On paper, Hollowtech II is a step forward to old 3-piece cranks. It’s lightweight, bottom brackets are easy to remove / replace, cranks are easier to pull out. Getting the Hollowtech II bottom bracket cups is very easy and foolproof thanks to sturdy tool; while “inboard” bottom brackets like the first Hollowtech, ISIS, or the classic, conventional threaded ones are harder to remove; because they tend to “fuse” with aluminium frames, where these threaded type bottom bracket bodies are steel. Galvanic corrosion due to two different metals touching together can also damage these parts, especially aluminium, in the long run. You can use copper grease to slow it down; but thats not a permanent way to stop it forever.
Hollowtech II (or similar) cranks are also lighter due to simpler bottom brackets. There is a common misunderstanding that Hollowtech II cranks are lighter. Thats not the case. In fact, if, say, an older Alivio crank was produced with same methods like a current Deore XT crank, but with threaded bracket, Alivio crank would be lighter, minus the bottom bracket. What generally makes these 2-piece cranksets is the bottom bracket; not the crank itself…Shimano had hard time in some models to reinforce the area that holds the rod on drive side. A real manufacturing nightmare…Its’ immensely hard to center it perfectly, make the crank lightweight, and have enough toughness.
Is it stiff enough?
I’ve seen a lot of people talk about crank stiffness. Lets face it: most of them are not fit enough to produce 150 watts. And they’re absolutely not Shaq. You cannot bend a perfect steel alloy axle. And you cannot flex an aluminium crank that is bonded together. It will simply crack. Yes; “ordinary” aluminium cranksets flex to some degree, but it is almost impossible to flex a Hollowtech II crankset. It is a patented construction method by Shimano, and quite hard to produce. Shimano chose a hard path to stop a crankset flexing, and believe me it works.
Its just a rumor that haunts bike talk for too long. Maybe some way of saying “I’m very strong!”. I do feel bikes flexing; but thats the frame, not the crankset! In fact, a 3000$+ Merida MTB bike in 2010s flexes so heavily that I occasionally check for a frame crack !
I talked about stiffness because if the crank as a whole flexes, it would damage bottom bracket cup bearing easier.
So whats the problem with exterior cup bottom brackets, like Hollowtech II ?
In order to make a solid axle that doesn’t bend, and to make it light, you need a big bore, hollow pipe. Wall thickness is not as critical as the bore. Of course, a solid, thick rod is better than a hollow bore, if you are OK with a crankset that weighs as much as a road bike.
First problem is, the most common frame shell standard, BSA threaded, is too narrow to handle a Hollowtech II type crank properly. With 24 mm spindle diameter, it should fit BSA shell, which is approximately 34mm only! Look’s proprietary BB95 cranks, that is far superior to anything else, fits in a 65mm shell!
In order to address the issue, Shimano, or SRAM for GXP, should have used gigantic cups that would protrude a lot to fit decent sized bearings, or ditch cassette bearings for a more proper type, like roller bearings. Shimano Hollowtech II bottom brackets use a derivative of 6805 bearings: 25mm inner, 37mm outer, and 6mm width. (“Original” 6805 have 7mm width, while Shimano uses 6mm ones that you cannot buy off-the-shelf! – Though I have 2 methods to replace those bearings)
So, the bearings are tiny. They are not meant to handle such loads! Besides, when you have smaller bearing balls, they exert much more pressure to races, a recipe for failing bearings. Manufacturing such small balls in tighter tolerances is a nightmare. Conversely, Shimano uses big balls in their hubs. When correctly set after a clean up, an ordinary Shimano hub will outlast the bike, probably you, too. Higher end ones, like XT and XTR ones, uses boron nitrate hardened steel for added strength. (“Ceramic coated”, or sometimes just “ceramic” is mostly boron nitrate added steel)
What surprises me is that, Shimano loves to use far superior cup & cone system everywhere, but not in bottom brackets, which they are absolutely needed most!
Other problem with Hollowtech II bottom brackets are, insulation.
In threaded bottom brackets, at least in good quality ones, you have 2 RS bearings (sealed on both sides), running in a sealed container.
Many times, after a slight rain, my Hollowtech II bottom brackets soaked. I’ve tested an XTR series lately for some time, which performed much better then a Deore, sealing wise. 2RS bearings is a nice precaution, but won’t stop water, because it is not intended to do so: sealed bearings are sealed for dust, and keeping the grease in, not keeping water out. If you need water tightness, you have to use labyrinth seals.
Am I missing something?
I’ ve seen some horrible Hollowtech II bottom brackets, because the crankset missing an o-ring. Yes; Hollowtech II cranks have an o-ring like face seal! Surprising number of people don’t have this seal on their cranksets, don’t know why. It serves 2 purposes: it helps to make a better water tightness, and pushes the plastic cap on Hollowtech II caps that attaches to bearings, so the plastic protector does not move inside the bearing – instead, crankset rotates the bearing, not the plastic cup.
Is it really doomed from the start?
The crank itself was a brave step forward. It’s a challenge to produce such a crank, back in decades ago. But the doomed bottom bracket is a bad implementation of a novel idea. SRAM also failed, and their implementation is actually far worse.
Yes; there are challenges, but BB’s could have been MUCH better.
Like, using roller bearings that are designed to handle such loads, instead of tiny, non-standard 6805’s that are joke.
6805’s use 3.3mm balls; while crappiest Chinese BB’s uses bearings twice the size. Look at the SKF threaded Octalink bottom bracket: roller bearings on one side, hefty balls on other.
Shall I buy an aftermarket one ?
Frankly, I did not come across decent ones, even Chris King or Cane Creek is highly dubious to me as they are using same style bearings, same bad design. Looks like their bearings are non-standard, too. So, the question is, “Shall I pay 5x-10x for a bad design, boutique BB, or less for a badly designed, cheaper one that needs to be replaced more often?”. I’d still stick to Shimano; at least its
cheap. Better yet, I mastered my own methods to repair them.
I wish SKF make those roller bearing versions for Hollowtech II, too.
If you consider bottom brackets as consumables, you’ll feel better about ruining them. It’s not your fault.
Unfortunately, bike industry heading to a wrong direction. High end parts are not guaranteed to last any longer anymore, at least, from mainstream manufacturers.
Maybe its a good idea to invest on a decent, square taper crankset like Rene Herse, with a hefty price tag like 500$.
Making it a little better ?
Hollowtech II bottom brackets fail due to water damage, mostly.
If water can access to your bottom bracket shell internally, the water will get into these bearings from the plastic retainer. Depending on the model / brand, these plastic parts either do not have, or have weak o-rings around the lip. So, if yours have a groove to fit an o-ring, its wise to replace this every time you remove the bottom bracket. Use a bigger size if you can, or add some teflon tape into the groove, so o-ring fits tight. If yours don’t have this groove, just silicone it.
For the outer cups, we will need to replace the bearings with quality, 2RS versions. 2RS means “sealed from both sides”. Original ones from Shimano does have face seals; uses an overlapping metal washer with some rubber instead. Use 61805-2RS bearings if you can; these are deep groove bearings that provides better contact area, and will be more robust then the originals.
I’ll describe the process step-by-step once I repack my cups 🙂
Advantages of Hollowtech II bottom bracket
Since these bottom brackets do not last long, they become a commodity item: you can get them cheap from AliExpress or eBay. I doubt they are much worse then boutique items or the authentic Shimano. Anyway, they are cheap. You can buy a dozen with the price tag of a XTR item. Or, better, buy some nice quality 6805 bearings from reputed manufacturers, and replace your bearings. This is the most eco-friendly way. There is a cheaper way still, if you can take a beating, which is replacing balls in a bearing cage – given the number of balls in there, its pretty crazy.
The other advantage is, you can use them as a GXP bottom brackets with a shim! In my country, GXP bottom brackets is exotic items, and ridicilously expensive if you can get one. Lately, I get a Sram X5 crankset with a GXP bottom bracket, because it’s almost impossible to buy 10 speed, XT – XTR level cranksets these days. I did not regret it yet, Sram front shifting is not the best; noisy, slow and have a clunky feel, unlike Shimano.
Being Shimano, you can buy a Hollowtech II bottom bracket anywhere in the world, for a fair price.
Hollowtech II for longer life, possible?
If you ride in rain, bottom bracket damage is inevitable. In fact, every bottom bracket will suffer eventually. That is why vintage bikes, and some newer city bikes have chain guard and fenders. Bike manufacturers and racer-wannabees don’t like them anymore. And they cannot be used on a MTB, as a small pebble can rip off fenders, if not cause a serious crash. %90 of MTB owners I know almost never ride in dirt. They buy MTB’s because they think suspension is a great comfort item. On a 100$ bike, it isn’t. MTB’s are cheaper, more abundant and looks more “modern”, that’s why people buy them, forgetting that the design does not fit their ride.
For a beater bike,or any other bike where 200-300 gr extra weight at same price point doesn’t make a difference, nothing beats square taper.