Quite a big subject. Essentially, they allow thinner shells to stay round. To a lesser extent, they permit the use of generous bearing edge forms on thinner shells too.
So, the upside is they open up more shell thickness options, accommodate different shell constructions that don't rely on multi layer glue bonds for structural integrity (i.e. steam bent single ply), & afford more choice in bearing edge forms.
The payoff is an increase in overtones that require managing/mitigating. Of course, some overtones can be beneficial, but some aren't.
Thank you for the replies.... I'm still a bit confused....... I initially was looking at the options on the Keller site for adding rings to their shells. It appears they are milling down an area about 2-3 inches down from the bearing edge. This option appears to be offered on all thickness options.
After reading KIS' response I went to the Guru drum site and it appears that their rings are support rings installed internally or externally...... are both the Keller method and the Guru method accomplishing the same thing?
Thanks for your responses and patience...... Terry
Actually it was not the keller site I was looking at but rather the drum maker.com site. In looking at the pictures of their rings closer it does appear to be a built up reinforced edge rather that a milled down ring as I originally thought.... Terry
With the Keller shell, I've heard the 6 ply shells are kinda on the thin side (of industry standard) and the reinforcement rings are recommended. That said, talking to an actual boutique builder might shed more light on the matter. The 8, 10, and 12 ply shells are thick enough to not need reinforcement rings ..... so adding them would be more for sound, rather than strength.
If you're looking for a warm, vintage type of sound, the reinforcement rings would be the way to go. If you're concern is more projection .... and a more modern sound ..... straight shells are what most companies use now. Ludwig switched to the straight shell, in 1976. Rogers, in 1978. Ludwig, DW, and Tama offer shells with reinforcement rings still, and you pay a premium price for them.