from Rookie’s keyboard,
Hello, friends
Good news. Another Bike Theory Lesson (BTL) is here.
Today, I will focus on bottom brackets based on the position of their bearings. There are two types – external and internal.
Internal Bottom Bracket (IBB) – a bottom bracket with bearings contained within the bottom bracket shell.
External Bottom Bracket (EBB) – a bottom bracket with bearings protruding outside of the bottom bracket shell.
The Pros of Internal BBs
Bearing Protection (that’s big)
The bearings of internal BBs are protected by the frame and the BB’s body. As a result, water and dust have a harder time entering the mechanism.
For that reason, the bearings of some internal BBs last ages.
Cheap (for people like me who work at warehouses)
Internal BBs such as the common square taper model are cheaper and thus often found on entry-level bicycles. If you’re looking to build a bike on a budget, an internal BB compliments the mission.
There are also high-end square taper bottom brackets with a long warranty that tend to cost a lot more. The average internal bottom bracket, however, isn’t very expensive.
Serviceable models (my favorite)
There are two main types of IBBs – 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 Cons of Internal BBs
More Difficult Crank Removal
The spindle of internal BBs is part of the bottom bracket itself. Therefore, both crank arms have to be attached to the respective side of the spindle separately.
This makes the installation and removal of crank arms more difficult and time-consuming.
If the crank isn’t “self-extracting”, it will also be necessary to purchase a crank puller and use it to remove the cranks.
In the case of external BBs, the spindle is press-fitted into the drive-side crank permanently. The spindle slides through the bottom bracket. Then, the non-drive side crank is secured to the spindle via a single bolt.
Thus, to remove or install a crankset designed for an external BB, you need to untighten or tighten one bolt on the non-drive side.
You may also need a mallet to tap the drive side crank out. The procedure is a lot simpler and doesn’t necessitate the use of a crank extractor. A bike multi-tool will suffice.
Non-compatible with Modern High-end Cranks
Currently, most high-end cranks are designed for external bottom brackets. If you want to equip your bicycle with a unit from that class, you have no choice but to get a matching bottom bracket.
The Pros of External BBs
Torsional Stiffness (Light and Strong Spindle)
The bearings of external BBs are positioned outside of the frame. Since their size isn’t restricted by the bottom bracket shell, they can have a more voluminous structure. The larger bearings allow the bottom bracket to be used with a spindle of a greater diameter.
The larger spindle diameter results in extra strength but also lightness.
How?
The spindle of a bottom bracket experiences a phenomenon known as torsion – the twisting of an object via the application of an outside force.
During torsion, the outer layer of a tube or a shaft experiences the most stress. The closer you get to the center, the less stress there is.
If a tube and a solid rod are of the same material and mass, the tube is more resistant to twisting because it has a thicker outer layer. The rod has the same amount of material but a lot of it is positioned closer to the center and doesn’t help much during torsion.
The diameter of a tube affects the resistance to torsion too. The larger the diameter, the more torsion stress a tube can withstand.
If two tubes are of the same wall thickness and material, the one with the larger diameter is stronger. In order for the tube with a smaller diameter to match the strength of the other tube, it will be necessary to add more material to the wall and consequently increase the tube’s weight too.
Thanks to the outlined principles, spindles of larger diameters can surpass the strength and stiffness of thicker spindles of smaller diameters while weighing less.
Or in simpler terms, hollow spindles of larger diameter are both lighter and stronger. And since the bike industry is obsessed with acquiring maximum strength for minimum weight, hollow spindles and consequently external bottom brackets are seen on higher-end bikes.
Larger Bearings = Longevity
Bigger bearings tend to last longer. Thus, the outboard bearings of external BBs have the potential to remain problem-free for a long time.
However, the fact that the bearings are exposed (outside of the frame) makes them more vulnerable to humidity and contaminations. Hence why in practice, the bearings may have a shorter lifespan than expected.
Increased Spindle Support
The larger outboard bearings provide more support for the spindle than the smaller bearings found in internal bottom brackets.
Potential Weight Savings
It is accepted that external bottom brackets save weight. This is true in some cases but not always.
The bottom bracket itself weighs less than an internal one, but we have to take into account that internal BBs come with a spindle too. Meanwhile, the spindle of external bottom brackets is part of the cranks.
Whether weight is saved by using an external bottom bracket depends on the crankset too. Thus, the two types of bottom brackets cannot be compared in isolation.
It’s also worth mentioning that the larger spindle of external BBs necessitates the use of a wider crank arm.
If a slim crank is used with a large spindle, the crank may break. The extra material makes the crank heavier.
The Cons of External BBs
Expensive
External BBs tend to be more expensive than internal ones. Since BBs are not terribly expensive as a component, the difference may seem slim, but if presented as a percentage it can be well over 100%.
That said, the true price boost comes from the cranks. (External bottom brackets cannot be coupled with the cheapest cranks.) Thus, if you’re looking to save money, and you don’t already have cranks that would fit an external bottom bracket, nothing beats a classic square taper combo.
Press-fit Bottom Brackets Suck
Many external bottom brackets are press-fitted into the frame. The downsides of press-fit bottom brackets are:
- New specialized tools are needed for the installation/removal.
- The bottom bracket can start rotating inside the bottom bracket shell and damage the frame. This is especially true for carbon frames.
- If the press-fit bottom bracket disintegrates, and the bike continues to be used, the chance for frame damage becomes exponential.
The main benefit of press-fit bottom brackets is that they remove the need to thread the bottom bracket shell and the bottom bracket itself. Thus, manufacturing corners are cut. But from the rider’s perspective, threaded bottom brackets use a superior retention mechanism.
Exposed Bearings
The bearings of external bottom brackets aren’t nearly as protected from the elements. As a result, it’s not uncommon for outboard bearings to disintegrate faster despite being larger than inboard ones.
There are situations when a basic square taper bottom bracket outlives a more expensive external model by 1000s of miles.
Brand-specific
Many external bottom brackets are brand specific and can be coupled only with cranks of the same brand. For example, it’s not possible to use Shimano cranks with a SRAM external bottom bracket by default.
Meanwhile, square taper bottom brackets are compatible with square taper cranks regardless of both units’ brands. This makes internal bottom brackets a lot more versatile.
Stiffess, really?
Truth be told, people who use their bikes for light-duty (e.g., commuting to the store) will have a hard time telling the difference between a cheap square taper bottom bracket and an expensive external bottom bracket when it comes to cranks/spindle stiffness.
That said, cyclists who demand top performance from their machines claim that there is a perceivable difference.
In order to truly know whether an external bottom bracket offers perceivable stiffness, it will be necessary to do a blind test. The participants will have to use a bike without knowing whether it has an internal or an external bottom bracket and then report their findings.
The test has to be done with the same frame. If a different frame is used for each test, the perceived flex might be coming from the frame itself.
Ultimately, however, most recreational riders won’t feel much of a difference when going between the two bottom bracket types.
TL: DR;
| INTERNAL BOTTOM BRACKETS | EXTERNAL BOTTOM BRACKETS | |||
|---|---|---|---|---|
| Benefits | Disadvantages | Benefits | Disadvantages | |
| Affordable package | Not as stiff | Maximal stiffness | Pricy when accounting for the compatible cranks | |
| Widely available | Harder to install and remove | Large bearings | Exposed bearings | |
| Bearings protected from the elements | Longer and more difficult crank installation | Compatible with high-end cranks | Often non-compatible with cranks made by a different brand | |
| Serviceable models | A potentially heavier bb + cranks combination | Potential weight savings | Cranks with a wider profile | |
| Compatible with slim cranks | Not compatible with high-end modern cranks | Problematic Press-fit bottom brackets | ||
| Greater compatibility between different brands | Smaller bearings |
Until next time,
Rookie
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