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Inherent Conflicts between Static Stability and Dynamic Stability
A manual wheelchair is complex device that can be setup in many configurations. There are many factors which influence its performance. The following are several factors that have multiple effects on the wheelchair’s performance characteristics. There exists neither a universally better nor universally worse wheelchair setup that applies to ALL manual wheelchair users given the wide variety of disabilities involved. A setup that works well for one person may work poorly for someone else with a different type of disability and with contrasting functional needs.
That being said, wheelchair performance is determined by the laws of physics. Changing and adjusting certain physical aspects of the wheelchair will produce predicable results. For every wheelchair user, there exists an optimum wheelchair setup in terms of overall everyday performance. This setup will have certain trade-offs where an advantage in one area may translate to a disadvantage in another area. What matters in the long term is that the overall benefits of a given setup exceed its disadvantages.
Rear Axle Position -
The farther back the rear axle: the greater rearward stability AND the more difficult it is to pop and maintain a wheelie AND the more difficult it is to push the rear wheel biomechanically AND the less availability to utilize the grabbing the rear wheel for upper body stability.
The more forward the rear axle: the more likely to flip over backwards inadvertently AND the more push force is lost to unwanted front caster lifting AND the easier it is to use grabbing the wheel for upper body stability.
Backrest
The higher the backrest: the greater its back support AND the more it interferes with arm/elbow movement AND the heavier the wheelchair AND the larger the wheelchair’s profile AND the more it prevents leaning backward for upper body stability when on a decline.
The lower the backrest: the less its back support AND the less it interferes with arm/elbow movement AND the lighter the wheelchair AND the smaller the wheelchair’s profile AND the more it enables leaning backwards for upper body stability when on a decline.
Armrests increase upper body stability AND provide body lifting handles AND interfere with access to the wheels and limit picking up objects on the ground AND increase the wheelchair’s profile AND add weight AND increase the wheelchair’s cost.
Anti-Tippers increase rearward stability AND prevent backward flipping AND prevent voluntary wheelies AND may strike the ground on uneven terrain AND increase the wheelchair’s profile AND add weight AND increase the wheelchair’s cost.
Front Caster Wheel Size
The larger the caster wheel diameter, the easier it rolls over uneven surfaces AND the less likely it is to stop the wheelchair upon striking a crack/bump AND the less likely it is to sink on soft terrain AND the more likely it is to flutter at high speed AND the more likely it is to strike an object when rotating from left/right or right/left.
The smaller the caster wheel diameter: the harder it is to roll over uneven surfaces AND the more likely it is to stop the wheelchair upon striking a crack AND the more likely it is to sink on soft terrain AND the less likely it is to flutter at high speed AND the less likely it is to strike an object when rotating from left/right or right/left.
Front Caster Wheel Position
The more forward the caster wheel position, the more forward tipping stability AND the less weight on the caster wheels AND the less likely they are to sink on soft terrain AND the less likely they are to stop the wheelchair upon striking a crack AND the greater the forward frame length of the wheelchair AND the larger the wheelchair’s profile AND the greater the force required to turn/spin the wheelchair.
The less forward the caster wheel position, the less forward tipping stability AND the more weight on the caster wheels AND the more likely they are to sink on soft terrain AND the more likely they are to stop the wheelchair upon striking a crack AND the less the forward frame length of the wheelchair AND the smaller the wheelchair’s profile AND the less the force required to turn/spin the wheelchair.
The examples above demonstrate that specifications that create an advantage in one respect create disadvantages in another areas. Therefore, the optimum wheelchair setup is highly dependent upon the specific circumstances, wants, and needs of the individual user.
Two Opposite Wheelchair Setups
The following example wheelchair setups are at opposite ends of the spectrum. Most wheelchairs fall somewhere in the middle and incorporate aspects of each to differing degrees.
Safe & Supportive Wheelchair Setup
A Safe & Supportive Wheelchair is defined as follows:
A wheelchair that is least likely to tip forwards or backwards.
A seating system that is least likely to result in pressure sores and skin abrasions.
A seating system that provides postural support.
A wheelchair that is easiest to transfer into and out of.
A wheelchair that provides the greatest degree of static stability
A wheelchair that provides safety mechanisms to reduce risk of falls.
Typical Safe & Supportive Wheelchair characteristics
High Supportive Backrest: Provides upper body postural support
High Above the Wheel Sideguards: Protects skin and clothes from spinning wheels.
Armrests: Provides support for sitting, transfers, pressure releases, and changing clothes.
Push Handles: Makes it easier for receiving able-bodied assistance for being pushed or carried.
Anti-Tippers: Prevents the wheelchair from tipping over backwards.
High Mount Parking Brakes: Easiest to access without leaning over.
No to Moderate Camber: Camber is typically not used for safety and/or support.
Moderate to Heavy Weight Frame: A heavy frame is more stable and usually cheaper to purchase.
Rigid or Folding Frame: Both types of frames meet the requirements.
Thick or Molded or Air Cushion: For the prevention of pressure sores. For posture support.
No to Minimal Seat Dump: For distributing sitting pressure away for tailbone.
Wide Seat Pan Width: For accommodating additional wheelchair user weight gain and lateral stability.
COG in Front of Rear Axle (rear wheels back): Wheelchair is less likely to tip over backwards.
Solid Rear Tires or Low Pressure Inner Tubes: For minimizing or eliminated flats.
Regular Tires with Standard Tread: For providing rear wheel traction on a variety of surfaces.
Large Diameter and Wide Front Caster Wheels: For rolling over cracks and soft surfaces without jolting or sinking.
Front Casters Positioned Well Forward of Rear Wheel: For reducing forward wheelchair tipping.
Wide Dual-Tube Front Frame with Wide Footplate: To make it easier to position legs and provide space for rear entry wheelchair transfers.
Seat belt: To eliminate the chance of the user falling forward out of his/her wheelchair.
Electrical assist device: To reduce muscle strain and physical exertion.
Minimalist & Dynamic Wheelchair Setup
A Minimalist and Dynamic Wheelchair is defined as follows:
Having the smallest profile with the least amount of material and components as possible.
Having the capability to navigate a wide variety of terrain as quickly as possible.
Having the capability to make tight and nimble turns.
Having the capability to negotiate small spaces.
Having the capability to accelerate and decelerate quickly.
Having the capability to navigate inclines, declines, and side slopes as easily as possible.
The wheelchair is easy for the user to pick up, breakdown, and transport.
Typical Minimalist & Dynamic Wheelchair characteristics
Low Light Weight Backrest: To provide freedom of arm and upper body movement for mobility. A low backrest enables the user to lean backwards to dynamically adjust his/her COG and also to counter upper body forward lean when on a decline.
Low Wheel Profile Sideguards: To allow access to grab the entire wheelchair wheel. To allow for sitting on the rear wheel to gain reaching height.
No Armrests: To provide for more freedom of arm movement, wheel access, and reduce weight. To create a smaller wheelchair profile.
No Push Handles: To reduce weight. To reduce chance of unwanted “help”. To increase freedom of arm movement.
Minimal to Moderate Camber: The use of camber reduces turning friction and provides lateral wheelchair stability.
No Anti-Tippers: To provide for wheelchair freedom of movement (wheelies), control of COG, ability to raise front casters as desired, and reduce weight.
No Parking Brakes or Low Mount Parking Brakes: To provide more freedom of hand movement when propelling and grabbing front frame. To reduce weight. To create a “sleeker” wheelchair profile.
Very Light Weight Frame: To reduce weight for pushing wheelchair and for lifting and transporting it.
Thin to Moderate Cushion: To reduce weight and cushion profile size.
Minimal to Maximum Seat Dump as Required: To provide sitting stability and reducing forward sliding.
Seat Width as Narrow as Possible: To reduce wheelchair profile size.
COG Over Rear Axle (rear wheels forward): To increase wheelchair freedom of movement. To increase mechanical advantage when pushing. To increase access to rear wheels. To reduce wheelchair rotation radius. To increase rear wheel traction. To reduce weight on front casters. To enable the low angle wheelie position.
High Air Pressure Rear Inner Tubes: To reduce rolling/tire friction.
Thin Tires with Minimal Tread: To reduce rolling/tire friction.
Small Diameter and Thin or Beveled Caster Wheels (single or dual): To increase caster rotation mobility and reduce caster flutter.
Tapered or Narrow or Mono-Tube Front Frame with Narrow Footplate: To reduce wheelchair front profile size. To secure the wheelchair user’s legs in a vertical position (not splayed) - (dual-tube only).
Front Casters Positioned Close to Rear Wheel: To reduce wheelchair turning radius and required turning force. To reduce wheelchair length profile.
No Seat belt: To increase wheelchair user’s bodily freedom of movement.
No Electrical Assist Device: To reduce wheelchair weight. To enable physical exercise and bodily movement.
Conclusion
The goal of this three part series (Part I, Part II) is to establish the importance of understanding and capitalizing upon the dynamics of user/wheelchair system stability in order to enable wheelchair users to experience the greatest degree of mobility relative to their individual needs, wants and physical abilities.
The able-bodied world and adaptive skiing have already established the template for success. It is not a matter of reinventing the wheel, it is a matter of emulating what already works. Wheelchairs must be viewed as performance based mobility devices in the same way that fighter planes, bicycles, skateboards, and skiing devices are viewed. Peak wheelchair mobility performance results from determining and utilizing the optimum setup in terms of both Static and Dynamic Stability for each individual.