The first thing I did when I got the tubes home was carefully and accurately mark them at key divisions, say every 6 or 12 inches, using a wax marker. It's a lot harder to measure these accurately once the tube has even a single bend in it!
|Be sure to mark the pipe before bending|
A four foot length of tubing turned out to be pretty much the perfect length. Width of the frame at the back needed to be around 12 inches, to give Kayla's back end plenty of clearance (and to make the cart wide enough to not topple too easily if she tried to turn while running). Since I planned to use large 14 inch diameter wheels, the axle height would already be 7 inches off the ground. So I decided to start by bending the tubing into a 12-inch wide U-shape.
I'm being a little vague on measurements, since every cart will be specific to the needs of the dog. Just be sure to provide clearance should your dog need to hop.
A word about pipe bending. I managed to find a decent pipe bender (a Ridgid 36132 model 408) on Ebay for relatively little, but then again I'm a tool freak. It's quite possible to bend the tubing without such a tool. Just be sure to take precautions against flattening the tube at the bend. An effective way to do this is to compact sand in the tube prior to bending. The sand will resist the temptation of the tube to collapse. Bend the tube a little at a time, around an appropriately curved object, such as a lamp post, power utility pole, or even one of those iron pipes they embed in your garage floor to stop you driving into your water heater or furnace. (That's how I put the gentle bend into the shoulder bar.) Once the bends are correct, empty out the sand. As an aside: my Dad's uncle Ern was a professional industrial pipe bender fifty years ago, bending huge pipes (many inches diameter) on a big jig by, you guessed it, first filling them with sand.
|Fill the pipe with sand to stop it from flattening|
Whether using a tube bender or not, be sure to work away from the center of the tube. In other words, once you've bent one side, say, by bending from a given wax mark, bend the other in a mirror-image of the first, so you're still bending from the equivalent wax mark on the other end. Why? This way, if the first bend isn't exactly as you expected - perhaps the bend radius stretched or shrank the tube very slightly - the second bend will make the whole thing symmetrical, so it's not the end of the world. This is also where your pre-measured wax marks will be a great help.
Keep all the bends on the same flat plane, aided by using a spirit-level. Minor corrective tweaks are possible afterwards so don't break into a sweat if the thing doesn't lay exactly flat first time.
|Keep the bends in the same plane|
Next, bend the tube to form the wheel axles. If you have enough tubing, be generous here, because some wheels have quite wide hubs - perhaps three or four inches. Also, if you intend to make your wheels splay outwards a little for reasons of stability (much like many modern human wheelchairs do), now is the time to tweak the bends slightly to achieve that. Bear in mind that some wheel hubs are not designed to take that kind of angular strain, but most will be fine. See the 'wheels' section for more information on hubs and bearings. Once you have finished bending your first frame tube, it should look something like this:
|The main vertical pipe and wheel axles|
Now to form the second part of the frame - the horizontal piece that should rest lightly on the dog's shoulders, with the shoulder bar vertically in line with the front legs to avoid any pressure on the back or neck. When you measure and bend this tube, hold it up against your dog as a sanity check that all is as it should be. My tube bender let me do some tight bends close to each other. If you are not using a bender, you can achieve similar but simpler geometry by putting a gentle bend into the long 'arms' of the frame rather than the 'up and over' close bends shown here. Also, the addition of a very gentle arc to the shoulder bar will help with comfort once the padding is on.
|The horizontal pipe for the shoulder harness|
Congratulations - the most nerve-wracking part is over!
Update: I revisited Ace Hardware to track down some info on the parts I used. I have no affiliation with Ace, I'm just supplying this info as a courtesy.
Steel tube (1/2" cold rolled)
SKU# 5014634 (in the hobby metal section)