Firstly, can I just say I'm in awe of your welding, pure artistry! I love the "garage" and your work and thanks for taking the time to post. I'm sure it pays off in sales in the long term!
In post 977 picture 23 and 24 there is small arch of framing yet to be welded in place that appears to brace the 2 upper rear axle supports together. Is that required structurally or stylistically? Or is it used to support something else? In my older bikes it was where the rear brakes hung off but with modern disc brakes are secured elsewhere.
I believe you are speaking of the seat stay brace? (here's another view of a different one - also a fat bike).
These can be welded in place, but over time I've noted that brazing them is a little better approach. Welding them draws the dropouts together and the dropouts need to be reset. Brazing adds a little less heat, so they do not get drawn together as much. Brazing them also allows me to create a little bit of a relief between stays and brace, so it gives that area a little more style - also gives a little "lug like" shore line too. Here it is by itself:
This also creates triangulation in the rear triangle, and ties the seat stays together a bit more (so adding some rigidity as well as structure). Without that brace, the rear end actually feels a little "soft" (I've built some early bikes with and without them - they ride very differently). The only time I add a brace between the chain stays is when the client requests fenders. On mountain bikes and fat bikes, that brace is a shelf for collecting mud/gunk which effects the drivetrain performance. It does add some stiffness down there in the bottom bracket area, but I prefer the feel / ride of a bike un-braced at the chain stays personally.
The other part you mention is the ISO Disk Brake mount's brace. If running disk brakes, this is a must add item. The rotational forces around that disk brake mount are significant - unbraced it would eventually tear off that seat stay. Here's two shots of the current build in the shop where I am fitting the brace and ISO mount (I do this by hand - it's much easier and quicker). This picture is the first standard which is ISO 51mm:
The exception being a pair of sliding dropouts where the disk brake mount is isolated and part of the dropout. Here's a recent example of that (this is a second standard Post Mount 74mm version):
Pretty much all suspension forks run 74mm Post Mount now. They all start (for the most part) at a 160mm Rotor, and to go up to 180/203mm rotors, you use an adapter. ISO 51mm is a mounting standard for different adapters to adapt to 140, 160, 180 and 203mm rotors. The claim of Post Mount is that it is reportedly "simple" and eliminates some bolts which also takes the mounting bolts out of sheer - it's also "stiffer". The supposed problem with ISO is that your it requires more hardware and the mounting bolts for the adapters are in sheer. But as long as I have been riding, I've never seen anyone sheer off a set of adapter bolts. The problem with Post Mount is that it gets built to a certain rotor size. If you start at 180, you cannot use 160. But if built to 160, you still need an adapter to get you to 180 or 203. ISO 51mm on a frame is still much lighter and much easier to work with (my opinion). So there are arguments for both but ISO 51 is still a bit more simple as well as a bit less complex to machine/make (everything is in one plane - Post mount is requires the shape to be cut and then the holes to be drilled/tapped in a second plane so technically a harder part to make). But both if used on a traditional rear dropout need to still be braced to transfer the loads up and through the frame.
In other news: This one just got back from powder and was built up yesterday. Still some final tuning to be done...
Few fab shots too...
One last thing: Interbike was this week. Kind of the Sema of the bicycle world. I had a bike at Wilderness Trail Bikes booth.
Here's a link to a process book I put together detailing the entire build from start to finish. Here's a snip from that book:
Little promo for the bike too: