Welcome back if you haven’t forgotten about the site (due mostly to my lack of posts lately). It’s finishing out to be quite the end of the season and the holidays creep closer and closer for all those waiting to open something that has to do with a bike. College kids come home for break and families visiting even stop in starting now for small repairs and the winter project rebuilds. Even so, the thirst for knowledge of bottom brackets awaits.
I covered square tapered BB’s because of their common existence, but there are a couple other BBs that look the same except for the spindle that your crank is attached to. There are many different thoughts on what is the best “interface” for a crank. That’s the space gizmo term for how solid it feels when you pedal the bike and how much it flexes when you really press down hard. One common type from Shimano is called an Octalink BB. As you may have guessed, it has eight “splines” that are equally spaced around a circular spindle. This part is the “interface” between the crank and the BB. The threads on the BB cups (in last post) and the opposing threads on the frame is the “interface” between the BB and the frame.
Octalink BBs have two types. Sometimes, it is difficult to determine which one it is. They look quite similar. One is mostly used for road bikes while the other is used for mostly mountain bikes. Both work well and last quite a long time (I have seen ones last nearly 15000 miles).
Another style of BB is what many people refer to as “outboard” BBs. The bearings are spaced apart much wider than the previous BBs mentioned. In this case, they sit on the outside of the frame and the spindle is actually part of the crank. Good compression and a tight fit between the bearing and the spindle was found in research some time ago to be a more efficient transfer of energy and durability. The spindle would last forever essentially and the bearings could be “popped” out and replaced any time the wear started to click or creak.
Here is an example:
This type of bottom bracket is still very popular in both racing and recreational cycling. The compatibility is very high from old to new frames and can completely transform a bike into a better ride that feels more confident and powerful. All three major drivetrain manufacturers (Shimano,. SRAM, and Campagnolo) have and still use this technology.
The cups thread in just like the above cartridge style BBs mentioned earlier. FSA, Chris King, and a few other manufacturers also make compatible cups and bearings as well as adapters.
Some component makers and frame designers started to realize that the spindle design of the outboard BB was a great technology and that, if the “cups” were built into the frame, it could be lighter and even stronger than it’s predecessor. This group of bottom brackets is named acronyms like BB30, BB90,m BB86, etc. Since the frame designates which crank and bearings you can use, it still is listed as a “proprietary BB.” However, some of the manufacturers making adapters for the outboard bearing BBs used the concept to create adapters that attach to the bearings and look like cups to accept older style cranks. In my opinion, it is a very efficient technology but not as adaptable as the outboard BBs.
That is a summary of modern BBs and the pro/cons about each type. If you have questions regarding technical specs and the like, send me a message or leave a comment and I will explain. This article is intended to explain the theory behind BB development rather than what BB will fit to your bike, etc. I hope it has been helpful.
– SNC
secondnaturecycling 