Bicycle Mechanics - In need of a primer on modern bottom brackets

I've done quite a bit of work on bicycles, but haven't had any experience with more sophisticated bottom brackets after a Shimano UN-54 I installed several years ago. That BB, and everything I worked on before it, had lots of size options and measurement/calculation was important in getting an appropriate chainline. It also was easier because square taper was most of what was available at my price point.

Now, I'm just about to start a new build on a Devinci Tosca SL 2 cyclocross frame. I haven't gotten it yet, but I suspect it's a standard BB shell based on the specs (http://www.devinci.com/bikes/browse_6#6_22_86), which says the complete bike comes with a Hollowtech II BB. 130mm rear spacing also, if that matters.

So, here are my questions:
1) What are some of the more important differences between the various BB designs out there?
2) Is square taper really that inferior to "outboard" designs? I've done a ton of riding on my 'old fashioned' bottom brackets, without any problems.
3) How can the user decide what sort of chainline to run with many BBs I've looked at which don't come in different lengths?? For example... (http://www.jensonusa.com/Hope-6873mm-Bottom-Bracket)

What all this boils down to is product selection for this build. I'm planning on running a 1x9 setup, not doing any serious competition, just for fun. I'm not picky about high performance, and value (note, 'value' to me does not necessarily mean cheap) is more important that a few grams of weight or bit of extra drag. Recommendations?

chainline is fixed for the most part on modern cranks. you have traditional threaded and press fit types. a few different press fit types that revolve around a 24mm spindle or 30mm spindle

chainline is fixed for the most part on modern cranks. you have traditional threaded and press fit types. a few different press fit types that revolve around a 24mm spindle or 30mm spindle

How is that an upgrade--inflexibility in allowing a user to select their chainline?? That just seems foolish to me.

How is that an upgrade--inflexibility in allowing a user to select their chainline?? That just seems foolish to me.
It's all new and improved marketing hype. Each manufacturer is trying to make theirs "stronger, stiffer, lighter" and all of the other catchphrases that sell the stuff. There was nothing wrong with the old square taper system.

It's all new and improved marketing hype. Each manufacturer is trying to make theirs "stronger, stiffer, lighter" and all of the other catchphrases that sell the stuff. There was nothing wrong with the old square taper system.

That sets my mind at ease. Thanks!

In need of a primer on modern bottom brackets



I just use rustoleum

The new external bearing cranks with their built in large diameter hollow spindle are indeed lighter and stiffer than their square taper predecessors. Is that important? Not to me.

However, they do take the ambiguity out of bb selection because if you use the crank as designed (road cranks on road frames and MTB cranks on MTB frames) the chainline is automatically correct. Do a search here for bottom bracket topics and you will find dozens of "how do I choose the right bb spindle length for my XX crank?" questions. Add in the ISO/JIS semi-mismatch and symetrical vs. unsymetrical spindles and square taper bb's weren't that simple either.

What is really confusing now is the myriad of new bottom bracket and frame "standards" with several variations on the press fit, thread in, and who knows what's next design.

What is really confusing now is the myriad of new bottom bracket and frame "standards" with several variations on the press fit, thread in, and who knows what's next design.

Agreed. No question that the newer stuff is lighter and stiffer for the most part, but I've recently counted eight different bottom bracket "standards" in our shop, and I probably missed something.

The word is that Shimano will introduce cranks with 30mm spindles soon, and that will throw a new twist into the oversized press-fit bb market as well. Just trying to keep up with it all is a challenge.

IMO the advantage in the new BB style is for the manufacturer- not the end user. There is the marketing hype, but also manufacturing costs are less than the "old stuff." For the end user the advantage would be less weight, and lower costs for aftermarket or replacement parts. Ain't nothing wrong with the old square taper, but it's getting expensive, somewhat due to scarcity. Price a Phil bottom bracket and a set of quality cranks. Adds up quick. If weight isn't an issue you can do a UN54, and choose from the huge supply of used square taper cranksets out there. Makes the newer stuff look pretty attractive though.

I would argue that the cost of a square taper JIS BB has never been cheaper. I can buy today, for $18, a Shimano UN55 which is like the UN54 only with metal cups on both sides and not plastic on one. Sealed cartridge bearings. Pretty darn smooth, and square taper. I can also buy a Vuelta BB in square JIS taper in a variety of widths. Sealed cartridge bearings, steel cups. $14. And I just bought a new Vuelta crank, compact crank set, 50-34 for just $30 and it's square taper. So it's not getting that expensive. It's not super high end, but it works great and it's compatible with Shimano. I picked up an SR cheap XCT v2 crank for around the same price, $30 and it supports a square taper.

The current issue of Bicycle Times magazine has a pretty comprehensive article on the features and compatibilities of the current configurations of bottom brackets.

I would argue that the cost of a square taper JIS BB has never been cheaper. I can buy today, for $18, a Shimano UN55 which is like the UN54 only with metal cups on both sides and not plastic on one. Sealed cartridge bearings. Pretty darn smooth, and square taper. I can also buy a Vuelta BB in square JIS taper in a variety of widths. Sealed cartridge bearings, steel cups. $14. And I just bought a new Vuelta crank, compact crank set, 50-34 for just $30 and it's square taper. So it's not getting that expensive. It's not super high end, but it works great and it's compatible with Shimano. I picked up an SR cheap XCT v2 crank for around the same price, $30 and it supports a square taper.

No argument here- but there are cases, and if weight is no object I'll choose square taper any day. It's really economical. But in the low weight class square taper BBs and cranks are expensive- more than OB bearing crank.

No argument here- but there are cases, and if weight is no object I'll choose square taper any day. It's really economical. But in the low weight class square taper BBs and cranks are expensive- more than OB bearing crank.

Well, I ended up getting a hollowtech II 105 BB for $20 and a tiagra crankset for $76. Not too bad. Though I'm reading the install details and getting a bit queasy. Do BB shells really have to get machined often???

BB shell "facing", IF needed at all, is only needed once. Basically, it's to make sure the BB shell is 'square', not lopsided on the exterior contact points with the external BB. The more expensive the frame, the more likely it was factory faced.

Most often, the "facing" only clears sloppy paint from the frame. Did that myself once with a file, had no issues.

ONE of my last three frames could have used the facing operation, and only for the external-BB crankset. I've been using an ISIS crankset for two years now, and IMO, it's better than anything external. ISIS and Octalink is where they should have stopped.

Here's a really great write up on the different BBs;

http://blog.treefortbikes.com/index.php/posts/1473