This post compares the advantages and disadvantages of Hollowtech and Octalink bottom brackets.
Definitions
Bottom bracket – a mechanism consisting of bearings and cups allowing the spindle connected to the cranks to spin smoothly inside the bike frame.
Octalink bottom bracket – A Shimano bottom bracket with internal bearings. The spindle of Octalink bottom brackets is splined and has a larger diameter than a square taper.
Hollowtech bottom bracket – A modern Shimano bottom bracket with external bearings. Hollowtech models have a much larger spindle than both Octalink and square tapers.
The Advantages of Octalink Bottom Brackets
1. Internal Bearings
The bearings of Octalink bottom brackets are well protected because they’re sealed inside the bottom bracket body.
The bottom bracket body does not protrude outside of the bottom bracket shell and is also protected to a greater degree by the frame and the cranks.
The Disadvantages of Octalink Bottom Brackets
1. Small Bearings
The main goal behind Octalink was to engineer a bottom bracket that’s lighter and stiffer than square taper models. To achieve that task, Shimano made the spindle of Octalink bottom brackets larger and thinner.
The new diameter of the spindle resulted in the use of smaller bearings because otherwise, it would have been impossible to fit the spindle inside the bottom bracket.
The smaller the bearings of a bottom bracket, the shorter their expected lifespan is. Consequently, some riders complained of the bearings’ longevity.
In different, the bearings of Hollowtech bottom brackets are outside of the bottom bracket shell and thus there’s no compromise with their size.
2. Overly Recessed Splines
The splines of Octalink 1 are about 5mm. This was enough for road bikes, but when the bottom bracket was installed on MTBs, the crank arms would sometimes fall off the spindle during extreme riding. The solution was Octalink 2 which comes with longer 9mm splines.
Or in other words, Octalink 1 bottom brackets are not recommended for bikes that will be used for stunts. This limits the bottom bracket options even further.
3. Limited Choice
Octalink bottom brackets are the middle child of the bottom brackets. They’re aren’t nearly as popular as square taper bottom brackets due to their downsides and the fact that only Shimano makes them.
Meanwhile, they are inferior to Hollowtech bottom brackets when it comes to weight and bearing size. Consequently, the incentive to invest in Octalink bottom brackets is minimal.
Since the demand for such a bottom bracket is low, the market has no choice but to reflect that.
4. Limited Crank Pairing
The number of Octalink cranks is quite low compared to the options for Hollowtech bottom brackets. Thus, if you’re looking for crank variety, Octalink bottom brackets are not a logical pathway.
5. Potential Damage to The Cranks During Installation
A major downside of Octalink is that the splines of the spindle are not visible once the crank arm is inserted. This creates the opportunity for inserting the crank improperly (out of the necessary groove).
If the user tightens the bolt when the crank arm isn’t properly fit, the spindle will dig new grooves into the crank and damage it. This could lead to failed splines and a loose connection between the cranks and the bottom bracket.
6. Heavier
The table below compares the weight of Octalink and Hollowtech bottom brackets.
Weight Comparison
| Octalink | Weight | Hollowtech | Weight |
|---|---|---|---|
| Shimano Dura-Ace BB-7700 | 173g | Shimano SM-BB71-41A | 72g |
| Shimano Deore LX ES51 | 262g | Shimano XTR SM-BB93 | 73g |
| Shimano Dura Ace BB-7710 | 244g | Shimano Deore XT BB-MT800 | 82g |
| Shimano 105 BB-5500 | 250g | Shimano SM-BBR60 | 77g |
| Shimano ES51 | 262g | Shimano Saint SM-BB80 | 95g |
| Shimano Dura-Ace SM-BB92-41B | 54g | ||
| Shimano Dura Ace BB-R9100 | 65g | ||
| Shimano BB-MT500-PA | 72g | ||
| Shimano SM-BB72-41B | 69g | ||
| Shimano SM-BB94-41A | 58g | ||
| Average: | 238g | Average: | 71.7g |
Conclusion: On average, Hollowtech bottom brackets are over 3 times lighter than Octalink models.
That said, it’s necessary to specify that Octalink bottom brackets include a spindle whereas Hollowtech bottom brackets do not. In the case of external bottom brackets, the spindle is part of the drive-side crank arm. Thus, if we add the spindle to the Hollowtech weights, the numbers will increase slightly.
It’s not possible to know by how much without removing the spindle from a crankset and adding its weight to the equation. Since the spindle is press-fit into the crank arm, the only way to do that would be to destroy the crank.
But even with the added weight of the spindle, Hollowtech bottom brackets will still be many times lighter.
For a more accurate comparison, one can also compare the weights of cranks designed for each bottom bracket type.
| Octalink | Weight | Hollowtech | Weight |
|---|---|---|---|
| Shimano FC-T521 (3x) | 827g | Shimano 105 FC-R7000 | 743g |
| Shimano FC-345 (2x) | 770g | Shimano Ultegra FC-R8000 | 681g |
| Shimano Acera 9-Speed (3x) | 972g | Shimano Ultegra FC-R8100 | 711g |
| Shimano FC-R450 (2x) | 790g | Shimano GRX FC-RX810-2 | 710g |
| Shimano Alivio FC-M430 (3x) | 890g | Shimano Dura-Ace FC-R9100 | 624g |
| Shimano XT FC-M8000-2 | 718g | ||
| Shimano Dura-Ace FC-R9200 | 692g | ||
| Average: | 849.8g | Average: | 697g |
Conclusion: On average, Hollowtech cranks are lighter. However, the Octalink column contains a few triple cranksets that could be made lighter by removing the lowest gear as well as the chainring guard. But even then, they won’t be as light as the models available for Hollowtech bottom brackets.
Ultimately, the Octalink system is simply heavier than Hollowtech. That said, the extra weight is important only if the rider is trying to build the lightest possible bike and/or looking for top performance. If the machine is used for light-duty, 100-300 extra grams are not going to change much.
The Advantages of Hollowtech Bottom Brackets
1. Compatible With the Latest Cranks
Hollowtech bottom brackets are the current standard and are therefore compatible with the latest cranks on the market.
2. Compatible With Cranks From Other Brands
Octalink is an exclusive Shimano system. Consequently, Shimano produces the vast majority of Octalink cranks.
Conversely, Hollowtech bottom brackets are compatible with cranks produced from other brands too.
3. Research and Development
Unlike Octalink, Hollowtech bottom brackets aren’t an abandoned system and have a greater chance of evolving in the future.
4. Lighter and Stiffer Spindle
During pedaling, the spindle experiences a phenomenon known as torsion which represents the twisting of an object via external force.
Most of the stress that torsion generates is on the outer layer of the spindle. Consequently, adding additional material closer to the center of the spindle does not greatly increase its strength.
For the same reason, if a pipe and a rod weigh the same and are made of the same material, the pipe has a greater resistance to torsion because all of the material is part of its outer layer/wall. The rod has the same amount of material, but a lot of it is positioned closer to the center and doesn’t increase the resistance to torsion by a notable amount.
The diameter of a tube has an effect on torsion resistance too. The larger the diameter, the greater the resistance to torsion. If two tubes are made of the same material and have a matching wall thickness, the tube with a greater diameter will be stronger.
Octalink bottom brackets take advantage of those principles and come with a hollow spindle larger than a square taper one. However, since Octalinks are internal bottom brackets, the diameter of the spindle is limited.
Hollowtech is also based on the same principles but fixes all the problems that Octalink has by using bearings outside of the bottom bracket shell. The outboard bearings are larger and thus more durable. Moreover, they allow the use of a hollow spindle of a much larger diameter. The end result is a stiffer and lighter spindle without compromising bearing size.
5. Easier to Remove
Hollowtech bottom brackets and cranks are easier to remove than their Octalink and square taper rivals for the following reasons:
- The bottom bracket is less likely to get stuck.
- One of the cranks is already attached to the spindle
- A crank puller is not needed to remove the crank arms
6. Multi-tool Storing
The hollow spindles of external bottom brackets make it possible to install a multi-tool into the cranks. One example of such a multi-tool would be ALL IN MULTITOOL V2.
The benefit of this system is that the multi-tool is concealed and doesn’t require a bag.
The Downsides of Hollowtech Bottom Brackets
1. Exposed Bearings
The bearings of external bottom brackets are positioned outside of the bottom bracket shell and are therefore more exposed to the elements than those of internal bottom brackets. For that reason, the longevity of external bearings is shorter than expected for their size.
2. Wider Cranks
The larger spindle of Hollowtech bottom brackets necessitates additional crank material near the connection point. Otherwise, the crank could break. As a result, Hollowtech bottom brackets aren’t compatible with ultra-slim cranks.
People who want their bike to have a more retro look via the use of elegant cranks would do better with a square taper bottom bracket.
Hollowtech = Winner
Octalink bottom brackets can offer solid performance, but they have far too many disadvantages when compared to Hollowtech and even square taper bottom brackets.
In short, the advantages of Hollowtech bottom brackets over Octalink are:
- Lighter bottom bracket and cranks
- Lighter, stiffer spindle
- The same bottom bracket can be used on different bike types
- Compatible with a greater number of modern cranks
- Easier bottom bracket and crank installation and removal
- The system continues to be supported
