I can only partially answer your questions because I'm relooking at every aspect of the pack in anticipation of making the next, lighter version on my way to the breaking point......which I haven't yet reached.
My strategy has been to make a version and use it every day going to the gym and grocery store (I'm retired) and backpacking with it as well. As I'm testing one pack I'm researching ways to make each component (frame, cross bar, waist belt, fabric, etc.) lighter and/or better. Many/most of the things I try don't work. Once a get a good set of things that do work, however, I make a new version and start testing it. I try to post things as I go along. Feels good to share and I get some good ideas and feedback from others.
Soooooo, keeping in mind that this is a moving target, here are my responses to your questions.
"Is this the fitting with model number ending in "Shark 400" for Skyshark 400 spars?"
"How do you now attach the top straps, side straps, pack, and waist belt to the frame now that you no longer use the T's?"
I haven't abandoned the Ts and may go back to them. For the end fittings shown in this thread I have tied them on with cord and/or used zip ties. I drilled out the string holes in the end fittings so a zip tie would slip through easily. The 40 lb testing was done after I drilled out the holes. Home depot has plenty of zip ties with breaking strengths up to 75 lbs. Not real keen on any of my methods of attaching to the end fittings yet. They are lighter than those very heavy(joke) quarter ounce plumbing Ts, however, so they are worth a look. Plus they can be glued to the spars with super glue. Neither nylon nor polypro plumbing Ts glue very well with anything I've tried.
"How are you attaching the cross piece now? Your earlier photos show the cross piece attached with T's."
Ts are still my favorite. I've experimented with a half dozen lighter ways of doing it but none of them really click for me. The cross piece needs to be stiff but not very strong. It's primary purpose is to keep the top of the vertical spars from coming together. As one goes weaker, however, the primary risk of breakage is grabbing the cross piece as a handle to lift the pack. It is so handy it is hard to resist.
"The cross piece is now lower than the top of the frame where the front webbing comes off? Does this give the frame better support, or give a better angle for the front webbing to come off?"
I'm not sure of the answer to either of these questions. I experimented with the position of the cross bar for a different reason. My wife would like to try the front bag. She has a very straight back and a bar at the top of the frame might hit her head. (I have a curved back and it doesn't bother me. So I've been experimenting with ways to put the bar 2-3" lower so it crosses near the curve of her neck rather than the back of the head. My experience has been that the front bag webbing should connect to the top of the two vertical frame stays. For me, having the frame stay tops at about the same level as the tops of my ears works well. The front bag webbings then kiss but leave no weight on my shoulders on their way to the front bag.
"You posted a while back that the additional buckles on the front pack allow you to use it by itself as just a day pack. How do you rig this without the frame?"
I think I'll have to add a photo to make this clear but here are some words to get you started. I make the frontbag-to-spartop straps extra long. While hiking I use these straps as arm wrests. For use as a day pack I disconnect the male half of the buckles from the spar tops (quick release buckles) and connect them to the female buckle halves on the bottom corners of the front bag. I then have a day pack with 1/2" inch shoulder straps.
"Have you ever tried an additional cross piece on the bottom of the frame?"
Yes. Seems to add no benefit and unless it is curved it rubs against my lumbar area.
"I've been experimenting with your design for the last year or so. Thanks for all your posts and information. I tried a mock up frame last year with wooden dowels, but couldn't really get it to work. So, I stripped down an old Camp Trails frame and hip belt, and attached a homemade pack and front bag per your design."
That's pretty much how I started about 10-20 years ago. My goal was to get the comfort and carrying capacity of my 5 lb MSR pack without all the weight.
" It's worked great for the last year. Total weight 3 lbs - not that bad for a full frame pack, padded hip belt, with no weight on the shoulders."
" The convenience of the front pack is lovely in and of itself. That it balances the back pack with no shoulder straps is marvelous."
I agree. It's hard to know the feeling without having experienced it.
"I have the Skyshark 400 spars on hand now, and some webbing. I'll see what I can do to copy your frame and hip belt. Thus the questions above. First I'm going to try just the carbon fiber frame, and attach it to my traditional hip belt."
That's what I did too. I think it is good to change only one component at a time on a pack that is working. Otherwise the variables can interact and leave one confused about what worked and what didn't. I changed several things at once just prior to a backpacking trip about 10 years ago. Nothing worked. The pack didn't even feel good at the end of the trip with no water or food in it.
"I recall you posting that getting a lightweight hip belt to work right is tricky - hard to keep it from kinking. I found this to be true using a soft webbing from the fabric store for my first trial."
The hip belt was the most challenging part and I left it til last. I think I have it licked now, however. I use Leno Loc Mesh (or the apparently unavailable Saran mesh). Thanks to Roger for steering me to the Leno Loc Mesh. A two inch wide good fitting belt with a fold-back-onto-itself velcro buckle comes in at less than 2 ounces and is as comfortable (for me) as the 8-12 ounce padded (Camptrails?) waist belt that was the last thing I give up on my road to lightweight insanity.
I'm pleased that you are working on this project. If I can get a few people up to speed on this project I'll soon be asking the questions.