Wheelchair-Scooter Dynamics - Erik Kondo

Updated: Sep 3

Wheelchair-Scooter fun!

Wheelchair scooterboarding allows wheelchair users access to some of latest electric powered mobility devices at a variety of prices. While many people are familiar with the general idea of using an electric scooter or detachable front wheel for powered mobility, few actually understand the dynamics of how they work. I will shed some light on the subject as I understand it.

A. One-Wheel Attachments

The traditional method is to attach a specially designed powered front wheel to the frame of the wheelchair. This method turns the wheelchair into an electric powered tricycle. Turning is initiated by steering the front wheel left/right. The more the front wheel turns on its axis (the steering column), the smaller the turning radius.

The UNAwheel Mini has a small front wheel. ($2,500)

A smaller the diameter the front wheel and the closer it is to the wheelchair’s rear wheels, the tighter its turning radius. A tricycle turns well and is stable at slow speeds. It is also stable at high speeds while going straight.

The Firefly by Rio Mobility has a medium sized front wheel. ($2,500)

The Batec has a large front wheel. ($8,000)

Tricycles are limited when it comes to turning at moderate to high speeds. When a tricycle is turned, centrifugal force (inertia) “pushes” the rider/tricycle to the outside of the turn. The higher the speed and the tighter the turn, the greater this force. When this force overcomes the downward force of gravity and the stabilizing effect of the outside wheel, the tricycle tips over to the outside of the turn. The higher the rider sits on his wheelchair, the less force that is needed to cause this tip.

Cornering during a tricycle race.

In the photo above, the riders are leaning to the inside of the turn as they corner, but the tricycles remain vertical. The more the riders lean into the turn, the faster they can corner. Note that a three wheel handcycle is also a tricycle, but it sits much lower which makes it more stable since the center of mass is lower.

A reclining handcycle has a low center of gravity making it more stable when cornering.

Another way to think about it. A child could push a person in a tall sitting wheelchair tricycle over from the side, while it would take a strong adult to do the same for a handcycle. The higher your center of gravity, the easier it is for you to flip over when cornering.

A tricycle is stable until it flips when corning or on a side slope.

The point at which a tricycle flips is a function of speed, turning radius, the location of center of mass, and the configuration of the tricycle. For people who will be riding at slow speeds, or NOT turning at high speeds, a tricycle works well. For those that like to engage in sharp moderate/high speed turning, a tricycle has limited functionality.

B. 2-wheel Scooters

People riding 2-wheel scooters are a common sight

2-wheel scooters are widely available at affordable prices. When a 2-wheel scooter is attached to a wheelchair using a fixed mount, it performs in the same manner as a traditional fixed one-wheel attachment with the same advantages and drawbacks. It creates a tricycle.

There are front wheel drive scooters and rear wheel drive scooters. A front wheel drive scooter is more likely to slip going uphill, and have better braking going downhill. A rear wheel drive scooter is more likely to have traction going uphill, and skid when braking going downhill. These dynamics are also affected by the placement of the mount, configuration of the wheelchair, and weight of the rider.

One of the difficulties in dealing with a 2-wheel scooter is that it is designed to be held up by an able-bodied person. Otherwise, it falls over. Therefore, 2-wheel scooters are awkward for wheelchair users to manage by themselves. If you ride a 2-wheel scooter without a fixed mount to your wheelchair, you will have to hold it up with your upper body while riding and turning (or it will fall over). Therefore, this method is most functional for wheelchair users with a higher degree of core strength and stability. It is easy to ride straight. The difficulty comes with turning.

Tyler is holding up an non-modified 2-wheel rental scooter. ($500)

2-wheel scooters are designed to be ridden like a bicycle. Which means that able-bodied riders lean into turns like a bicycle. As the rider leans inside the turn, the bicycle leans too, with both acting as a single unit.

Bicyclist leans into the turn with his bicycle.

Note that there are times that riders use angulation while turning, but those dynamics are beyond the scope of this document.

The significant difference between a bicycle and a tricycle is the leaning of the device. In both case the rider leans into the turn, but the tricycle doesn’t lean while the bicycle (2-wheel scooter) does. In terms of going straight and stopping, these devices perform similarly.

C. Articulating 3-wheel Scooters

This type of scooter is less common than the 2-wheel scooter because most able-bodied people do not need the extra stability created by having two rear wheels. For those that do require more stability, this setup works well. The scooter can now stand vertically on its own. Instead of having to hold up the handlebars, a wheelchair user with limited trunk stability can use the handlebar to increase stabilization.

KXD 3-wheel scooter with articulating rear wheels. ($260)

The front wheel does the steering and the rear two wheels are designed to articulate in order to NOT create a fixed tricycle effect. Therefore, this scooter performs dynamically like a 2-wheel scooter except that it does not fall over as easily. It is more stable which means it cannot be leaned as much as a 2-wheel scooter. Thus, all else being equal, a 3-wheel scooter has similar high speed turning capability as a 2-wheel scooter.

Articulating tricycle prototype

A 3-wheel scooter is essentially an articulating tricycle with enables the tricycle to lean while cornering similar to a bicycle. In this manner, a leaning tricycle has the turning capability of a bicycle along with slow speed and stopped stability. Whereas bicycles are not stable at slow speeds.

For wheelchair users, moderate/high speed turning is limited not by the scooter, but by the configuration of the wheelchair. While a wheelchair user can lean his or her body, the wheelchair itself does NOT lean. Therefore, whenever a wheelchair is turning at anything but a low speed, the user must lean into the turn to keep from tipping to the outside of the turn, just like a tricycle. A wheelchair is unlike a 2-wheel scooter or articulating 3-wheel scooter as described above. Despite having four wheels, wheelchairs perform dynamically like tricycles.

Wheelchairs are designed for tight slow speed turning.

Note the inherent incompatibility between the dynamics of leaning scooters and wheelchairs.

If you use a fixed mounting system for a 3-wheel articulating scooter you are creating a tricycle. The fixed mount will prevent the scooter from leaning which nullifies the articulation.

D. Lean-to-Steer Scooters

This type of scooter has two wheels in the front and one or two in the rear. It turns, not by rotating the front wheel along the steering column axis, but by leaning the steering column sideways left/right which mechanically causes the two front wheels to turn like a skateboard. It is in effect, a skateboard with a handle on it where the handle causes the footboard to tilt. As a result of using a truck to turn, the turning radius is larger than a regular scooter (i.e. it’s not as maneuverable at slow speeds).

The Cycleboard is a 3-wheel Lean-to-Steer scooter. ($1,500)

The only way to turn is by leaning, if you do not lean, you do not turn. The more you lean into the turn, the tighter the turning radius (just like a skateboard). This type of configuration allows for high speed turning while leaning.

Why would people want to use such a scooter? The reason is the feeling created by the combination of balancing leaning and turning which is UNLIKE a tricycle and different than a regular scooter. In addition, the ability for the scooter to lean enables corning at higher speeds than a tricycle.

I am tipping the Cycleboard to tighten it's turning radius.

E. The Flexi-Wheelchair-Scooter Concept

The Flexi-Wheelchair-Scooter is my name for a wheelchair/scooter combination that does not use a rigid mounting system. It uses a flexible mounting system. If the scooter is rigidly attached to the wheelchair, the overall configuration becomes a tricycle regardless of the style of scooter. Having some type of a flexible attachment which enables the scooter to lean from side to side makes it a Flexi-Wheelchair-Scooter (FWS).

The flexible attachment allows the scooter to lean (as it was designed to perform) while the wheelchair does not (as it was designed to perform). When cornering, the rider leans into the turn with the support of the steering column. This physical support provides riders who have decreased muscular trunk control/stability the ability to lean to a greater degree than unsupported. This greater leaning capacity translates into being able to corner at higher speeds. Remember, wheelchair dynamics require users to lean into the turn when going fast, otherwise the wheelchair will flip to the outside of the turn.

Flexi-Wheelchair-Scooter turning.

If you look closely at this photo, you can see that the scooter’s two rear wheels are articulating to the left. The front wheel is leaning to the left since it is parallel to the steering column. I am leaning to the left while supported by the scooter’s handlebar. Yet, my wheelchair is not leaning (since wheelchairs do not lean). The next result is that I can corner at a higher speed than I would be able to with a rigid mount.

When going straight, the flexible mounting has no advantage.

The flexible mount makes a difference when turning. When going straight, the dynamics between rigid and flexible are similar. A rigid mount is more secure which may make the rider feel more comfortable.

The type of scooter and mounting system a person chooses is dependent upon their personal preferences and lifestyle. Each setup will have advantages and disadvantages.

My personal preferences are as follows:

1. I want to be able to corner at as high speed as possible.

2. I want to be able to get on and off the scooter as fast as possible.

3. I want an affordable, detachable, and portable electric mobility device.

4. I don’t want to transform my wheelchair into a tricycle.

5. I don’t want to add any type of mounting hardware to my wheelchair.

6. I don’t want to require help transporting my device.

I primarily ride my scooter for fun, not out of transportation necessity. I expect that other people will have different priorities. The point of this document is to provide people with information that will help them decide what type of wheelchair-scooter setup is best for them.

Riding the flexi-wheelchair-scooter at home.

Red Pill Innovations
1 Broadway, Arlington MA 02474
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