Introduction: All Terrain Jogging Stroller
Sutton is a 7 year old with Cerebral Palsy who loves to spend time with his family. To that end his family was looking for a mode of all-terrain transportation for Sutton that would allow him to go off-roading. This would allow him to follow his family off of the cement where his wheelchair is king of road and explore rougher terrain.
- Bob Alterrain Pro
- Accessible Stroller Chair
- Either 12 or 16 Gauge Metal, or a 1.5" Block of Metal
- 1/2" aluminum tube, (about 36" long)
- 1/4" sheet of aluminum, 2' square will be plenty
- Solid Surface (2' square will be plenty) - this could also be any number of materials, it will be used to make the middle piece of a bracket for the stroller
- Blue locktight
- Cutting oil
- Screw drivers
- 3D Modeling Software and 3D Printer for Prototyping
- CNC Machine
- Drill with Drill Bits
- Angle grinder and sanding pad
- Sand paper
- Water jet
- Deburring bits
- Threading taps
- Dremel tool
- Drill Press
Step 1: Pick the Brains of Professionals
We started our project with a strong team of professionals who gave us a strong knowledge base to start with.
Sutton's physical therapist invited us to one of Sutton's physical therapy appointments. She helped Sutton us how much more support he has when his pelvis and knees are at 90 degrees. We could feel how much more stable he was when he was supported with a solid base. She also showed us how much fun Sutton has playing with his family when she helped Sutton play basketball with his brother.
We learned from Sutton's family that taking his existing wheelchair off of cement doesn't happen much. The small wheals work great on concrete but not much else. We learned that 1 front wheel would work best and that it being able to walk wasn't hugely important. We learned that the family was most excited about being able to explore the backyard, local paths and the beach. Additionally, having a sunshade to shade Sutton was super important. They also described the wheelchair van that they use to transport Sutton's existing wheelchair and would also use with the stroller.
Sutton's wheelchair and posture specialist taught us about how to provide that support to Sutton. We learned that laterals on either side of Sutton help keep him upright and that he would need support on either side of his hips. Sutton's wheelchair and posture specialist topped sharing all this knowledge with us by giving us a retired wheelchair seat that we could attach to the stroller. He could then modify that seat to fit Sutton perfectly.
We also had support from a local entrepreneur who has worked with stroller significantly. He told us about how he developed his early prototypes, advised us on what he would look for in a stroller and pointed us toward brands that would be mostly likely to support a kiddo who has grown out of the toddler phase.
Finally, our classmates, professor and our universities Project Innovation Hub manager were critical in every stage. They critiqued our ideas and pointed us toward ideas that were realistic with our resources. This became even more critical when we transitioned to distance learning mid-semester.
Step 2: Find the Perfect Stroller
With the help of a local entrepreneur with lots of experience working with strollers we started looking for the perfect stroller that we could modify and that would have all the features we were looking for.
We looked at bike trailer/jogger conversions because they have a lower base of support which would decrease the left to right tilting Sutton would experience. Unfortunately they had a wider base of support which would make transportation more difficult. We also didn't like that the fabric surrounding the strollers would close Sutton off from his family.
We looked for jogging strollers because their single front wheel leads to easier navigation and because their suspension systems are designed for taking bumps in stride. We wanted to give Sutton the most comfortable ride we could. We also wanted a stroller with a foot brake, similar to the one on Sutton's wheelchair as that is what Sutton's family and caregivers are must accustomed to using.
The BOB Alterrain Pro had the best combination of the features we were looking for.
Step 3: Put the Stroller Together
The family purchased the stroller and our team picked it up and put it together. Assembly was quick and easy.
Step 4: Remove the Existing Stroller Seat
The existing stroller seat needed to come off so that we could replace it with the wheelchair seat that would give Sutton all the support that he needed.
The existing seat was fabric and was held on with screws to a channel along the edge of the stroller frame. Follow the perimeter of the fabric seat and find all the screws connecting it to the frame, some are on the top and some are underneath the stroller. Also trace where the brake line passes, on our stroller the brake line traveled with the seat and had to be disconnected before the seat was removed. The stroller can then be pulled up and removed from the frame.
Having a flat surface to attach mounting brackets for the new seat will be very helpful so the next step is to use an angle grinder of a dremel tool to remove the channel that held the original seat to the frame. It's a small space and angle grinders can be a challenge to control so we we used a dremel tool. Be prepared for it to heat up quite a bit, possibly burn through a blade or two and take some patience.
There were also some rivets that held on the seat that needed to be drilled out of the frame. They were in the area where we planned to attach a bracket and wanted as flat a space as possible. We used a drill with a drill bit slightly bigger than the rivet to drill out the rivets.
After the channel has been removed sand the inside surface of the frame flat. We started with ~60 grit sanding pad on an angle grinder and used that in the open spaces. Around the suspension bracket and in the tight spaces we used a sanding attachment for the dremel and did some sanding by hand to get it as close to flat as we could.
The final step is to paint the frame black. Cover the parts of the frame that don't need to be painted (in particular the padded handle and the fabric pouch at the bottom of the stroller) and tape off the area that needs painted. We used black Rust-Oleum spray paint and gave the frame about 3 coats.
Step 5: Design a Mounting System for the New Seat
Our team did a lot of brainstorming at this step. We wanted to design a bracket to attach the wheelchair seat (the new seat donated by Sutton's wheelchair and posture specialist) to the stroller that would still allow the stroller to fold up but would securely hold the seat to the frame. We also wanted the chair to attach to the frame in a way that the center of gravity of the seat would fall around the same place as the center of gravity of the stroller so that the stroller would be stable.
The team experimented with attaching the seat to the stroller from the back first. This backfired as a rod across the top of the stroller blocked to headrest from getting where it needed to be. The team decided the seat would be set onto the frame from the front of the stroller.
At this point the team ended the semester working on 3D printing prototypes of potential designs so that the team could get a feel for what worked. We started the prototyping process using Fusion 360 to model the options and then used a 3D printer to get a feel for how they fit the stroller and chair. We did several 3D prototypes before we were ready to fabricate the final pieces.
You'll see in the images some of our brainstorms, as well as the .stl files for one of our 3D prints.The design we settled on was 3 pieces that sandwiched the frame. For each of the sides of the frame then we had an outer, middle and inner piece. The middle piece sat on top of the frame (red piece) while the inner and outer pieces (black pieces) were on the outsides of the frame to gain stability from the frame.
There is also an image of the side profile of the wheelchair seat and a picture of the original wheelchair it fit into. The seat had a central knob that slotted in to the original wheelchair and carried most of the weight as well as a pin at the bottom that allowed the wheelchair's angle to be adjusted. The pin is pulled in when attaching the seat and and the chair is moved to the desired angle and then released into a slot. We wanted to mirror this functionality in our design so that the wheelchair angle could be adjusted. We'd learned from Sutton's therapists that Sutton preferred to be tilted backward at about 15 degrees. The middle and inner pieces contained a channel for the central knob to slide into which was chamfered to match the profile of the knob and the inner piece had holes for the bottom pin to slot into.
Step 6: Fabricate Mounting System
Once deciding on the design for the bracket the next step was to fabricate all 6 pieces.
The inner and outer pieces of the bracket were the most important structurally so the team decided to fabricate them from 1/4" aluminum. Those four pieces (left inner, left outer, right inner and right outer) were cut on a waterjet from the attached .dxf file (StrollerBracket.dxf).
After cutting the pieces out they were cleaned up by sandblasting the faces of the 4 pieces. The edges were rounded slightly by filing the edges. The holes in the aluminum also needed to be tapped. The holes were drilled using the bit corresponding to the die that fit the size of bolt we wanted to use. It is super important that the holes be tapped straight up and down.
After the aluminum pieces were prepared we gave them several coats of the black Rust-Oleum to match the frame of the stroller.
The middle piece was made from Corian solid surface, an acrylic polymer commonly used for countertops. The middle piece does not bear as much of the load and could be made from any number of materials. The team's original plan was to 3D print the piece from ABS but this option was most convenient in this situation. A CNC was used to fabricate the middle piece. The Corian piece was 1/4" thick so two pieces of Corian were planed down and CNC and glued together to produce the middle pieces. The attached dwg file (MiddlePieces_7.dwg) has all four pieces that were cut out to produce the final left and right middle pieces. The letters on the pieces refer to which piece they are. R_L refers to the left side of the right middle piece and R_R refers to the right side of the right middle piece and so on. The dimensions that are listed on the dwg are the depths that the CNC blade cut. For example, on the R_L piece 0.008" was planed down from the original 0.25" to produce the final thickness of that piece.
The curved profile of the top surface of the stroller frame was challenging to match using the CNC so the middle pieces were sanded down to lift them up off of the stroller frame. Use a piece of tape to mark where the piece should be sanded to or use sharpie to mark a reference line on the tape.
Step 7: Modify Wheelchair Seat
There were two modifications that we made to the wheelchair seat that was donated by Sutton's wheelchair positioning and posture specialist.
- Extend the central knob to better fit into the channel created in the middle and inner bracket piece. We accomplished this by using a longer bolt to attach the knob to the frame and filling the distance between the central knob and the frame with washers. In the image you can see the washers added, but the bolt is still sticking out beyond the central knob. When the number of washers we wanted to add was finalized we shortened the bolt to the length we needed and then used blue locktight to secure the bolt.
- Extend the pin that determines the angle the chair sits at. The existing pin was removed by unscrewing the attachments on the bottom of the chair and a new pin was made using an aluminum dowel of the same diameter and securing it in the same position
These modifications allowed the wheelchair to fit more securely into the bracket we fabricated on the chair.
Step 8: Modify Stroller Frame
The bar at the top of the stroller frame in front of/below the handle was a limiting factor on how much the wheelchair seat could rotate and made inserting the wheelchair seat into the frame a challenge. The seat's headrest hit the bar when leaning the chair back.
The stroller handle was removed by drilling out rivets on the stroller frame near where the handle met the rest of the stroller frame. This made it easier to get the bar in and out. The bar was removed by drilling out rivets on the bottom surface of the bar, near where it met the rest of the frame. In the last image, the red arrows indicate where the original bar was removed from the stroller frame (a metal colar on the stroller frame accepts the bar), the blue line indicates where the stroller handle detaches from the frame and the pink arrows indicate where to look for the rivet that holes the stroller handle onto the frame.
An aluminum pipe the same diameter of the original bar was used to create a new one with a curve in the middle that allowed more room for the seat's headrest. A 1/2" aluminum pipe, 36" long was purchased and bent with pipe bending tools. The same curves were done on both sides of the center of the pipe to keep the bar symmetrical and then the pipe was cut to the same length as the original bar. Finally, the team applied several coats of black Rust-Oleum to protect the new aluminum bar.
The bar was inserted back into the stroller frame. The team chose not to put new rivets into the hole to secure the new bar because the stroller frame kept it in place and leaving the rivets out allows the user of the stroller to rotate the bar out of the way for folding.
The stroller handle was then replaced in the stroller frame. Bolts the same diameter as the rivets that originally held the stroller handle in place were cut down to less than the width of the stroller frame tubing. They were screwed into the hole where rivets had originally attached the stroller handle. Rivets could also be used for this but this allowed us to avoid buying a larger rivet gun.
Step 9: Attach Mounting System to Stroller
The first step is to do a test fit with all the bracket pieces and the bolts that did not go through the stroller frame. Line up all the pieces on top of the frame and manipulate as needed until you get a perfect fit. Put the wheelchair seat into the brackets and make sure that everything fits as expected (first image). We put the bolt heads on the outside of the bracket so that the inside would be smooth (since the holes were tapped we didn't need nuts). This made inserting the wheelchair seat into the frame easier and creates fewer sharp edges on the inside that could get in Sutton's way.
The next step was to mark on the stroller frame where the holes needed to be drilled through the stroller frame. Start drilling from the outside and use the holes in the outside bracket piece as a template to drill pilot holes through just the outside edge of the stroller frame. Remove the bracket pieces and use a drill guide to drill the correct size hole all the way through the stroller frame. It is very important that these holes be drilled straight.
Put the bracket pieces back on the stroller frame and test fit with the bolts. We found that we had to use a slightly larger drill bit on the stroller frame holes to get the bolts to fit through the stroller frame.
When everything is on the stroller frame as it should be use blue locktight to secure the bolts in every hole on the bracket and then use a torque wrench to tighten the bolts down to about 60 lb-ft. Note that it is every hole except the little holes in the bottom of the bracket that accept the pin to change the angle of the seat.
Step 10: Adjust Seat to Fit Child
Here we are once again indebted to the team of professionals that support Sutton. The wheelchair seat was then sent to Sutton's wheelchair seating and posture specialist. He adjusted the pieces of wheelchairs such as the height of the headrest and the length of the footrests. He also made a custom seat for Sutton to sit on.
The before and after pictures are attached - we could hardly recognize it!
Step 11: Testing
The next step for us was to make sure that everything worked as we expected. We put the wheelchair seat onto the stroller frame and then added some heavy vices on the seat to simulate Sutton's weight. We pushed the chair around in the lab and then took the stroller outside and pushed it over some grass and gravel.
Step 12: Sunshade
Sutton is very sensitive to light so we know it will be important to have an option to protect him from the sun. When we changed the seat and added the attachment bracket we covered the bracket that attached the stroller's sunshade to the stroller frame.
We used this Sport-Brella Versa-Brella Umbrella from Academy as a short term solution because it has a very adjustable clamp to attach the sunshade to the stroller. It was very adjustable and would need to be adjusted to block the sun based on the sun's position. We were very glad to have it on the sunny evening we delivered the stroller. Unfortunately its large size meant that it also blocked a lot of the view of the person pushing the stroller.
We hope to develop a new bracket to attach the original sunshade and move it higher since Sutton's head sits higher than the original stroller chair and plan to 3D print a new bracket to deliver to the family.