TRANSLATION / ROTATION

Walter & Group........

Sometimes we can get too complicated and confuse an issue.  I've been guilty of that before.... so why not now ?

Lewis Hinks came in with this message:-

 

Hi Gordy,

 

    I guess I'll have to talk to Dennis some more. It appears I still do not have the concept of translation and rotation clear in my head.

 

Lewis

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My reply, designed to simplify a concept which, frankly, is not easy for all to understand :-

Lewis....

 

Try to move your rod with no rotation at all.  Your hand moves at exactly the same rate as your rod tip.

 

THIS IS TRANSLATION.

 

 

 

Now, keep your hand in one place.  Rotate at the wrist.  Your hand didn't move forward or back at all .... but your rod tip moved a long way.

 

THAT IS ROTATION.

 

Another way to look at rotation is that it achieves ROD ARC.  This is the change in the angle of the butt section of the fly rod from the beginning of the cast to its end.

 

 

Trying to do one without the other isn't easy.  That is what Ally pointed out in today's messages.

Gordy

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His reply:   

Thanks, I'll let that settle in my mind. Actually, this is what I thought I understood the terms to mean.

 

I agree that the pick up cast has translation, but it seems to me that during the pickup part of the cast, there is some arc of the rod tip, from low to high,(albeit minor) before rotation at the completion of the back cast, and a small change in the angle of the butt of the rod during this stage, both of which are against the definition of the term translation. I am probably a) nit picking (bad thing for someone just learning these terms and b) trying your patience.

 

I'll let it rest for a bit. In the end, a bit a both are required in most casts.

 

Cheers,

Lewis

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His followup note, as the light shined through :- 

Hi again Gordy,

 

I just reread your last message and one line about rotation jumped out:

 

This is the change in the angle of the butt section of the fly rod from the beginning of the cast to its end.

 

This is a key point for me. Then all casts must have some of each, translation to give stroke length and rotation to give stoke arc.

 

N'est-ce pas?

 

Lewis

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                                                           HE'S GOT IT, exactly !!!!

Now, you see ...... if I'd only been more clear in the first place, I'd have been a better teacher !

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From Jim Valle :-

Gordy,

Glad you are ok

 

Pick up translation ….just a thought on teaching basics

 

I actually teach lifting the rod up horizontally to minimize circular movement of the rod tip at the initiation of the pick up ( a vertical translation if you like) to the nail knot….Then as direct line as possible to the backcast stop position ( I would also note I teach aiming the backcast high) ( a SLP with a high back trajectory). Translation is tensioning the casting system.

I would also agree there is some rotation by virtue of physiology in most normal translation( but this is minimal when compared to the rotation of the power snap)  … I would also note that as the arm is moved to more horizontal (Lefty style) there is less rotation.

 

Hope this helps,

 

Jim

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NOW LET'S SEE HOW WE'VE FIRED UP OUR PHYSICISTS AND MATHEMATICIANS OF THE GROUP :

 

From Troy Miller.  (My comments in red italics) :-

I think somehow we've morphed our definition of translation too restrictive.  If we're saying that translation is ONLY linear displacement with no rotation permitted, then I think we miss the spirit of the term.  I'm not sure if anyone before me used the word translation in describing the hand/wrist movement during a casting stroke.  I don't say that to sound arrogant, I just never heard anyone say that before some of our early writings on Guy Manning's list.  To me, from a scientific standpoint, translation occurs when the COG of an object is displaced from Point A to Point B in 3-D space.  Doesn't matter if it spins or heats up or changes color while it's doing it.  If it moves, it has translated.  I'm not really even certain that it has to be linear motion between the two. 

 

This being the case, I think there can only be three possibilities of movement during a stroke:

 

Pure Translation -- as per the definition quoted below (except for caveat below).

 

Superimposed Translation/Rotation-- simultaneous linear displacement and rotation of the cork (may be more of one than the other, and changing the mix with time)

 

Pure Rotation -- COG stays in one place, and object turns about a stationary axis only.

 

Most  real life flycasts involve only the mixture.  Only on the extremes of applications do we use the extremes in motion type (pure T or R).

 

I agree completely !  Actually not easy at all to demo one without the other.  In the real world of fly casting, the admixture of each prevails.    G.

 

One more thing -- I don't believe that "translation" or "rotation" terms should be used to describe RTP.  I believe that the words describe the movements of the grip throughout the stroke, stop, rebound, mends, drift, followthrough, or any other time the grip is moving.  That might be a minor point, but a rod tip BETTER not be rotating or I've got a repair to do...

 

Again, I agree.  Rod tip path (RTP) is not at all described by either the movement of the hand or the butt section of the rod, and so is not even remotely described by tranlation and rotation.

 

In fact, with an efficient cast, we strive to achieve a straight line path of the rod tip with the admixture of movements at the rod handle.  That couldn't really be achieved without the flexibility of the rod itself !      G.

 

JMO, of course.  I'm not on the Definitions Committee.

Regards -- TAM

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Walter Simberski's self described, "rant"...

I've been meaning to write to you about this earlier. (I'm very glad to hear that Gustav passed by with no damage by the way).

 

It must have slipped by me earlier but the idea that rotation and translation are mutually exclusive is not a good way to look at things.

I have a concern that there are many perfectly good definitions for some of these things in the world of physics and mathematics but

we struggle when it comes to applying these definitions to fly casting. First of all, these definitions are centuries old and have stood the

test of time. Second, trying to change the definitions to suit fly casting is going to make it extremely difficult, if not impossible, to apply

mathematical and physical analyses to casting.

 

This will get really messy when we look at multiple rotations and translation within a single, complex action. If we fix

the elbow in space and cast by rotating the combined lower arm and rod around this point then we have pure rotation. But if

we now add some rotation at the wrist we now have a more complex system. Add in the rotation of the elbow around the

shoulder and we have a very complex system. We can't just simplify this by saying there is only one point of rotation and that

translation can't possibly occur at the same time. It's nice to try to simplify this for our students but as Master candidates I think

we need to accept that the fact that this is a complex system and learn to deal with it. Each of us will develop and share our

own mental images to help with personal understanding but trying to exclude part of the picture because we find it too difficult

to comprehend would be the wrong approach. Many of us as children would have liked to be able to ignore the fact that

there is a world of mathematical analysis beyond counting to 10. Fortunately, we weren't allowed to limit ourselves.

Now back to the idea that rotation and translation could not occur together. Imagine that I'm false casting and using only my wrist.

I think everyone can agree that this is pure rotation. But what if I'm standing inside of a moving railway car. Can I say that the

railway car can't possibly be moving because I'm rotating my wrist and railway cars don't typically rotate? I don't think so. On the

surface this may appear as a frivolous or argumentative position but what if we replace the movement of the railway car with rocking

the body or stepping forward and backward during the cast? Obviously we now have a combination of tranlsation and rotation and

we are now talking about a realistic casting scenario.

 

Take it a step farther - can we have translation of the hand due to arm/body motion at the same time as rotating our wrist or

elbow?

 

Just as the stop is not an instantaneous event, rotation and translation can not alway be separated.

 

I'm done ranting now.  :)

 

Thanks

 

Walter

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Walter...

 

In different words, you have just sealed the very conclusions which most of us have had .... that pure translation and pure rotation are not usual events in the real world of fly casting.  For almost all casts, they are mixed in differing proportions.

 

Gordy