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This post compares the advantages and disadvantages of square taper and Hollowtech bottom brackets.

Definitions

Bottom Bracket – a mechanism of cups and bearings allowing a spindle connected to the cranks to spin smoothly. The bottom bracket is installed in the so-called bottom bracket shell – the point where the seat tube and the downtube meet.

Square Taper Bottom Bracket – a bottom bracket operating with internal bearings and a spindle with square tapered ends. Square taper bottom brackets have been used on bicycles for decades.

Hollowtech Bottom Bracket – A Shimano bottom bracket operating with outboard (external) bearings allowing the use of a hollow spindle of a much larger diameter.

The Advantages of Square Taper Bottom Brackets

1. Budget-friendly

The price of square taper bottom brackets varies from USD 5 to USD 100+. That said, even the cheaper models offer acceptable performance, especially when the bicycle is used for light-duty (e.g., commuting), and the user isn’t after minimal weight and maximal spindle stiffness.

Moreover, square tapered cranks are among the cheapest. Thus, if you’re in the process of building a budget bicycle, square taper bottom brackets fit the bill. Hence why you see them on most entry-level machines.

2. Protected Bearings

The bearings of square taper bottom brackets are shielded from the elements on two levels:

1. The bearings are sealed inside the bottom bracket body.
2. The bottom bracket body does not protrude out of the frame and is therefore protected by the bottom bracket shell. The cranks add a layer of protection too.

For those reasons, many high-end square bottom brackets come with a multi-year warranty. For example, the SKF square taper bottom bracket is made in Germany to a very high standard and comes with a 10-year warranty.

That said, even much cheaper bottom brackets have surprising longevity. The square taper bottom bracket on my hardtail is a low-end Shimano one and survived 20,000km before needing a replacement.

3. Widely Available

Since square taper bottom brackets have been the standard for a great number of years, they’re widely available in most bike shops. People who travel all over the world and want to be able to service their bikes even in less developed areas might find this property beneficial.

4. Compatible with Slim Cranks

The spindle of square taper bottom brackets is fairly slim. Consequently, it can be coupled with narrower cranks. In different, external bottom brackets such as Hollowtech operate with larger spindles and thus more material has to be added to the crank near the connection area. Otherwise, the crank would break due to the lack of support.

People who want to use a set of retro cranks will find this feature important.

5. Serviceable Models

Square taper bottom brackets come in two kinds – sealed cartridge and cup and cone (image above).

Sealed cartridge bottom brackets are the norm today. Their bearings are sealed and thus not serviceable. When the performance of the bottom bracket isn’t satisfactory, the user is expected to replace the entire unit.

The cup and cone bottom brackets are found on older bikes from the 80s or earlier and are completely serviceable just like a cup and cone hub. Theoretically, a cup and cone bottom bracket can last forever for two reasons:

• The unit can be disassembled, cleaned, and lubricated.
• The parts that wear down are the spindle and the ball bearings. Those can be replaced with new ones for cheap.

The downside of cup and con bottom brackets is that they aren’t maximally sealed and thus contaminants can reach the ball bearings. Also, it takes a bit of time and experience to adjust the cup and cone to the point where the spindle is rotating freely without play.

The Disadvantages of Square Taper Bottom Brackets

1. More Complicated Crank Removal

Since the spindle is part of the bottom bracket, each crank arm has to be attached/detached separately. To do this, the user needs a specialized tool known as a crank puller and a wrench (if the puller doesn’t come with a handle.)

Conversely, cranks designed for Hollowtech bottom brackets are easier to install for two reasons:

• The spindle is part of the drive-side crank. Thus, the user has to attach only one crank to the spindle.
• The disassembly can be done with a single hex key.

2. Non-compatible With High-end Modern Cranks

If the user wants to stay with a square taper bottom bracket, it won’t be possible to take advantage of the latest cranks available on the market. For high-performance riders, this could be a big disadvantage.

Currently, most brands aren’t investing in the production of high-end square taper cranks. There are top units available but in much smaller quantities and variety.

Also, the price of many high-end square taper cranks is similar to that of some hollow cranks. Thus, the user is unlikely to save a lot of money unless a good deal is found on the second-hand market.

3. Worn Crank Tapers

With time the square taper of the crank arms can get rounder if the unit is removed frequently or operating without sufficient tightening. In that situation, the crank has to be replaced.

This happens because the crank arms are often made of aluminum whereas the spindle is steel. Since steel is denser/harder than aluminum, the spindle can take material from the crank if there’s movement and friction at the connection point.

FAQ: Will Square Taper Bottom Brackets Go Extinct?

Unlikely. The square taper bottom bracket has been operating well for many decades. It’s still the norm for track bikes and entry-level models. At the end of the day, square taper bottom brackets just work and can sustain a lot of abuse without complaining. Their performance is top-notch for the money.

The Advantages of Hollowtech Bottom Brackets

1. Lighter and Stiffer Spindle

The top feature of Hollowtech bottom brackets is the ability to install a spindle of a much larger diameter.

Since the bearings are outside of the frame, the diameter of the spindle isn’t nearly as restricted.

The larger diameter of the spindle allows it to be both light and strong.

Here’s how that happens:

1. The application of twisting force onto the spindle is known as torsion and is the main stress on the unit during pedaling.

2. During torsion, most of the stress is on the outer layer/wall of the spindle. Consequently, adding more material near the center of the spindle will not increase its resistance to torsion significantly.

For the same reason, hollow pipes are stronger than solid rods of the same material and weight.

Why? In the case of the pipes, all of the material goes into the wall which provides most of the resistance. In the case of the rods, the material is spread over the entire area. Thus, the rod has a thinner wall and poorer torsion resistance.

The extra diameter allows even further removal of material. The greater the diameter of a pipe, the more resistant it is to torsion.

If two pipes have the same wall thickness and are of the same material, the pipe with a larger diameter is stronger.

For the reasons above, the spindles of hollow cranks are strong and stiff while also being as light as possible.

The additional stiffness is considered beneficial because it results in greater power transfer.

2. Lighter

Hollowtech bottom brackets are much lighter than square taper units. The weight of the two is compared in detail below.

Weight Comparison

Hollowtech and other external bottom brackets are a lot lighter than square taper bottom brackets if we compare only the bare units.

The table below contains the weight of some Hollowtech bottom brackets and that of many square taper models regardless of brand.

Conclusion: When we compare the bare units, Hollowtech models are about 3.4 times lighter than square taper bottom brackets.

However, it’s necessary to take into account that square taper bottom brackets come with a spindle whereas in the case of Hollowtech models the spindle is part of the cranks.

This makes the comparison a bit uneven. But since the spindle of cranks designed for hollow bottom brackets is press-fit into the drive-side crank, it’s not possible to isolate its weight without breaking the crank.

To make the comparison more complete, it’s necessary to compare the weight of cranksets designed for each bottom bracket type.

The table below contains the weight of 2x cranks. In the first column, the cranks are square tapered. In the second, the cranks are designed for a Hollowtech bottom bracket.

* The cranks with an asterisk are high-end Shimano cranks from the 80s. The reason for using them is the limited amount of high-end square taper cranks produced today.

Comparing the weight of a Shimano Dura-Ace (692g) hollow crank to that of Shimano Tourney FC-A070 (1370g) square taper cranks, for example, will present a massive difference, but that difference comes from the fact that Dura-Ace components are high-end whereas Tourney is entry-level.

The crank class rather than the bottom bracket mounting system is the main reason for the massive discrepancy in weight.

Conclusion: Modern cranks designed for Hollowtech bottom brackets are lighter than the entry-level square tapers available on the market. That said, it’s still possible to find very light square taper cranks by going for less mainstream brands or by purchasing retro crank models.

In that case, the weight difference between the two bottom bracket types becomes smaller.

Example:

Dia-Compe ENE Ciclo cranks weigh 626g. If you couple them with Shimano BB-UN300 (267g), you get a total weight of 893g.

Shimano 105 FC-R7000 cranks weigh 743g. If you couple them with Shimano XTR SM-BB93 (73g), you get a total weight of 816g.

The difference between the two combinations is only 77 grams and is therefore irrelevant to most people.

That said, Hollowtech bottom brackets and cranks make it easier to acquire a light setup. Hence why people after top performance simply go for the new technology.

3. Compatible with the Latest Cranks

Hollowtech bottom brackets are compatible with the latest high-end cranks. If you want your bicycle to be up to date, a Hollowtech bottom bracket fits the bill.

4. Easier Crank Removal/Installation

As mentioned above, hollow cranks are easier to mount because you don’t need a crank puller. Also, only one side has to be mounted to the spindle.

5. Compatible with Multi-tools

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 Disadvantages of Hollowtech Bottom Brackets

1. New Tools Needed

To install and pre-load a Hollowtech bottom bracket, you will need a new tool such as Park Tool’s BBT-9. Some people may consider the extra expense a negative.

2. More Expensive Crank + BB Combo

If you’re on a tight budget, nothing beats a square taper bottom bracket and cranks. Hollowtech bottom brackets are not expensive, but when you add the cranks, the price may be too much for some situations.

3. Exposed Bearings

The bearings of external bottom brackets are outside of the frame and are therefore not nearly as well protected from the elements as those of square taper bottom brackets.

4. Brand-specific

In general, hollow bottom brackets are brand-specific. For example, you can’t couple a Shimano Hollowtech bottom bracket with a SRAM crank because the bottom bracket is designed for a 24mm spindle.

Meanwhile, SRAM GXP spindles are 22mm on the non-drive side and 24mm on the drive side. To make the set-up work, one will need an adapter to fill the 22mm side.

In different, most square taper cranks and bottom brackets are compatible with each other. There’s one major exception, however – Campagnolo.

The two types of square tapers are known as ISO (Campagnolo) and JIS (Shimano, SRAM…etc.) The JIS system is wider than the ISO. As a result, an ISO crank won’t go far enough on the spindle of a JIS square taper bottom bracket.

5. Questionable Benefits to Recreational Riders

The main pros of Hollowtech is the lighter system plus the extra stiffness of the spindle.

Those qualities are beneficial to riders who want top performance, but if a bike is used for basic tasks (commuting to the office), the gains are not nearly as important.

FAQ: Is a switch from a square taper bottom bracket to a Hollowtech an upgrade?

It depends. If the bike is already using fairly light cranks, then switching to a Hollowtech system will not be seen as a fundamental upgrade.

However, if the user is relying on low-end square taper cranks, switching to modern Hollowtech cranks and bottom bracket will represent a major change.

That said, switching from a well-operating square taper bottom bracket to a Hollowtech one simply for the sake of it is not needed. A more strategic plan would be to wait for the bottom bracket or the cranks to complain before making the change.