So, I need a new BB. I know that I currently have:
- an ISO/English 68mm shell,
- somewhere around a 122.5mm axle length, [I'll measure more accurately before I buy.]
- a square taper chankset
- a triple crankset.

My first question is what, even just getting a basically matching replacement, does one look for in a BB. There are what seem to be compatible replacements from Shimano's UN26 and UN55 through various offerings from Suntour, Sunlite, FSA, IRD, etc. to expensive ones from Hope, Campy and Velo Orange. Even excluding the expensive ones, is there any practical reason to choose an IRD QB55 over a Shimano UN55 or UN26 besides price, convenience? This is for an inexpensive bike, so cost is an issue.

My second question is what considerations should I make if I'm considering trying a compact double? Should I get a 118mm (2.25mm shorter per side) spindle as a compromise between the double and triple chainlines?

Finally, is there a reason to dump square-taper and move to ISIS or something else as a "future-proofing" option so that if I find cranks I really like I can use them on a more modern frame later? Or is the BB standard smorgasbord on modern frames enough to prevent predictably finding cranks that one can move between frames?

Firstly. What exact crankset do you have? There's Campy taper and then there's JIS taper.

If you're going to change to a compact doublt, you need to get the right spindle width BB for it...mixing the wrong spindle width screws the chainline and then nothing works well.



If you're considering just buying a contemporary crankset BB system...measure your BB shell with a caliper-it needs to be damn close to 68mm to work right. All the post-square-taper BB/crankset formats have minimal if any room to compensate for a shell out of spec by more than a tenth of a millimeter. Most of them the crank and axle can drop into any BB cups designed to work with it-and those cups can be threaded 68mm English or PF or what have you.

Sq taper is the most universal BB, UN 26 or 55 are around $20 and last a long time. Sq taper will be around as long as they make metal frames out of tubing.
CF frames allow the wizards to go wild, hence the proliferation of BB bearing/axle/spline variants. ISIS is a bit of a kluge and not recommended, all the
disadvantages and no advantages. There are several sq taper 'tapers' but Japanese is the most common. Campy differs a bit. HollowtechII will be around for
awhile and is relatively 'future proof'.

If it's a cheap bike, just get a UN-26 of the same length.

If you change cranks, you get the BB that works with that crank. Just going 2.5mm (or whatever) shorter probably won't work.
Cranks use widely varying BB lengths. Older cranks tend to use longer ones then newer cranks.

Get a Shimano UN5x or UN72, rather than a UN26. The UN26 NDS cup is plastic, not metal, and you don't want that. The plastic will eventually break down. A UN51, having a lipless NDS cup, will allow for tighter adjustment than the lipped cup of a UN55.

Given about 2000 miles per year, with a heavy rider, won't the bearings need replacing before the plastic oxidizes? Or is the UN55 generally more durable?

Any thoughts on the so called "universal" BB386 from FSA? It seems to work with BSA, BB30, PF30, MegaExo, BBRight frames and a reasonable array of cranks. Is this actually practical or just marketing hype?

At which point you're talking about buying a new bike instead of just replacing a crankset or frameset.

Remember. Right now you have a 68mm threaded BB. You can't just take a 68mm threaded BB386 and plop it in a pressfit frameset.

"Universal" Isn't really at all universal. Also, many (like me) are of the opinion that 68mm English thread BB interfaces are still superior to any of the pressfit nonsense frames come with today. Pressfit mind you was invented to save frame makers money in tooling-not because it works better in any way for the consumer.

Inexpensive bike = UN-26. That's already a step up from what that bike probably came with originally. That nylon thing that everybody calls a "cup" is really just a bushing that keeps the cartridge from tipping inside of the bottom bracket shell. I've yet to see one have any kind of problem.

Crank sets and bottom bracket interfaces are evolving faster than I'm able to grasp. No way would I try to predict what I might want on some future bike.

I am in the middle of doing this with one of my bikes. For "conventional" screw in bottom brackets with spindles, the things you need to consider are: 1) Shell width 2) Thread standard 3) Taper standard 4) Spindle length.

If you're keeping your current crankset it should be an easy process. Buy a bottom bracket that has the same dimensions and standards as the current.

If you're changing cranksets things get a little sketchy. This is what I am going through right now. The taper standard must be compatible and the new crankset will likely require a different spindle length. Hopefully you can find the spindle length dimension for your new cransket online. Otherwise it's a process installing the new crank and determining the required spindle length to get the correct chainline.

Probably reads more daunting than it actually is. Good luck!

I kinda assumed that since since Shimano would not likely risk the reputational damage from BBs falling apart, even on the cheapest part of their line. Plastic on no-name parts I'd worry about, but Shimano seems to know just how to maximize margins without risking excessive failure rates. Thanks.

Part of my problem is that I may be moving soon and am not sure what kinda hills to expect in the near future. Triples are a major convenience for serious hills, but kinda pointless on the level. I'm also still enough of a novice cyclist that my leg strength is still increasing, so hills that were granny ring for me last year and now quite manageable on the middle ring. That is why I sometimes obsess about compatibility / interchangability -- I'm still learning what works for me and trying new things. Being able to do so without spending a fortune is important to me.

Even if the ghost of Tullio Campagnolo popped out of a lamp and granted me whatever $10000 bike I wanted; I'd be unable to pick anything that I knew would be "right" for me long term.

And, as you note, it is sometimes frustrating trying to dig up the recommended axle lengths on discount cranksets. Which makes planned purchasing annoyingly challenging.