By Mike Johnson M.D.
Why does the Walk’n’Chair morph into so many configurations?
- Because people have different mobility limitations.
- Because people have different environments to contend with.
And some serious engineering.
The outdoor environment is not always conducive to mobility aids.
As the outdoor terrain becomes more irregular, it is second nature to take more care and go slowly.
Theoretically, increasing the rolling resistance of the wheels and increasing the turning radius as terrain gets more irregular mimics what people do when the going gets rough. They slow down and take wider turns.
Rolling resistance is related to the tire material, wheel radius and load on the wheel base (Lin et al., 2015; Mediola et al., 2014; Sprigle and Huang 2015). Most of the resistance to rolling is due to “tire scrub” and “caster shimmy” not bearing resistance2.
How Walk’n’Chair changes Rolling Resistance
Changing the configuration of the Walk’n’Chair alters the weight distribution of the load relative to the front and back wheels. The large wheels have much less rolling friction than to the smaller caster wheels (due to size, materials, caster shimmy).
When we change the Walk’n’Chair from a Smooth Terrain Walker to a Rough Terrain Walker, the center of gravity moves from the large wheels (low resistance) to the casters (high resistance)
This increases the rolling resistance to match the caution one normally uses in rough terrain. Increased resistance for rough terrain gives the Walk’n’Chair a solid feel.
Turning Radius (Maneuverability)
The Walk’n’Chair turning radius is increased for the
Rough Terrain Walker configuration. This in combination with increased rolling resistance makes the Rough Terrain Walker configuration a sturdy match for irregular terrain.
Effects of Wheel Camber
The inclination of the large rear wheels (camber) is usually reserved for athletics and wheelchair sports (Tsai et al., 2012). Camber increases lateral stability; the effects are significant from 0-15o. Camber allows increased turning velocities. Camber from 8 to 15o increases
rolling resistance (Tsai et al., 2012). A camber of 6o was found to be optimal in terms of lateral stability on smooth terrain (Mediola et al., 2014).
The Walk’n’Chair large wheel camber is 12o. This enhances stability especially when moving over lateral slopes.
The largest obstacle height safely traversed is related to the wheel radius. A curb half of the radius of the forward wheel is easily and safely crossed.
The Rough Terrain Walker and Rough Terrain Wheelchair configurations allow the traverse of rough terrain and curbs.
Several principles of wheel chair design were extracted from recent engineering literature1-8. These principles were applied to the Walk’n’Chair.
The Walk’n’Chair modifies the center of gravity to match rolling resistance to the terrrain. Turning radius is modified to optimize for smooth indoor or rough outdoor terrain. Stability is increased by reducing turning radius and adding camber to the rear wheels. Rough terrain features are traversed with larger wheels.
- Caspall JJ, Selgsohn E, Dao PV, Springle S (2013) Changes in inertia and effect on turning effort across different wheelchair configurations. J Rehabil Res Dev 50(10):1353-62.
- Lin JT, Huang M, Springle S (2015) Evaluation of wheelchair resistive forces during straight and turning trajectories across different wheelchair configurations using free-wheeling coast-down test. J Rehab Research Devel 52 (7): 763-774.
- Mediola FO, Elui VMC, Santana CDS, Fortulan CA (2014) Aspects of manual wheelchair configuration affecting mobility: A review. J Phys Ther Sci 26:313-318.
- Sauret C, Bascou J, de Saint Rémy N, Pillet H, Vaslin P, Lavaste F. (2012) Assessment of field rolling resistance of manual wheelchairs. J Rehabil Res Dev. 2012;49(1):63-74.
- Springle S, Huang M (2015) Impact of mass and weight distribution on manual wheelchair propulsion torque. Assist Technol 27(4) 226-235.
- Thomas L, Borisoff J, Sparrey CJ (2017) Quantifying the effects of on-the-fly changes of seating configuration on the stability of a manual wheelchair. Conf Proc IEEE Eng Biol Soc 2017: 1897-1900.
7. Tsai CY, Lin CJ, Huang YC, Lin PC, Su FC (2012) The effects of rear-wheel camber on the kinematics of upper extremity during wheelchair propulsion.
The effects of rear-wheel camber on the kinematics of upper extremity during wheelchair propulsion. Biomed Eng Online. 2012 Nov 22;11:87.
8. Zepeda R, Chan F, Sawatzky B (2017) The effect of caster wheel diameter and mass distribution on drag forces in manual wheelchairs. J Rehabil Res Dev 53(6) 893-900
Check out the Walk’n’Chair at www. walknchair.com
Or email us: USA firstname.lastname@example.org
Or call us: USA 1(406) 925 5663
Canada 250 324 1285