8. Pedal Spar (AKA bottom bracket, AKA front bracket) - crunchysteve/TriceratopsTwo GitHub Wiki

Parts

There are 2 main parts to this component...

  1. BSA standard 40mm x 68mm bottom bracket shell. You could source one from a donor bike, but you can get them cheap enough online. For example, the Trisled one, mentioned in a previous chapter, or on eBay, AliExpress or even your local bike shop.
  2. 375mm of 30x1.6 SHS steel tubing. this slips inside the front tube of the main frame member and is clamped by 2 M8 grub screws, tightend through 2 M8 nuts welded to that part, as shown in an earlier chapter

Construction

Most trike riders still call this part the "bottom bracket" because it does the same job as the bottom bracket on any standard road, gravel or mountain bike. I prefer "front bracket" because it's out the front, not down at the bottom of the frame. If anything, our steerer head is at the bottom, but that already has a more appropriate name. Call it what you prefer, but note what I mean when I say "bottom bracket shell" (the name of the part to be combined with the square tube) or "front bracket"/"front spar". (The whole assembly.)

While a simple part, good alignment is critical, so I've designed a jig, that centres my 30mm square tube on my 40mm diameter by 68mm wide BSA bottom bracket shell, as per... BB_Brace

The piece to the front supports the tail of the square tube, the T-piece supports the front of the square tube against the side of the bottom bracket shell.

The square tube needs a cylindrical cut in each side, to closely match the outer diameter of the bottom bracket shell. Jig the two parts, check they're square to each other, then make the top edge weld, to join the back side of the bottom bracket shell to the top-front edge of square steel tube. Remove the jig and continue the weld all the way around the joins between the square tube and the bottom bracket shell. You should have something like this... frntBrkt

Coating Advice

It's worth getting this part galvanised or chromed at a plating company, rather than painting it. As it slips in and out of the main, front tube, to help with chain adjustment, paint will get scuffed and dirty. My preference is for galvanising, as this will reduce the rust potential, especially if done inside and out. (Although, complete galvanising may mean you'll need to clean up the bottom bracket thread.) You can still mask and paint the bottom bracket end to 50mm up the square tube if you want, but I wouldn't take it much further.

I'm not a fan of chrome finishes for bicycles. I have a beautiful, chromed fixie that is developing tiny rust spots under it's rose tinted clear lacquer because chromium and steel make a "battery." This bike is never used in the rain and lives in a dry and well ventilated bicycle shed when not in use. Steel and zinc play a little nicer together, hence the widespread use of galvanising to protect steel.

Important Note

Also, don't rush out and paint any parts yet, except this one, if you must. The main frame needs brackets welded to it, the whole trike needs to be assembled for rolling/steering/alignment testing, to check everything's running "true." Only start disassembly and preparation for painting it after the "billy cart" testing.