This post is additional material to my post of 15 June 2015. In that post, I did not say enough about the interface of the reel foot and the reel seat sliding ring. Here again I am discussing “cap and ring” type seats. These problems do not occur with screw locks seats, in my experience.
A discussion on The Classic Fly Rod Forum, “Nice fish make my reel fall off” http://classicflyrodforum.com/forum/viewtopic.php?f=93&t=81255, addresses the problem. I replied, saying that the radius on the top of the foot should closely match the inner radius of the ring. The reply following mine was probably closer to the target, pointing out that the ring should be as compliant as possible; i.e., deform with a low spring rate. A compliant ring is less likely to loosen from shock and vibration.
I want to point out that the relative compliance of the ring has a lot to do with the radius of the top of the foot. If the foot top radius is larger than the inner radius of the ring, then the ring is loaded at three points.
In the lower sketch, the load at the bottom of the ring is contact with the reel seat insert.
But when the foot top radius is less than the ring inner radius, the ring is loaded at two diametrically opposite points:
In this latter case, the ring is much more compliant, and therefore better able to hold when subject to shock.
It should also be apparent from the sketches that contact stress between the ring and foot is much less in the latter situation. So the foot is less likely to be scarred by contact with the ring.
A more recent discussion on the forum is “How do you make your reel feet?” http://classicflyrodforum.com/forum/viewtopic.php?f=93&t=27895 . Several replies show fixtures that are simple straight, cylindical rods. Such a rod will be 0.70 inch diameter in order to fit the foot bottom. When the top surface is turned, the top radius of the foot will be about 0.39 inch out at the tip, and will increase toward the foot center to about 0.50 inch (the foot top is a portion of a cone). Rings that I have seen are in the range of 0.67 to 0.73 inch inner diameter, and so fit such a foot poorly, causing three point contact between ring and foot and damaging the foot.
It is possible to turn the foot top radius so that it is 0.35 inch radius (or even less) over its entire length, but this requires a more complex fixture (see my blog post of 24 July 2016, “A Foot Fixture for the Lathe”.)
I still own a few commercially made reels, and they have different foot top shapes. My Orvis reel has a foot top that is conical, and is greater than 0.35 inch radius everywhere. It fits both ring and cap poorly and is damaged by contact with a sliding ring.
My Sage reel has a foot top that is 0.35 inch over its entire length (is cylindrical in shape). It fits both ring and cap nicely and there is no significant damage to the foot.