What makes Snap Casts (Interesting)

[Dirk le Roux]

Not on an ipad

[VGB]

Hi Dirk

Looking at the fly leg velocity history, the acceleration rate between 0.15 -0.7s appears to be reasonably constant, yet the acceleration trace for the same time period is varying quite considerably. In fact at 0.3s, the velocity trace shows an acceleration, whilst the acceleration trace shows a deceleration. Why do think that is?

Regards

Vince

[Dirk le Roux]

Hi Vince

Hi Dirk

Looking at the fly leg velocity history, the acceleration rate between 0.15 -0.7s appears to be reasonably constant, yet the acceleration trace for the same time period is varying quite considerably.

You should more closely look at the slope of the tangents.

EXAMPLE AT AFTER 0.3s

Also, I applied different moving average periods to both the velocity and acceleration graphs, the minimum in my opinion to each respectively for readability. From the raw consolidated velocity data, I used 3 period moving average and included only every second value in the plot. From the raw acceleration data, calculated from the raw consolidated velocity data, I used 5 period moving average for the base trace (otherwise it is distractingly noisy) and further smoothing along the lines of that described for the velocity smoothing above for the trend line on the overall graph.

In fact at 0.3s, the velocity trace shows an acceleration, whilst the acceleration trace shows a deceleration. Why do think that is?

A deceleration on the fly leg acceleration trace at 0.3s? On the snap of that plot you provided, I read just under 20m/s² acceleration at 0.3s. Are you sure you are not confusing negative slope on acceleration trace as deceleration?

Can I send you my Excel file?

Regards,
Dirk

[VGB]

Dirk

I think your smoothing has degraded the correlation between velocity and acceleration, it’s even more pronounced on the rod leg. From 0.05-0.18s, the rod leg velocity goes from 0 to about -22m/s at a reasonably constant rate of acceleration, the acceleration chart reduces from about 160-40 m/s^2 in that time. Then from about 0.2-0.7s, the rod leg decelerates reasonably smoothly from 22 to 8 m/s, a deceleration of about 28 m/s^2. The acceleration trace is showing some quite large variation over the same time period.

I won’t be able to look at any Excel data before the end of the month, I’ll be either working away from home or in Switzerland and Austria until then and can only use my iPad for this stuff. However, I would suggest deriving the acceleration plots from the smoothed velocity data would provide a better outcome.

Regards

Vince

[Geenomad]

You (and anybody else) are welcome to pm request my Tracker and Excel files if you want to check.

Hi Dirk
Ok thanks. What I was missing was the technical frame of reference for acceleration/deceleration. Appreciate your offer mate but I couldn't do anything with the data, much less a better job than you have.

Cheers
Mark

[Geenomad]

You must ignore the loop nose velocity if you're looking at the fly leg only. The markers on the fly leg are moving upwards after beginning with zero velocity. That means there is acceleration upwards in the fly leg.

The downwards velocity of markers on the rod leg decreases once the tip hits the ground, so that means acceleration is upwards for a period.

Yep. Thanks Graeme. I now understand the technical frame of reference. It is not, of course, my usual one in which acceleration means speeding up and deceleration means slowing down.

Even with the necessary adjustments I am still struggling to grasp the exact significance of the data and graphs for acceleration of the fly leg in a standard overhead cast. Snap casts being pullback on steroids and a bit different to how i typically deliver a fly to a target.

I am just back from casting practice and while there I played with an idea for a bucket science experiment to get a handle on tension between line legs and acceleration of the fly the leg by the rod leg in a "normal" casting scenario. Probably best to start another thread rather than lay it out here.

Cheers
Mark

[Dirk le Roux]

Hi Vince

I think your smoothing has degraded the correlation between velocity and acceleration, it’s even more pronounced on the rod leg. From 0.05-0.18s, the rod leg velocity goes from 0 to about -22m/s at a reasonably constant rate of acceleration, the acceleration chart reduces from about 160-40 m/s^2 in that time. Then from about 0.2-0.7s, the rod leg decelerates reasonably smoothly from 22 to 8 m/s, a deceleration of about 28 m/s^2. The acceleration trace is showing some quite large variation over the same time period.

I won’t be able to look at any Excel data before the end of the month, I’ll be either working away from home or in Switzerland and Austria until then and can only use my iPad for this stuff. However, I would suggest deriving the acceleration plots from the smoothed velocity data would provide a better outcome.

Here is a same scale graph of the loop front's travel velocity and acceleration. I use this as example because the loop front travel data can be taken directly from Tracker without having to piece together various markers (can continuously track the loop front in Tracker), so to limit what can be "lost in translation".

As you can see, the acceleration plot of a pretty innocuous velocity plot does show quite large variation. The brown line is of acceleration calculated in Excel from the smoothed velocity data as you suggested. The grey line is acceleration taken directly from Tracker, then smoothed using the exact same routine as for the smoothed velocity shown here.

The two acceleration approaches yield some small differences, which I ascribe to what happens to the data by smoothing, but you have to admit the gist, and pronounced variation of acceleration shown by both, is pretty much the same.

Regards,
Dirk

[Dirk le Roux]

Hi Mark

Yep. Thanks Graeme. I now understand the technical frame of reference. It is not, of course, my usual one in which acceleration means speeding up and deceleration means slowing down.

The rod leg is indeed slowing down, decelerating (downward). I don't know of another way of showing this deceleration downwards than the way I have. Acceleration graphs, just like velocity graphs, can show both positive and negative values. So in this sense acceleration, just like velocity, does not per se mean "positive".

Two cars drive in opposite directions. The one slams on the gas and the other slams on the brakes. At that time their acceleration is in the same direction. Hope this makes sense.

Regards,
Dirk

[Dirk le Roux]

Edit "Two cars drive in opposite directions. The one slams on the gas and the other slams on the brakes. At that time their acceleration is positive in the same direction.

[Dirk le Roux]

Hi Mark

I am just back from casting practice and while there I played with an idea for a bucket science experiment to get a handle on tension between line legs and acceleration of the fly the leg by the rod leg in a "normal" casting scenario. Probably best to start another thread rather than lay it out here.

I'm working on a similar velocity and acceleration Tracker study (but a simpler one) of a normal cast footage received from Graeme, so am quite interested to hear more about this experiment idea of yours...

Regards,
Dirk