Introduction: Cat Cart (Rear Leg Paralysis)
Our multidisciplinary team at RIT (Rochester Institute of Technology) was tasked with designing a cart for paralyzed cats. Our goal was to create a cart that would increase the cat's mobility while remaining safe, comfortable, and low-cost.
We spent two semesters designing, assembling, and testing the cart. The final iteration is contained within these instructions. Feel free to modify and change steps to customize your cart to your cat or pet.
Step 1: Materials, Tools, and Printing Instructions
The following materials are recommended to construct the cart. Within the parentheses listed after each item are approximate price and potential distributor.
- 5 Aluminum arrow shafts ($15, Lancaster Archer Supply)
- 10 3D printed joints ($95, Shapeways, see instructions below)
- 12 8-32 x 5/16" set screws ($4, Lowe's)
- 4 Rubber stoppers - #00 ($5, Lowe's)
- 2 Model airplane wheels, ~3.5 inches in diameter ($13, hobby/craft shop)
- Front mesh cat harness (1, $6, Amazon. com)
- Rear strap ribbon - ($3, Joann Fabrics)
- Rear fleece fabric ($7, Joann Fabrics)
- Various screws, washers, and nuts
- Parachute clips
The following tools are recommended to assemble the cart
- Dremel or Saw
- Hot glue gun
- Allen wrench
- Tape measure
- Sewing kit
Shapeways Ordering Instructions:
1.) Download attached STL files below
2.) Go to Shapeways.com
3.) Create an account
4) Go to My Workshop -> Models on the left side of the page
5.) Upload all the STL files into "My Models" (this will take a few minutes so be patient. After 5 minutes if it's still processing go ahead and refresh the page to confirm the files were uploaded)
6.) Add all the STL files into your cart
7.) Select Strong & Flexible Plastic in any color (polishing doesn't matter)
8.) Set Units to millimeters
9.) Change quantity of Cross Beam Joints to 4
10.) Order the parts!
Step 2: Build Frame
Begin by cutting the arrow shafts to length. Our lengths and quantities included:
- 2 Rear Legs (RL) - 6.125" (6 1/8") - 155 mm
- 2 Front Legs (FL) - 8.5" (8 1/2") - 215 mm
- 2 Side Bars (SB) - 11.75" (11 3/4") - 300 mm
- 1 Front Cross Bar (FCB) - 4.33" (4 5/16") - 110 mm
- 1 Rear Cross Bar (RCB) - 4" - 100 mm
- 4 Drop Bars (DB) - 2" - 50 mm
- 2 Kick Stands (KS) - 2.25" - 57 mm
Note that these lengths are suitable for a cat of approximately 8-10 lbs. If you have a larger cat, lengthen all of the dimensions with the exception of the kickstands. The cart can be dynamically sized on each axis (L, W, H) so it's best to overestimate and then trim parts to the correct length later. Length and Height can be easily adjusted with set screws but Width requires you to get it right the first time or else you'll have to redo the hot glue.
Once all the arrow shafts are cut, glue the arrow shafts into joints using a hot glue gun. We did so in the following order:
1.) Glue the RL, KS, and FLShafts into the Axle Connector Joints. The RL shafts should be glued on the same side of each joint as the Kick Stands.
- The two finished pieces should be mirror images of each other.
2.) Place the FL shaft through a Front Leg Joint, and a corresponding RL Shaft through a Rear Leg Joint.
- A through-hole on each of the Leg Joints should line up in such a way that a Side Bar Shaft can be pushed through both (if not, the two Leg Joints are not meant to be on the same side).
- Once the SB is through both pieces, the assembly should be a big triangle (see frame images).
- Make sure the two holes left on the Leg joints face down and inward, opposite the "nub" sticking out of the Axle Connector (which should be pointing outward).
3.) For the moment insert and screw all of the set screws into the Leg Joints until flush (but not too tight).
- This will thread the plastic and prime it for when you do your final cart sizing once the harness is built and installed.
- Take care to screw them in straight because the angle you initially use is the angle they'll always use.
- Additionally, the threads will wear with repeated use (plastic is soft) so try to adjust these lengths no more than 5-6 times. If you're concerned of the set screws loosening you can always lock them in with a dab of hot glue.
4.) Glue each of the Drop Bar Shafts into the remaining holes in the Leg Joints.
Now it gets somewhat difficult...
5.) Place one of the assemblies on its side with the DB Shafts pointing "upward". Place the two Cross Bar Shafts (in no particular way) into two different Cross Beam Joints without gluing.
6.) Glue the Drop Bars of the laying down assembly into the other holes of the two Cross Beam Joints.
- Make sure before gluing that the unglued Cross Bars are parallel to each other, pointing upward, perpendicular to the assembly's Side Bar Shaft.
7.) Repeat Step 6 for the opposite assembly, reusing the same Cross Bars as guides.
- Glue the Rear Cross Bar (the shorter of the two) into the Cross Beam Joint which is connected to the Rear Leg Joint.
- Glue the Front Cross Bar into the Cross Beam Joint which is connected to the Front Leg Joint.
8.) Line up each half of the assembly and glue them together into a completed cart frame!
Step 3: Create Harnesses
The front harness is very simple.
Take a purchased cat harness (a link to a recommended harness is provided in the materials) and two sets of parachute clips. Using nylon ribbon attach the two sets of parachute clips to the collar of the harness. Be sure to place the clips such that they would be aligned along the back of the cat and are separated slightly. A simple sewing kit or sewing machine can be used to fix the clips to the harness.
Take your piece of elastic and make sure that it is sized appropriately for the cart, you should have 1-2 inches of slack between the cart's posts. Loop each end of the elastic around the cart's poles. Record the line where the elastic end meets the remaining elastic. Sew each end to the length of the elastic such that there is a small loop on each end of the elastic band. With the nylon ribbon sew the opposite ends of the parachute clips to the rear of the elastic band.
The rear harness will require patience and practice to make sure that it is properly constructed.
Align your two pieces of fleece on top of one another and begin sewing along the sides. Do this for three of the four sides, then turn the 'pillow' inside-out. You may choose to add batting to the inside of the pillow before continuing. Roughly 1 inch of batting is recommended. Sew the remaining opening close by turning the lip of the fleece inwards. Once completed, hand sew the fabric ribbon to the underside of the pillow. Do so such that the horizontal length can loop around the other side of the pillow and there is enough space to encompass the torso of your cat. On each end of the horizontal strap, attach your remaining parachute clip. Attach the Velcro to the vertical fabric ribbon running on the underside of the pillow.
Step 4: Attach Harnesses to Frame
1.) Loosen set screws on frame which hold the Side Bar Shafts in place, then slide the shafts out.
2.) Add rubber washers to the side bars such that they will be located "behind" the adjustable Leg Joints.
3.) Put the Side Bars back in place and re tighten the set screws.
4.) Loop Velcro strips around side bars between the front and back Leg Joints and the corresponding washers.
5.) Place a parachute clip around the attached Velcro in order for the attachment loops to stay in place.
Step 5: Put Cat in Cart
The cat we tested our cart with, Trey, has full paralysis in her rear legs. To install her, one has to put the front harness around her shoulders and neck. This harness straps into two clips that are attached to the elastic band that spans the width between the front of the harness. Since the rear harness is essentially a seat belt, we simply placed Trey on it and buckled her in.
Initially we placed her hind legs beneath her body, but after several tests we found out she was significantly more comfortable with her legs splayed out as seen in the first picture. Every cat is different and they're not prone to give informative feedback, so be patient and expect proper sizing to take a fair amount of experimentation.
It's important to keep the spine of your cat either parallel to the floor or at a very slight negative angle (rear is lower than front of cat). It will take time for your cat to become accustomed to the cart and learn how to maneuver with it. The first picture demonstrates an ideal spine angle. You might have to adjust the kickstand lengths if the cat is having trouble sitting back in the cart.
The cat may decide to lay down in the cart, heavily arching their back in the process. This isn't good for the cat and it should be encouraged to sit up (leaning back on the kickstands) instead. If the cat continues to lay down it may be an indication that the kickstands are still too long and the cat cannot sit properly. The second picture shows what happens when the kickstands are too long (notice the wheels are off the ground).
We wish you the best of luck in improving your cat's mobility, and thank you for using our cart design!