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Discussion Starter · #1 ·
I need to replace the BB on my daughter's Hotrock. At the same time, I'd love to lose as much weight off the bike as possible. I'm eyeing new cartridge style square taper BBs, and there are a couple of options.

Shimano UN55 - Probably the most reliable and comes in exactly the correct length, insuring a good fit.

Origin8 - Shaves some weight off the shimano, still probably reliable, a little more expensive but still within reason. Narrower spindle but I think that will be a good thing.

Ebay titanium - Anybody have experience with these? It's narrower than stock, but I don't think that will significantly impact the chainline on a 6spd bike. It's ultralight, which is great, and my six year old doesn't need a super strong set of bearings nor axle. I figure why not, if I have to replace the BB anyways, may as well upgrade.

I can't find any info on these BBs anywhere. I assume they are decent enough, I like that they have external fittings for what appears to be a park tool wrench, but I also don't want to buy a part that won't fit and will be hard to return. Any takers?
 

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IDK about these, but many years ago (30-40?) they quit making Ti square taper spindles because they'd break. As an example you can look at pedal spindles where Ti spindles generally have more weight restrictions than steel. You put your full weight on the pedal but it's magnified by the leverage of the crank arm when it reaches the crank spindle.

A shorter spindle moves the rings and crank arms inward. This affects the chain line and clearance between the rings and crank arms and the chain stay. It can also affect the front derailleur if equipped. Check these things before deciding on a shorter spindle.
 

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I run a ifx cheap chiner crank set, with a chiner pf30 BB. Both good quality and both work great and are pretty light for the price. 50 bucks for 630g probably 70 with a BB included.

I would not do the titanium square end axle
 

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Discussion Starter · #4 ·
IDK about these, but many years ago (30-40?) they quit making Ti square taper spindles because they'd break. As an example you can look at pedal spindles where Ti spindles generally have more weight restrictions than steel. You put your full weight on the pedal but it's magnified by the leverage of the crank arm when it reaches the crank spindle.

A shorter spindle moves the rings and crank arms inward. This affects the chain line and clearance between the rings and crank arms and the chain stay. It can also affect the front derailleur if equipped. Check these things before deciding on a shorter spindle.
Thanks for the reply. I'm aware of the relative weakness of ti, although I actually had a ti spindle BB back in the day on my STS at 170lb without issue. In this case, however, the BB is for my six year old daughter who weighs 48lb and will run 127mm cranks, so I don't think breakage will be an issue for her. That's probably my lowest concern with such a purchase.

As for the spacing, the original spindle was 127.5mm on a 20" hotrock with a single crank, bash and dual roller chainguide. I can run a 110mm spindle on the same bike without clearance issue, but it rubs on the guide at that length, and the chainline is less than ideal. I'm thinking carving 4-5mm off per side won't be a big deal with a 6sp drivetrain and probably no guide....but if you really think going from a 128 to a 122 is going to foul everything up, then I'll listen to your advice.
 

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FWIW: In my experience the measured length of the spindle (with calipers) is always a mm or so longer than the nominal length.

I can't say if 122.5 nominal wouldn't work in your case. I was just pointing out the things to check before making your decision. Agree that at 48 lb a ti spindle will very likely be fine. Very few of the old ones actually broke.
 

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Discussion Starter · #6 ·
Thanks. I measured the original spindle at 128mm, retail they all appear to be 127.5. I went and ordered the Origin8- it's lighter than the shimano, will likely fit without issue in terms of chainline and guide fit (If I keep it), and I can return it if it somehow goes wrong. Plus, being on Amazon, I'll get it before her birthday. Might be a long time for the Chinese parts to get here.

Had a bit of a wake up call the other day as I'm trying to lighten this bike up for her- she was more interested in streamers and pink saddles than the lightweight bike with front suspension at the bike shop....
 
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