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Mathematics and Fly Fishing

The "Impact factor"

Page 242.

"While it is possible to calculate the impact factor, the Mathematics are very involved. Garry recognized this and instead performed actual load tests. From this information, he determined that a resonable figure (4) was appropriate. This merely means that the rod will deflect four times as much under a moving load than it will under a static load. This is important since the stress within the fibers of the bamboo in the rod is proportional to the amount of deflection."

There seems to be some divergent opinions of what the impact factor is, and from Garrison's explanation (page, 242) I would expect, that he too had flunked Rocket Science.
On the other hand I cannot find any faults in Garrison's calculation of the tapers, but the explanations are not comprehensible.

The blue part is incorrect, but it is worth noting that Garrison associates the impact factor with the deflection.

Looking at how the factor is applied we find, that it is always used for calculating the moments. The moments are forces times a distance. Now applying a factor to the distances does not make sense, so the factor must be associated with the forces. Here the forces are masses times acelleration. This is not obvious when we calculate in the Anglo-Saxon unit system. It is a convenient system for engineering calculations, and the unit for forces is the gravitational force that acts on the mass 1 lb. at a place where the acelleration of gravity has a strictly defined value, 32.174 feet/second².

Working with vertical static loads it makes 1 lb(force) numerical equal (or very close) to 1 lb.(mass).
If we want to be 1 lb(force) equals l lb.(mass)/32.174 times the value of the local acelleration of gravity.

But Garrison is not working with static loads. In order to throw the line (or anything) we have to acellerate the mass, and a horizontal acelleration of 1 G would not be enough, and he figured that maybe 4 G would do the trick. So what Garrison calls an impact factor is actually a G- factor.

But ......... now to the interesting part.
As far as Garrison lets us into the his system for calculating the tapers, the value of the factor is of no magnificence at all!

The "impact factor" is a common factor to all moments. So we may calculate a Garrison stress curve for a rod using any value of the impact factor we like. And from that curve we can calculate the tapers for the rod if we use the same impact factor.

We do not even have to redraw Garrisons stress curves to use a factor 1. Just change the scale on the Y-axis (divide by 4).

Back to the quotation on top of this page.
It is now tempting to conclude that the first part of the quote is just as meaningless as the second part.
Let us resist the temptation and look at the value the factor, it is not a universal konstant. It is determined by the force (acelleration) we apply when we cast. And for a given rod, there is a range in which the rod will function properly.

So you can change your favorite rod calculation program to calculate a rod from a Garrison stress curve (calculated with a factor 4), but with a new "impact factor". The effect of changing the "impact factor" is more poverful than changing the tip impact, so keep it close to 4. Avalue lower than 4 will give you a slower rod and a value higher than 4 will give you a faster rod.
I am not serious, but it does make just as much sense as changing the tip impact by changing the lenght of line.
The computer is a marvelous means for calculating. You do not even have to understand the theory.

As to the value of the G-factor. It is propably too high. A car with that kind of acelleration would hit 175 miles/hour in 2 seconds flat and 2 seconds is just about the time it takes to make a cast.

The question is now: Why did Garrison use the g-factor 4 when he could have achieved the same thing using a factor 1?

We do not really have to worry about exeeding the allowable stress in bamboo?

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