I have been building rods and teaching rod building for a few years. When I get to guide placement I use the same process I think most of us do. Place the guides where we think they belong, then test to see if the "arc" looks right and adjust from there. I have been doing that for probably 30+ years of building. But in all those years I never knew (or really tried to find out) why I want the guides placed so the flats are equal. So I put it to everyone, why is it so important, what exactly is it that the spacing controls. The action? The feel? Casting distance? Etc. I mainly build fly rods and am on the third session of a new fly rod building class I am teaching for the Nebraska Game and Parks and Project Healing Waters, and we will be measuring and placing the guides on the rods. I would like to give my class some reasons for placement other than "thats the way its done", or "it makes the rod cast better" neither of which really satisfy a curious student.
Thanks.

Interesting question. I'm going to guess the guide spacing which causes, under tension, the curve of the line to most closely parallel the curve of the blank would provide the best rod performance in distance and lifting power.. Proving that by observation and comparative measurements should be possible but might not be acceptable to fly casters seeking "smoothness" or a "soulful feel", etc..

Here is something I found a while back. Just plug in the length of you rod, number of guides,where you what the first guide and where you want the stripper guide and it calculates the spacing for you. Basically you want to distribute the load evenly along the length of the blank. This app is part of the rod cents site.
[www.rodcents.org]
Norm

Norman hit it right on the head !! It is to equally distribute the load on the rod and this make for efficiency when the rod is bent either in casting or fish fighting.

John,
For me, guide placement is dependent on the flex of the rod in a given area of the blank. The more flex the closer the guides need to be to each other and the line will follow the contour of the blanks flex. Another aspect is keeping the line "off" the blank so that is when the guide height comes into play. With micro guides you need more of them in order to do this. Also the ring size over the last 15-20 years has been understood better and this will also effect the placement. It still though, when taking into account these factors, comes down to putting the guides on and seeing what works best. The variables are too great to come down to one working formula at least for me. Try different set ups that are not conventional and see how they work. Do it with each class, you have a unique situation. Make what you know works but also do a rod unconventional and compare to your standard.
Tight Lines,
Lance

Think on your question in terms of a worse case where there is a very large gap between two guides about a third of the way down from the tip. Then imagine what might happen when the rod is placed under stress fighting a fish. Might the stress tend to concentrate in the gap? Might the concentration of stress result in a rod failure in the gap under extreme pressure?

There is an old admonition to give a rod what it needs and not a lot more. What the rod needs in terms of guide location to perform its best is for the line to parallel the bend of the rod thereby spreading stress over the length of the rod consistent with the rod's taper (its natural bend).

You could secure the butt of a blank, put a large sheet of paper behind it, weight the tip of the blank until it is deflected 1/3 of its length, and then trace the curve of the blank on the paper. Then use elastic thread to secure guides to the blank, secure the blank in the same position as before, run a line through the guides and tip-top, pull straight down on the line until the blank tip covered your first tracing. The elastic thread would allow you to move your guides up and down the blank until its bend Precisely matched the natural bend of the blank. I'm unsure of the practical value of such measures.

After 30 years of fly rod building - I don't think john is contemplating placing guides so far apart to cause rod failure.
But, what might be the result if a guide is moved a inch one way or the other causing one flat spot to be longer than optimal or 1/4" farther from the blank than optimal. It certainly won't cause blank failure. But will it not cast as well? Perhaps it will not look as good under load. On the beginning of the forward cast - the line may rub on the blank.

There are a lot of guide sizing/spacing tables recommending spacing for the "average" fly rod. Rods built from tables probably cast as well as the ones that we build using flex-testing to decide where a guide will be placed. Probably we, as builders, have a bit of OCD in us.
That is why we use the structural epoxy we do - why we flex-test - why we spine a blank - why we don't use masking tape for arbors - why we use the lightest components available - why we use corrosion free/resistant components for the salt.
Nothing can just be "good enough". It should be the "best.
Herb

I really appreciate everyone's response. I think I understand a lot of things now I never considered before. That's one reason I truly appreciate this forum, I can get expert advice on a wealth of subjects. I never considered the rod under stress discussion. We don't catch many really large fish here in Nebraska, so never really considered the stress angle. Line slap is one issue I have encountered recently in a rod I purchased. I get a lot of slap and have been trying to figure out why. Think I'll take a close look at guide spacing.

Again, thanks everyone.

Rod slap can be controlled with ring size and rapid reduction, this applies to all rods. Herb pretty much summed it all up.

One key element in "line slap" is the line itself. Air temperature and water temperature play a BIG part in how supple a fly line is and how much it will "slap" in the guide train. Many line manufacturers label their lines with the ambient temperature range for which they best perform. One benefit of belonging to a fly-fishing club is free access to a variety of different fly lines you can test cast and match to your fly rod at different temperatures.

On guide spacing I use small clothes pins with Velcro loop on each. You simple place a quantity of them on the blank and spline and put on tip also. Using a fly line of color you can see thread thru all pins then put bend on rod and start at tip and slide pin till that section of line is mirror image to arc that section of rod. You will see the pins in the tip section are about same distance apart and more frequent. As you move down rod they will start to get farther apart. Just now doing a 5wt 9’ and have fourteen guides on it. Try this it works great and it all fits in pint size zip bag. If anyone wants to see it contact me and we can FaceTime I do this all time with interested people.