The main tube can be aluminum40 to 50mm square. A wall thickness of 3 or 4mm is a good compromise between strength and weight. 5mm Wall would be on the heavy site.
If you are going to use steel of course it will be less thick.
At some places it is important to reinforce the tubes with washers or plates we make ourselves. But don't overkill. Keep the weight down.
The total length of the 40 or 50mm tube is 550+850+250+250=1900. Make that 2mtrs. It is advisable to order half a mtr extra. We can make little braces and brackets out of the extra material.
Notice the cross member tube ( cross-t) and rear forks are not part of the 2.5mtrs tubing. If you are going to use the same tube sizes for those as well add another 2mtrs.
Personally I think 50mm rear forks look a bit bulky and for the cross-t 40mm gives a little more room for the steering. But there is no real reason why you can't have them 50mm as well.
The front part of the main tube is longer then the rear level piece. This may sounds awkward. But it's how a tadpole trike is.
The front piece is 850mm or 85cm. This will easily be long enough for riders 2mtrs tall. You can always cut it back later. Or if you're taller make the sliding boom longer.
The rear horizontal piece is 55cm. ( I'll use centimeters describing the frame sizes on regular base) This will allow for a very reclined seat angle in combination with a 26" rear wheel. If your rear wheel is going to be 20" that rear piece and the forks can be much shorter. The wheelbase of a all around 20" trike is about 105cm for general use. Less if you want it very compact with a more upright seat like 35 degrees. 35 Degrees is still pretty sporty though. A small persons trike I made has a wheelbase of only 90cm and it works. The 26" trike is kept longish at 130cm. The reason for that is beside the seat angle it will also be the perfect length for a possible full tear drop shaped fairing. If you don't plan a fairing you can shorten the 26" trike by 10cm without a problem.
Cut a tube of 55cm at a 7 to 7.5 degree angle. Rotate the piece, hold it to the uncut tube, and check if combined it is a 14 to 15 degree angle. If not file one or both ends so together it will be the required angle. Now cut off a second piece 85cm long measured from the longest distance, straight at 90 degrees.
( notice if you mess up you will have cut the shortest one first so most is left intact)
Give the edges a medium touch-up with file and sandpaper.
Make the plate for the connection like on the picture.
Or give it a shape you like. Keep in mind there have to go 2x4 m5 or m6 bolts through the sides and 2 m8 bolts top to bottom. But it doesn't really matter if they stick out a bit and they're larger or smaller then the main tube diameter.
Copy the first side plate and file them to exactly the same shape in the vise.
Hold them against the tube and mark out where to drill the holes. Do the drilling of the plates while clamped together.
Now clamp both plates to the long front main tube. Drill one or two holes from both sides and check if the holes line up. Put the bolts in and drill the other two holes.
Next take the short piece and check the angle! Drill one hole form both sides. Put a bolt through and check the angle again. The tubes don't have to exactly touch. A small gap is pretty normal. It won't weaken the structure.
Please take your time with this main tube connection. It could easily be a 2 day job. Make the plates as nice and smooth as you can because we're not gonna touch it again. We could, but we don't have to.
Continue with the plates on top and bottom for extra strength.Notice the bottom plate is slightly longer then the top one! You cannot drill both plates simultaneously. Measure out the holes as good as you can using an L-square. First the top plate. Then drill the bottom plate. If the bolts go through at an slight angle don't worry. Run the drill through a couple of times and the bolts will slide in.
Again take your time. You don't want to do this part twice. If the angle is off by 1 or 2 degrees you'll be safe. If it's more we can correct it with the rear forks. All is not lost.
This connection is much stronger then you think. Don't use dozens of gigantic bolts.
Cut the cross-T. For a 75cm track it will be roughly 57cm. It depends on how close the hubs are going to be to the frame. Trikes used to have 80cm tracks as a standard. More recent models like Greenspeed, Catrike and ICE use 75cm. If there are no narrow bike paths and passages near your place and you are a larger person, you can add a couple of cm. This will help achieving a tighter turning circle because the handlebars are further away from the seat. If you want a very narrow track make the cross-T 52cm. This will result in roughly a 70cm track. Which helps getting it through doorways.
Once cut round off the edges to a smooth finish. Measure the exact middle and drill a 10mm hole.Use the L-square to mark it on the other side. Here it is more important that the holes top and underside perfectly line up. You can drill a smaller size first and check. When the smaller bolt does not go through at a straight 90 degree angle you can correct it with a small file. If it's only one mm well, it won't be the end of our tube. In the worst case scenario when you really mess up you have to turn the tube and use the undrilled side as top and bottom.
The cross-T will be attached 30cm from the bottom of the main tube connection.
The 'stabilizer' strip comes halfway at about 15cm. People have asked me why it has only one. Well two of those strips are fine. But the seat will eventually help keeping the cross-T in place. The one strip is simply easier.
Make a strip long enough and cut it back later. The material not too thick. You must be able to bend it. 3 to 4mm at most. Steel is also okay. Drill one hole for attachment to the main tube. And one where it will be on the cross-T at roughly a 45 degree angle. Don't drill into the cross-T yet!!
Now take much care the cross-T is exactly 90 degrees to the main tube. Tighten the bolt a bit so it'll stay in place.
And now you can mark the hole in the cross-T. First drill through the top. Mark it out on the underside so it'll be straight.
The good thing: If you attach the stabilizer strip and the cross-T is not perfectly 90 degrees, you can make a new strip. Leave it for the time being. Get on with the rest.
If you want to drill the cross-T first and stick a marker through to the stabilizer strip it's fine with me. But it is a bit more awkward through the small hole. With a drill maybe? .
Here is what our holy cross should look like. .
The cross-T is taken off again for the steering. So enjoy the frame while taking a break.
I may add some info on this page. Let me know when something isn't clear.