The term center of mass — sometimes referred to as center of gravity — refers to a single point that exists within an object, or in the space that surrounds the object, around which all forces are balanced.

Center of mass is a common design consideration for complex rehab wheelchairs, but seating and wheeled mobility professionals might be surprised to learn that center of mass also plays a huge role in the effective design of a Mobile Shower Commode Chair (MSCC). That design can greatly impact function, from how maneuverable the chair is (especially in tight spaces typical of many bathrooms), to how it rolls in larger spaces, how users transfer in and out, and how efficient the chair is to use for both the wheelchair user and the caregiver.

Center-of-mass concerns are normally addressed by considering a side view of the wheelchair or the Mobile Shower Commode Chair (MSCC), and how the device’s and user’s weight (combined) is apportioned between the front and rear wheels.

How Center of Mass Impacts Function

Why is the location of the center of mass so important from a functional viewpoint? Propulsion of an MSCC will be easier if the bulk of the weight is put onto the rear wheels. There are three reasons why:

• Larger-diameter wheels will roll over bumps (thresholds, for example) more easily than smaller-diameter wheels;

• Casters have two functions: They roll for movement and they swivel for direction. The swiveling effectively “scrubs” the tire on the floor material. The less weight there is on the casters, the less the friction (and therefore, less resistance) produced when swiveling;

• The force vectors of mobility devices are such that during forward motion, the forces are acting downward and forward, essentially pushing the load onto the casters. At the same time, the forces acting on the rear wheels are forward and up. Consider how you can pull a bundle buggy or cart up a curb, but you cannot push one up a curb.

When Weight Is Concentrated Rearward

For these reasons, propulsion is made easier when the weight in an MSCC is biased toward the rear and away from the front.

For example, to facilitate propulsion, Raz Design’s Raz-SP Mobile Shower Commode Chair has rear wheels that have been moved forward. As a result of that design:

• The rearward stability of the chair will decrease; anti-tippers can eliminate any safety concerns.

• Space for lateral transfers will be reduced.

• Side access for hygiene and personal care will be reduced.

• The chair may be able to roll further over a toilet before the rear wheels contact the wall behind the toilet.

• Maneuverability will be improved due to a shorter overall length and a shorter wheelbase.

The ideal shower commode chair will be easy to maneuver and easy to use for both the chair user and family members or caregivers; for example, the Raz-SP is the only self-propelled MSCC that offers adjustable rear wheel position for optimal performance and function. An MSCC should also offer plenty of adjustability and/or options to allow fine-tuning and a custom fit for every user, while also meeting the needs of caregivers in environments that can be quite difficult to maneuver.

Center of mass is a major consideration for complex rehab wheelchairs. Why shouldn’t it also be a major consideration for MSCCs?

This story is sponsored by Raz Design.