Square taper → hollowtech BB?
 

Hello,

On a 20" wheeled folding bike with a single chainring that I need to lighten up a bit after replacing its original mechanical disk brakes with hydraulic brakes… I'm thinking of replacing its original cheapo square taper bottom bracket with a dual ring hollowtech crankset:

• Chainline: 55mm
• Square taper BB: BSA 68mm, 127mm (Kenli KL-08A), with a 3mm spacer used on drive side

For instance, would the 118mm spindle be long enough on this kind of entry-level crank — I know it's heavy at 1,130g, so I would at least replace its steel disks with aluminum parts, if not get eg. a Shimano combo instead? Should I look at road or MTB BBs?

I'm concerned the spindle might be too short, with the derailleur hitting the frame while on the small ring.

Thank you.
https://cimg4.ibsrv.net/gimg/bikefor...afc8858ef2.png

They say that includes a BB91 bottom bracket. Which is not for a 68mm wide shell.

I might be wrong. Let's wait till some others weigh in on that. However spindle length on a 2-piece does not equate to spindle length on a square taper. Chain line and perhaps tread/q-factor will give you a better idea if it will fit without the rings or arms hitting the stays. And if the BB for the crank is made for the width and threading of your shell, then spindle length isn't something you should compare.

External bottom bracket cranks have a fixed spindle length. They are made to fit 73mm bottom bracket shells. They are adjusted to fit on a 68mm bottom bracket shell by using 5mm of spacers which are arranged on the outer part of the bottom bracket bearings to provide the correct chain line.

Some are, but tons of road Hollowtech II cranks have no room for spacers and work for 68mm shells only. Which seems like something you already know, so maybe you didn't mean this as it is written?

It looks like the only way to know if a retrofit works on that bike… is to order the combo (BB + crank) and see for myself.

"Shimano HollowTech II : Spindle length varies depending on type of crank (road, Mountain, Triple, etc.)."
https://wheelsmfg.com/pages/crankset-tech

That crankset is going to have a road chainline, which would be between 43 and 45mm. You would be better off getting an MTB crankset, which will have a chainline of around 50mm.

Investigating

It appears that BCD104 can accomodate a 50T big ring, so looking good
https://cimg1.ibsrv.net/gimg/bikefor...ca24293609.png

I would bet it won't work, because your bike is unlikely to have room for an inner chainring and the spindle is not long enough to add 3mm of spacers. It seems like this comes up all the time with folding bikes and is always a disaster.

I just upgraded my Bike Friday from a square taper bb with 2x to Hollowtech II with Shimano Cues 2x. I added the two included 3mm spacers but that did not leave enough of the spindle for the crankarm to attach to securely and for the crankbolt threads to catch the spindle. So I removed the spacers and now everything is working fine.


https://cimg2.ibsrv.net/gimg/bikefor...416b878d30.jpg
Shell width suggested that the spacers were required.
https://cimg1.ibsrv.net/gimg/bikefor...d563ac544a.jpg
But these threads would not catch the spindle
https://cimg0.ibsrv.net/gimg/bikefor...ff945197ac.jpg
Removing the spacers fixed things

I'm no expert, but I'd imagine that going from square taper to Hollowtech will push the chainrings away from the frame, and thus reduce any interference between the drivetrain and the frame. But it will also affect the chainline, which is more critical in folders with shorter-than-normal chainstays.

Question about using a mountain crankset - would the external cup bb Hollowtech II + mountain crankset push the chainline out further than the original square taper setup? Would Hollowtech II + road crankset counteract that a little?

Yes, mountain externals push the crank further out. That’s because the chainline for mountain bikes is typically wider than road bikes. I run mountain cranks on my touring bike and commuter and have to stack all of the spacers on the left side to get the chainline in far enough to use a road front derailer.

That assumption does not make sense. HT has a somewhat fixed chain line, but you can manipulate chain line on a square taper quite a bit, and sometimes that's why square taper is used on modern bikes: Because sometimes a wider chain line is necessary to clear chain stays.

I'm curious: Why is that?

Am I correct in understanding that MTBs usually use a 73mm BB shell while road bikes usually come with 68mm shells, which is why hollowtech BBs sometimes/often come with 5mm worth of spacers?

Yes. for the most part. But they do specify a 70mm shell also for road bike bb's also.

Not sure what you didn't understand, but the spacers per the DM go on both sides of the BB. At least the few for mtn bikes that I've looked at do. Some people alter their chain line ever so slightly by putting them all on one side of their bb shell if it's smaller than 73mm which wouldn't get spacers installed.

Folded size