An aspiring mountaineer, who wakes up every morning with a back pain is wondering what might be some of the most supportive backpacks suitable for mountaineering use that can carry 45lbs loads without being excessively bulky. I see that the dead bird company has a 65-liter pack with an interesting swinging waist suspension, but (apart from the price point) it's classified as backpacking, not climbing use, and not very clear to me that it can work well with a harness. Plus, it's too damn large! I was hoping for something closer to my Alpha FL 45, but with a better distribution of load away from my back, and onto my hips. Any suggestions?

You are going to want a pack with some type of suspension system that transfers the weight to the hip belt via some combination of metal stays and/or plastic frame sheet. Completely frameless packs like the Wild Things Andinista, Cold World Chaos, and Arc'Teryx FL series packs lack this feature and are therefore uncomfortable carrying loads heavier than 35 pounds. I have used the WT Andinista, ArcTeryx FL 45, and CCW Chaos packs. The argument is that one cannot climb hard alpine routes with more than 35 pounds on the back, which I agree with, but most climbing objective require some length of approach during which far more than 35 pounds needs to be carried. My recommendations, therefore, include packs that have some type of hard suspension that transfers weight off the shoulders onto the hip belt.

CRUX AX-50 or AK-47. https://www.crux.uk.com/product/ax50 These are both light, tough, and purpose built for alpine climbing. They use two vertical titanium stays to support loads. FWIW, I own the AX-50 and I find it small for '50 liter' pack, maybe even smaller than my trusty BD Shadow 45 L, but incredibly light. My two gripes are lack of a crampon patch and smallish top lid pocket.

CiloGear Worksacks http://www.cilogear.com/45lws.html use a plastic framesheet and aluminum stay to provide support. I have used these packs and like them for alpine climbing, they carry and climb very well. I do find the design to be inherently less durable then other packs and have never seen such beaten down, torn packs. They are designed to be configurable and one can strip everything down, including the top lid, frame, hip belt, and the straps which leads to accidentally lost straps. That said, if you don't mind replacing your ~ $300 pack every couple of years, they are a good choice.

McHale packs. http://www.mchalepacks.com/ultralight/index.htm These are custom made packs so the fit is incredible and you can have it built as simple as you want. McHale packs use two aluminum vertical stays to transfer weight to the hips. Most of the photos of their packs on their (terrible) website have extra pockets and zippers and what not. I have a SARC that I had Dan build with a small hip belt, no zippers or pockets on the body, and the lightest materials available at the time. The carrying ability/comfort and climbing ability are outstanding, as is the durability. You will not find a more durable pack. My SARC pack is now 20 years old and has been dragged all over North America, including three Alaska Range trips and still looks brand new. A couple of suggestions should you decide on a McHale; order your pack with a small, single buckle hip belt ("better than an Andinista or some such shit" in Dan's words), no zippers or pockets in the pack body, and made of Dyneema grid fabric for a light weight, stream lined pack. Additionally, \have the pack built with out a top flap, or tabs and handles on the top lid which allow it to be configured as a fanny pack. Useless extra weight.

A couple of other packs that I really like are older, used ArcTeryx Nozones or the Gregory Alpinisto 50.

Having backpacked and climbed with a number of packs with 45+ lb loads overs the years, and assuming you have to carry that much, I would strongly suggest you consider a McHale. They're not cheap, but a dialed-in McHale SARC or CM II against anything else is night and day, in my opinion.

Not knowing the cause of your back pain makes suggesting a particular pack a bit challenging as how the hip belt padding is designed can potentially aggravate a lumbar instability. I've got a L4/L5 spondilolythesis that has shifted 23.4 mm, and will require some hardware at some point. As such I can no longer use any pack that has any substantial hip belt padding that puts pressure on the L4/L5/S1 area. As such nearly all my packs now are Deuter, that have their complete spine channel design (shown below). I've carried 45+ lb on splitboarding/mountaineering trips with my Glide 35+ and Guide 45+ with good results. YMMV