Originally Posted by
3alarmer
...could you explain that for me in a little more detail, please ? My experience (mostly from the bikes donated to the local co-op), and everything I've read about them, indicates that external bearings are actually les durable (on average). It's usually attributed to the fact that they are out there, hanging on the edge of the BB shell, and more exposed to the elements and the bad things that can happen in that more exposed position.
I guess I'm uncertain what you mean by "external bearings".
We are in agreement about what constitutes external bearings. Let me find my notes, this has been brought up enough that I copied it. Your concern is mentioned below:
Some useful info I posted on a different thread; Advantages and disadavantages of internal cartridge BB, versus external bearings with hollowtech II style crankset:
"Internal cartridge" bottom bracket, design been around many decades. It replaced "cup and cone" BBs which had loose or caged balls, solid axle (usually square taper ends), and bearing cups (outer races that threaded into the BB shell).
Advantages:
+ Huge labor savings for service; no need to remove and clean bearings, repack with grease, reassemble. The cartridge just screws in and you're done.
+ For mountain biking, some have said the internal cartridge bearings seemed to be more durable for them. I am guessing because those seals, while still plenty exposed to dirt and splashing, are a good distance from the crank arms, whereas with external bearings, the crank is right against them, so any dirt trapped between might grind at the seals.
Disadvantages:
- Once the cartridge starts to "loosen up" from wear, there is no way to adjust it out. It'll still be a while until the BB fails, but the axle will be looser, which stretches the seals (letting in rain if applicable), accelerates wear on the bearings (because a loose bearing only loads 2 or 3 balls at a time, whereas a "snug" bearing loads about half, 180 degrees of the bearing balls under load), and makes the chainrings wiggle.
- Cost; More expensive in parts than cup and cone bearings which can be rebuilt for ten cents in grease, two dollars for new bearing balls if needed. For this reason, cup and cone bearings still dominate low-cost bikes, and most especially bikes in the third world.
The other style BB bearings that you may have seen are "external", 2 parts, each a bearing that mounts between the BB shell end faces and the crank on each side.
Advantages:
+ Allows a larger diameter, hollow BB axle, which is stronger, stiffer, and lighter.
+ Like internal cartridge, quick to replace.
+ Because external, bearing balls are larger and/or more of them, making the bearings more durable.
+ Bearings are closer to each crank arm, reducing bending moment on axle and loads on bearings, making the bearings more durable.
+ Unlike internal cartridge, ***slack over time can be adjusted out, to back like new***, you just loosen the left crank arm, adjust preload, replace crankarm. Restoring the preload reduces load on individual bearing balls. This makes the bearings more durable. (Are you sensing a theme?)
+ Backwardly compatible! Can fit in BB shells whose design dates back 100 years.
+ Can remove the crank (even in the field) by only loosening two allen-head clamp screws on the left arm, I don't need a "crank tool".
Disadvantages:
- Requires replacement of the entire crankset for (one example) "Hollowtech II" style; This has hollow (pipe) BB axle permanently attached to the right crank arm, it slides through both bearings, the left arm attaches with a clamp around splines on the end, and there is a cap on the end to adjust bearing preload.
- Requires a special wrench for the outside splines on the bearings, which vary greatly, but the most common standard is "ISO External"; I bought a 4-way wrench that has that pattern, as well as 3 others, in case I need in future.
The good news is, Hollowtech II style is now available generically, so a whole double chainring crank with chainrings, and ISO-External bearings, cost me USD$65, cheaper than just the chainrings would cost. I ordered a 2nd one for spare parts, cheaper.
For any of the above, be careful:
- The cup or bearing on the left (non-drive-side) of the bike is RIGHT HAND THREAD (normal; righty-tighty, lefty-loosey).
- The cup or bearing on the right (drive-side) of the bike is LEFT HAND THREAD, left rotation (anti-clockwise) tightens, right rotation (clockwise) loosens.
This helps keep the bearings tight. This assumes you have a BSA*/English BB shell standard, the most common.
There are more rare BB designs (such as "Italian") where the right cup is right hand thread, and they have a habit of coming loose.
* Birmingham Small Arms (yes, that Birmingham, UK); Guns, bicycles, motorcycles, etc.