a narrow flat object. When the lure does not tip off the CG is inside the edges of the object. If this is done in two opposite ways the CG is the point on the lure that was on the object both times. If the above method do not work with a lure (due to the shape of the lure). The CG can be found by weighing both ends of a lure on a fine scale, while holding the lure level. Then the CG can be calculated. This method does not give the height of the CG in the lure. Finding the center of lift The center of lift (or buoyancy). Is found by finding the point where the bait displaces half of it’s volume, in two different axis’s (like finding CG by the first method described above). If it is a bait of a consistent material, with no hardware installed, the CG and CL is the same point. The baits level in the water If we leave out the hydrodynamics. The level of the lure when it sits in the water, will be with the CG directly below the CL (a steel leader or movement of the lure might change this). Changing the CG with ballast If the weight, CL, CG, the desired weight and the desired level in the water (resulting CG) is known. Then the weight of the ballast is calculated by subtracting the actual weight of the lure, from the desired (this way do not account for loss in weight when drilling). The place to put that ballast is found in the following way: Now back to moment of inertia. We have the amount of ballast figured out, and where in the lure we want the CG of that ballast. If the entire ballast is placed at the place of the CG the lure will have a low moment of inertia, and change direction easily. If that is not wanted and a slow action is desired, the weight needs to be split out. When splitting the weight and keeping the same CG, the weight multiplied with distance (to CG) of both weights need to be the same: d1*B1 = d2*B2 If the distances is desired the weights to place at each point is calculated this way: B1 = BTOT*d2/(d1+d2) B2 = BTOT*d1/(d1+d2) (d1/d2 being the distances, BTOT being the total weight of the ballast and B1/B2 being the weight of the individual ballasts) Hope it gives a little sense, both theory, and the entire text (I'm from Denmark, and I'm better at mathematical stuff, than with grammar, and foreign languages) 1 Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 Very good article, well explained. The only part that I question is the relationship between the inertia and the speed of the lure. The speed of the cycle is wholly determined by the vortex fluid dynamics. The cycle speed can be roughly calculated with simple maths, using Strouhal equation. The inertia determines how far the lure can move in its allotted time. To back up my point of view; consider a lipped crank built light with no hooks or ballast. It will swim at say 6 cycles per second. Add the hooks and ballast and the cycle rate remains the same, but the width of the cycle will be reduced due to the extra inertia of the ballast and hooks. Dave Quote Link to comment Share on other sites More sharing options...
Loft Posted June 16, 2015 Author Report Share Posted June 16, 2015 Very good article, well explained. The only part that I question is the relationship between the inertia and the speed of the lure. The speed of the cycle is wholly determined by the vortex fluid dynamics. The cycle speed can be roughly calculated with simple maths, using Strouhal equation. The inertia determines how far the lure can move in its allotted time. To back up my point of view; consider a lipped crank built light with no hooks or ballast. It will swim at say 6 cycles per second. Add the hooks and ballast and the cycle rate remains the same, but the width of the cycle will be reduced due to the extra inertia of the ballast and hooks. Dave Hi I actually didn't put that much thought into it, i was just thinking about overall movement. I'll see if i can rewrite it to be more correct Thank you for the reply Loft Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 Hello Loft I appreciate you putting this together and sharing it . Really good stuff. No engineer here but I can follow what your talking about very well. Usually takes some doing for me to wrap my head around this sort of thing but the way you have written here it is easy to understand. I am having some trouble though with parts of the equation. NCG = 2.5 As I understand, this is the new desired CG. Where did you get 2.5? And the equation at the end there dCGB = (NCG - d Wr/Wt) Wt Are we supposed to put 2.5 into the equation here for NCG? Thank you again!! Vic Quote Link to comment Share on other sites More sharing options...
Loft Posted June 16, 2015 Author Report Share Posted June 16, 2015 (edited) My mistake. Forget about the 2.5 i just forgot to erase it after testing the equation. Loft Edited June 16, 2015 by Loft Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 Loft - I would leave it as it is. It is a very good article for discussion. Dave Quote Link to comment Share on other sites More sharing options...
mark poulson Posted June 16, 2015 Report Share Posted June 16, 2015 I only know moment of inertia from residential construction, and shear forces. For that, the moment of inertia is how much force is needed to move a weight up above the ground, like a roof or second floor, at the same time as the ground moves. In other words, how stiff the walls need to be to move the high weight with the ground without bending, to overcome the moment of inertia. In lure building, I guess moment of inertia comes into play for me in top water baits, and glide baits. They need to overcome the moment of inertia both when the walking action is first initiated, and again at each change of direction. For top waters, the further back past the mid point, head to tail, the center of gravity is located, the harder the baits are to walk, but the better they cast. My small poppers and smaller walking baits hang down almost vertical, because they are so light it's easy to get them up and walking. But my bigger walking baits, like Lunker Punker knock offs, sit just slightly tail down at rest. They are so big that having them hang down like a popper would make them a bear to get up to walk all day, and would make a slow glide almost impossible. I'm sure it is involved the cranks and jointed swim baits, too, but I don't really think of it with them. 1 Quote Link to comment Share on other sites More sharing options...
Loft Posted June 16, 2015 Author Report Share Posted June 16, 2015 Mark The moment of inertia also keeps your glidebaits going straight. Loft 1 Quote Link to comment Share on other sites More sharing options...
mark poulson Posted June 16, 2015 Report Share Posted June 16, 2015 Loft, I always struggle with one piece glide baits, because they do seem to want to go straight, and I'm looking for a side to side glide, like with my Punker surface gliders. I've tried putting the ballast just behind the center of gravity of the baits, and it works, but it is finicky. Too close, and no side to side. Too far back, and it tail drags, which looks bad. I have had to play around with each bait so much to get it right that I've quit trying to make them. Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 (edited) Loft/ Anyone I would like to use this in a practical way. The first hurdle is finding / making a fine scale to measure our extremely light lures. I am having trouble finding anything in a pull scale with a range small enough. Anyone have any ideas ? Vic This is the only thing Ive found and like but look at that price . Not practical for my budget. http://www.scalesgalore.com/imadafbps.htm Edited June 16, 2015 by littleriver Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 I would like to use this in a practical way. The first hurdle is finding / making a fine scale to measure our extremely light lures. I am having trouble finding anything in a pull scale with a range small enough. Anyone have any ideas ? Can you explain in more detail what you intend to try. Dave Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 Dave , I would like to weigh a lure as described in Loft's drawings. But the scale required is extremely small.Vic Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 (edited) Here is another interesting option and at only $184 may be the bargain of the day but not for my wallet. Accurate to .01g http://www.scalesgalore.com/ocentdl.htm Edited June 16, 2015 by littleriver Quote Link to comment Share on other sites More sharing options...
Loft Posted June 16, 2015 Author Report Share Posted June 16, 2015 You should be able to find a reloading scale much cheaper than 184 USD If it is a digital like a kitchen scale, then an improvised stand should do And don't use that kinda money based on my theories, i'd be ashamed if it didn't work How do other guys manage the extreamely light lures? Loft Quote Link to comment Share on other sites More sharing options...
scrubs Posted June 16, 2015 Report Share Posted June 16, 2015 Vic, I've seen articles on making beam balances. If you want crude remember a penny is 2.5 grams and a nickel is 5 grams. try this set of links: https://www.google.com/?gws_rd=ssl#q=homemade+balance+beam+scale bill 1 Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 Vic, I've seen articles on making beam balances. If you want crude remember a penny is 2.5 grams and a nickel is 5 grams. try this set of links: https://www.google.com/?gws_rd=ssl#q=homemade+balance+beam+scale bill Finally something that hits my price point. Under Actually! Thanks! Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 Loft I have been using the looks right, it must be right rule . Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 Here is one I like . http://sci-toys.com/scitoys/scitoys/mathematics/microgram_balance/balance.html Quote Link to comment Share on other sites More sharing options...
JRammit Posted June 16, 2015 Report Share Posted June 16, 2015 At the risk of sounding like an inappropriate comment... Ive been wondering if a finger scale (like the kind you would find at a "tobacco shop") would be accurate enough for our needs.... I think they run in the neighborhood of $5 and measure down to 1/2 gram Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 I don't see why not but not sure of the resolution. Mail scales are also very cheap . Mail scales read to 1 gram . Here is a way to convert the scale I have to mg scale and it is cheap. So I am liking this solution. Reload scales is another option and i like that to but i am going to start with materials I already have. Here is the link. http://www.rcgroups.com/forums/showthread.php?t=904951 Quote Link to comment Share on other sites More sharing options...
littleriver Posted June 16, 2015 Report Share Posted June 16, 2015 (edited) Going cheaper yet and doing no math but i am stuck with 1g resolution of my digital mail scale. This lure weighs 12 grams sitting on the scale . Here is what i get using my version of the Loft method. @Loft Will this work? Edited June 16, 2015 by littleriver Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 I wrote a TU article on how to make a beam balance. I am sure this could be adapted for this application. Cost - free. Dave Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 Here is the article about the scales. The images are lost, but I still have them on my computer, and the spreadsheet. If you are interested, I will re-post the images. The scale in the article is designed for weighing a general range of lures and so no good for your purpose, but the scale can be designed for a specific application and be very accurate. I believe 0.01g is possible. Dave 1 Quote Link to comment Share on other sites More sharing options...
Travis Posted June 16, 2015 Report Share Posted June 16, 2015 Hmmmm.....do any of these equations equal designing a better crank at catching more fish? Quote Link to comment Share on other sites More sharing options...
Vodkaman Posted June 16, 2015 Report Share Posted June 16, 2015 Travis - comments like that certainly don't! Quote Link to comment Share on other sites More sharing options...
