Building a Cart
(LOTS OF GRAFIX SO BE PATIENT!)
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Disclaimer: Note that any dog should be diagnosed by a qualified veterinarian for diagnosis of what leads to them going down in the rear. A cart might not be suitable for all problems. I'm sure a dog with spinal problems around the shoulder area may find a cart very painful and further injury could occur due to the pressure of the harness carried there.

There are several companies that professionally make carts that can be found on the internet. Eddies Wheels, K9 Carts, and Doggon Carts. A professionally made cart is likely to be much better than the cart described here as they've been in use longer to find out about small matters like pressure points, etc, that my limited experience with a home-made cart might not take into account. However, not everyone can afford a professionaly made cart.

The cart below cost approximately $40-50. The material used is suitable for a Pembroke Welsh Corgi or dog of similar weight. The PVC and sling material is not sturdy enough for a larger dog such as a German Shepherd, but if you're good at building things, you can make something similar. Also note that a dog must have a decent amount of musculation left in it's front to be able to use a cart. The person building the cart must have use some thought as to sizing the various lengths of PVC and sling material to suit their individual dog's needs and measurements.

Materials:
-1 schedule 40 PVC, 10' long, 1/2" diameter
-6 elbows for above PVC
-6 tees for above PVC (2 elbows may be used in place of 2 tees on top back of cart if wish)
-2 endcaps for above PVC
-2 lawnmower type wheels, lighter is better but smaller diameter isn't better
-2 carridge bolts and matching 2 nuts, long enough to go thru axel of wheel, nut, and width of one PVC elbow.
-dog harness to fit dog
-electrical ties at least 50lb capacity
-7 1-5/8"eyebolts plus 2 nuts each (long enough to go thru 1/2" PVC and 2 nuts)
-5 hooks sized to hook to eyebolts above
-8 grommets (don't need special tools to insert, just a hammer)
-6 self tapping screws ca 1/2" long
-PVC glue
-Material for dog sling (see text).
Equipment:
-hacksaw
-drill
-glue gun
-Dremmel (dog nail grinder) or file

Wheels- drill a hole in the middle of the width of the angle of a PVC tee big enough for the bolt to smoothly fit thru (see pix). You don't need a drill bit as big as the hole needs to be, a smaller drill can do a good enough job with multiple punctures and "filing" the moving bit back and forth in the hole. Insert bolt thru wheel, tee, and put nut on so it's as tight as possible while still allowing the wheel to turn freely. You may have to flatten the side of the tee a bit (using file, dog's Dremmel nail grinder) for better movement of the wheel. When nut position is figured out, put a bit of glue (or Lock-tite) on the bolt then crank the nut on to it's proper position so it doesn't unscrew on it's own over time. Glue gun a small pad of fake sheepskin over the bolt so the dog's hocks don't bang the nut/bolt end.

Harness. I just bought a cheap harness from Petsmart. It would be best if the strap going around the dog's chest isn't immediately behind the elbow so the front points of the cart don't interfere with the dog's elbow movement there. Conversely, you don't want it much further back as the weight should be not too far behind the dog's withers. Padding the top strap is best for the dog's comfort. I simply glue-gunned the fake sheepskin material that I used for the sling around it.

Working with PVC. All PVC lengths insert into PVC joints ca 5/8" (plus/minus) for this dimension of PVC, so take this into account when fitting/cutting. PVC can easily be cut with a cheap hacksaw. Note that PVC glue sets in a micro second and you have just about zero time for readjusting. If you don't want to risk a mis-glue, all parts can be screwed together with self-tapping screws. Be sure screw head isn't in a location where it rubs against dog and the screw is short enough so tip doesn't go thru other side of PVC. It's nice to have a drill with a screwdriver bit to put screws in, instead of muscle and a screwdriver.

General. Adjustments to height of sling in relation to dog can be made by attaching front of cart frame higher on harness or by increasing length of cart leg that goes to wheel.

Main Frame. If the dog can't stand by itself, it helps to have a 2nd person holding dog up for fitting and measuring. Put harness on dog, and make lengths of PVC to fit dog in relationship to dog's body as you seen in photos and attach with joints as shown. These lengths vary from dog to dog so giving exact measurements from my cart won't help. I would start with the leg to the wheel being longer in proportion to the cart shown as I went a bit low. It can always be cut shorter later. The rearmost 2 joints are connected by a very short section of PVC. Option: the tees at the top rear cross bar can be replaced with elbows instead, but I like the flexibility of perhaps being able to attach something else to these tees in the future. The PVC should not go much past the front of the harness attachment or it'll interfere with the dog's elbow movement. The legs of the frame should splay the wheels out a bit to the side away from the dog's feet so they don't hit it. PVC is cheap, so if you make a mistake or later have to go back and shorten/lengthen a piece, it's easy to correct before gluing/screwing. Don't glue any thing till you're sure things are correct, tho note that the whole frame will be a bit floppy till attached. Screwing will give you more flexibility for corrections.

Attaching harness to frame. Drill a hole for each eyebolt right behind the cap end of the PVC, the hole preferably just a little smaller than the bolt so it screws a bit, then screw in and attach the nut. These eyebolts should be inserted in PVC with eye end halfway between horizontal and vertical so the nut/end of bolt doesn't stick into the dog's side. Sew any sturdy material thru the bolt onto the harness, or perhaps a small hook. I used a small piece of thin nylon strap from some cheap old camera. I attached it bit too low on Ty's cart.

Sling. This material must be sturdy, non-stretching, non-raveling, yet comfortable on one side against the dog's delicate belly skin and groin areas. I found a good material (crate pad) made of fake wool skin at Petco. Best would be fake woolskin sewn onto a sturdy backing but I have no sewing skills.

Fitting the sling is the tough part. I'd make it bigger, and slowly cut down as you fit it each time. It might be best to do an initial template out of a sturdy different material first, especially if you do a 2 material/2 sided sling. The photos show a Boy sling with pee hole in sling. The back of the boy pee hole should actually not go as far back as I put it, so the V cut for the poop hole could extend further forward. Note that for girls the forward pee hole wouldn't be there and the V cut towards the back would extend farther forward for girl parts. The sling material might have to be reinforced in this rear V area by sewing on nylon strapping, depending on the strength of the material used. Also, the rear attachment to the frame for a girl might have to be further forward. One can always drill another hole in the frame for the rear attachment if the 1st one doesn't work out.

Attaching sling. I found some very easy to use big blue plastic grommets, simply cut a hole in the sling material (following grommet directions for hole size), put grommet in, hammer with anything heavy, and they're in. I attached the sling to the harness with electrical ties, keeping them looser at 1st then tightening as needed. Option: my sling is attached permanently with the ties to the right side of the frame, but one could also use more eyebolts/hooks instead to remove and wash the sling. The left and rear of the sling have the ties going thru the grommet and a hook, and the hooks hook onto eyebolts that are bolted onto the frame. These eyebolts should be inserted in PVC with eye end halfway between horizontal and vertical so the nut/end of bolt doesn't stick into dog's side. Drill a hole for the bolt, preferably just a little smaller than the bolt so it screws a bit, then screw in and attach the nut.

Attaching PVC parts. When happy with all parts, other attachments and fitting, you can use PVC glue to glue the parts together. Do this the very last as it's permanent. The cart will be a bit wobbly till then. However, I would use screws on the PVC tee that attaches the main frame to the frame's legs to the wheel, for later possible adjustments.

Cart wheel attachment, woolskin padding hides nut going thru PVC tee.