Taper Research #2

Diagonal analysis is the best taper design and comparison method I'm aware of at this point. As I understand it, John Bokstrom introduced it to the rod making community in the Controlled Modification strategy. Larry Tusoni has included John’s taper modification program in RodDNA. This slide illustrates diagonal values for a Payne 101 taper as I’m calculating them. This may not be exactly the same as John’s calculations. It’s been a while since I’ve studied the Controlled Modification strategy.

The diagonal value calculation starts by converting the taper to diameters by percentage of length in 5% increments. That makes it possible to compare tapers of any length. Then the slope at each 5% point is calculated and all slope values are averaged. The average slope is used to extrapolate a trend line starting with the tip diameter and adding the average slope to the cumulative diameter at each 5% point. The taper diameter is then subtracted from the trend line diameter. The difference is divided by the trend line diameter to compute the diagonal values. The last step is to average all the diagonal values. The average diagonal value is used to stratify the taper database into tapers with similar design characteristics.

Generally speaking, a negative average diagonal value indicates that the mid section of the rod is smaller than the trend line. A positive value indicates that the mid section is larger than the trend line. That does not indicate whether the rod is “fast” or “slow”…more on that in a later post. However it does indicate how most rods in each respective category will load (bend). I’m going to try to avoid naming tapers. That just seems to add confusion to a fairly simple subject. However, I’ll digress into names for a minute. I think it might help in this case. Visualize the load characteristics of a negative average diagonal value as “mid flex”, a zero value as “progressive” and a positive value as “full flex”.

This slide charts the 444 tapers summarized in Taper Research #1 by line weight and average diagonal value. Although there are a wide variety of taper designs for each line weight, it is clear that heavier rods favor the “full flex” design. The 8wts appear to be the exception in this chart, but there really aren’t enough 8wt tapers in the database to draw any conclusions. It’s probably fair to conclude that 2wt and 8wt tapers are beyond the practical limits of bamboo.

Here's one more slide on diagonal values. This one is by Maker. It’s not all the Makers in the database, just the ones that have several tapers with their name on them. This slide includes 4, 5 & 6wt tapers. It should be noted that just because a taper has a particular Maker’s name on it doesn’t mean that it’s the original version of the taper. Most of the tapers in the database were measured over finished rods with estimates for the thickness of the varnish. In some cases, there are multiple versions of the same taper with slightly different measurements.

With that disclaimer, this is still an interesting slide. I thought most Makers would have specialized in certain taper designs. There are a few that did...like Hardy, Constable, Young and Pezon. Most of their tapers are in the full flex category. In fact, most of their tapers are greater than 5%. Those tapers are commonly referred to as “parabolics”. Thomas is another maker that stayed pretty close to home with his tapers. He stayed in the 0% to 5% category. Just about everyone else has made them all.

So…what’s the point of all this? It seems to me that there are clearly defined limits to the functional design of a bamboo fly rod. Average diagonal values of -5% to 5% are the general rule with a few Makers pushing the upper limit to 10%. By line weight, functional limits appear to be in the 4 to 6wt range with a notable number of tapers pushing the limits in the 3wt and 7wt categories. It appears to me that bamboo rules in the 4 to 6wt categories for small game. Light weight composite rods are better suited to 7wt+ big game hunting.

In the next post I’ll dig a little deeper into the 4 to 6wt taper designs with an analysis of three of the “Masters”.

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