I was about to order a new square tapered BB for some folding bike.

Normally I would opt for Shimano UN55, since they tend to last a long time. However I noticed that some new variants were offered: UN300 and UN100.

Since I could not find any detailed info on Shimano's page (nor got a response from the webshop), I ask here.

  • Does anyone have press-releases regarding these UN300 and UN100 parts?
  • Are UN300 as durable as UN55?
  • Do the Chain-case and E-bike variants indicate a lateral offset? (how much?)


After a bit of digging I bring my preliminary conclusion (partly based upon comments), although nothing is confirmed:

  • UN55 and UN26 are no longer available so probably UN100 and UN300 will be the replacements
  • UN300 have a "stabilizer", which is probably just the small plastic spacer on the axle, that will line up flush with the crankset (on the images it looks like UN100 have this too)
  • UN100 is so far only available with 122.5 mm axle, so they probably only target the most-used axle length (and for low-end bikes you might not care much about the chainline)
  • The 3x variants (Normal/K/E) seem to differ in the how the right (drive-side) of the shell looks at the outermost part (see the image). Normal version have more axle exposed, and the E-bike variant have a few mm without threading (to allow for a sensor?). Furthermore the chain-case seems to provide a spacer (see the exploded view)
  • UN300 have aluminium shells on both sides, UN100 have resin shells
  • In terms of price: UN100 6,2 €, UN26 ~8 €, UN300 10,5 €, UN55 15 € (bike-discount.de 2020-06-09)

My guess it that UN300 is a minor upgrade from UN26, by having alu shells and some convenient improvement with spacers (to me it seems unrealistic that new product in this category is 30% cheaper and more durable). I kind of doubt Shimano will introduce a new "high end" square BB on par with UN55.

I think I will buy one of each, and perhaps bring an initial impressions update. Pictures from Shimano's product page lined up (all are 123mm axle)

Update after receiving UN300s and UN100

In general it looks like a production evolution of the previous designs like UN55 and UN26.

TL;DR: UN300 looks to be a well-made unit with good seals, but maybe not as tough as UN55. UN100 looks identical, except for plastic shells.

Let's get the weight out of the way:

  • UN300 270g for a 113mm axle
  • UN100 285g for a 123mm axle... so kind of identical.

The shells look like they are fitted/glued on. On the old UN55 the main unit with drive-side looked as if it was machined from a single piece of steel. The new UN300/100 is 1x steel axle, 2x shells (alu/plastic), 2x cartridge bearings and one middle piece (steel), although the drive-side shell is not immediately removeable.

The bearing on both models are 6903RS (ID17mm, OD30mm). I bought 2x UN300 and 1x UN100, on one UN300 I felt some subtle irregularities/bumps when turning by hand, but I guess that goes away after a bit of use.

The UN300 has the same white powder coating like UN55, and I guess this is similar to blue loctite you find on many bolts. The UN100 does not have this, but for plastic you do not have to care about corrosion anyway.

The finish of UN300 looks great, and has laser etched name + tighten direction.

The shells have seals that attach to the axle and it looks like an effective way of keeping water and dirt out.

  • 3
    Looks like the UN101 and UN300 are Shimano's only current square taper offerings, so presumably replace the UN55. The 300 has far more axle length options. See productinfo.shimano.com/#/spec/MTB/Bottom%20Bracket – Argenti Apparatus Jun 8 at 18:05
  • were you able to confirm of how it compares to the UN55? literally in the same position as you in trying to find a new BB like the UN55 – sidehits Aug 8 at 5:44
  • 2
    This looks like a good write up, but have you put the answer in the question? It's better to add the information into an answer instead, and it's ok to answer your own question! – Swifty Aug 8 at 20:14
  • @steffen can you also add a small answer saying "Updates in the question" or similar? I want to reward this with a bounty but can't do so to the question, only to an answer. – Criggie Aug 8 at 23:33
  • I believe the "E" designation is for use with an "E-type" front derailleur ("E-Type front derailleurs are made to be clamped between the frame and the bottom bracket. This type is less laterally adjustable than the others. Each model can only be paired with the specific chainring sizes it was designed to work with. Fortunately, Shimano is the only company which makes E-Type front derailleurs.") – Armand Aug 9 at 3:08

On the german Shimano site the UN300 is a bottom bracket designed for (e-)MTB. It is advertised as available in 3 versions: standard, E-type and closed gearbox (I think it is the spindle&co. without the housing). Different lenghts are offered. It is advertised as part of the "Shimano Acera" family of products.

The UN100 is advertised as part of the "Shimano Altus" and "Shimano Tourney" family of products. It explicitly mentions being compliant with two specifications only (D-NL and LL123)

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  • Good point regarding UN100 only being available in one spindle length. I do not get what you wrote about "spindel&co without housing". Honestly i can not use the groups (Altus vs. ...) to determine if UN55 quality is better/worse, since UN55 is not really part of group. – Steffen Villadsen Jun 9 at 17:59
  • @SteffenVilladsen I agree on the group affiliations ... it is just the way it appears on the site and I was mentioning it for completness. One of the 3 versions of the UN300 is the "closed gearbox" version, if I translate correctly from German. I do not know what is exactly the meaning, so I suppose it is only the mechanical parts, without the housing. I don't know. – EarlGrey Jun 9 at 18:58
  • I have not seen that translation, please post the original german terms if you can find it. I have only been able to find chain-case/chain-guard (kettenkasten in german) – Steffen Villadsen Jun 9 at 21:53
  • @SteffenVilladsen wrong translation, you are right, it is the chain-guard – EarlGrey Jun 10 at 4:47

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