Some questions on American style sidemount system, any ideas?

[stanleychan]

Just finished my sidemount course 1 year ago, as the sidemount dives more, the more I realised that a better understanding of the design concept of a sm system is important. My course was conducted in warm water with Hollis sms 75, but my instructor used a Razor, and that course just taught you basic skills and principles, no further introduction on the design of Hollis sms 75. So even now I don't know the functions of some parts of my sms 75.

For example, what's the function of rail system on the butt plate and armpit Drings? Some people say the rail system is just to attach more cylinders but I wonder that is it only be used to take more cylinders? And, American style sm often have 4 Drings on the shoulder strap, but Razor and some DIR BP/W only have 2 on it, so I want to know its function.

In addition, we know that Razor and xDeep have a butt mount pouch, but I don't see Hollis, Dive Rite and Halcyon sell that pouch. Is butt mount pouches common in the cold water areas using American style sidemount systems? Or is US style sidemount users have their own ways to carry backup items?

Hope have a discussion, thanks

 

[tbone1004]

The Two Schools of Sidemount Diving Heritage
ADVANCED DIVER MAGAZINE
read and report back

basically the rails are for hanging heavy steel tanks that don't float. They need to be attached far enough back so they don't hang below you.
2 sets of d-rings are for storing things, quite often the top d-ring will be used for backup lights mounted up, and the lowers will be used for cylinder attachment. Uppers are placed in the "normal" position, and the second set are placed just below the armpit.

the sidemount purse is a function of most wetsuits not having pockets and needing a place to put the stuff. If you have pockets on your suit, the purse isn't required. The butt rails and the extra d-rings on the back also provide a great attachment point for reels, spools, etc.

 

[DevonDiver]

The Two Schools of Sidemount Diving Heritage

Rails are an optimal method for attaching steel tanks.... as they don't transition in buoyancy during the dive. They are not optimal (at all) for aluminum cylinders.... as they have to be shifted forwards once positive. Rails are lower on the torso than front D-rings....so this pushes the cylinders forwards.... valves pushing out/down beyond the armpits (gorilla style....) and that destroys the ability to retain proper cylinder trim.

American (Florida Cave) versus British (Mexican Cave) design is primarily about the influence of steel versus aluminum cylinders.

American rigs generally prefer comfort-style harnesses....including multiple fixed d-rings. The Mexico rigs are more minimalist.... attempting to translate Hogarthian principles (DIR etc) into sidemount design.