The best bicycle bottom bracket standard?

[BikeGremlin]

A question I got in a YouTube video comment, regarding what I recommend to get, and what to avoid (posting the answer below, because it's a good question, but comments and replies get lost on YouTube):

Thanks. Any thoughts on bottom brackets?

 

[BikeGremlin]

The full answer:

Shimano Hollowtech II threaded are probably the least bad (or best, depending on whether your glass is hafl-full or half-empty ).

How to inspect HT II bearings for wear:

Why square taper is not perfect:
https://bike.bikegremlin.com/10123/unsafe-cycling-patents/#3.2

Why Octalink sucks:
https://bike.bikegremlin.com/1634/bicycle-bottom-bracket-standards-explained/#4

Other 2-piece crank standards are merely a less-than-optimal work around the Shimano-patent-protected 24 mm steel axle, and I consider them to be slightly inferior despite the marketing.

Relja

 

[lucas-mior]

I have research a lot and still don't understand the mode of failure for each BB standard. Is it true that square tape axle may break? How to avoid it? What exactly is it that make octalink bad? Hollowtech 2 seems the most safe, but it also has a shorter life span. How is that?

 

[BikeGremlin]

I have research a lot and still don't understand the mode of failure for each BB standard. Is it true that square tape axle may break? How to avoid it? What exactly is it that make octalink bad? Hollowtech 2 seems the most safe, but it also has a shorter life span. How is that?

Let's discuss each type, one by one:

Table Of Contents (T.O.C.):​

• Square taper
• Octalink (v1 and v2)
• Hollowtech II (Shimano)
• Conclusion

1. Square taper​

I have explained the problem (and inherent risk) of the square taper patent in the article "Unsafe and bad cycling industry patents", in section "3.2. Square taper crank axles".

Briefly put, fretting can cause cracking of the axle (especially the left side) in the section that is not visible without removing the crank(s). A snap can happen without prior warning (excessive play or similar). Removing cranks each week to check is highly impractical.

More info in the above linked section - no need to duplicate it all here (I prefer to keep information up-to-date in one place).

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2. Octalink (v1 and v2)​

Shimano made the Octalink system where cranks are mounted without any preload on the splines. Without preload, when you put some load (and cause the inevitable elastic deformation), you effectively have play!
My website article's section about Shimano Octalink and ISIS

When you pedal, the torque on the right crank is not transferred through the axle, but when you press on the left crank (pedal to be more precise), the torque is transferred through the axle, to the right crank and the chainring.

The effect of the pedalling torque on the left crank is the same you apply when you are coasting, with pedals parallel to the ground, your weight on them, and the left pedal/crank facing forward. If you coast like this most of the time (most people do for some reason), you won't have any problems with Octalink.

If, on the other hand, you usually coast with the right pedal/crank facing forward, then you are exerting the opposite torque to the one exerted when pedalling. With no preload on the splines, you get some back-and-forth movement. This movement inevitably causes the crank tightening/mounting bolt to start unscrewing.

When you have play, bolted connections tend to move in the direction of less tension. You can visualise that using a tray with some small parts on it, tip it to a small angle and keep it stationary. The parts will stay put. But, if you introduce some vibrations, some movement, the parts will slowly, gradually, start sliding down-hill (in this case dragged by gravity, not by tension, but the principle is the same).

Shimano's engineers apparently didn't understand the cause of the cranks loosening problem, so they designed Octalink V2 with deeper splines - only to see the same problem reoccur. Sigh.
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3. Hollowtech II (Shimano)​

Hollowtech II is easy to check and inspect (as I showed in the video), and if you don't go with superlight ("high-end") models, you might also get a pretty strong axle and cranks.

It is lighter, stiffer, without being less safe.

However, the bearings are mounted outside of the frame's BB shell. That leaves them exposed to dirt and water, shortening their life span. Despite the bearings being larger and more widely spaced compared to the internal square taper and Octalink bearings.

If you ride a lot in the rain and mud, or even "just" dust, you are likely to see your Hollowtech II bearings last 1/4 of the usual lifetime of square taper bearings (which can often last for a decade of commuting).

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Conclusion​

• Square taper is durable and low-maintenance, but it hides fatigue cracks and can fail without warning.
• Octalink was meant to be easier to use (and a bit "lighter and stiffer"), but the lack of spline preload leads to play, bolt loosening, and crank damage - especially if you don't coast "correctly" (see above for the explanation).
• Hollowtech II offers more stiffness, lower weight, and easy inspection. The main downside is shorter bearing life in harsh conditions - but replacement is simple, and parts are widely available (though not as easily as square taper).

What I do:

• Square taper for separate-bicycle-path commuter bicycle (Novi Sad has pretty good cycling infrastructure).
• Threaded Hollowtech II for all else (my travel, weekend ride and offroad/gravel bicycle).

↩ Back to TOC

Relja

 

[lucas-mior]

Thanks for the answer. Unfortunately, it is impossible to only ride in separate-bycicle-path where I live. I also note that hollowtech 2 cranks and BB are more expensive than square taper.

Why do you think that checking on a weekly basis is necessary? I am highly skeptical that, using a bike for commuting, not many jumps or long climbs, will cause the spindle to break on such short period of time, given how rare it is. On the other hand, bearing damage on external BB bearing seems to be widely aknowledged and unavoidable.

Isn't it more practical to just replace a square taper BB every year, given how cheap it is?

Also note, since high end bikes don't come with square taper anymore, they should no longer be prioritizing weight over durability either.

 

[BikeGremlin]

Thanks for the answer. Unfortunately, it is impossible to only ride in separate-bycicle-path where I live. I also note that hollowtech 2 cranks and BB are more expensive than square taper.

Why do you think that checking on a weekly basis is necessary? I am highly skeptical that, using a bike for commuting, not many jumps or long climbs, will cause the spindle to break on such short period of time, given how rare it is. On the other hand, bearing damage on external BB bearing seems to be widely aknowledged and unavoidable.

Isn't it more practical to just replace a square taper BB every year, given how cheap it is?

Also note, since high end bikes don't come with square taper anymore, they should no longer be prioritizing weight over durability either.

That is a reasonable, logical, good question that I forgot to address in my original reply. Exellent, thank you (no irony, I mean it).

Square taper axle and interface design​

Square taper BB was never about much weight savings - there's no room to make a thinner axle or make it hollow. So, the best one can do is make it as strong as the steel allows.

Some weight saving could be done with cranks (aluminium instead of steel), hollow etc, but cranks are on the outside and clearly visible. Which leads us to the next section:

Square taper axle's mode of failure​

Mode of failure is drastic, sudden, and relatively quick. Similarly to the quill stem, anything short of weekly (or at most monthly) checks is not safe or frequent enough to catch a failure when it starts propagating, before it fails (you are relying on luck, basically, in that case - since it can fail within 10 days of riding).

Riders that are strong and heavy, are more likely to cause even the good, manufacturing defect free square taper axles, but there is no guarantee it won't happen to a lighter rider too.

Conclusion​

I would be lying if I said that I'm confident about square taper axle being safe if it's less than one year (or even less than one month) old.

They don't fail often, but my worry is the mode of failure, and that is something I take into consideration when riding bikes with the square taper axle.

I also think it is fair to let people know of the risk(s).

It is not fear mongering, but more of a curse - the less you know, the less you suffer - as I like to joke at my expense.

Relja