I am wanting to change from old school bottom bracket to Shimano cartridge . My current bb uses a 3U spindle . 124.5 mm long overall. Per The Sheldon Brown chart , a symmetrical cart. bb would need to be 133mm. As far as I can tell, no such exist. So, what is an option besides changing the crank?

Thanks

I'd not change just to be changing, unless you are going to go for new cranks and then I'd get the bb that is appropriate for the cranks you want.

Whether cartridge types or the old style spindles with races, I never cared for square tapers. They give you too many numbers and other gotcha's to worry about when changing things up. I much prefer the shimano hollowtech II and similar products from other mfrs.

I'm not entirely certain the symmetrical number means anything for you picking a bb cartridge with square tapers on it. But I'll let those that use and change their square taper bb's address that.

Ill chime in with the opposing viewpoint .. a square taper cold forged aluminum crank is fine

for chainline, the left side is not significant

right side fixed cup face to spindle end .. match that.
.
Shimano's UN types work well... (and low cost)

@ $150. , probably not willing to spend up for SKF


IRD is about half that ..

Phil Wood is USA Made . they are very fine tunable .. in chainline ..





....

just get any english bb, all shimano goes into all shimano english/bsa bb. unless you have italian. then you get italian bb and shimano whatever cranks.

A 127mm spindle will be about 3mm short on the DS.
IF your FDER will go in that much, by backing out the L screw (3mm at least?) AND you are willing to suffer a chain line that is about 1/2 cog in too far, it should work.

That Sheldon Brown chart is a joke!!!! I've tried it several times now and it's always been wrong. Use Bill and the Addict's advice and if that don't work, shim and adjust the front derailleur to get your alignment. The UN-26 is a good cheap alternative to loose bearings. I pick them up on the bay for around $10.US Can't go wrong.

And what chart do you think I used to base my advice on?

I have used cartridge BBs in place of loose bearings a few times and have always had good success. It is one way to make the drive train of a crappy Chinese or bigbox store much smoother.
We had a similar occurrence with our Chinese road tandem. The first set of BBs were too narrow so I picked up another wider set that worked great.

I noticed also Bill that you're dimensional advice is different from the Addicts.... Hmmmm

Since he's on my ignore list, I have no idea what he suggests.
However, my other 2 spindle charts confirm what I said.
A "U" has a 40.5mm offset on the DS. A 127mm spindle is 39mm on the DS.

Whatcha got now?

Bill's calculations are correct.

Maybe I need to learn how to use the chart? It's always suggested a much longer BB than I've needed and caused me problems so I've grown to be a tad miscalcontent over it.

Symmetrical equivalent is simply-
52mm +(2X DS length)
Actual length is 52mm +DSL+NDSL. The "D" length is what you should be looking at.
Since the OP has the longest spindle made (DSL) and since Shimano doesn't make anything over 127mm (to my knowledge) a 127mm is about the only option unless one wants to get into the far more expensive "adjustable etc." BB's. One then has to hope the FDER has enough adjustment left to accommodate the closer position of the chain rings with the 1.5mm shorter DS spindle length.

According to the chart, the longest DS length available is 39mm vs the 40.5mm of the "U".

Thanks for the input. Since the bike I am currently working on is a vintage tourer, I will probably go with the old school setup. But all the info posted will help me with future use of cartridge bb.