Final Scooter Report

Electric Scooter Modifications

Electric Scooter Modified for Boy with HemiplegiaKevin Ko, Aya Eguchi, and Eric Schwartz

Duke UniversityDurham, NC 27708

ABSTRACTA Schwinn S180 electric scooter has been modified for safe and comfortable use by an eleven-

year-old boy with right-side hemiplegia. The modifications include 1) a stabilizing mechanism to aid in maintaining balance, 2) a foot guard to keep the right foot from sliding off the scooter, 3) an adapted seat with armrest to support the torso while riding, 4) a handlebar extension to facilitate handling and steering, and 5) a speed governor to limit scooter acceleration. As a result, the client can safely and comfortably ride the scooter. These modifications are specific to this scooter but can be applied to other mobility devices to assist individuals with other motor disabilities.

KEYWORDS: scooter; assistive mobility; armrest; training wheel; speed governor; handlebar extension

BACKGROUNDThe client is an eleven-year-old boy with right-side hemiplegia as a result of a stroke incurred at

birth. Hemiplegia is a form of cerebral palsy, a disability that affects an individual’s motor abilities as a result of brain damage (1). While the left-side of his body functions normally, the client’s right body has decreased motor coordination and strength, limiting his ability to ride the Schwinn S180 battery-powered scooter. His right arm does not extend to the handlebars, causing problems when steering and maintaining posture. His right foot also slides off the scooter floor, posing a danger while he rides.

While projects have addressed similar modifications in the past, most of them have involved the transformation of a commercial, two-wheel scooter into one with three wheels (2). Although a tricycle guarantees stability, training wheels better fit the client’s needs as they more closely resemble a two-wheeled bicycle and provide the opportunity to learn balance. The specified alterations provide the comfort and stability necessary for the client to ride his scooter and also maintain its aesthetic appearance.

PROBLEM STATEMENTThe goal for the project was to improve the comfort, safety, and usability of a battery operated

scooter. Functional specifications included improving scooter balance, protecting the right foot from falling off the scooter, providing comfortable support for the right arm, facilitating control of the scooter, consisting of non-permanent modifications, and being easily transportable.

RATIONALEThe modifications will allow the client to ride his scooter safely for the first time under minimal

supervision. Aside from a tricycle that he used as a child, the altered scooter will be the first vehicle that the client can operate on his own, allowing for a greater degree of mobility while engendering independence and confidence. The final product consists of several features that can be implemented in other designs for assistive mobility, benefiting a larger population of people with motor disabilities.

DESIGN AND DEVELOPMENTDevelopment of the device began with extensive consultation with our client and his therapist.

Through these meetings, the following areas were identified for modification: 1) a foot guard to prevent the right foot from falling off the scooter, 2) a modified handlebar to facilitate one-handed control, 3) a


stabilizing mechanism to help our client balance, 4) a modified seat and right arm support to maintain the client’s posture while riding, and 5) a speed governor to maintain a safe operating speed (Photo 1). -----------------------------------------------Insert Photo 1 Here: Finalized Scooter-----------------------------------------------1. We created a foot guard out of aluminum sheet metal, which we molded to the contour of the floor

board’s right, outer edge (Photo 2). The metal wraps around the under side of the floor board and is attached by four 3/16” flat headed bolts. The top edge of the foot guard extends 1.5” from the bottom edge, which is sufficient to block a foot from sliding off that side of the scooter. Initially we decided on thermoplastic, but it proved to be either too difficult to mold or deformable in hot weather.

----------------------------------------Insert Photo 2 Here: Foot Guard----------------------------------------2. We modified the handlebars to sit in closer to the client’s body and facilitate steering with one hand.

Initially, we also considered developing a footbrake so that the client would not have to control both the throttle and the brake with his left hand. After additional testing, we decided that the footbrake would be difficult to implement and would not vastly improve our client’s ability to control the scooter. The handlebars are only modified on the left side of the scooter. Two segments of aluminum tubing, 8” long are used to form a handlebar extension (Photo 3). The throttle and hand brake are reattached to this extension effectively bringing the scooter controls 8” closer to the user’s body.

----------------------------------------------------Insert Photo 3 Here: Handlebar Extensions----------------------------------------------------3. We implemented a stabilizing mechanism to assist the client with overall balance of the scooter.

Several mechanism designs were considered, such as a tricycle design and a “side car” design. We ultimately decided on installing Fatwheels, a robust set of training wheels intended for off-road use. One-inch axle extenders were added to properly attach the Fatwheels (Photo 4 & 5). During testing of this mechanism, excessive stress on the training wheels caused the mounting bracket to bend. To counter this possibility, an additional triangular plate was welded to the bracket, and a larger axle extender was created to help absorb stress.

---------------------------------------------------------------------------Insert Photo 4 Here: Training Wheel Attachment from Above Insert Photo 5 Here: Training Wheel Attachment from Side---------------------------------------------------------------------------4. We modified the current bicycle seat because the client found it difficult to maintain his posture. We

purchased an 18” highback seat from Freedom Concepts, Inc., which came with side-supports to help contain the client’s body within the seat (Photo 6). Seat straps were removed for safety purposes. The backrest was mounted to the scooter with an aluminum rod, which we painted black and screwed into the rear fender, which we then reinforced with two sheets of aluminum. An armrest was fabricated from a sheet of aluminum, slightly curved to an arm’s contour, lined with an adhesive foam and Poly-Fil, and covered with black vinyl. The armrest is supported by two aluminum shafts, one attached to the backrest, and one attached to the bottom of the seat. A grip ball was attached to the end of the armrest to provide support for the right hand (Photo 7).

--------------------------------------------Insert Photo 6 Here: Modified SeatInsert Photo 7 Here: Arm rest-------------------------------------------- 5. We implemented a speed governor to limit reckless driving while the client learns to ride safely. We

contacted a manufacturer for electric speed governors for Schwinn scooters but found that only the


older motor models could support such governors. We thus decided to create a mechanical speed governor by placing set screws in three different locations on the throttle to physically inhibit full rotation of the throttle (Photo 8). This alteration provides for three adjustable levels of speed control.

--------------------------------------------------------Insert Photo 8 Here: Throttle/Speed Governor--------------------------------------------------------

EVALUATIONWe conducted several test rides with the client and determined that the modifications are effective

in enhancing the comfort, safety, and usability of the scooter. The client has little difficulty maneuvering the scooter, as observed while he rode through a simulated obstacle course consisting of cones and uneven surfaces. In addition, we tested the client operating the scooter outdoors on variable terrain and inclines: the client had very little difficulty braking at the bottom of hills, but experienced some difficulty climbing steep hills while the speed governor was in place. Some testing was performed at the client’s primary residence, which is where he will normally operate the scooter. The only problem encountered there was an inability to drive training wheels over the graveled driveway. As a result, the client must walk his scooter to the road before operating, but the client has confirmed that this is not a problem.

DISCUSSION AND CONCLUSIONSThe scooter modifications have met our objectives and allow the client to safely, comfortably, and

independently ride the scooter. Major advantages of the modifications include the scooter’s enhanced stability, one-handed steering, ergonomic seating and armrest, the ability to keep the right leg within the scooter frame, variable speed control, and a non-permanent design. One major disadvantage is that the scooter has a difficult time with steep inclines due to the added weight and frictional force from the modifications. Another disadvantage is that the scooter is prone to slipping and tipping around turns at high velocities. Future work could include devising a mechanism for reverse driving, implementing a shock suspension system for the training wheels, and making the handlebar and armrest adjustable. REFERENCES1. Zisook, Sheldon Oliver. Cerebral Palsy: Ask the Doctor. Lawyers Inc., P.C. 13 Sept. 2004

http://www.about-cerebral-palsy.org/index.html.2. Follensbee, Debra, Dean Markiss, and Cary Munger. “Tri-Scooter: A Mobility Device for a Child

with Arthrogryposis.” in National Science Foundation 1989 Engineering Senior Design Projects to Aid the Disabled, J.D. Enderle, NDSU Press, (1989): 32-33.

ACKNOWLEDGMENTSThis material is based upon work supported by the National Science Foundation under Grant No.

0118558. We would like to thank our supervisor, Jodi Petry at Lenox Baker Children’s Hospital; our machinist, Joe Owen at Duke University; and our supervising professor, Dr. Laurence Bohs. Each of them provided immense guidance throughout the design procedure. Author Contact Information: Kevin Ko Aya Eguchi Eric Schwartz 7 White Clay Lane 7507 Oldchester Road 9335 Sprinklewood LaneWest Grove, PA 19390 Bethesda, MD 20817 Potomac, MD 20854

http://www.about-cerebral-palsy.org/index.html


GRAPHICS PAGE---------------------------------Photo 1: Finalized Scooter---------------------------------

Alternative Text Description for Photo 1: Finalized ScooterImage shows the scooter in its finalized form with the training wheels, handlebar extension, speed governor, modified seat with armrest, and foot guard. The aesthetics of the scooter have been maintained with the alterations.

-------------------------Photo 2: Foot Guard-------------------------

Alternative Text Description for Photo 2: Foot GuardImage shows the top view of the foot guard attached to the scooter floor. The guard is constructed out of a thin metal rod and rises 1.5 inches high. It curves around the side of the floor and is secured in place by several flaps that curve under the floor and are screwed into the floorboard.

Handlebar extension

Modified seat

Arm rest

Training wheels

Foot guard

Scooter floor

Foot guard


--------------------------------------Photo 3: Handlebar Extensions--------------------------------------

Alternative Text Description for Photo 3: Handlebar ExtensionsImage shows the handlebar extension for the left side. The extension is made from two electrical conduit rods that are attached to the existing handlebar and to each other by two L-brackets. At the end of the second rod is the throttle/brake system. A battery-operated light is also added to the existing handlebars.

-------------------------------------------------------------Photo 4: Training Wheel Attachment from Above-------------------------------------------------------------

Alternative Text Description for Photo 4: Training Wheel Attachment from AboveImage shows the top view of the training wheels. The wheels were ordered from FatWheels, Inc. and attached to the scooter frame by manufactured steel brackets and axle extenders. The wheels are attached at slight angles so that the scooter can more easily maneuver turns.

L-brackets

Electrical conduit rods

Front lights

Existing handlebars

Hand brake

Throttle

Bracket

10” Fat Wheel

Scooter floor


----------------------------------------------------------Photo 5: Training Wheel Attachment from Side----------------------------------------------------------

Alternative Text Description for Photo 5: Training Wheel Attachment from SideImage shows a side view of the training wheel attachment. The Fat Wheel is connected to the scooter frame through the axle extender and steel bracket frame. The height of the wheels can be adjusted for variable assistance by placing the bolt at differing height along the bracket frame.

-----------------------------Photo 6: Modified Seat-----------------------------

Alternative Text Description for Photo 6: Modified SeatImage shows a frontal view of the highback seat with the side supports and the accompanying arm rest. The seat was ordered from Freedom Concepts, Inc., and the seat straps that came with it were removed for safety purposes. The seat is supported by two steel posts, both of which allow height adjustment.

Axle extender

Bracket frame

Adjustable wheel attachment

Training wheel

Arm rest

Grip ball

Attachment brackets

Arm rest support


-----------------------Photo 7: Arm Rest-----------------------

Alternative Text Description for Photo 7: Arm restImage shows a side view of the seat with the arm rest. The arm rest is supported by two aluminum rods: rod 1 curves around from the back of the seat where one end is screwed into the back panel and joins rod 2 that extends vertically. A third rod that is screwed into the bottom panel of the seat attaches to this second rod through L-bracket 1. The actual arm rest is attached to rod 1 by two attachment brackets, and a grip ball is attached at the end by another short rod extending out of L-bracket 2.

------------------------------------------Photo 8: Throttle/Speed Governor------------------------------------------

Alternative Text Description for Photo 8: Throttle/Speed GovernorImage shows the top view of the throttle and speed governor setup. The speed governor is fabricated by placing a set screw in one of three different locations for three different speed settings. As the throttle rotates counterclockwise, it hits the set screw and is prevented from making a full rotation.

Arm rest

Rod 1

Rod 2

Grip ball

Attachment brackets

L-bracket 1

L-bracket 2

Throttle

Speed governor set screw

Speed governor set screw

Handbrake


APPENDIX A: SUMMARY OF IMPACT

Our project will allow our client to use his electric scooter comfortably, independently, and safely. The modifications will allow him to ride with family and friends during trips to the beach and foster a greater sense of self-confidence. It will also offer him an enjoyable method of stimulation which will help him to overcome the impact of his disability. The client has expressed gratitude and excitement in the final project, as he informed us in a letter: “Thank you very much for all the hard work you all did on my scooter. I will be able to ride with my brothers and sister and not be left sitting out watching them have all the fun. I won’t have to work as hard as them! I’ll be the coolest kid in the neighborhood with the coolest ride! Thank you.”

APPENDIX B: MECHANICAL DRAWINGS AND ELECTRICAL SCHEMATICS

Figure B-1: This drawing shows the placement and attachment of the custom foot guard constructed for the floor board of the Schwinn S180 Electic Scooter. The footguard is constructed from 1/8” inch thick aluminum sheet metal. The footguard is 1.5” high when measured from the outer edge. 3/16” flat-headed bolts were used to secure the footguard to the floor board.

Figure B-2: This drawing shows the modified handlebars for the modified electrical scooter. An L-bracket connects the existing handlebars to a 7/8” aluminum tube, 8” in length. Another L-bracket connects the aluminum tube to another 7/8” aluminum tube, 8” in length so that it runs parallel with the existing handlebar. The throttle and brake handle were placed on this parallel piece of tubing (not shown).

Figure B-3: This drawing shows the armrest which was added to the high-back seat purchased from Freedom Concepts. A 7/8” piece of aluminum tubing approximately 3 ft. in length was bent to conform around the seat back and bend 30° past perpendicular. This tube is secured by 2 1/4" screws in the back of the seat. The bar is further supported using 2 aluminum tubes and an L-bracket. This support system is secured with 2 1/4" bolts to a metal support brace underneath the seat. The armrest length was tailored to our client’s exact position as he sat on the scooter. An L-bracket was placed at the end of the armrest bar where a soft hand rest was placed on top of a short segment of aluminum tubing. The arm rest cushion was constructed from a metal frame placed on top of a rectangular piece of plywood. A layer of padding was placed over the metal frame followed by an additional layer of polyfill (details not shown). The cushion was attached to the aluminum rod using two brackets.


Figure B-1: Schematic of the Footguard

Figure B-2: Schematic of Handlebar Modifications


Figure B-3: Schematic of the Armrest

APPENDIX C: QUANTITATIVE ANALYSIS

The objective of this quantitative analysis is to determine the minimum length that the training wheel brackets need to be to allow the scooter to safely turn without either slipping or tipping. We determined the bracket length by analyzing the effects of centripetal force on a force-body-diagram of the scooter.

Theory:

Centripetal force is the component of force acting on a body in curvilinear motion that is directed toward the center of curvature or axis of rotation. Centripetal force is necessary for an object to move with circular motion (1).


Figure C-1: Centripetal force diagram

In the figure above, the point mass, of mass m, is moving around a circular path of radius R at a constant velocity, v. The mass, velocity, and radius are all related to each other by the equation for centripetal force, which always points perpendicular to the velocity and towards the center of the circular path.

An object moving in a circular path can fail in two ways: slipping and tipping. For an object to slip, the centripetal force must overcome the static friction between the object and the surface. Static friction is the frictional force between two bodies at rest that responds to any external force tending to slide one body along the surface of the other (2). As the external force increases, the opposing static friction increases, as well, until a maxim value is reached at

where µS is the coefficient of static friction. Any external force greater than this maximum frictional force value, will force the object out of its resting position in that direction.

For an object to tip, the centripetal force must create a moment around the inner-side of the object, with respect to the circular path, that overcomes the moment around that same pivot point from the object’s center of mass around. A moment is is a quantity that represents the magnitude of force applied to a rotational system at a perpendicular distance from the axis of rotation (3) and is described in the equation

where M is the moment, F is the force, and d is the perpendicular distance between the force and the pivot point. If the sum of the moments at a pivot is non-zero, the object will rotate at the pivot point.


Analysis:

The force-body-diagram that will determine the minimum training wheel-bracket length can be found in Figure C-2 below:

Figure C-2: Force-body-diagram with centripetal force added

In the diagram above, the scooter is drawn in abstract form to simplify calculations. It is assumed that the scooter is nearly symmetrical with respect to the vertical axis. Acting against each tire is a normal force, N, and a frictional force, f. At the center of mass, the entire weight of the scooter plus the client, W, is acting downwards. In addition, the centripetal force, F_C, acting on the scooter as it is making a turn is drawn in the figure, which also acts at the center of mass. The center of mass is located ỹ inches above the ground. Finally, the measurement we are interested in is the length, d, of the training wheel-brackets.

First, we determine the center of mass by dividing the scooter into its main components. The table below shows the weights and location of the main components:

Component Weight(lbs)

Height (in. from ground)

Scooter frame 45 6.625Highback seat 20 21.375Training wheels (x2) 6 5.000Client 70 32.750

Table C-1: Data for center of mass calculation


The center of mass is calculated using the following formula:

Slipping:

We can now analyze the force-body-diagram with respect to slipping. An object is stable if the net force and net moment on the force-body-diagram are both zero. By summing up the forces in the vertical direction we get that

We do the same analysis in the horizontal direction and we get that

We use “a less than or equal sign” in place of an “equal sign” because we want to make sure that the centripetal force cannot overtake the frictional force and cause the scooter to slip. Substituting the equation for centripetal force, we get that

This equation shows what turning radius is needed to prevent the scooter from slipping when going velocity, v, around the turn. The scooter was originally built to operate at a maximum speed of 15 miles/hour, or 22 ft/sec. Through our modifications, we have added not only extra weight to the scooter, but added additional frictional force from the training wheels. As a result, the scooter is unable to reach its maximum speed. To be safe though, we will assume the scooter can in fact reach its maximum speed of 22 ft/sec. The static coefficient for a tire on pavement is about 0.9 (4). Inserting all of the values into the equation, we get that R must be greater than


Figure C-3: Turning radius is limited by street width

The average lane width in the state of North Carolina, where the client’s primary residence is located, is 12ft (5). This is the maximum radius we want the scooter to have to operate in order to make the turn (See Figure C-3). If we reverse the equation and solve for velocity, this relates to

Given that we added a lot of weight and there is additional friction force provided by the training wheels, the scooter should run well below 12.675mph at its maximum. Therefore, the scooter will not likely slip while the client is riding a turn.

Tipping:

We analyze tipping by summing up the moments around the inner-tire on a turn. For example, we will assume that we are making a right turn and the right wheel is the inner-wheel. We will analyze the point when the two outside wheels leave the ground (at the start of the tip). Because neither the middle nor left wheel are on the ground


We now sum of the moments around the right wheel

Inserting the equation for centripetal force, ensuring that the centripetal force is less than center of mass force, we get that

Supplementing the variables for values like we did before, with max velocity of 22ft/sec and max turning radius of 12ft, we get that

Having training wheels extend well over 2ft in both directions is unpractical. The client insisted that the scooter be able to fit through doorways and be easily transported in a car’s trunk. As a result, we made the training wheels extend 11.25 inches, or 0.9375 feet, on either side. Reversing the equation for velocity, this results in a maximum velocity of

As was noted before, because of the additional weight and friction incorporated in the final product, the maximum velocity of 15mph is inaccurate and a maximum speed of 9.638mph is much more reasonable.

It is important to note that the scooter will likely fail by tipping before it would by slipping because the maximum velocity accepted for tipping (9.638mph) is less than that for slipping (12.675mph). However, in rainy, icy, and snowy conditions, the static coefficient of friction between the wheel and the asphalt decreases and it is highly possible that slipping will in fact occur before tipping. The client has been made aware of the increase risk in driving the scooter during inclement weather. In addition, due to the


inherent danger of riding a scooter, it is important to note that incorporated within the final product and its design, is a speed governor that prevents the client from accessing high speeds before his parents have determined that he is ready and responsible to access them.

References:

(1) http://www.thefreedictionary.com/centripetal%20force (2) http://efunda.intota.com/multisearch.asp?mode=&strSearchType=all&strQuery=static+friction (3) http://en.wikipedia.org/wiki/Moment_(physics) (4) http://regentsprep.org/Regents/physics/phys01/friction/default.htm (5) http://www.fhwa.dot.gov/ohim/hs97/hm33.pdf

APPENDIX D: HAZARD ANALYSIS

Likelihood of Occurrence: Frequent, Probable, Occasional, Remote, Improbable, Incredible

Severity of Consequence: Catastrophic, Critical, Marginal, Negligible

Risk Assessment: Intolerable Risk (I), Undesirable Risk (II), Tolerable Risk (III), Negligible Risk (IV)

Potential Hazard LikelihoodSeverity of

ConsequenceRisk

AssessmentPlan

Loose balance and topple over

Ocassional Marginal III Prevent training wheels from loosening by installing a stopper; also, installed speed governor to prevent dangerous speeds around turns

Cut foot on foot guard

Improbable Marginal IV Encourage client to wear shoes while riding the scooter

Stab body with armrest

Remote Negligible IV Armrest was built to the side of the client’s body; cushion is made of foam and poly-fil, which would soften a blow between it and the body

Stab body with handlebars

Remote Marginal III Ample space was left between the client and the handlebars; also, client is encouraged to sit as far back as possible in the seat


Fly off the scooter as a result of a sudden brake

Improbable Marginal IV Re-mounted the brake on the handlebars and decided not to create a footbrake; also, the highback seat has side supports that help contain the client within the seat

Shock or burn from battery

Incredible Critical IV Encouraged client not to remove scooter floor, where the batteries and wiring lie; also encouraged to always wear shoes when riding scooter

Fall backwards down a hill due to insufficient power

Probable Negligible III Encouraged to not use speed governor if planning to climb hills; also, lowered seat enough so that the client can stop the scooter with his left foot if need be

Hit by another vehicle

Remote Critical III Encouraged not to ride at night; encouraged not to ride in traffic; and encouraged to wear helmet and pads

Table D-1: Risk likelihood, consequence, and assessmentAPPENDIX E: FINAL BUDGET

The list below is our development cost for the scooter modifications:

Item AmountTraining wheels $ 51.00Highback seat 231.00Steel tubing 30.00Aluminum rod 25.00Aluminum tubing (x2) 37.00Axle extenders 30.00L-brackets (x4) 34.00Armrest padding 10.00Helmet/pads 30.00Lights 16.00Total $494.00

Table E-1: Expenses


An estimated amount for the replacement cost for the scooter modifications is as follows:

Item AmountTraining wheels $ 51.00Highback seat 231.00Aluminum rod 25.00Aluminum tubing (x2) 37.00Axle extenders 30.00L-brackets (x4) 34.00Armrest padding 10.00Helmet/pads 30.00Lights 16.00Total $464.00

Table E-2: Replacement costs

APPENDIX F: USER’S MANUAL

See attached file “scooter_manual.doc”.

Final Scooter Report

Documents

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