Machine Design

Comfortable seating for underwater workouts

Think those spinning classes at the health club are tough? Try them at deep-ocean depths.

A diver fully submerged and in full gear, rides the Spongy Wonder bike with corrosion-resistant, high-strength stainless bike seat at Defence Research and Development Canada's Toronto test facility.
The modular Spongy Wonder bicycle seat adjusts to rider size. It has a steel frame, dual stainless platforms, and removable, closed-cell foam cushions.

That's how divers at the Experimental Diving Unit (EDU) Section of Defence Research and Development Canada (DRDC) simulate the stress of working far beneath the surface. Rebreather-trained divers in full gear must first pedal for 5 min on the bottom of a hyperbaric diving chamber, rest for 5 min, then repeat the sequence over varying spans.

Key to a comfortable ride on the underwater exercise bikes is a dual-platform, closed-cell foam seat. Unlike conventional bike seats, it puts no pressure on the rider's urological system. The seat, developed by Spongy Wonder Inc., Riverview, NB, Canada (, also eliminates damaging pressures to the tailbone and rectal area that have been linked to the development of prostate cancer.

Robert MacLean, diving-systems technician for EDU, says the original mild-steel seat frames worked well on land but didn't stand up under water. The frames fell apart after two dive series and began to corrode. This threatened to contaminate the water and foul expensive equipment being tested in the chamber.

The solution was a frame made from Type 304 SH (strain-hardened) stainless. EDU's McLean says seats made with the material have been in continuous service for eight months with no sign of corrosion or seat pad damage, to simulated depths of 81 msw (meters seawater).The strain-hardened 304 stainless steel comes from Carpenter Technology, Reading, Pa. ( It boosts ultimate tensile strength (UTS) 30%, to 135 kpsi, versus 104 to 108 kpsi for standard Type 304 stainless. Its polished finish doesn't corrode and resists scratches, marring, chipping, and discoloring.

The frame fabrication starts with a pneumatic machine that cuts stainless-steel bar to length. Five bends later the ends are welded together and ground smooth. The frame then mounts in a jig for the final bend. Additional sanding and grinding take place before the frame undergoes a two-stage polishing process, using a tumbling buffer.

Seat inventor Jeff Dixon worked with Carpenter metallurgist Bob Mohr to find the right steel for the frame. Another Carpenter alloy called A-286 will probably be the alloy of choice once Spongy Wonder retools its equipment for the material's higher UTS of 169 kpsi.

-- Jean M. Hoffman

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