Suspension Information

The Basic Function of Suspension

Front suspension is designed to dissipate road vibrations so they don't enter the frame of the wheelchair and transfer to the rider. It also absorbs the higher impact energy created when the front caster wheels encounter obstructions while rolling over everyday surfaces.

A study conducted by the Department of Health & Human Performance at Iowa State University determined that 80% of all the vibration within the frame of a wheelchair originates from the front casters. Their research showed that adding Frog Legs front suspension eliminates 76% of these vibrations.

Rear suspension, on the other hand, primarily reduces spinal compression experienced during curb drops.

Mass and Motion in Rigid vs. Suspended Chairs

The main difference between rigid and suspended wheelchairs lies in how much mass needs to be moved in a short period of time.

In a rigid chair, when the small front caster wheel strikes an object, the wheel must move up at least the height of the object while carrying its own weight plus the entire weight of the system (chair and rider combined).

With a suspended caster fork, however, the wheel striking the same object only carries its own weight plus the weight of the bottom of the fork tine. The difference in mass can be significant—sometimes thousands of times less. Additionally, since the suspended fork and wheel have so little mass, they can move out of the way and return to their position in a fraction of a second, often before the chair and rider even notice.

Combining Motion with Vibration-Absorbing Materials

Quick motion combined with vibration-absorbing materials is the best way to achieve safe and comfortable transportation. For front suspension, it's important to allow the wheel to travel not only upward but also backward, providing more time to respond to obstacles.

Frog Legs forks accomplish this with a simple pivot, allowing the wheel to travel on an arc rather than moving straight up and down. Made from aluminum, these forks are paired with our lightweight caster wheels (approximately 160 grams for a 4-inch wheel, bottom fork tine, axle bolt, and fasteners) to ensure quick response.

The Role of Polyurethane Dampeners

Within the hinge, Frog Legs use a polyurethane dampener. Polyurethane is the primary material used to eliminate vibrations in most industries. Similar to a spring, it has shape memory—it can be compressed and quickly return to its original size and shape after being distorted. However, unlike a spring that retains energy and reflexes it back into the system, polyurethane dissipates energy as heat, effectively eliminating it from the system.

Advantages of Frog Legs Suspension

Better Performance with Smaller Casters

Another advantage of allowing the wheel to move back and up is that it enables the use of a smaller, tighter-turning caster while still gaining the benefits of a larger wheel when rolling over obstructions. Typically, obstructions stop smaller wheels because they hit the object higher on the rim, forcing a steep climb to get over the top. Frog Legs suspension forks allow the wheel to hit the obstruction at a lower point, providing more leverage to roll over it. This design minimizes sudden stops and prevents the rider from being pitched forward out of the chair.

Improved Stability and Weight Distribution

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. This improved weight distribution allows the rider to roll over uneven surfaces with less effort.

Suspension Protects the Rider’s Body

A study (012700) conducted by Gerald Weisman and Dryver R. Huston of the Vermont Rehabilitation Engineering Research Center at the University of Vermont found that whole-body vibration caused by wheelchair use could lead to fatigue and decreased proficiency after just one hour outdoors.

Many users of Frog Legs front suspension report a significant reduction or complete elimination of their need for anti-spasmodic medications after using our product.

Another study by Rory Cooper, at the Human Engineering Research Laboratories, VA Pittsburgh Healthcare System, found that the vibrations and shocks wheelchair users encounter daily may be sufficient to cause injury. The study concluded that suspension caster forks reduce the shock and vibration exposure for manual wheelchair users.

Suspension's Impact on Health

Although the impact on wheelchair users has not been extensively studied, research into whole-body vibration in workers across industries like trucking, aircraft, maritime, and construction has shown a correlation between vibration exposure and the risk of discogenic back injuries. It’s reasonable to assume that unattenuated vibrations may be absorbed by tissues in the body, including skin, muscles, bones, and ligaments, particularly in the spinal column and areas in contact with the chair.

An article in The (Colorado Springs) Gazette notes that suspension in bikes, like in cars, is meant to protect riders from the jarring effects of traveling over rough terrain. The same principle applies to wheelchairs. The bicycle industry agrees that suspension reduces constant pounding, easing the rider’s body, decreasing energy expenditure, and fatigue while increasing performance.

John Seifert of Saint Cloud State University studied off-road cyclists and found that suspension systems result in less muscular trauma and better performance. Trail shock detracts from speed and endurance, but suspended bicycles reduce fatigue and increase comfort—an insight that applies equally to wheelchair suspension.