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Kitten wheel harness – part II

February 28, 2015

What a week. A week of many lessons learned.

5 faulty designs for the pipe connector until I finally found one that will hold the pipes in place and won’t break.

A failed attempt to put the harness on the kitten, giving some clues to required (but yet missing) body supports.

And the best lesson – a visit to a vet specializing in rehabilitation medicine.

In case you have no idea what I’m talking about, you should probably read part I.

Problems with the 3D printed pipe connector and how I finally solved ’em

The wheel harness’ frame has 3 pipes connected to each other, with two longitudinal pipes attached to one transverse pipe.

The aluminum pipes’ round profile makes them quite strong for their weight, but the roundness also makes it harder to hold the pipes in place. The simplest way to connect the pipes is to screw or weld them to each other, but that renders the connection permanent and non-adjustible.

My aim is to create an adjustible wheel harness that can fit cats and dogs of various sizes, so I designed an adjustable 3D printed connector that squeezes on the pipes to hold them in place.

I had some experience using this method with aluminum connectors and it worked quite nicely. Aluminum seemed like overkill here though, since the frame won’t have to withstand too much force. In addition, it was quite difficult to make that connector, and required a mill and a lathe (not your everyday DIY equipment).

3D printing to the rescue. I designed a minimalistic connector, and Inspired by some other designs I saw online, I added a socket for embedding bolts in the plastic.

Connector Design #1

However, this connector broke rather quickly. It could hold the pipes in place rather well, but preventing them from rotating required a much stronger grip, and trying to tighten the bolt to accomplish that broke the connector.

Unfazed, I changed the design and tried adding thickness, widening the contact area between the pipe and connector and making the gap between the “lips” larger to increase the pressure on the pipe. The design gradually improved but nothing worked quite well enough, until I made a rather rigid, blocky design (leftmost in the image below).


Initially I was pretty happy with it. It held the pipes beautifully in place, and the pipes did not budge in any direction. I could apply quite a bit of pressure by tightening the nut and bolt and it didn’t break. Success? almost. It did have one major flaw…


The rigid structure proved to be a double edged sword, and it made the connector fragile in a particular axis. When I pushed a pipe inside forcefully or yanked it sideways so that it applied pressure outwards, cracks showed up and the whole thing could split into two pieces.

Further investigation showed that the very nature of 3D printing was to blame here. The piece’s weakest point was the adhesion between layers themselves. The vertical layering weakened the structure and when enough force was applied, the layers just separated and the whole structure split.

The solution – rotating the whole structure by 45 Degrees

After consulting with my start-up partner Dudu, he suggested to change the direction of the layering, by rotating the whole piece by 45 degrees. This way, the vertical layers will not be parallel to the forces applied by the pipe to the connector, and the structure will maintain its rigidity without sacrificing cohesivity.

While it’s easy to rotate the part in 3D software, printing the rotated piece is another matter. From a nice flat piece, I now had an overhanging unsupported mass. I print with Repetier and Slic3r, and the latter never seems to generate stable supports. You can see the automatic supports it generated below:


I tried to print this, but the supports didn’t stick to the bed or just collapsed way before they even had to suport anything, and I was forced to terminate the print job.

You know what they say, “if you want something done, do it yourself”. So I generated my own supports in Blender, with a column field that provided excellent support but was a pain to extract the pieces from


It took me around 10 minutes per piece and a scalpel, rotary tool, scissors, wire cutter, electric drill and pliers to clear up all the supports, but the piece was perfect – didn’t break and withstood enough pressure to hold the pipes in place beautifully. A reliable solution at last!


It took a lot of trial and error until I found a design (and print method) that works reliably, but I’m happy with the result (rightmost above).

Fitting the harness to the kitten (or rather trying to)

There’s one element crucial to the harness that’s almost invisible in all the images you can see online, unless you know what to look for.

When animals are strapped to the harnesses, you can easily see the frame, the wheels, and the front harness. Sometimes you can even see a belly support strip. But one of the most important parts is the rear harness or saddle, and since its integral to the frame and lies flat, it’s really hard to spot.

Which is why I didn’t really include one in my design…


Without a rear harness, the kitten couldn’t use the cart at all, and the whole thing just anchored him to the floor. Poor thing mewed in frustration and I felt a pang of premature desparation. First time that happened, I didn’t even know that was the reason it failed, and suspected the frame was simply too short. While that was true, the real insight came later when we went to visit a specialist vet, who’s an expert in rehabilitation at “Ha Kfar Ha Yarok” veterinary hospital.

The vet loved my design and the lightness of the frame compared to the Walking Wheels commercial harness they offer to clients at the clinic. But she immediately pointed out the lack of a rear harness.

At first I was flummoxed, but then I saw her demonstrate how to train the kitten in spinal walking using a wiry front harness as a rear harness. I suggested we try to use it for the wheel cart, and it was a pretty sound solution that held Benji’s hind legs nicely in place.

But then an additional problem manifested itself – the wheels were to low, even extended to the full height (around 15cm above ground) using my adjustable design.

The harness has to lie straight across the cat’s body, in order to avoid applying pressure on his back, which means the wheels have to be precisely the right height.

Since I had no way of adding height without preparing longer aluminum bars for the wheels, it was game over for the night. To see how it’s supposed to work, we strapped the commercial harness on Benji, but it was way to big for him so it didn’t help much except for giving me a few ideas for improving the usability of my design.


The Walking Wheels harness is pretty awesome. It’s fully adjustible, sturdy and has great build quality. The only downsides are that It’s pretty heavy and quite expensive (250-500$ if you buy it directly from them). I guess the quality of the product justifies the price, but some people just can’t afford it, and it doesn’t fit small cats. Another proof that a cheap, versatile and effective design is in needed.

What’s next

Since I posted about the DIY harness on Facebook, I got 3 more requests for helping another disabled cat and two disabled dogs. One dog needs a front harness, which is a completely different matter.

But I shouldn’t get ahead of myself. After all, this harness is not yet finished. I still need to prepare longer bars for the wheels, and attach a permanent rear harness to the frame. Both tasks are pretty straightforward, especially since I’m not the one making the rear harness! A friend’s mother, an experienced seamstress, is making a beautiful rear harness for Benji.

Which makes me worry a bit about the simplicity of the DIY solution I’m trying to devise. If you need to find a seamstress to make a harness for you, in addition 3D printing and cutting and drilling into aluminum pipes and bars, it’s starting to get complicated.

But I believe I’ll be able to find a solution for this as well. I’ll try making the next rear harness myself, and to again combine readily available materials with 3D printing to keep it simple.

But first, I want to see Benji walk. Tomorrow I’ll be heading back to the makerspace to prepare the new wheels, and later try fitting the harness to Benji again. Hope I’ll have exciting news to share!

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