6. Main Frame Construction Part B - crunchysteve/TriceratopsTwo GitHub Wiki

Choosing A Rear Triangle

I can't stress enough that everything about building any cycle frame is 100% about alignment. Your front and rear axle heights have to be the same, relative to the horizontal. Your left-to-right alignments, angles, track lines and heights, have to be perfect or the cycle will pull to one side. With a trike, the track line of your back wheel has to be perfectly parallel to your front wheel track lines and perfectly centred between them. If you build your own rear triangle, all of this gets compounded by having to get the dropouts perfectly aligned to the ends of your hubs and perfectly perpendicular to your trike's track lines.

If this is your first build, and you're dead reckoning with wood working tools like squares, clamps, etc, then you may find liberating the backend of a BMX or a short seat tube mountain bike may be easier. A word of warning, though, make sure you have the bits and pieces, if not proper tools, for getting that rear triangle perfectly vertical and axle height matched, relative to the frame sections we've already built and have a true, flat work surface to do it on.

If you're no stranger to metalwork and welding, by all means attempt to build your own rear end. If you haven't done this part of a cycle before, have a look at the plans for Atomic Zombie's more conventional backends. Avoid the warrior for now, it's an "open-ended" rear, springy and bendy. This site's plans are only US$16 or so and they're a great source of ideas for dead reckoning this kind of alignment. I want sturdier supports than buckets, but they work if you are absolutely careful and have 3 identical buckets to work with. I'd rather use mechanic's axle stands and cut a groove in the up-flares to hold a dummy axle at the rear and your stub axles at the front.

If you're dead reckoning the construction of a rear end, use square tube, especially for your first attempt. Maybe you can support your square stays so that the diagonals are vertical and horizontal, rather than the faces. This might give a nicer, more elastic feel to the machine's ride. However, alignment of all parts is as critical as the final alignment of triangle and frame.

The golden rule in trike building is alignment, alignment, alignment!

Donor Bike Rear

A few years back, while walking as part of my recovery after surgery, I found an old Mongoose dual suspension mountain bike on hard rubbish. I know it was part of the hard rubbish pile and not just parked there, because it was at the bottom of an enormous pile of stuff and took an hour to rescue. The wheels were flogged out, the gears and chain were rusted and it was a noughties model, so a lot of bits, like the rear shock, are hard to replace. (Everything is wider, now!) It's also a 26er and nobody rides those much anymore. So it sat in my bike shed for a long time.

When I decided to build the trike, especially as I'm using 35 x 1.6 SHS tubing, the rear end of this old Mongoose has 3 major advantages...

1. It's chromoly with ISO disk brake mounts.
2. The suspension pivot bracket is offset to the left such that the right-hand side flange inner face is exactly 17.5mm from the centreline.
3. No major grind-off of where other tubes met it. It stands alone. It'll need a sand off and paint, but so will the rest of my trike. Meh.

Authors Note: I've just spent a "delightful" afternoon with a flat and round file, "machining" a millimetre off all around the inside of the axle slots of the dropouts, so I can have a modern, thru axle rear. The front is 12mm thru, now the rear is... And I am buggered. Drop forged dropouts are as hard as a spanner! As hard as a bag of spanners!!! It was worth it, though, the rear hub sits sweetly!

I'm using 35mm square tube, what's half (ie the centre!) of 35? 17.5mm! I have a flat surface, at the right location, to clamp the rear to the front and guarantee front and rear wheels will run parallel when not cornering! There's also a bracket 2/3 of the way up the centre tube at the front of the framelet. Again, centred, providing an alignment reference and a place to weld to. So, for me, using this half a frame was a no-brainer. And being a disk back I could design for 20", 26" or 27.5". Also, even though it's 135 QR rear over locknut, it actually comfortably seats a 142 thru, so it's getting modded for that - 1mm very carefully filed off each side of the axle slots in the dropouts.

Think I'm crazy? See Sheldon Brown's bike upgrades page, particularly with regard to putting 126mm hubs in 120mm frames, or 135 in a 130 frame. 142mm hub in a 135mm frame? 3.5mm wider each side. I didn't even really have to force my hub in. It dropped in under what would have been the weight of a bike on it, like it was made for the space between the dropouts.

Also, the linear brake brackets provide an anchor point for a custom pannier rack, there are derailleur cable and disk brake hose brackets that start at the front, very near the exit from the main part of the trike frame. Internal routing, you betcha. If this triangle has a problem, it's the asymmetric chain stays, but in this case, the centreline details mean I don't have to rely on the chain stays as supports when jigging. These are the swings and roundabouts of backend selection when going the donor parts route.

I tell you the story of my donor rear frame because finding the right donor is important. All those bonus bits make my donor rear potentially a killer trike backend. Look for a donor that enhances your project but simplifies your alignment. If you're building a trike, you probably don't need a rear triangle from a frame for a 7 foot tall basketballer. If you're using a 20" rear wheel, like me, a peewee mountain bike or 2 will render a heap of useful frame parts, like steerer head (don't go old school, you really do want 1" & 1/8 threadless), bottom bracket and a rear end. Except in rare cases, like with my frame's pivot and suspension brackets, a symetrical frame is probably easier to align.

About 3km from my place is a bike shop called Bicycle Recycle. This place has buckets of salvaged parts, a pile of frames good for donor duty and they rarely charge more than a few bucks for any such items. They also carry new-old-stock retro parts (more expensive) and order in new, too. If you have a similar shop nearby, start there. A ready built rear end is so much easier to work with because it's already true. It's not hard to mount it on a workspace, bracket it to the right heights at axle and bottom bracket, level it and align it, than to build one that's true, then do all of the other stuff.

Avoid 6005 or 7005 aluminium! Avoid aluminium altogether! Stay with chromoly. You can't easily, or cheaply, heat treat a frame like this at home. Needless to say, trashed carbon fibre bikes are, well, trashed. Not even good enough for garbage. CF is a great material to ride from new, but it's not salvage grade.

Build Your Own Rear

If you have the confidence and the tools, jig up and cook up. Don't let me discourage you. Building your own trike (or bike) is an adventure and a challenge. We ride for the challenges, why not build for them, too?

The usual rules apply, measure twice cut once. Double check your designs before you start cutting. Triangulate and keep things straight. If you have a conventional bicycle frame jig, building your own back end will be easy. The front of a trike is a different kettle of vegan fish, but in this scenario, you'll be starting with as near as you get to a store bought rear triangle. (Beginners, you can't buy a rear triangle as a store bought part, sorry. It was a figure of speech.)

I will be designing a rear triangle for my disabled friend's e-trike, and will be putting that up in the repo. Meanwhile, though, if you have the confidence to build, have the confidence to design. I use OpenSCAD, but FreeCAD is a powerful, architectural and engineering design software, with a graphical user interface and stacks of tutorials on the tubes. It can also import my OpenSCAD designs for you to tweak or even copy in a more native format. I'm a huge fan of free software, it aligns with my collectivist politics. If you use a ware, and like it, contribute to the project, either by donation or, if you code, offer improvements.

So, enough philosophy, it's time to weld front to back, bolt a few bits together and billy cart test this beast!