Springback: A DIY Progressive Suspension Anti-Tipper – Erik Kondo

Springback

See Springback in Action on Youtube.

The Use of Wheelchair Anti-tipping Devices

An active wheelchair with the drive wheels placed in the forward axle position (which maximizes maneuverability and self-propulsion power) is subject to flipping over backwards. There is a trade-off between mobility and stability. A comparison can be made between the difference between the mobility performance of a bicycle and a tricycle. A bicycle is inherently more mobile than a tricycle. The third wheel of the bicycle provides stability at the cost of mobility performance.

A wheelchair with rear placed drive wheels is similar to a tricycle in terms of increased stability at the expense of mobility. The purpose of an anti-tipping device is to prevent the wheelchair from flipping over backwards so that the drive wheels can be placed more forward in order to increase the wheelchair’s maneuverability and self-propulsion.

The problem is that the benefit of preventing the wheelchair from flipping over backwards also inhibits the advantages of the wheelchair wheelie. Standard anti-tipping devices are like training wheels on a bicycle. Training wheels prevent a bicycle from falling over sideways, but they greatly limit the bicycle’s performance. Anti-tippers prevent a wheelchair from flipping over backwards, but they stop the wheelchair from purposely tipping backwards to deal with ground obstructions, curbs, and wheelies when going down steep inclines.

Therefore, most wheelchair users who use anti-tippers configure them such that they prevent the wheelchair from flipping over but still allow for minimal raising of the front casters. They combine the anti-tipper with moderately rear placed drive wheels to increase system stability. While this configuration mitigates the extreme disadvantages of a rearward axle position or a wheelchair with no ability to raise the front casters, their wheelchair still performs sub-optimally.

Specifically, the wheelchair’s maneuverability and self-propulsion is limited by the less then optimal drive wheel position and the wheelchair will fall backwards until reaching the “hard stop” of the anti-tipper. This means that the wheelchair user will have the unpleasant sensation of falling backwards (helplessly) while knowing that there is the possibility that the anti-tipper will fail and they will flip over. The end result is a wheelchair that is less mobile than it could be and that also feels unstable.

Existing Anti-tipping Devices

Standard Anti-tipping Devices

Standard anti-tipping devices consist of two small plastic wheels attached to a metal bar which is placed on the rear frame of the wheelchair. Typically, these basic anti-tipping devices cost a few dollars wholesale and are made in China. The Medicare standard of reimbursement for an anti-tipping device is $58. Therefore, there is no financial incentive for wheelchair manufacturers to produce a better performing anti-tipping device, so they don’t.  As a result, high-end wheelchairs costing $10,000 or more still use these cheap and low performing anti-tipping devices.

Despite the fact that many wheelchair users (across the world) would greatly benefit from an improved anti-tipping device, wheelchair manufacturers don’t make them. Their focus is on increasing profits rather than the wheelchair user’s functional mobility.

A Progressive Suspension Anti-tipping Device

The solution to all of these problems is to have an anti-tipping device that enables the wheelchair’s drive wheels to be optimally placed in the forward position for mobility performance. By using progressive suspension, the anti-tipping wheel “pushes back” against backward tipping with increasing force. This action allows the wheelchair user to still use the wheelie function to raise the front casters over ground obstacles and for dealing with low curbs, but the wheelchair cannot flip over backwards. Since the anti-tipping wheel is always in contact with the ground, it prevents the sensation of falling backwards. In addition, the device should also be easily removable to deal with steps and stairs.

The concept of using suspension with wheels for increasing stability and mobility is not new. They currently exist in all sorts of wheeled devices. The only reason they are not in use for manual wheelchairs is because they would cost more to produce than the existing versions (which have not been improved in decades).

Springback

Springback, the DIY anti-tipping wheel I created is more of a proof of concept and less of a product. A professionally designed and manufactured product would be cleaner and lighter. But as I have explained previously, in general, suspension based anti-tipping wheels are not a new invention. Wheelchair manufacturers could make them as standard options. They chose not to.

Springback requires the wheelchair to have a round metal camber tube of approximately 1 ¼” in diameter and a seat support bar in front of it. The stability caster fork is mounted to a metal rod which pivots around the camber tube. The other end of the rod is attached to the seat support bar via springs. When the wheelchair begins to tip backwards, the stability caster wheel resists (via the springs) yet still moves upward. This action allows for a movement constrained wheelie. The higher the wheelie, the harder the springs “push back” until they reach their full extension and the wheelie is stopped. At this point, the wheelchair cannot flip over backwards and there is a strong (spring) force which is pushing the wheelchair back to upright equilibrium.

I am using a 4” wheelchair front caster wheel fork which is bolted to a section of 1"x1" Zinc square tubing. This bracket is bolted to two small pieces of 1/2" plywood squares. The plywood pieces are also bolted to a short section of 1” O.D. pipe (use saddle washers) which is inserted into a longer metal pipe of 1” I.D. A hole is drilled through both pipes. A metal pin is placed in the hole to keep the caster fork pipe locked in place. When the pin is removed, the shorter caster fork pipe can be removed from the longer pipe.

The longer section of metal pipe is bolted to a tube clamp which is connected to the wheelchair’s camber tube. The tube clamp allows the metal pipe to “see-saw” up and down. The end of the metal pipe is connected by a metal hose clamp to two springs. The other end of the springs connect to the wheelchair’s support bar using hose clamps.

The exact positioning of the components (and size/length of the springs) depends upon the geometry of the wheelchair and the weight of the wheelchair user. The DIY maker of a Springback will have to make the exact determination during their build.

I am sure there are other ways to make a device that has similar functionality. This is the way I made it based on the materials I had available and the geometry of my test wheelchair.

Please note that I do not use an anti-tipping device myself because I have full wheelie control and rarely fall over backwards. My drive wheels are optimally placed for mobility. Therefore, an anti-tipping device would have few advantages and many disadvantages FOR ME.

Anti-tipping devices are for those who need and want them. If you need and want an anti-tipping device, then it should improve your overall wheelchair mobility rather than degrade it. Hence the need for Springback or a similarly functioning device.