The Basic Function of Suspension:
There are two types of suspension – Front Suspension and Rear Suspension.
Front suspension is designed to dissipate road vibration so that it does not enter the frame of the chair and transfer it to the rider. It must also absorb the higher impact energy created when the front caster wheels encounter obstructions while rolling over everyday surfaces.
Rear suspension is designed primarily to reduce spinal compression experienced during curb drops.
Critical Design Considerations:
The main difference between rigid and suspended chairs is how much mass has to be moved in a short period of time.
In a rigid chair, as the small front caster wheel strikes an object, the wheel has to move up at least the height of the object carrying its own weight plus the entire weight of the system (chair and rider combined).
On a suspended caster fork, the wheel striking the same object only has its weight plus the weight of the bottom of the fork tine. The difference can be thousands of times. Also because a suspended fork and wheel has so little mass/weight it can move out of the way and return in a fraction of a second before the chair and rider even notice.
Combining quick motion with vibration absorbing materials is the best way to accomplish safe and comfortable transportation.
For front suspension it is important to allow the wheel not only to travel up but also to travel backward to give more time to respond to the obstacles. The Frog Legs forks have accomplished this by using a simple pivot so that the wheel would travel on an arc instead of along a line(straight up and down). The Frog Legs forks are designed out of aluminum and our caster wheels are as light weight as possible(approximately 160 grams for a 4 inch wheel and bottom fork tine plus axle bolt and fasteners) to give quicker response.
Within the hinge Frog Legs use a polyurethane dampener. Polyurethane is the primary material used to eliminate vibration in most industries. Urethane is similar to a spring in that is has shape memory – it can be compressed and then return quickly to its size/shape after being distorted. However, when a spring is compressed it retains energy and reflexes it back into the system. When a polymer is compressed, it dissipates the energy as heat thereby completely eliminating it from the system.
Another advantage of moving the wheel back and up is that it allows the rider to use a smaller, tight turning caster but gives them the advantage of a larger wheel to roll over obstructions. Obstructions stop a small wheel because they hit it at a higher point on its rim forcing it to climb steeply to get over the top. When the suspension fork pulls the wheel up in an arc. This allows the wheels hit the same obstructions at a lower point, thus having more leverage to roll up and over. This makes the wheelchair less likely to stop suddenly and pitch the rider onto the ground. The third advantage of front suspension is that all the wheels are more likely to stay on the ground keeping the rider’s weight more evenly distributed in the seat of the chair. This improved weight distribution allows the rider to roll over uneven surfaces with less effort.
Suspension will protect the rider’s body:
Many of the users of Frog Legs front suspension have reported that their need for anti-spasmodic was significantly reduced or completely eliminated with use of our product.
A study conducted by Rory Cooper, Human Engineering Research Laboratories, VA Pittsburgh Healthcare System, argued that the vibrations and shocks individuals encounter on a daily basis while propelling their wheelchair may be sufficient to cause injury and concluded that:
"Suspension caster forks reduce the shock and vibration exposure to the user of a manual wheelchair".
"Although the impact on users of wheelchairs has not been extensively studied, secondary injuries due to whole-body vibration in workers in the trucking, air craft, helicopter, maritime, and construction industries is well defined and these studies have correlated vibration exposure with the risk of discogenic back injuries. It is certainly reasonable to assume that vibrations that are left under attenuated may be absorbed by tissues of the body. These tissues include skin, muscle and vessels in parts of the body in contact with the chair; bone, ligaments and intervertebral discs in the spinal column and any active muscle tissue used to maintain head and trunk posture."
According to an article in The (Colorado Springs) Gazette, "Suspension in bikes exists for the same reason as suspension in cars: to protect the rider from the jarring effects of traveling over rough terrain" and "In this day and age almost all mountain bikes have suspension." It is generally agreed within the bicycle industry that "…suspension….eases the constant pounding that tires your body and decreases energy expenditure and fatigue allowing for increased performance." (active.com/mountain biking/Proper suspension improves all aspects of your ride).
John Seifert of Saint Cloud State University reviewed a group of trained off-road cyclists and stated, "This information, in addition to the heart rate and creatine kinase results, indicate that riding a front or fully suspended bicycle resulted in less muscular trauma and better performance than a rigid bicycle. Trail shock detracts from speed and endurance…..suspended bicycles will reduce physiological fatigue and increase comfort."
Areas that Suspension helps a wheelchair rider: