Did your Fixed Cup have a cartridge BB attached to it?

Nope, but the technique is similar.
It had been in there for 71 years though.

Method I used, explained in Cracking stuck BB using rear wheel as winch, will almost certainly work. It gives you 15x mechanical gain, like a ten-foot cheater bar - with no need to clamp the bike, because the big forces are all contained within the frame. The tool you need to make is just a short piece of chain with some strong cord attached.
https://cimg6.ibsrv.net/gimg/bikefor...f9ffe7ac96.jpg

You think you put 225 pounds of force through that light nylon cord?

It's Dyneema - UHMWPE yachting cord; "the world's strongest fiber...15 times stronger than steel, yet light enough to float on water. Think it sounds too good to be true? Think again.": https://www.dyneema.com/.
That 1/16" cord has a breaking strength of 500 pounds, and it's doubled, so 1000# left me with a pretty good safety factor.

So - yeah. 225# being a conservative estimate. You... um... didn't know there were plastic fibers stronger than nylon? Ultra-high molecular weight polyethylene: very long polymer molecules to start with, and then they make it super-strong by stretching the extruded resin to align the polymer molecules. You can sometimes experience the same effect by stretching a plastic bag, and finding that it becomes stronger and resists stretching further: what was a chaotic jumble of orientations gets pulled into an organized system - all the strength on one axis.

Regarding research skills: that info was in the source thread.

Edit: so I checked, and see that you left a derogatory comment in the source thread, saying a cheater bar would be simpler. Wasn't for me - that chain-and-cord whip was already in my toolkit, so using it was an absolute cinch - and I thought it was worth mentioning the method so that people such as OP of this thread might be spared some grief. Others were drilling holes in 2x4s to bolt them to wrenches & so forth; tying some cord to a bit of chain seems easier to me.

(above ) UHMWPE (AmSteel, Dyneema, et al), yep, amazing stuff, not just strong, but ultra low stretch, and better UV resistance than Kevlar. Excellent for making a Deltech cable on a folding frame.

But to the task at hand...

Wrenching on it with handle on one side only, has the drawback of wanting to bend the spine socket out of engagement, although bolting it in place should prevent that. But you are then still putting shear and bending moment into the threads, which will increase friction.

What you want is a "couple"; Either a huge 2-handed wrench, or as others have said, clamp the spline socket in a beefy vice* and grab the bike frame with both hands to unscrew it, and of course, making sure turning direction is correct. This will apply a "pure torque" with no increase in friction due to bending and shear loads.

* If the spline socket is clamped vertically in the vise, the vise may spin, unless the base has really strong clamp levers or does not rotate. If this is a problem, you may need to rotate the vise sideways and clamp the spline socket horizontal so is not trying to rotate the vise; The only drawback of that is that the vise clamps best at jaw center, not at the edge of the jaws.

True remarks, Duragrouch - but irrelevant if the method works (easily!), and the forces generated by asymmetry are not large enough to damage threads. Huge 2-handed wrenches are, to say the least, seldom on hand - and I reckon people who have a bench vise and clamp the spline socket in it seldom worry about gripping the bike frame on both sides to avert one-sided loads which all normal wrenches generate and which almost never matter.

Oh I agree, always try easy first. Wanna be rich? Get rich quick is better than get rich slow. :)

But I think in this case, the OP may be operating at the fringes of capability, and could use all the help possible; A few percent here, a few percent there, it adds up, sometimes just enough to push over the finish line. I'd be worred that a one-sided wrench could torque things enough to misalign the bottom bracket, but with two chainstays, a down tube, and a seat tube, and the wrench center is not that far offset from BB centerline, I think there's enough beef there to resist that. But a really lightweight frame, like racer steel or carbon fiber... I'd be careful about inputting a bending moment that is greater than that exerted by a stout rider. I think your chain wrench assist is within that margin, but I'd do a quick calc to know at what rim tangential force do I exceed that, and not do so.

"Missions are won or lost in the planning stages." - Neil Burnside, D/Ops

I have always had success by not bothering about fancy wrenches, just get a long piece of metal pipe (more than a meter) and put it over the handle and get someone to help. One person holds the tool on and the other turns the wrench. That has always generated more torque than I have ever needed

I was interested in this. Any update? Have you been able to get the BB disassembled?

Dan

UPDATE:
I took the bike to a bike shop, where they used a powertool to remove my bottom bracket in a matter of seconds! And didn't even even charge me for it :)

Now I have a new issue: I asked the dude for the size of the bottom bracket so I could get a new one and he told me 122. I'm finding it hard to believe that's correct and unfortunately I didn't bring the old one home with me. See, the part of the frame where the bottom bracket goes into is 70mm wide, so 122 seems way too wide, even considering the part that sticks out. Is there a way to figure out the size of the bottom bracket based on those 70mm?

70mm is the shell width for an Italian road bike. 122 would be a common length for some triple cranks, and is specified for the particular crank you removed. If that is the crank you are putting back on, what model is it?

122mm is the axle length to put your crank arms and chainrings at the proper distance from the frame. Look for a bottom bracket designed for a 70mm wide shell and matching thread spec (likely Italian, but not 100% definite) with a 122mm long axle. What make/model is your bike and your crank? How many chainrings?

The bike is Trek Zektor Zero. I'm not sure about the frame size, if that makes any difference.
The crank says "Bontrager #070225." I think it's Bontrager Nebula, 46T
One chainring.

Here's a picture of the crank and one of the bottom of the shell, if it helps:
https://cimg3.ibsrv.net/gimg/bikefor...46367ca13e.jpg
https://cimg4.ibsrv.net/gimg/bikefor...63749f3643.jpg

A Trek is going to have either a 68mm BB shell width or 73mm. You'll have to measure again to find out what you have.

That crankset doesn't have a listed BB spindle length anywhere. It certainly could be 122 for that style crank. Is there a reason you reject that length?

I just felt like 122 seemed way too long compared to what I remember it looking like.

But ok! According to the specs of my bike, what I need is VP BC-73C, 68x122.5mm, Square Taper
And here it is: VP Components Square Taper Bottom Bracket - Trek Bikes (DK)

I can't find it in Denmark. Interestingly, on the picture of the item that I linked to above, the part itself only says "VP BC-73;" that is, without the "C" at the end. I do see in some other places, like here, that it is called "VP BC-73C:"
https://i.ebayimg.com/images/g/P7EAA...DK/s-l500.webp

But I can't really order from outside the EU or it'll become expensive due to import tax.

So I'm wondering what does the "C" at the end mean? Is it important at all, or should I just focus on the 68x122mm part?
And if so, then I suppose I could order one of these (different brand)?
Shimano Krankboks Firkantet BB-UN300 BSA - Cykelgear
or
Shimano BSA Krankboks, 68mm

Any 68 x 122 will work. Cheap is fine.

Apparently not.

I got this Shimano BSA Krankboks, 68mm and it doesn't fit.

This stupid metal tube is in the way:
https://cimg2.ibsrv.net/gimg/bikefor...90ffc3daff.jpg
Dumb

So what do I do now?

If you can't dislodge that tube, you could use a traditional loose-ball bottom bracket instead of a cartridge.

Are you sure that it's not a fixed part of the frame?

No, I'm not certain that it is removable, but I feel that there's a non-zero chance that it might be. Tap on it with a drift punch and see if it moves at all. If it does, you should be able to drive it all the way out.

That is almost assuredly the center spacer from the original bottom bracket cartridge.

Alright, it's just that I tried to bank it out and it didn't move.

There is a cable guide under the BB shell. It is held on by at least one screw. That screw might be long enough that it is digging into that tube, preventing it from moving.

Loosen or remove that screw.

it's either rusted in place, or someone put glue in there, for some odd reason.
drip REAL penetrating oil in there (WD-40 is not a REAL penetrant oil)., around the edges, and also drip some through the drain hole on the bottom of the frame BB... re-soak it before bed time... let it set overnight.
then knock that remnant of the BB bearing assembly out...

i run into those tubes being stuck fairly often... the last stuck one was a PLASTIC tube... Rusted in place.
that stuck tube in your frame was one reason that the Cups didn't want to move easily, and is a symptom of WHY that tube is also stuck... RUST.
Sea Water does Bad things to bicycles, and other metal objects.

i find it rather odd that the bottom bracket underside pic shows a cable stop for a front derailleur, but the Trek Zeckor Zero doesn't have a front derailleur... and you should measure that BB width again, more carefully... i bet it's a 68mm width... the only treks i've seen with a 7omm width BB are special ordered ones that were specified to have Italian Campy parts installed by the ordering person...

for reference... meet the Zecktor Zero Trek bike... https://billigcykel.dk/images/Trek_Z...l-2017-2-p.jpg
another pic, and specs...https://99spokes.com/bikes/trek/2016/zektor-zero-dk